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Spinal Cord Anatomy
Spinal Cord Anatomy K Chandar, University Hospitals Case Medical Center, Cleveland, OH, USA; and Case Western Reserve University, Cleveland, OH, USA BK Freeman, Case Western Reserve University School of Medicine, Cleveland, OH, USA r 2014 Elsevier Inc. All rights reserved. This article is a revision of the previous edition article by R E Burke, volume 4, pp 339–348, r 2003, Elsevier Inc.
Introduction
their synaptic interconnections, called the neuropil. However, unlike the brain, the white matter of the spinal cord is on its exterior, completely surrounding the gray matter. This ‘inside-out’ construction is presumably related to the two major functional roles played by the spinal cord: to convey bidirectional information to and from the rostral brain and to provide the neuronal machinery for semiautonomous functions such as reflexes and other basic patterns of action (i.e., reflexes and certain rhythmic movements) that can respond rapidly and efficiently to changes in external conditions. The ascending and descending axonal tracts expand in bulk from caudal to rostral along the cord, as descending tracts
The spinal cord (Figure 1(a)) is the oldest part of the vertebrate central nervous system (CNS). Its basic organization, dictated by the vertebrate body plan, has changed relatively little during evolution in comparison to the dramatic alterations found in supraspinal structures. Indeed, the structure of the spinal cord is fundamentally different from the rest of the CNS. Throughout the CNS, the regions that are composed of large bundles of myelinated and unmyelinated axons, called the white matter from its gross appearance, are segregated from the gray matter regions that contain collections of tightly packed neurons and C1
Representative segments
T1
Cervical enlargement
Cervical enlargement
Brachial plexus
Lumbosacral enlargement
Dorsal horn L1
Cauda equina
Mid-thoracic
Lateral horn Lumbar plexus
Ventral horn Lumbosacral enlargement
S1 Sacral plexus
(a)
(b)
Ventromedian fissure
Figure 1 (a) Lateral view of the human spinal cord within the vertebral column. The long nerve roots that exit between the vertebrae below the L1 level form the cauda equina (literally the ‘horse’s tail’). The nerve roots that innervate the upper and lower limbs form the complex brachial, lumbar, and sacral plexuses that eventually ‘sort out’ into peripheral nerves. (b) Cross-section diagrams of the cervical, thoracic, and lumbosacral portions of the cord showing the relative sizes of white and gray matter at these levels.
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terminate and ascending tracts are added (Figure 1(b)). In the spinal cord, the gray matter lies interiorly and is surrounded by the white matter. This arrangement is in contrast to that in the cerebrum and cerebellum, where the gray matter lies mostly on the outside and the white matter is located mostly internally. In the brainstem, gray matter and white matter are intermixed.
Gross Anatomy The spinal cord in most vertebrates is a long cylinder of tissue, approximately oval in cross-section that is continuous with the caudal medulla at the foramen magnum. The spinal cord lies within the vertebral canal formed by the articulated bony vertebral arches dorsal to the vertebral bodies proper. The spinal cord is surrounded by three protective membranes; from inside outward, these are the pia mater, the arachnoid mater, and the dura mater. Rostrally, these are continuous with those membranes surrounding the brain. Although the pia mater closely invests the surface of the spinal cord, there exists a space between it and the arachnoid mater, called the subarachnoid space. The subarachnoid space is continuous with the space of the same name in the cranial cavity; it is filled with cerebrospinal fluid (CSF) and contains the arteries and veins that supply the spinal cord. The spinal cord is no wider than 12 mm, being widest at the levels of the cervical and lumbosacral enlargements, and 42–45 cm in length. Just before the spinal cord terminates at the lower border of the body of the L1 vertebra, it tapers into the conus medullaris; the conus contains most of the sacral and the single coccygeal spinal cord segments (Figure 1(a)). The pia mater that intimately invests the spinal cord continues caudally in the vertebral canal as a ‘ligamentous’ or ‘threadlike’ structure, the filum terminale, which attaches caudally to the coccyx. (One can think of the filum terminale as an ‘anchor’ for the caudal end of the spinal cord.) In contrast to the pia mater, both the arachnoid mater and the dura mater continue caudally in the vertebral canal and do not terminate until the level of the second sacral vertebra; here, these layers ‘insert’ into the filum terminale. The absence of the spinal cord below the level of the L1 vertebra and the continuation of the subarachnoid space to the level of the S2 vertebra result in a relatively large space, known as the lumbar cistern, containing the CSF. The spinal cord is divided along its length into multiple functional segments, each related to the sensory and motor innervation of a particular and distinct portion of the body. There are 31 such segments in the human spinal cord. The segmentation of the cord is not obvious externally; however, 31 pairs of spinal nerves can be observed to enter/exit the spinal cord along its length with considerable regularity. Each spinal nerve is formed by the union of a dorsal root (containing the axons of sensory neurons) and a ventral root (containing the axons of motor neurons); these roots unite just before the spinal nerve exits the vertebral canal via its designated intervertebral foramen. The fact that the spinal cord is shorter than the vertebral canal and the fact that each spinal nerve must exit the vertebral canal at its appropriate intervertebral foramen results in a collection of dorsal and
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Dorsal roots
Dorsal root ganglion
Ventral roots Figure 2 Diagram showing the dorsal and ventral rootlets, respectively, entering and emerging from the spinal cord. Central processes of the axons of sensory neurons enter the spinal cord, the peripheral processes of axons of sensory neurons enter the peripheral nerves and distribute to their receptors, and cell bodies of sensory neurons are located in the dorsal root ganglion.
ventral spinal nerve roots, the cauda equina, in the vertebral canal below the level of termination of the spinal cord (Figure 1(a)). The spinal cord also has a patent central canal that is a continuation of the ventricular system of the brain. The fatty tissue outside the dura mater provides additional mechanical padding along the length of the spinal cord. The rootlets of the dorsal roots of the spinal nerves enter the cord at the dorsolateral sulcus (Figure 2) and carry the incoming (afferent) central projections of the sensory nerve cells that are located in the dorsal root ganglia. Each sensory ganglion, called the dorsal root ganglion, is located in or very close to the intervertebral foramen, just proximal to the union of the dorsal and ventral roots to form the spinal nerve. Each spinal segment has a corresponding sequence of ventral rootlets that emerge from the ventrolateral surface of the cord and unite to form a ventral root. These carry the outgoing (efferent) axons of somatic motoneurons and autonomic preganglionic neurons whose cell bodies are located in specific parts of the ventral and lateral areas of the gray matter, respectively. In animals with appendages, the cervical and lumbar segments that supply innervation to the limbs are larger in cross-sectional area than those in the intervening regions that serve the trunk (Figure 1(b)). Each spinal segment receives topographically organized sensory information from the skin and deep tissues in strip-like regions called dermatomes around the trunk and along the limbs. Although there is some overlap between dermatomes, they are clinically important in locating the level of damage in cases of spinal cord injury. Their existence is all too obvious to patients with painful herpes zoster eruptions, which can affect a single dermatome. The number of spinal cord segments depends on the number of vertebrae in a given species. The primitive lamprey cord has approximately 100 segments, whereas the human spinal cord has a total of 31 segments: 8 cervical (C), 12 thoracic (T), 5 lumbar (L), 5 sacral (S), and 1 coccygeal. The positions of the cervical segments correspond to those of the associated vertebral bodies, whereas the more caudal segments do not (Figure 1(a)). The more caudal spinal cord segments lie at the level of the last thoracic and first lumbar vertebral bodies (Figure 1(a)). The segments that innervate the upper
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extremity (C5–T1, often referred to as the cervical enlargement) and the lower extremity (L2–S2, the lumbosacral enlargement) are expanded in bulk compared with the intervening thoracic region in order to accommodate the additional gray matter required to innervate these appendages (Figure 1(b)).
the anterior spinal artery is significantly larger than the others. It often arises from a left lower posterior intercostal artery at or close to the T10 cord segment and is called the Artery of Adamkiewicz, or the Great Anterior Medullary Artery of Adamkiewicz.
Internal Organization
Blood Supply of the Spinal Cord The spinal cord receives its blood supply from two pairs of arteries originating from the vertebral arteries, soon after they enter the intracranial cavity through the foramen magnum. A pair of anterior spinal arteries arises from the vertebral arteries just above the foramen magnum; they turn caudally and fuse almost immediately, and travel the length of the spinal cord as the single anterior spinal artery in the ventromedian sulcus. The posterior spinal arteries arise just above the origin of the anterior spinal arteries; they remain singular and travel the length of the spinal cord in the socalled dorsolateral sulci, just medial to the entry zones of the dorsal rootlets of the spinal nerves. The anterior spinal artery supplies approximately the anterior two-thirds of the spinal cord, with the exception of the dorsal columns and dorsal horns, which are supplied by the posterior spinal arteries. The anterior and posterior spinal arteries are relatively small vessels and, beginning at lower cervical levels, they are supplemented along their length by a variable number of segmental arteries (also called medullary arteries) that arise from the spinal branches of the vertebral, deep cervical, intercostal, and lumbar arteries. These segmental spinal arteries nourish the dorsal and ventral roots as well as the cord itself. One of the segmental spinal arteries that contribute to
White Matter Except for the small region in which dorsal roots enter the cord (Figure 2), the gray matter of the spinal cord is enveloped by white matter that can be divided into specific regions called funiculi, composed of particular ascending and descending groups of axons, called tracts (Figure 3). For historical and technical reasons, most of the descending white matter tracts in the spinal cord are named according to the supraspinal nuclei or regions in which they originate. On the contrary, the ascending white matter tracts that originate within the spinal gray matter are also named according to their supraspinal destinations because their origins within the spinal gray matter are difficult to identify.
