- Email: [email protected]
The brain: functional divisions
PHYSIOLOGY
The brain: functional divisions Chris J D Pomfrett
Anaesthesia is not the same as natural sleep. The basis of contemporary balanced anaesthesia is the use of the CNS as a target, specific regions of which yield a component of good anaesthesia. Hypnosis (elimination of consciousness) and amnesia are achieved by disrupting the normal function of the cerebral cortex and underlying brain structures. Progressively greater depths of anaesthesia cause progressive reduction in the global metabolic rate of the brain. The adult brain (Figure 1) weighs 1250–1450 g within a volume of 1400 ml. It comprises the: • telencephalon (cerebral cortex, basal ganglia and olfactory bulb) • diencephalon (thalamus, hypothalamus and habenula) • mesencephalon or midbrain (structures around the cerebral aqueduct, superior and inferior colliculi and the cerebral peduncles)
The adult brain Olfactory bulb
Cerebral cortex Corpus callosum Diencephalon Midbrain Cerebellum Pons Medulla Spinal cord
1
Chris J D Pomfrett is Non-clinical Lecturer in Neurophysiology Applied to Anaesthesia at the University of Manchester, UK. He qualified from Queen Mary College, and The Medical College of St Bartholomew’s Hospital, London. He has conducted research into determining the depth of anaesthesia and diagnosing prion disease using measurements taken from the brainstem.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 6:5
150
© 2005 The Medicine Publishing Company Ltd
PHYSIOLOGY
Limbic system The limbic system (Figure 2) is responsible for the recall and expression of emotions such as fear, anger, pleasure and contentment. It is a prime target for anaesthesia, because elimination of emotion and the recall of unpleasant events is the basis of adequate hypnosis. Damage to different regions of the limbic system results in a characteristic loss of a component of emotion or memory. The limbic system is also a vital component of consciousness. Consciousness is not located in one particular region of the brain, though localized lesions in the solitary nucleus of the medulla oblongata (brainstem) or the anterior cingulate gyrus (a component of the limbic system) lead to coma. Consciousness appears to be the result of coordinated firing of large assemblages of neurons, whereas subconscious behaviour can be initiated and sustained by groups of relatively few neurons. Fast binding of relevant sensory inputs with large assemblage activity is probably required for the expression of consciousness and components of the limbic system are probably involved. Two types of consciousness have been suggested: core consciousness controls homeostasis whereas extended consciousness controls social interaction. Clinical sedation progressively abolishes extended consciousness, and general anaesthesia abolishes core consciousness.
• metencephalon (pons and cerebellum) • myelinocephalon (including the medulla oblongata). The most prevalent excitatory neurotransmitter in the brain is glutamate. γ-aminobutyric acid (GABA) receptors are the most important inhibitory receptors in the brain, apart from the brainstem, where glycine is also an important inhibitory neurotransmitter. There are many subtypes of receptor protein. This means that although a specific brain region may have a particular type of receptor, there is no guarantee that the receptor performs in the same manner as a slightly differently configured subtype of the same receptor in another region. For example, the potency of propofol and volatile anaesthesia at GABAA and glycine receptors is conferred by the protein receptor configuration directed by two amino acids; if those amino acids are substituted, the receptor is no longer sensitive to hypnotic anaesthetic agents. Cerebral cortex The cerebral cortex comprises millions of neurons. One of the most common is the pyramidal cell (Betz cell, see page 149). Axons from single pyramidal cells may terminate many centimetres from the cell body, against targets such as other pyramidal cells or, in the case of the corticospinal tract, even spinal interneurons linked to motor neurons. Graded potentials from thousands of pyramidal cells near recording electrodes are the basis for the electroencephalogram (EEG). Single neuron recordings from the sensory regions of the cerebral cortex, such as the primary visual cortex, have been performed since the 1960s and have revealed much about sensory processing. Histology of the cerebral cortex reveals six distinct layers. Layers I–IV receive input from other regions of the cerebral cortex and brain, mainly from the thalamus (layer IV). Layers V and VI house the cell bodies of cortical efferent interneurons. In the most heavily studied region, the primary visual cortex (V1 or Brodmann’s area 17), much initial processing is performed by ‘simple cells’ (i.e. neurons sensitive to visual stimuli such as a line moving at a particular orientation of movement). ‘Complex cells’ receive input from simple cells along with parallel direct input from the lateral geniculate nucleus, and are selective to movement in a particular direction (not just orientation) and spatial frequency; they are actively inhibited by movement at other orientations. Complex cells are often ‘end stopped’ (hypercomplex) whereby only stimuli of a specific length, orientation and spatial frequency are encoded into action potentials. Subtle changes in action potential frequency, and hence the sensitivity of these neurons to different characteristics of visual stimuli, have been observed with increasing depths of volatile anaesthesia. Visual neurons in the primary visual cortex are arranged in columns of similar orientation selectivity. These columns have been directly visualized by the use of voltage-sensitive dyes on the surface of the exposed cerebral cortex. Complex cells are also often sensitive to textured visual stimuli (e.g. wood grain pattern on a desk, or a ‘hidden eye’ visual illusion), to which simple cells are largely insensitive. Some have suggested that this is evidence for parallel processing of information in the primary visual cortex, rather than a strict hierarchy of function proposed by earlier researchers. It is likely that other sensory regions of the cerebral cortex have a similar organization, and most regions have been topographically mapped to the environmental spatial or frequency position of stimuli.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 6:5