Major descending fiber tracts The locations of the major tracts that descend into the spinal cord are shown on the right side of Figure 3. The corticospinal tract (CST) arises in the cerebral cortex. Other descending, or ‘motor’, pathways arise from brainstem locations – the red nucleus, the tectum, the vestibular nuclei, and the reticular formation. These descending pathways are also known as ‘upper motor neurons.’ Although they do not innervate the
Fasciculus cuneatus Fasciculus gracilis Lissauer’s tract Dorsal spinocerebellar tract
Fasciculus proprius Lateral corticospinal tract Rubrospinal tract
Fasciculus proprius
Ventral spinocerebellar tract
Anterolateral system
Lateral reticulospinal tract
Ventral corticoLateral vestibulospinal tract spinal tract Medial vestibulospinal Medial reticuloand tectospinal tracts spinal tract
Figure 3 Diagram of a spinal cord section at C4 illustrating positions of major ascending (labeled on left) and descending (labeled on right) tracts.
Spinal Cord Anatomy
skeletal muscle, they modulate the activity of those motoneurons that innervate the skeletal muscle (alpha and gamma motoneurons) that are usually called ‘lower motor neurons.’ Upper motor neurons chiefly terminate upon local circuit neurons (LCNs) and, these, in turn, synapse with the alpha and/or gamma motoneurons. The CST originates in neurons that are widely scattered in the primary sensory and motor cortices, including the premotor areas in primates. Although the course of CST axons differs in some species, in most mammals approximately 85% of the fibers cross in the caudal medulla and enter the contralateral dorsolateral fasciculus; this bundle is called the lateral CST (LCST). These axons project to LCNs in laminae V and VI (Figure 5). In primates and humans, CST terminal branches also enter lamina IX, where they synapse directly (monosynaptically) onto the alpha motor neurons, especially those that innervate the flexor muscles of the fingers and thumb, that is, the musculature concerned with fractionated fine movements. Interestingly, such direct corticomotoneuronal connections also exist in the cervical segments in raccoons, which exhibit considerable forepaw dexterity. Approximately 15% of corticospinal fibers remain uncrossed in humans, and these travel in the ipsilateral ventromedial fasciculus (the ventral CST; Figure 3, right). These fibers eventually do cross; they regulate, via LCNs, axial and proximal limb musculature (in some cases, bilaterally). The myelinated axons that make up the CST have a wide range of sizes and conduction velocities, with most being less than 2 mm in diameter. The large-diameter fibers originate in large pyramidal neurons (Betz cells) in the primary and secondary motor areas and have fast conduction velocities. These are the CST fibers that directly excite specific alpha motor neurons innervating the distal flexor musculature. The majority of CST axons with smaller diameters appear to project mainly to spinal LCNs concerned with sensory–motor reflexes. The major function of the CST is to control a wide variety of voluntary motor actions. Interruption of the CST in primates produces serious and permanent defects in hand and finger dexterity. Rarely, patients with strokes that affect the CST may recover near normal strength in the upper extremity but may be left with marked difficulty with fine movements of the digits (threading a needle, buttoning and unbuttoning, manipulating table utensils, etc.). The rubrospinal tract (RST) (Figure 3, right side) originates in somatotopically organized cell groups within the red nucleus in the midbrain; it crosses immediately to descend into the dorsolateral funiculus of the spinal cord, where it lies just ventral to the LCST and reaches only up to the cervical and upper thoracic levels of the spinal cord. Most RST axons are large-diameter myelinated fibers that conduct rapidly. They project mainly to LCNs in laminae V and VI (Figure 5), although in primates they also make monosynaptic contact with some motoneurons in lamina IX. The function of the RST is largely exerted indirectly through its influence on LCNs projecting onto the alpha motor neurons of flexor muscles of the upper extremity. Although the RST is overshadowed by the great development of the CST in higher primates and humans, it remains an important motor pathway in humans. The function of the RST becomes obvious in patients with lesions
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of the CST above the level of the red nucleus in whom the upper extremity assumes a posture of generalized flexion, due to the unopposed action of the RST, whereas the lower extremity assumes a position of generalized extension (unopposed countervailing influence of the lateral vestibulospinal tract (LVST)). The tectospinal tract (TST; Figure 3, right side) originates in the superior colliculus. Its axons cross the midline and are found close to the midline in the ventral white matter of the cord, terminating on LCNs in the cervical and upper thoracic segments. These LCNs synapse with the alpha motor neurons that innervate the musculature concerned with orienting the eyes and head to visual, auditory, and somatic sensory stimuli. There are two reticulospinal tracts (Figure 3, right side). The medial reticulospinal tract (MRST) is derived from the pontine reticular formation and descends close to the midline in the ventral white funiculus of the spinal cord. It projects to LCNs in laminae V and VI, and occasionally, lamina VII, influencing those alpha motor neurons that innervate the axial and proximal muscles concerned with maintaining balance and posture. The MRST is activated in response to any anticipated disturbance in balance or posture. (Its function is opposite to that of the LVST, which is activated in response to any disturbance in posture, as in tripping.) The lateral reticulospinal tract (LRST) is derived from the medullary reticular formation. It descends in the ventral white funiculus just lateral to the MRST. The LRST also projects to LCNs in the spinal cord, and activation of these particular LCNs results in the generalized hypotonia that occurs during rapid eye movement sleep. There are two vestibulospinal tracts (VSTs) (lateral and medial) (Figure 3, right side) that originate from the vestibular nuclei. The vestibular nuclear complex is situated at the pontomedullary junction and the nuclei receive proprioceptive information via the vestibular nerve (of cranial nerve VIII) from the cristae ampullares of the semicircular canals and the otolith organs in the inner ear. The LVST is derived from the lateral vestibular nucleus and descends ipsilaterally through all levels of the spinal cord in the ventral white funiculus, slightly lateral to the midline. The LVST, usually via LCNs in laminae V and VI, and occasionally in lamina VII, influences the alpha and gamma motor neurons that innervate the axial and proximal extensor muscles. Like the reticulospinal tracts, this tract is also concerned with the maintenance of balance and posture; however, whereas the role of the RSTs in this function is anticipatory, the role of the LVST is reactive, in response to any perturbance in balance, as in tripping. The medial VST is derived from the medial vestibular nucleus and descends bilaterally in the ventral funiculus of the cord very close to the midline. The axons of this tract project to LCNs in the cervical and upper thoracic segments only. It serves to stabilize images on the retina during rotational head movements, as may occur during walking. There are a number of other tracts that descend from the brainstem sites into the spinal cord. Most notable are a descending fiber system from the solitary and retroambiguus nuclei that provides supraspinal drive to spinal LCNs and motoneurons that generate respiration and monoaminergic systems that regulate other motor and autonomic activities.
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Major ascending fiber tracts There are a variety of identified axonal systems that originate from neurons in the spinal cord and project upward to supraspinal destinations. The white matter locations of the major ascending tracts are shown in Figure 3 (left side). There are three categories of sensory pathways; these are the dorsal column/medial lemniscal and anterolateral systems (ALSs) and the spinocerebellar pathways. They convey sensory information to the brain, although not all of them carry signals that are experienced consciously. Many of the ascending tracts produce collaterals that project to neuronal groups within the spinal cord, making it likely that they can also contribute to sensory–motor reflexes.
Dorsal columns/medial lemniscal system The gracile and cuneate fasciculi are large, compact bundles of mostly large, myelinated axons that are known as the dorsal columns (Figure 3, left side). These fasciculi actually consist of the central processes of axons of large, somatic sensory neurons whose cell bodies are in the dorsal root ganglia and whose peripheral processes end in relation to mechanoreceptors in the skin, muscle, tendon, and joint capsules. The central process that enters the spinal cord branches into a larger ascending branch and multiple secondary branches. The larger branch climbs rostrally to reach either the nucleus gracilis or nucleus cuneatus located dorsally in the caudal medulla. Neurons in these nuclei project contralaterally to the nucleus ventralis posterolateralis (NVPL) of the dorsal thalamus via the medial lemniscus. Thalamocortical projections reach the primary somatosensory cortex in the parietal lobe. The smaller branches mentioned earlier project onto the neurons in laminae III and IV (nucleus proprius) and to neurons in lamina VII (Clarke’s nucleus), whereas still others project directly to lamina IX to synapse on alpha motor neurons, providing the substrate for the myotatic, or stretch, reflex. Incoming afferent fibers that enter the dorsal columns in relatively caudal segments of the spinal cord (carrying information from the lower extremity) lie more medially than those that enter from more rostral segments (carrying information from the upper extremity). Thus, large branches of afferents that enter the sacral, lumbar, and lower thoracic segments form the more medial gracile fasciculus with the most medial fibers representing sacral segments, the more lateral fibers representing lumbar segments, and the most lateral fibers representing lower thoracic segments. The same process continues from the midthoracic segments upward, but the large branches of the entering afferents here form the cuneate fasciculus, in which the more medial fibers carry information from upper thoracic segments and the most lateral fibers carry information from the cervical segments (Figure 4). Collaterals of some axons in the medial lemniscus reach parts of the reticular formation as well as the superior colliculus. Axons in the dorsal columns originate from proprioceptive receptors and from several other varieties of mechanoreceptors in the skin and deeper tissues. The dorsal column–medial lemniscal system is concerned with localization of fine touch, two-point discrimination, vibration, recognition of size, shape, and texture of objects (stereognosis),
C T L S CT
LS Flexor muscles Distal muscles
S
L T C Extensor muscles Proximal muscles
Figure 4 Diagram of the fifth cervical spinal cord segment showing somatotopy of the ventral horn cells and lateral corticospinal tract on the right, and the dorsal columns and anterolateral system on the left. Spinal cord levels indicated by letters: C, cervical; L, lumbar; S, sacral; T, thoracic.
and conscious proprioception (e.g., recognition of limb position at rest and during movement). Medial lemniscus axons that reach the thalamus carry sensory information that, ultimately, reaches the postcentral gyrus and results in the conscious perception of the above-mentioned sensory modalities. There is controversy among clinicians whether proprioception and vibration travel solely in the dorsal columns. Proprioception travels cephalad in multiple pathways; dorsal columns and spinocerebellar tracts (described below) are the known ones, with the possibility of the existence of still another unknown pathway. Isolated lesions in the dorsal columns in some individuals and in primates did not result in loss of proprioception. This result can be explained by redundancy, the fact that proprioception travels via multiple pathways.
Anterolateral system Another major sensory pathway is called the anterolateral system (ALS), also called the spinothalamic tracts (Figure 3, left side). This sensory system carries information about noxious stimuli, temperature stimuli, and deep touch. Axons of the primary sensory neurons that bring information about such modalities to the CNS are unmyelinated ‘C’ fibers or thinly myelinated A-delta fibers. The central processes of these dorsal root ganglion (DRG) neurons enter the spinal cord via the dorsal root (lateral in position relative to the larger axons that will, among other options, travel in the dorsal columns) and branch extensively. Certain of these branches, immediately after entering the spinal cord, either ascend or descend via Lissauer’s tract for a variable number of segments, thus providing the adjacent spinal cord segments with identical information concerning pain, temperature, and deep touch. The main central branches terminate in laminae I and II (marginal layer and substantia gelatinosa, respectively). In addition, other branches reach the nucleus proprius and laminae V and VI. Axons originating in laminae I and II, with some contribution from laminae V and VI, cross to the opposite side of the cord nearly horizontally in the ventral white commissure, then climb diagonally for one or two segments
Spinal Cord Anatomy
before coalescing to form the ALS (spinothalamic tract). This pathway ascends through the brainstem to the NVPL of the dorsal thalamus; then, thalamocortical projections from this nucleus reach the primary somatosensory cortex in the postcentral gyrus of the parietal lobe. The somatotopic pattern in the spinothalamic tract is opposite to that in the dorsal columns. An examination of this tract at the cervical level would reveal that fibers arising from sacral levels of the cord are most lateral and fibers originating at cervical levels are most medial (with lumbar- and thoracicoriginating fibers occupying an intermediate position) (Figure 4).
The spinocerebellar tracts The spinocerebellar tracts carry unconscious proprioceptive information gleaned from muscle spindles, Golgi tendon organs, and joint capsules to the cerebellum. The cell bodies of the primary sensory neurons that bring this information from such receptors to the spinal cord are located in the dorsal root ganglia. There are three spinocerebellar tracts, viz. anterior and posterior spinocerebellar and cuneocerebellar tracts (Figure 3, left side). The anterior spinocerebellar tract carries unconscious proprioceptive information from those musculoskeletal structures innervated by cord segments caudal to (and including) the second lumbar segment. The first-order neurons carrying information from proprioceptive receptors terminate in laminae V–VII; the axons of the second-order neurons decussate to assume a position immediately ventral to the dorsal spinocerebellar tract at the periphery of the lateral funciulus. The anterior spinocerebellar tract ascends through the cord and brainstem as far as the rostral pons/caudal midbrain. Here, the tract crosses the midline again to enter the cerebellum through the superior cerebellar peduncle. Axons synapse in the vermal and paravermal regions of the cerebellum called the spinocerebellum. The posterior spinocerebellar tract carries unconscious proprioceptive information from the lower extremity and trunk. The central processes of the relevant neurons in the dorsal root ganglia project to the prominent dorsal nucleus of Clarke located in the medial part of lamina VII between the T1 and L2 segments of the spinal cord (Figure 5(b)). Central processes of axons carrying this information from below the level of L2 ascend in the fasciculus gracilis in order to reach the dorsal nucleus of Clarke. Axons originating from this nucleus then form the ipsilateral posterior spinocerebellar tract, which is located in the periphery of the dorsal funiculus of the cord, just dorsal to the anterior spinocerebellar tract. The tract enters the ipsilateral spinocerebellum through the inferior cerebellar peduncle. The cuneocerebellar tract carries unconscious proprioceptive information to the cerebellum from the upper extremity, that is, from those musculoskeletal structures innervated by segments rostral to, and including, T1. The central processes of the dorsal root ganglion cells carrying this sensory information ascend through the cuneate fasciculus into the caudal medulla and synapse on the accessory cuneate nucleus, which lies just lateral to the cuneate nucleus. Axons originating from this nucleus reach the ipsilateral spinocerebellum via the inferior cerebellar peduncle.
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Propriospinal tracts Collaterals from the central processes of axons carrying all sensory modalities from the periphery to the spinal cord and higher levels synapse with neurons in the nucleus proprius, in laminae III and IV. Axons originating from the nucleus proprius leave the gray matter and, after ascending and/or descending for a variable number of segments, reenter the gray matter and make connections with other LCNs. For the most part, these tracts closely envelop the spinal gray matter on all sides, including the inner borders of the dorsal columns (Figure 3). These propriospinal fibers can interconnect many types of neurons in various spinal cord segments and are critical to coordinating both the sensory and motor functions between and among the cord segments (both ipsilaterally and contralaterally).
Lissauer’s tract Collaterals of unmyelinated and thinly myelinated axons of small primary sensory neurons (C and A-delta fibers, respectively) both ascend and descend for at least a few cord segments in Lissauer’s tract (Figure 3, left side). Intermingled with them are axons of neurons whose cell bodies are located in laminae I and II and they also ascend and descend for a few segments. The neurons and axons that make up Lissauer’s tract are involved mainly in transmitting information from painand temperature-sensitive afferents to the neighboring segments of the spinal cord. The broadcast of information via Lissauer’s tract can be said to underlie intersegmental activity with regard to behavioral responses to noxious stimuli.
Gray Matter The spinal gray matter is a compact, bilaterally symmetrical structure, with an ‘H’-shaped cross-section, that contains all the neurons in the spinal cord. The gray matter is completely surrounded by the white matter. When viewed in crosssections (Figure 1(b)), the spinal gray matter can be divided into dorsal and ventral horns. In the thoracic and upper lumbar segments there is also a small lateral horn. There are four basic types of neurons in the spinal gray matter: motoneurons, autonomic preganglionic neurons, LCN, and projection neurons. The cell bodies of motoneurons are located in the ventral horn of the gray matter; they have axons that leave the cord via the ventral roots and run out in peripheral nerves to innervate the skeletal muscle fibers. There are two subtypes of motoneurons: large alpha motoneurons that exclusively innervate the large extrafusal muscle fibers that make up the bulk of the muscles and produce forces on the skeleton and/or connective tissues, and smaller gamma motoneurons that exclusively innervate the specialized intrafusal muscle fibers within the muscle spindle stretch receptor organs located inside the muscles. Gamma motor neurons control the length of the intrafusal fibers and thus regulate the sensory input from the muscle traveling toward the spinal cord. The alpha motor neurons are also termed lower motor neurons. Their activity is modulated by the LCNs as well as by the upper motor neuronal tracts originating from the cerebral cortex and the brainstem. As all the movements are carried out by the lower
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Marginal nucleus I
II III
Substantia gelatinosa IV
Nucleus proprius V VI X VII
VIII
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(a)
Medial and lateral motor nuclei
Marginal nucleus I II
Substantia gelatinosa III IV V VI
Nucleus proprius Dorsal nucleus of Clarke X
VII
VIII Intermediolateral column
IX Medial motor nuclei (b)
Figure 5 (a) Diagram of a spinal cord section at C6 illustrating the gray matter, showing division into Rexed’s laminae (left) and recognized nuclear groups (right). (b) Diagram of a spinal cord section at T2 illustrating the gray matter, showing division into Rexed’s laminae (left) and recognized nuclear groups (right).
motor neurons by innervating the skeletal muscles, Charles Sherrington, the prominent British physiologist, called the lower motor neurons the ‘final common pathway’. The alpha motor neurons have a medial to lateral, as well as a ventral to dorsal, somatotopic organization within the ventral horn (Figure 4). More medially placed alpha motor neurons innervate the axial muscles. Immediately lateral to these motor neurons, in the cervical and lumbosacral enlargements, motoneurons innervate the proximal limb muscles, followed by progressively more distal muscles, so that the most laterally placed motor neurons innervate the distal most muscles concerned with fine movements. Furthermore, the ventral group of alpha motor neurons supplies the extensor muscles of the extremities, whereas the dorsal group supplies the flexor muscles. Autonomic preganglionic neurons also give rise to axons that leave the cord via ventral roots. Cell bodies of sympathetic preganglionic neurons are located in the lateral horn
(intermediolateral column) of the spinal cord segments T1–L2; these project to the postganglionic neurons in the paravertebral and prevertebral sympathetic ganglia. Cell bodies of preganglionic parasympathetic neurons are located in the intermediolateral column of spinal cord segments S2, S3, and S4. Axons of these neurons project to the postganglionic parasympathetic neurons located in the viscera. Both types of autonomic preganglionic neurons are located in the lateral part of lamina VII. LCNs are the most numerous cells in the spinal gray matter. The axons of these cells project to targets at various distances within the spinal cord proper. Like the alpha motor neurons, LCNs also display medial to lateral somatotopic organization. Those located medially project to alpha motor neurons over multiple segments, sometimes traversing the entire length of the cord, and their axons cross over to the opposite side and are thus concerned with maintenance of
Spinal Cord Anatomy
posture. The laterally placed LCNs project to alpha motor neurons over a few segments only and are concerned with fine movements. LCNs receive synaptic inputs from primary afferent fibers that enter the dorsal roots, from descending fiber systems, and from other LCNs. Although a few spinal LCNs may have axons that terminate locally within the gray matter, most project into the fasciculi proprii that surround the gray matter, where they run longitudinally for variable distances (Figure 3). To facilitate contraction of agonist muscles, a certain group of LCNs makes inhibitory connections with alpha motoneurons that innervate antagonistic muscles. To terminate the activity of alpha motoneurons that have just fired, collaterals of the axons of these alpha motoneurons activate another group of LCNs called the Renshaw cells; the Renshaw cells project back to and inhibit the alpha motor neuron that just fired, terminating the ongoing activity. Certain neurons of the dorsal horn gray matter send axons rostrally to the brain and may be called projection neurons. (Other researchers have called such neurons ‘tract neurons’.)
Laminar organization Like many regions in the brain, the spinal gray matter exhibits a degree of laminated organization based on the density and size of the constituent neurons. This lamination (Figures 5(a) and (b)), originally described by Rexed, has guided studies of the functional organization of neurons in the spinal cord. Rexed separated the spinal gray matter into 10 laminae, numbered using Roman numerals. The dorsal horn contains laminae I–VI, which are quite obvious in Nissl-stained sections that reveal differences in neuronal densities. Lamination is much less clear in the lateral and ventral horns; the ventral horn is dominated by groups of large cells, most of which are motoneurons that are located in the lateral and ventromedial portions (called lamina IX). The lateral horn contains the cell bodies of preganglionic autonomic neurons.
Dorsal horn From a functional standpoint, the neurons in the dorsal horn are mainly devoted to processing sensory information that arrives via the central processes of dorsal root ganglion cells (Figure 2) from the sensory receptors distributed throughout the body. Most dorsal horn neurons are LCNs that process afferent information before it is delivered to the motoneurons. Some larger neurons in the dorsal horn, called projection neurons, project to the brain. The layers of the dorsal horn have a complex cytology. Lamina I is the thinnest and most dorsal layer. It is also called the marginal layer, or lamina marginalis (Figures 5(a) and (b)). The marginal layer is composed of small to large neurons that receive input mostly from small-diameter, myelinated, Ad nociceptor afferents from the entire body (Figure 6). These fibers run rostrocaudally along Lissauer’s fasciculus after entering in the dorsal roots, giving rise to complex terminal arborizations in lamina I. Some of the larger neurons in lamina I project to the thalamus via the ALS. Lamina II, also called the substantia gelatinosa, is composed of densely packed small neurons that result in the gelatinous appearance of this lamina in fresh tissue (Figures 5(a) and (b)). Lamina II contains a wide variety of neuron types, some of which give rise to axons that project long distances
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Proprioceptive Ia and II Proprioceptive Ib Cutaneous A and A C A
Figure 6 Diagram of a low cervical segment of the spinal cord illustrating the distribution of various types of afferents in the laminae of the gray matter. Group Ia and II proprioceptive fibers terminate in lamina IX, sending collaterals to the medial part of lamina VII. Group Ib proprioceptive axons synapse in lamina VI and the medial part of lamina VII. Large-diameter cutaneous afferents (Aa and Ab) branch into laminae III–VI. Nonmyelinated C fibers terminate in laminae II–IV, whereas thinly myelinated Ad fibers terminate mainly in lamina I. Collaterals of C and Ad fibers also ascend and descend for short distances via Lissauer’s tract.
and others that project both locally and to other dorsal horn laminae. The input to cells in lamina II is primarily from fine, unmyelinated C fiber afferents that run in Lissauer’s tract, many of which are sensitive to noxious stimuli and temperature (Figure 6). Some of the larger neurons in lamina II project to the thalamus via the ALS. Laminae III and IV are together known as nucleus proprius (Figures 5(a) and (b)). They contain somewhat widely spaced neurons that receive input from secondary branches of larger diameter myelinated afferents from mechanoreceptors in the skin subserving fine touch, as well as secondary branches of thinly myelinated afferents concerned with thermal and noxious stimuli (Figure 6). Axons from neurons in the nucleus proprius enter the fasciculus proprius. Laminae V and VI extend across the base of the dorsal horn between the dorsal columns and the lateral white matter (Figures 5(a) and (b)). Afferents to laminae V and VI come from secondary branches of large sensory axons that bring information to the CNS from muscle spindles, Golgi tendon organs, and joint capsules (Figure 6). Additional afferents to these laminae come from secondary branches of thinly myelinated and unmyelinated fibers subserving noxious and thermal stimuli. LCNs in laminae V and VI receive abundant input from upper motor neurons in the descending pathways (CST, RST, TST, VSTs, and medial and lateral reticulospinal tracts). Neurons in laminae V and VI contribute to the formation of the anterior spinocerebellar tract. Lamina V also receives terminals from visceral nociceptors. Convergence of somatic and visceral nociceptors on neuron cell bodies in this lamina is an accepted theory regarding referred pain.
Lateral and ventral horns The lateral and ventral horns of the spinal gray matter include laminae VII–IX, which are in fact less laminae than irregular ‘regions’ of groupings of neurons, or nuclei. In most segments, the nuclei of motoneurons, collectively known as lamina IX, are easily identifiable; however, in the thoracic and upper
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lumbar segments, two additional and distinctive groups of neurons – the dorsal nucleus of Clarke and the intermediolateral column – can readily be identified in lamina VII (Figures 5(a) and (b)). Most of the ventral horn is concerned with efferent mechanisms that control movements. The lateral horn contains autonomic neurons. The boundaries of lamina VII are irregular because it is essentially the region of the ventral horn not included in laminae VIII and IX (Figures 5(a) and (b)). It contains a wide range of neurons that can be localized with variable precision. Lamina VII occupies an intermediate position in the spinal gray matter and also extends into the ventral horn. It includes the intermediolateral column of the lateral horn present in the spinal cord segments T1–L2, as well as the intermediolateral column of cells located in cord segments S2–S4. These regions house preganglionic sympathetic and parasympathetic neurons, respectively. The preganglionic sympathetic neurons project to sympathetic ganglia via the ventral nerve roots, innervating cardiac muscle as well as smooth muscle and glands throughout the body. The preganglionic parasympathetic neurons, located in the lateral part of lamina VII, project to the parasympathetic ganglia that, in turn, innervate the smooth muscle and glands in the distal third of the gastrointestinal tract and the pelvic viscera. Located in the medial part of lamina VII in cord segments T1–L2 is a well defined group of cells called Clarke’s nucleus or Clarke’s column of cells. These cells receive collaterals of axons of proprioceptive afferents from muscle and tendon and from endings in and around joint capsules. The axons of these cells constitute the dorsal spinocerebellar tract, which projects to the spinocerebellum via the inferior cerebellar peduncle, carrying information about unconscious proprioception. A less well-defined group of LCNs, sometimes called the intermediate nucleus, occupies the medial part of lamina VII dorsal to the central canal. This region receives a rich synaptic input from collaterals of large-diameter muscle, cutaneous, and deep tissue receptors. Neurons in this region play important roles in a wide range of reflex activities. Two other recognized groups of LCNs are localized in lamina VII. LCNs that receive monosynaptic input from group Ia muscle spindle afferents are located just dorsomedial to the motor nuclei of lamina IX (Figure 6). These cells directly inhibit motoneurons and account for the well known reciprocal inhibition that is part of the stretch reflex. Renshaw neurons, the first category of LCNs to be functionally defined, are located more ventrally along the medial border of lamina IX. Lamina VIII contains a disparate collection of LCNs of various sizes. Some have contralaterally projecting axons that are important in coordinating motor activities in the two halves of the cord. Others receive projections from descending tracts originating from the brainstem motor areas, for example, reticulospinal tracts and VSTs. Lamina IX contains the motor nuclei that are collections of motoneurons that innervate skeletal muscles. Its limits are best defined in the cervical and lumbar enlargements, where the profusion of motoneurons that innervate the complex musculature of the limbs is located in the expanded lateral portion of the ventral horn (Figure 1(b)). The somatotopic organization of the alpha motor neurons innervating different
groups of muscles in the extremities, from medial to lateral and ventral to dorsal, has already been described. Motoneurons that innervate a particular muscle are collected into an elongated grouping that occupies a predictable location, with little overlap with other motor nuclei. Alpha and gamma motoneurons belonging to a single muscle are intermingled within its motor nucleus. Lamina X comprises the gray matter surrounding the central canal. The identities and functions of the neurons in this region are not well understood.
Functional Considerations of Spinal Cord Anatomy From a physiological perspective, the spinal cord has been intensively studied because its afferent inputs and motoneuronal outputs have clearly understood functions. It performs the first stages of information processing in both somatic and visceral sensation and the final stages of computation in the control of movement. These tasks are accomplished mainly by spinal LCNs, although there is only rudimentary information on how they actually accomplish these tasks. The relative lack of clearly defined nuclear groupings of LCNs that match specific functions is a significant obstacle. It remains a major challenge to combine spinal cord anatomy with physiology in order to unravel the functional identities and connectivities among spinal cord LCNs. In spite of these shortcomings, clinicians find systematic application of the current overall knowledge of anatomy and physiology of the spinal cord of considerable help in localizing lesions of the spinal cord, with great precision in the great majority of patients with spinal cord disease. Diseases of the spinal cord are quite common and these include trauma, tumors (extradural metastatic tumors much more common than primary intramedullary), multiple sclerosis, motor neuron diseases (e.g., amyotrophic lateral sclerosis), epidural abscess, vitamin B12 deficiency, etc.
See also: Brain Anatomy. Central Nervous System, Overview. Cordotomy. Skull. Spinal Cord Diseases. Spinal Cord Transection. Spinal Roots. Vertebrate Nervous System, Development of the
Further Reading Brodal A (2010) The Central Nervous System Structure and Function, 4th edn. New York: Oxford University Press. Cramer G and Darby S (2005) Basic and Clinical Anatomy of the Spine, Spinal Cord, and ANS, 2nd edn. St. Louis: Elsevier Mosby. Gilman S (2002) Joint position sense and vibration sense: Anatomical organization and assessment. Journal of Neurology, Neurosurgery, and Psychiatry 73: 473–477. Kandel ER, Schwartz JH, and Jessel TM (2013) Principles of Neural Science, 5th edn. New York: McGraw-Hill. Kuypers H (1981) Anatomy of the descending pathways. In: Brooks VB (ed.) Handbook of Physiology, Section 1: The Nervous Systems, Vol. II, Motor Control, Part 1, pp. 597–666. Bethesda, MD: American Physiological Society.
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Mancall EL and Brock DG (2011) Gray’s Clinical Neuroanatomy: The Anatomic Basis for Clinical Neuroscience. Philadelphia: Elsevier Saunders. Purves D, Augustine GJ, Fitzpatrick D, et al. (eds.) (2008) Neuroscience, 4th edn. Sunderland, Massachusetts: Sinauer Associates, Inc.
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Rexed B (1952) The cytoarchitectonic organization of the spinal cord in the cat. Journal of Comparative Neurology 96: 415–496. Ross ED, Kirkpatrick JB, and Lastimosa A (1979) Position and vibration sensations: Functions of the dorsal spinocerebellar tracts. Annals of Neurology 5: 171–176.
Introduction
their synaptic interconnections, called the neuropil. However, unlike the brain, the white matter of the spinal cord is on its exterior, completely surrounding the gray matter. This ‘inside-out’ construction is presumably related to the two major functional roles played by the spinal cord: to convey bidirectional information to and from the rostral brain and to provide the neuronal machinery for semiautonomous functions such as reflexes and other basic patterns of action (i.e., reflexes and certain rhythmic movements) that can respond rapidly and efficiently to changes in external conditions. The ascending and descending axonal tracts expand in bulk from caudal to rostral along the cord, as descending tracts
The spinal cord (Figure 1(a)) is the oldest part of the vertebrate central nervous system (CNS). Its basic organization, dictated by the vertebrate body plan, has changed relatively little during evolution in comparison to the dramatic alterations found in supraspinal structures. Indeed, the structure of the spinal cord is fundamentally different from the rest of the CNS. Throughout the CNS, the regions that are composed of large bundles of myelinated and unmyelinated axons, called the white matter from its gross appearance, are segregated from the gray matter regions that contain collections of tightly packed neurons and C1
Representative segments
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Cervical enlargement
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Cauda equina
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(a)
(b)
Ventromedian fissure
Figure 1 (a) Lateral view of the human spinal cord within the vertebral column. The long nerve roots that exit between the vertebrae below the L1 level form the cauda equina (literally the ‘horse’s tail’). The nerve roots that innervate the upper and lower limbs form the complex brachial, lumbar, and sacral plexuses that eventually ‘sort out’ into peripheral nerves. (b) Cross-section diagrams of the cervical, thoracic, and lumbosacral portions of the cord showing the relative sizes of white and gray matter at these levels.
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terminate and ascending tracts are added (Figure 1(b)). In the spinal cord, the gray matter lies interiorly and is surrounded by the white matter. This arrangement is in contrast to that in the cerebrum and cerebellum, where the gray matter lies mostly on the outside and the white matter is located mostly internally. In the brainstem, gray matter and white matter are intermixed.
Gross Anatomy The spinal cord in most vertebrates is a long cylinder of tissue, approximately oval in cross-section that is continuous with the caudal medulla at the foramen magnum. The spinal cord lies within the vertebral canal formed by the articulated bony vertebral arches dorsal to the vertebral bodies proper. The spinal cord is surrounded by three protective membranes; from inside outward, these are the pia mater, the arachnoid mater, and the dura mater. Rostrally, these are continuous with those membranes surrounding the brain. Although the pia mater closely invests the surface of the spinal cord, there exists a space between it and the arachnoid mater, called the subarachnoid space. The subarachnoid space is continuous with the space of the same name in the cranial cavity; it is filled with cerebrospinal fluid (CSF) and contains the arteries and veins that supply the spinal cord. The spinal cord is no wider than 12 mm, being widest at the levels of the cervical and lumbosacral enlargements, and 42–45 cm in length. Just before the spinal cord terminates at the lower border of the body of the L1 vertebra, it tapers into the conus medullaris; the conus contains most of the sacral and the single coccygeal spinal cord segments (Figure 1(a)). The pia mater that intimately invests the spinal cord continues caudally in the vertebral canal as a ‘ligamentous’ or ‘threadlike’ structure, the filum terminale, which attaches caudally to the coccyx. (One can think of the filum terminale as an ‘anchor’ for the caudal end of the spinal cord.) In contrast to the pia mater, both the arachnoid mater and the dura mater continue caudally in the vertebral canal and do not terminate until the level of the second sacral vertebra; here, these layers ‘insert’ into the filum terminale. The absence of the spinal cord below the level of the L1 vertebra and the continuation of the subarachnoid space to the level of the S2 vertebra result in a relatively large space, known as the lumbar cistern, containing the CSF. The spinal cord is divided along its length into multiple functional segments, each related to the sensory and motor innervation of a particular and distinct portion of the body. There are 31 such segments in the human spinal cord. The segmentation of the cord is not obvious externally; however, 31 pairs of spinal nerves can be observed to enter/exit the spinal cord along its length with considerable regularity. Each spinal nerve is formed by the union of a dorsal root (containing the axons of sensory neurons) and a ventral root (containing the axons of motor neurons); these roots unite just before the spinal nerve exits the vertebral canal via its designated intervertebral foramen. The fact that the spinal cord is shorter than the vertebral canal and the fact that each spinal nerve must exit the vertebral canal at its appropriate intervertebral foramen results in a collection of dorsal and
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Dorsal roots
Dorsal root ganglion
Ventral roots Figure 2 Diagram showing the dorsal and ventral rootlets, respectively, entering and emerging from the spinal cord. Central processes of the axons of sensory neurons enter the spinal cord, the peripheral processes of axons of sensory neurons enter the peripheral nerves and distribute to their receptors, and cell bodies of sensory neurons are located in the dorsal root ganglion.
ventral spinal nerve roots, the cauda equina, in the vertebral canal below the level of termination of the spinal cord (Figure 1(a)). The spinal cord also has a patent central canal that is a continuation of the ventricular system of the brain. The fatty tissue outside the dura mater provides additional mechanical padding along the length of the spinal cord. The rootlets of the dorsal roots of the spinal nerves enter the cord at the dorsolateral sulcus (Figure 2) and carry the incoming (afferent) central projections of the sensory nerve cells that are located in the dorsal root ganglia. Each sensory ganglion, called the dorsal root ganglion, is located in or very close to the intervertebral foramen, just proximal to the union of the dorsal and ventral roots to form the spinal nerve. Each spinal segment has a corresponding sequence of ventral rootlets that emerge from the ventrolateral surface of the cord and unite to form a ventral root. These carry the outgoing (efferent) axons of somatic motoneurons and autonomic preganglionic neurons whose cell bodies are located in specific parts of the ventral and lateral areas of the gray matter, respectively. In animals with appendages, the cervical and lumbar segments that supply innervation to the limbs are larger in cross-sectional area than those in the intervening regions that serve the trunk (Figure 1(b)). Each spinal segment receives topographically organized sensory information from the skin and deep tissues in strip-like regions called dermatomes around the trunk and along the limbs. Although there is some overlap between dermatomes, they are clinically important in locating the level of damage in cases of spinal cord injury. Their existence is all too obvious to patients with painful herpes zoster eruptions, which can affect a single dermatome. The number of spinal cord segments depends on the number of vertebrae in a given species. The primitive lamprey cord has approximately 100 segments, whereas the human spinal cord has a total of 31 segments: 8 cervical (C), 12 thoracic (T), 5 lumbar (L), 5 sacral (S), and 1 coccygeal. The positions of the cervical segments correspond to those of the associated vertebral bodies, whereas the more caudal segments do not (Figure 1(a)). The more caudal spinal cord segments lie at the level of the last thoracic and first lumbar vertebral bodies (Figure 1(a)). The segments that innervate the upper
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extremity (C5–T1, often referred to as the cervical enlargement) and the lower extremity (L2–S2, the lumbosacral enlargement) are expanded in bulk compared with the intervening thoracic region in order to accommodate the additional gray matter required to innervate these appendages (Figure 1(b)).
the anterior spinal artery is significantly larger than the others. It often arises from a left lower posterior intercostal artery at or close to the T10 cord segment and is called the Artery of Adamkiewicz, or the Great Anterior Medullary Artery of Adamkiewicz.
Internal Organization
Blood Supply of the Spinal Cord The spinal cord receives its blood supply from two pairs of arteries originating from the vertebral arteries, soon after they enter the intracranial cavity through the foramen magnum. A pair of anterior spinal arteries arises from the vertebral arteries just above the foramen magnum; they turn caudally and fuse almost immediately, and travel the length of the spinal cord as the single anterior spinal artery in the ventromedian sulcus. The posterior spinal arteries arise just above the origin of the anterior spinal arteries; they remain singular and travel the length of the spinal cord in the socalled dorsolateral sulci, just medial to the entry zones of the dorsal rootlets of the spinal nerves. The anterior spinal artery supplies approximately the anterior two-thirds of the spinal cord, with the exception of the dorsal columns and dorsal horns, which are supplied by the posterior spinal arteries. The anterior and posterior spinal arteries are relatively small vessels and, beginning at lower cervical levels, they are supplemented along their length by a variable number of segmental arteries (also called medullary arteries) that arise from the spinal branches of the vertebral, deep cervical, intercostal, and lumbar arteries. These segmental spinal arteries nourish the dorsal and ventral roots as well as the cord itself. One of the segmental spinal arteries that contribute to
White Matter Except for the small region in which dorsal roots enter the cord (Figure 2), the gray matter of the spinal cord is enveloped by white matter that can be divided into specific regions called funiculi, composed of particular ascending and descending groups of axons, called tracts (Figure 3). For historical and technical reasons, most of the descending white matter tracts in the spinal cord are named according to the supraspinal nuclei or regions in which they originate. On the contrary, the ascending white matter tracts that originate within the spinal gray matter are also named according to their supraspinal destinations because their origins within the spinal gray matter are difficult to identify.
Major descending fiber tracts The locations of the major tracts that descend into the spinal cord are shown on the right side of Figure 3. The corticospinal tract (CST) arises in the cerebral cortex. Other descending, or ‘motor’, pathways arise from brainstem locations – the red nucleus, the tectum, the vestibular nuclei, and the reticular formation. These descending pathways are also known as ‘upper motor neurons.’ Although they do not innervate the
Fasciculus cuneatus Fasciculus gracilis Lissauer’s tract Dorsal spinocerebellar tract
Fasciculus proprius Lateral corticospinal tract Rubrospinal tract
Fasciculus proprius
Ventral spinocerebellar tract
Anterolateral system
Lateral reticulospinal tract
Ventral corticoLateral vestibulospinal tract spinal tract Medial vestibulospinal Medial reticuloand tectospinal tracts spinal tract
Figure 3 Diagram of a spinal cord section at C4 illustrating positions of major ascending (labeled on left) and descending (labeled on right) tracts.
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skeletal muscle, they modulate the activity of those motoneurons that innervate the skeletal muscle (alpha and gamma motoneurons) that are usually called ‘lower motor neurons.’ Upper motor neurons chiefly terminate upon local circuit neurons (LCNs) and, these, in turn, synapse with the alpha and/or gamma motoneurons. The CST originates in neurons that are widely scattered in the primary sensory and motor cortices, including the premotor areas in primates. Although the course of CST axons differs in some species, in most mammals approximately 85% of the fibers cross in the caudal medulla and enter the contralateral dorsolateral fasciculus; this bundle is called the lateral CST (LCST). These axons project to LCNs in laminae V and VI (Figure 5). In primates and humans, CST terminal branches also enter lamina IX, where they synapse directly (monosynaptically) onto the alpha motor neurons, especially those that innervate the flexor muscles of the fingers and thumb, that is, the musculature concerned with fractionated fine movements. Interestingly, such direct corticomotoneuronal connections also exist in the cervical segments in raccoons, which exhibit considerable forepaw dexterity. Approximately 15% of corticospinal fibers remain uncrossed in humans, and these travel in the ipsilateral ventromedial fasciculus (the ventral CST; Figure 3, right). These fibers eventually do cross; they regulate, via LCNs, axial and proximal limb musculature (in some cases, bilaterally). The myelinated axons that make up the CST have a wide range of sizes and conduction velocities, with most being less than 2 mm in diameter. The large-diameter fibers originate in large pyramidal neurons (Betz cells) in the primary and secondary motor areas and have fast conduction velocities. These are the CST fibers that directly excite specific alpha motor neurons innervating the distal flexor musculature. The majority of CST axons with smaller diameters appear to project mainly to spinal LCNs concerned with sensory–motor reflexes. The major function of the CST is to control a wide variety of voluntary motor actions. Interruption of the CST in primates produces serious and permanent defects in hand and finger dexterity. Rarely, patients with strokes that affect the CST may recover near normal strength in the upper extremity but may be left with marked difficulty with fine movements of the digits (threading a needle, buttoning and unbuttoning, manipulating table utensils, etc.). The rubrospinal tract (RST) (Figure 3, right side) originates in somatotopically organized cell groups within the red nucleus in the midbrain; it crosses immediately to descend into the dorsolateral funiculus of the spinal cord, where it lies just ventral to the LCST and reaches only up to the cervical and upper thoracic levels of the spinal cord. Most RST axons are large-diameter myelinated fibers that conduct rapidly. They project mainly to LCNs in laminae V and VI (Figure 5), although in primates they also make monosynaptic contact with some motoneurons in lamina IX. The function of the RST is largely exerted indirectly through its influence on LCNs projecting onto the alpha motor neurons of flexor muscles of the upper extremity. Although the RST is overshadowed by the great development of the CST in higher primates and humans, it remains an important motor pathway in humans. The function of the RST becomes obvious in patients with lesions
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of the CST above the level of the red nucleus in whom the upper extremity assumes a posture of generalized flexion, due to the unopposed action of the RST, whereas the lower extremity assumes a position of generalized extension (unopposed countervailing influence of the lateral vestibulospinal tract (LVST)). The tectospinal tract (TST; Figure 3, right side) originates in the superior colliculus. Its axons cross the midline and are found close to the midline in the ventral white matter of the cord, terminating on LCNs in the cervical and upper thoracic segments. These LCNs synapse with the alpha motor neurons that innervate the musculature concerned with orienting the eyes and head to visual, auditory, and somatic sensory stimuli. There are two reticulospinal tracts (Figure 3, right side). The medial reticulospinal tract (MRST) is derived from the pontine reticular formation and descends close to the midline in the ventral white funiculus of the spinal cord. It projects to LCNs in laminae V and VI, and occasionally, lamina VII, influencing those alpha motor neurons that innervate the axial and proximal muscles concerned with maintaining balance and posture. The MRST is activated in response to any anticipated disturbance in balance or posture. (Its function is opposite to that of the LVST, which is activated in response to any disturbance in posture, as in tripping.) The lateral reticulospinal tract (LRST) is derived from the medullary reticular formation. It descends in the ventral white funiculus just lateral to the MRST. The LRST also projects to LCNs in the spinal cord, and activation of these particular LCNs results in the generalized hypotonia that occurs during rapid eye movement sleep. There are two vestibulospinal tracts (VSTs) (lateral and medial) (Figure 3, right side) that originate from the vestibular nuclei. The vestibular nuclear complex is situated at the pontomedullary junction and the nuclei receive proprioceptive information via the vestibular nerve (of cranial nerve VIII) from the cristae ampullares of the semicircular canals and the otolith organs in the inner ear. The LVST is derived from the lateral vestibular nucleus and descends ipsilaterally through all levels of the spinal cord in the ventral white funiculus, slightly lateral to the midline. The LVST, usually via LCNs in laminae V and VI, and occasionally in lamina VII, influences the alpha and gamma motor neurons that innervate the axial and proximal extensor muscles. Like the reticulospinal tracts, this tract is also concerned with the maintenance of balance and posture; however, whereas the role of the RSTs in this function is anticipatory, the role of the LVST is reactive, in response to any perturbance in balance, as in tripping. The medial VST is derived from the medial vestibular nucleus and descends bilaterally in the ventral funiculus of the cord very close to the midline. The axons of this tract project to LCNs in the cervical and upper thoracic segments only. It serves to stabilize images on the retina during rotational head movements, as may occur during walking. There are a number of other tracts that descend from the brainstem sites into the spinal cord. Most notable are a descending fiber system from the solitary and retroambiguus nuclei that provides supraspinal drive to spinal LCNs and motoneurons that generate respiration and monoaminergic systems that regulate other motor and autonomic activities.
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Major ascending fiber tracts There are a variety of identified axonal systems that originate from neurons in the spinal cord and project upward to supraspinal destinations. The white matter locations of the major ascending tracts are shown in Figure 3 (left side). There are three categories of sensory pathways; these are the dorsal column/medial lemniscal and anterolateral systems (ALSs) and the spinocerebellar pathways. They convey sensory information to the brain, although not all of them carry signals that are experienced consciously. Many of the ascending tracts produce collaterals that project to neuronal groups within the spinal cord, making it likely that they can also contribute to sensory–motor reflexes.
Dorsal columns/medial lemniscal system The gracile and cuneate fasciculi are large, compact bundles of mostly large, myelinated axons that are known as the dorsal columns (Figure 3, left side). These fasciculi actually consist of the central processes of axons of large, somatic sensory neurons whose cell bodies are in the dorsal root ganglia and whose peripheral processes end in relation to mechanoreceptors in the skin, muscle, tendon, and joint capsules. The central process that enters the spinal cord branches into a larger ascending branch and multiple secondary branches. The larger branch climbs rostrally to reach either the nucleus gracilis or nucleus cuneatus located dorsally in the caudal medulla. Neurons in these nuclei project contralaterally to the nucleus ventralis posterolateralis (NVPL) of the dorsal thalamus via the medial lemniscus. Thalamocortical projections reach the primary somatosensory cortex in the parietal lobe. The smaller branches mentioned earlier project onto the neurons in laminae III and IV (nucleus proprius) and to neurons in lamina VII (Clarke’s nucleus), whereas still others project directly to lamina IX to synapse on alpha motor neurons, providing the substrate for the myotatic, or stretch, reflex. Incoming afferent fibers that enter the dorsal columns in relatively caudal segments of the spinal cord (carrying information from the lower extremity) lie more medially than those that enter from more rostral segments (carrying information from the upper extremity). Thus, large branches of afferents that enter the sacral, lumbar, and lower thoracic segments form the more medial gracile fasciculus with the most medial fibers representing sacral segments, the more lateral fibers representing lumbar segments, and the most lateral fibers representing lower thoracic segments. The same process continues from the midthoracic segments upward, but the large branches of the entering afferents here form the cuneate fasciculus, in which the more medial fibers carry information from upper thoracic segments and the most lateral fibers carry information from the cervical segments (Figure 4). Collaterals of some axons in the medial lemniscus reach parts of the reticular formation as well as the superior colliculus. Axons in the dorsal columns originate from proprioceptive receptors and from several other varieties of mechanoreceptors in the skin and deeper tissues. The dorsal column–medial lemniscal system is concerned with localization of fine touch, two-point discrimination, vibration, recognition of size, shape, and texture of objects (stereognosis),
C T L S CT
LS Flexor muscles Distal muscles
S
L T C Extensor muscles Proximal muscles
Figure 4 Diagram of the fifth cervical spinal cord segment showing somatotopy of the ventral horn cells and lateral corticospinal tract on the right, and the dorsal columns and anterolateral system on the left. Spinal cord levels indicated by letters: C, cervical; L, lumbar; S, sacral; T, thoracic.
and conscious proprioception (e.g., recognition of limb position at rest and during movement). Medial lemniscus axons that reach the thalamus carry sensory information that, ultimately, reaches the postcentral gyrus and results in the conscious perception of the above-mentioned sensory modalities. There is controversy among clinicians whether proprioception and vibration travel solely in the dorsal columns. Proprioception travels cephalad in multiple pathways; dorsal columns and spinocerebellar tracts (described below) are the known ones, with the possibility of the existence of still another unknown pathway. Isolated lesions in the dorsal columns in some individuals and in primates did not result in loss of proprioception. This result can be explained by redundancy, the fact that proprioception travels via multiple pathways.
Anterolateral system Another major sensory pathway is called the anterolateral system (ALS), also called the spinothalamic tracts (Figure 3, left side). This sensory system carries information about noxious stimuli, temperature stimuli, and deep touch. Axons of the primary sensory neurons that bring information about such modalities to the CNS are unmyelinated ‘C’ fibers or thinly myelinated A-delta fibers. The central processes of these dorsal root ganglion (DRG) neurons enter the spinal cord via the dorsal root (lateral in position relative to the larger axons that will, among other options, travel in the dorsal columns) and branch extensively. Certain of these branches, immediately after entering the spinal cord, either ascend or descend via Lissauer’s tract for a variable number of segments, thus providing the adjacent spinal cord segments with identical information concerning pain, temperature, and deep touch. The main central branches terminate in laminae I and II (marginal layer and substantia gelatinosa, respectively). In addition, other branches reach the nucleus proprius and laminae V and VI. Axons originating in laminae I and II, with some contribution from laminae V and VI, cross to the opposite side of the cord nearly horizontally in the ventral white commissure, then climb diagonally for one or two segments
Spinal Cord Anatomy
before coalescing to form the ALS (spinothalamic tract). This pathway ascends through the brainstem to the NVPL of the dorsal thalamus; then, thalamocortical projections from this nucleus reach the primary somatosensory cortex in the postcentral gyrus of the parietal lobe. The somatotopic pattern in the spinothalamic tract is opposite to that in the dorsal columns. An examination of this tract at the cervical level would reveal that fibers arising from sacral levels of the cord are most lateral and fibers originating at cervical levels are most medial (with lumbar- and thoracicoriginating fibers occupying an intermediate position) (Figure 4).
The spinocerebellar tracts The spinocerebellar tracts carry unconscious proprioceptive information gleaned from muscle spindles, Golgi tendon organs, and joint capsules to the cerebellum. The cell bodies of the primary sensory neurons that bring this information from such receptors to the spinal cord are located in the dorsal root ganglia. There are three spinocerebellar tracts, viz. anterior and posterior spinocerebellar and cuneocerebellar tracts (Figure 3, left side). The anterior spinocerebellar tract carries unconscious proprioceptive information from those musculoskeletal structures innervated by cord segments caudal to (and including) the second lumbar segment. The first-order neurons carrying information from proprioceptive receptors terminate in laminae V–VII; the axons of the second-order neurons decussate to assume a position immediately ventral to the dorsal spinocerebellar tract at the periphery of the lateral funciulus. The anterior spinocerebellar tract ascends through the cord and brainstem as far as the rostral pons/caudal midbrain. Here, the tract crosses the midline again to enter the cerebellum through the superior cerebellar peduncle. Axons synapse in the vermal and paravermal regions of the cerebellum called the spinocerebellum. The posterior spinocerebellar tract carries unconscious proprioceptive information from the lower extremity and trunk. The central processes of the relevant neurons in the dorsal root ganglia project to the prominent dorsal nucleus of Clarke located in the medial part of lamina VII between the T1 and L2 segments of the spinal cord (Figure 5(b)). Central processes of axons carrying this information from below the level of L2 ascend in the fasciculus gracilis in order to reach the dorsal nucleus of Clarke. Axons originating from this nucleus then form the ipsilateral posterior spinocerebellar tract, which is located in the periphery of the dorsal funiculus of the cord, just dorsal to the anterior spinocerebellar tract. The tract enters the ipsilateral spinocerebellum through the inferior cerebellar peduncle. The cuneocerebellar tract carries unconscious proprioceptive information to the cerebellum from the upper extremity, that is, from those musculoskeletal structures innervated by segments rostral to, and including, T1. The central processes of the dorsal root ganglion cells carrying this sensory information ascend through the cuneate fasciculus into the caudal medulla and synapse on the accessory cuneate nucleus, which lies just lateral to the cuneate nucleus. Axons originating from this nucleus reach the ipsilateral spinocerebellum via the inferior cerebellar peduncle.
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Propriospinal tracts Collaterals from the central processes of axons carrying all sensory modalities from the periphery to the spinal cord and higher levels synapse with neurons in the nucleus proprius, in laminae III and IV. Axons originating from the nucleus proprius leave the gray matter and, after ascending and/or descending for a variable number of segments, reenter the gray matter and make connections with other LCNs. For the most part, these tracts closely envelop the spinal gray matter on all sides, including the inner borders of the dorsal columns (Figure 3). These propriospinal fibers can interconnect many types of neurons in various spinal cord segments and are critical to coordinating both the sensory and motor functions between and among the cord segments (both ipsilaterally and contralaterally).
Lissauer’s tract Collaterals of unmyelinated and thinly myelinated axons of small primary sensory neurons (C and A-delta fibers, respectively) both ascend and descend for at least a few cord segments in Lissauer’s tract (Figure 3, left side). Intermingled with them are axons of neurons whose cell bodies are located in laminae I and II and they also ascend and descend for a few segments. The neurons and axons that make up Lissauer’s tract are involved mainly in transmitting information from painand temperature-sensitive afferents to the neighboring segments of the spinal cord. The broadcast of information via Lissauer’s tract can be said to underlie intersegmental activity with regard to behavioral responses to noxious stimuli.
Gray Matter The spinal gray matter is a compact, bilaterally symmetrical structure, with an ‘H’-shaped cross-section, that contains all the neurons in the spinal cord. The gray matter is completely surrounded by the white matter. When viewed in crosssections (Figure 1(b)), the spinal gray matter can be divided into dorsal and ventral horns. In the thoracic and upper lumbar segments there is also a small lateral horn. There are four basic types of neurons in the spinal gray matter: motoneurons, autonomic preganglionic neurons, LCN, and projection neurons. The cell bodies of motoneurons are located in the ventral horn of the gray matter; they have axons that leave the cord via the ventral roots and run out in peripheral nerves to innervate the skeletal muscle fibers. There are two subtypes of motoneurons: large alpha motoneurons that exclusively innervate the large extrafusal muscle fibers that make up the bulk of the muscles and produce forces on the skeleton and/or connective tissues, and smaller gamma motoneurons that exclusively innervate the specialized intrafusal muscle fibers within the muscle spindle stretch receptor organs located inside the muscles. Gamma motor neurons control the length of the intrafusal fibers and thus regulate the sensory input from the muscle traveling toward the spinal cord. The alpha motor neurons are also termed lower motor neurons. Their activity is modulated by the LCNs as well as by the upper motor neuronal tracts originating from the cerebral cortex and the brainstem. As all the movements are carried out by the lower
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Marginal nucleus I
II III
Substantia gelatinosa IV
Nucleus proprius V VI X VII
VIII
IX
(a)
Medial and lateral motor nuclei
Marginal nucleus I II
Substantia gelatinosa III IV V VI
Nucleus proprius Dorsal nucleus of Clarke X
VII
VIII Intermediolateral column
IX Medial motor nuclei (b)
Figure 5 (a) Diagram of a spinal cord section at C6 illustrating the gray matter, showing division into Rexed’s laminae (left) and recognized nuclear groups (right). (b) Diagram of a spinal cord section at T2 illustrating the gray matter, showing division into Rexed’s laminae (left) and recognized nuclear groups (right).
motor neurons by innervating the skeletal muscles, Charles Sherrington, the prominent British physiologist, called the lower motor neurons the ‘final common pathway’. The alpha motor neurons have a medial to lateral, as well as a ventral to dorsal, somatotopic organization within the ventral horn (Figure 4). More medially placed alpha motor neurons innervate the axial muscles. Immediately lateral to these motor neurons, in the cervical and lumbosacral enlargements, motoneurons innervate the proximal limb muscles, followed by progressively more distal muscles, so that the most laterally placed motor neurons innervate the distal most muscles concerned with fine movements. Furthermore, the ventral group of alpha motor neurons supplies the extensor muscles of the extremities, whereas the dorsal group supplies the flexor muscles. Autonomic preganglionic neurons also give rise to axons that leave the cord via ventral roots. Cell bodies of sympathetic preganglionic neurons are located in the lateral horn
(intermediolateral column) of the spinal cord segments T1–L2; these project to the postganglionic neurons in the paravertebral and prevertebral sympathetic ganglia. Cell bodies of preganglionic parasympathetic neurons are located in the intermediolateral column of spinal cord segments S2, S3, and S4. Axons of these neurons project to the postganglionic parasympathetic neurons located in the viscera. Both types of autonomic preganglionic neurons are located in the lateral part of lamina VII. LCNs are the most numerous cells in the spinal gray matter. The axons of these cells project to targets at various distances within the spinal cord proper. Like the alpha motor neurons, LCNs also display medial to lateral somatotopic organization. Those located medially project to alpha motor neurons over multiple segments, sometimes traversing the entire length of the cord, and their axons cross over to the opposite side and are thus concerned with maintenance of
Spinal Cord Anatomy
posture. The laterally placed LCNs project to alpha motor neurons over a few segments only and are concerned with fine movements. LCNs receive synaptic inputs from primary afferent fibers that enter the dorsal roots, from descending fiber systems, and from other LCNs. Although a few spinal LCNs may have axons that terminate locally within the gray matter, most project into the fasciculi proprii that surround the gray matter, where they run longitudinally for variable distances (Figure 3). To facilitate contraction of agonist muscles, a certain group of LCNs makes inhibitory connections with alpha motoneurons that innervate antagonistic muscles. To terminate the activity of alpha motoneurons that have just fired, collaterals of the axons of these alpha motoneurons activate another group of LCNs called the Renshaw cells; the Renshaw cells project back to and inhibit the alpha motor neuron that just fired, terminating the ongoing activity. Certain neurons of the dorsal horn gray matter send axons rostrally to the brain and may be called projection neurons. (Other researchers have called such neurons ‘tract neurons’.)
Laminar organization Like many regions in the brain, the spinal gray matter exhibits a degree of laminated organization based on the density and size of the constituent neurons. This lamination (Figures 5(a) and (b)), originally described by Rexed, has guided studies of the functional organization of neurons in the spinal cord. Rexed separated the spinal gray matter into 10 laminae, numbered using Roman numerals. The dorsal horn contains laminae I–VI, which are quite obvious in Nissl-stained sections that reveal differences in neuronal densities. Lamination is much less clear in the lateral and ventral horns; the ventral horn is dominated by groups of large cells, most of which are motoneurons that are located in the lateral and ventromedial portions (called lamina IX). The lateral horn contains the cell bodies of preganglionic autonomic neurons.
Dorsal horn From a functional standpoint, the neurons in the dorsal horn are mainly devoted to processing sensory information that arrives via the central processes of dorsal root ganglion cells (Figure 2) from the sensory receptors distributed throughout the body. Most dorsal horn neurons are LCNs that process afferent information before it is delivered to the motoneurons. Some larger neurons in the dorsal horn, called projection neurons, project to the brain. The layers of the dorsal horn have a complex cytology. Lamina I is the thinnest and most dorsal layer. It is also called the marginal layer, or lamina marginalis (Figures 5(a) and (b)). The marginal layer is composed of small to large neurons that receive input mostly from small-diameter, myelinated, Ad nociceptor afferents from the entire body (Figure 6). These fibers run rostrocaudally along Lissauer’s fasciculus after entering in the dorsal roots, giving rise to complex terminal arborizations in lamina I. Some of the larger neurons in lamina I project to the thalamus via the ALS. Lamina II, also called the substantia gelatinosa, is composed of densely packed small neurons that result in the gelatinous appearance of this lamina in fresh tissue (Figures 5(a) and (b)). Lamina II contains a wide variety of neuron types, some of which give rise to axons that project long distances
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Proprioceptive Ia and II Proprioceptive Ib Cutaneous A and A C A
Figure 6 Diagram of a low cervical segment of the spinal cord illustrating the distribution of various types of afferents in the laminae of the gray matter. Group Ia and II proprioceptive fibers terminate in lamina IX, sending collaterals to the medial part of lamina VII. Group Ib proprioceptive axons synapse in lamina VI and the medial part of lamina VII. Large-diameter cutaneous afferents (Aa and Ab) branch into laminae III–VI. Nonmyelinated C fibers terminate in laminae II–IV, whereas thinly myelinated Ad fibers terminate mainly in lamina I. Collaterals of C and Ad fibers also ascend and descend for short distances via Lissauer’s tract.
and others that project both locally and to other dorsal horn laminae. The input to cells in lamina II is primarily from fine, unmyelinated C fiber afferents that run in Lissauer’s tract, many of which are sensitive to noxious stimuli and temperature (Figure 6). Some of the larger neurons in lamina II project to the thalamus via the ALS. Laminae III and IV are together known as nucleus proprius (Figures 5(a) and (b)). They contain somewhat widely spaced neurons that receive input from secondary branches of larger diameter myelinated afferents from mechanoreceptors in the skin subserving fine touch, as well as secondary branches of thinly myelinated afferents concerned with thermal and noxious stimuli (Figure 6). Axons from neurons in the nucleus proprius enter the fasciculus proprius. Laminae V and VI extend across the base of the dorsal horn between the dorsal columns and the lateral white matter (Figures 5(a) and (b)). Afferents to laminae V and VI come from secondary branches of large sensory axons that bring information to the CNS from muscle spindles, Golgi tendon organs, and joint capsules (Figure 6). Additional afferents to these laminae come from secondary branches of thinly myelinated and unmyelinated fibers subserving noxious and thermal stimuli. LCNs in laminae V and VI receive abundant input from upper motor neurons in the descending pathways (CST, RST, TST, VSTs, and medial and lateral reticulospinal tracts). Neurons in laminae V and VI contribute to the formation of the anterior spinocerebellar tract. Lamina V also receives terminals from visceral nociceptors. Convergence of somatic and visceral nociceptors on neuron cell bodies in this lamina is an accepted theory regarding referred pain.
Lateral and ventral horns The lateral and ventral horns of the spinal gray matter include laminae VII–IX, which are in fact less laminae than irregular ‘regions’ of groupings of neurons, or nuclei. In most segments, the nuclei of motoneurons, collectively known as lamina IX, are easily identifiable; however, in the thoracic and upper
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lumbar segments, two additional and distinctive groups of neurons – the dorsal nucleus of Clarke and the intermediolateral column – can readily be identified in lamina VII (Figures 5(a) and (b)). Most of the ventral horn is concerned with efferent mechanisms that control movements. The lateral horn contains autonomic neurons. The boundaries of lamina VII are irregular because it is essentially the region of the ventral horn not included in laminae VIII and IX (Figures 5(a) and (b)). It contains a wide range of neurons that can be localized with variable precision. Lamina VII occupies an intermediate position in the spinal gray matter and also extends into the ventral horn. It includes the intermediolateral column of the lateral horn present in the spinal cord segments T1–L2, as well as the intermediolateral column of cells located in cord segments S2–S4. These regions house preganglionic sympathetic and parasympathetic neurons, respectively. The preganglionic sympathetic neurons project to sympathetic ganglia via the ventral nerve roots, innervating cardiac muscle as well as smooth muscle and glands throughout the body. The preganglionic parasympathetic neurons, located in the lateral part of lamina VII, project to the parasympathetic ganglia that, in turn, innervate the smooth muscle and glands in the distal third of the gastrointestinal tract and the pelvic viscera. Located in the medial part of lamina VII in cord segments T1–L2 is a well defined group of cells called Clarke’s nucleus or Clarke’s column of cells. These cells receive collaterals of axons of proprioceptive afferents from muscle and tendon and from endings in and around joint capsules. The axons of these cells constitute the dorsal spinocerebellar tract, which projects to the spinocerebellum via the inferior cerebellar peduncle, carrying information about unconscious proprioception. A less well-defined group of LCNs, sometimes called the intermediate nucleus, occupies the medial part of lamina VII dorsal to the central canal. This region receives a rich synaptic input from collaterals of large-diameter muscle, cutaneous, and deep tissue receptors. Neurons in this region play important roles in a wide range of reflex activities. Two other recognized groups of LCNs are localized in lamina VII. LCNs that receive monosynaptic input from group Ia muscle spindle afferents are located just dorsomedial to the motor nuclei of lamina IX (Figure 6). These cells directly inhibit motoneurons and account for the well known reciprocal inhibition that is part of the stretch reflex. Renshaw neurons, the first category of LCNs to be functionally defined, are located more ventrally along the medial border of lamina IX. Lamina VIII contains a disparate collection of LCNs of various sizes. Some have contralaterally projecting axons that are important in coordinating motor activities in the two halves of the cord. Others receive projections from descending tracts originating from the brainstem motor areas, for example, reticulospinal tracts and VSTs. Lamina IX contains the motor nuclei that are collections of motoneurons that innervate skeletal muscles. Its limits are best defined in the cervical and lumbar enlargements, where the profusion of motoneurons that innervate the complex musculature of the limbs is located in the expanded lateral portion of the ventral horn (Figure 1(b)). The somatotopic organization of the alpha motor neurons innervating different
groups of muscles in the extremities, from medial to lateral and ventral to dorsal, has already been described. Motoneurons that innervate a particular muscle are collected into an elongated grouping that occupies a predictable location, with little overlap with other motor nuclei. Alpha and gamma motoneurons belonging to a single muscle are intermingled within its motor nucleus. Lamina X comprises the gray matter surrounding the central canal. The identities and functions of the neurons in this region are not well understood.
Functional Considerations of Spinal Cord Anatomy From a physiological perspective, the spinal cord has been intensively studied because its afferent inputs and motoneuronal outputs have clearly understood functions. It performs the first stages of information processing in both somatic and visceral sensation and the final stages of computation in the control of movement. These tasks are accomplished mainly by spinal LCNs, although there is only rudimentary information on how they actually accomplish these tasks. The relative lack of clearly defined nuclear groupings of LCNs that match specific functions is a significant obstacle. It remains a major challenge to combine spinal cord anatomy with physiology in order to unravel the functional identities and connectivities among spinal cord LCNs. In spite of these shortcomings, clinicians find systematic application of the current overall knowledge of anatomy and physiology of the spinal cord of considerable help in localizing lesions of the spinal cord, with great precision in the great majority of patients with spinal cord disease. Diseases of the spinal cord are quite common and these include trauma, tumors (extradural metastatic tumors much more common than primary intramedullary), multiple sclerosis, motor neuron diseases (e.g., amyotrophic lateral sclerosis), epidural abscess, vitamin B12 deficiency, etc.
See also: Brain Anatomy. Central Nervous System, Overview. Cordotomy. Skull. Spinal Cord Diseases. Spinal Cord Transection. Spinal Roots. Vertebrate Nervous System, Development of the
Further Reading Brodal A (2010) The Central Nervous System Structure and Function, 4th edn. New York: Oxford University Press. Cramer G and Darby S (2005) Basic and Clinical Anatomy of the Spine, Spinal Cord, and ANS, 2nd edn. St. Louis: Elsevier Mosby. Gilman S (2002) Joint position sense and vibration sense: Anatomical organization and assessment. Journal of Neurology, Neurosurgery, and Psychiatry 73: 473–477. Kandel ER, Schwartz JH, and Jessel TM (2013) Principles of Neural Science, 5th edn. New York: McGraw-Hill. Kuypers H (1981) Anatomy of the descending pathways. In: Brooks VB (ed.) Handbook of Physiology, Section 1: The Nervous Systems, Vol. II, Motor Control, Part 1, pp. 597–666. Bethesda, MD: American Physiological Society.
Spinal Cord Anatomy
Mancall EL and Brock DG (2011) Gray’s Clinical Neuroanatomy: The Anatomic Basis for Clinical Neuroscience. Philadelphia: Elsevier Saunders. Purves D, Augustine GJ, Fitzpatrick D, et al. (eds.) (2008) Neuroscience, 4th edn. Sunderland, Massachusetts: Sinauer Associates, Inc.
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Rexed B (1952) The cytoarchitectonic organization of the spinal cord in the cat. Journal of Comparative Neurology 96: 415–496. Ross ED, Kirkpatrick JB, and Lastimosa A (1979) Position and vibration sensations: Functions of the dorsal spinocerebellar tracts. Annals of Neurology 5: 171–176.