Anterior cingulate gyrus Ablation of the prefrontal cortex or cingulate gyrus eliminates concern over chronic pain, and pain no longer elicits powerful emotions. Functional imaging studies of the anterior cingulate gyrus have implicated it in the presence of conscious awareness during light sedation. Activity in the left anterior cingulate proportionally decreases with increasing depth of anaesthesia and appears to correlate with the level of hypnosis. Hypothalamus and thalamus The hypothalamus and thalamus form the diencephalon. The thalamus is the main relay for ascending information entering the cerebral cortex. The dorsomedial thalamus has to be active during light anaesthesia for explicit recall of events to occur; it is important
Limbic system Prefrontal cortex
Association cortex
Cingulate gyrus
Hippocampal formation
Amygdala
Anterior thalamic nuclei
Mammillary body Hypothalamus
2
151
© 2005 The Medicine Publishing Company Ltd
PHYSIOLOGY
for the formation of explicit memory and its subsequent retrieval. The dorsomedial thalamus is one example of the thalamic regions, each of which demonstrates varied functionality. Unconscious regulation of autonomic function is integrated between the autonomic, somatic and endocrine systems by the hypothalamus, the main function of which is to keep internal conditions constant. The hypothalamus is responsible for homeostasis, especially by directing motivation (e.g. temperature, feeding, thirst). Control of core temperature is an important role performed by the hypothalamus, the posterior region of which compares information from relatively warmer neurons on the anterior hypothalamus with cold receptors in the skin. Internal heat is increased by raising the metabolic rate in combination with a decrease in peripheral blood flow in the skin. When core temperature rises, sweating is promoted and dilation of blood vessels in the skin promotes cooling. Anaesthesia disrupts homeostatic control originating from the hypothalamus, and this is a difference between natural sleep and anaesthesia. During natural sleep, the set point of temperature control falls, but it is still under active control by the hypothalamus. Natural sleep is regulated via the hypothalamus, partly by the release of endogenous sleep-promoting hormones.
opposed to ionotropic or within beat) heart rate variability. Brainstem and cerebral laterality of autonomic function is a fascinating and clinically important topic that is surprisingly poorly reported in the literature. Brainstem sensory nuclei are arranged in columns in a similar manner to sensory regions in the posterior horn of the spinal column. The tract of the solitary nucleus (nucleus tractus solitarius) is large and responsible for many crucial functions. The solitary nucleus lies within the visceral sensory column. The special sensory column and vestibular and auditory nuclei of the vestibulocochlear (VIII) nerve receive input from the vestibular organs and the cochlea. Auditory evoked potentials derived from these regions are largely resistant to the effects of hypnotic anaesthesia, their effects being more noted on the auditory cortex. The somatic sensory column receives input from the vagus (X), facial (VII), glossopharyngeal (IX), and trigeminal (V) nerves. The presence of noxious stimuli from the head, face and neck is transmitted through these nerves and via the trigeminal spinal nucleus, as is fine touch, which also uses the main sensory trigeminal nucleus. Vibration and position are encoded by the mesencephalic nucleus of the trigeminal nerve. Midbrain: the midbrain primarily controls sensory and motor function, particularly the control of eye movement and the coordination of visual and auditory reflexes. The midbrain provides much of the processing necessary to maintain posture, walking, positioning and jumping behaviours. However, the midbrain does not initiate these behaviours. An animal with an intact brainstem, including the midbrain but no higher brain, exhibits righting reflexes (i.e. it can restore its normal posture from an unusual starting point) and normal muscle tone (though visual righting reflexes are eliminated in an animal with no higher brain). Without the midbrain, the righting reflexes disappear and the animal exhibits high muscle tonus (decerebrate rigidity) but can still remain upright because the inputs from labyrinth and neck proprioceptors are still intact.
Hippocampus The hippocampus is involved in the acquisition of memory from experience, and is important for the recall of spatial position. Functional imaging has suggested that people with enhanced mapping skills have greater activity in the hippocampus. Lesions to specific regions of the hippocampus have resulted in subjects who are unable to recall any events that occurred after the lesion, even though memory of events up to the trauma is secure. Such subjects have proved that long-term memory is not necessary for consciousness; as long as subjects still have a functional working (real-time) memory they exhibit normal consciousness. Amygdala The amygdala comprises five nuclei in three groups: the central nucleus; the cortical and medial nuclei; the basal and lateral nuclei. There is extensive connectivity between the amygdala and the telencephalon, diencephalon, midbrain and brainstem (solitary nucleus). The amygdala integrates information from the senses and from within the body (visceral afferents) and associates it with the memory of past experience. Amygdala-driven aversion is particularly profound. Patients with amygdala lesions can see no wrong in people or places that have caused them particular emotional distress in the past. Paradoxically, such behaviour is considered to be mature by many cultures. Recent research suggests that memories mediated by the amygdala are disrupted profoundly during propofol anaesthesia.
Pons: the pons is the site of origin for the facial motor cranial nerves and their nuclei (e.g. trigeminal (V) nerve and its nucleus). The frontalis electromyogram (fEMG) has been studied extensively because it declines with increased anaesthetic depth, especially at relatively light levels of anaesthesia; the fEMG reflects activity in the pons. The pons falls within the territory of the medial branch of the basilar artery. Medulla oblongata: the medulla is a region of the brainstem responsible for autonomic control and falls within the territory of the anterior spinal artery. Damage to the anterior medulla results in ‘locked-in’ syndrome in which a patient is unable to move or communicate but is otherwise conscious. Damage to the posterior medulla often results in profound coma. Less severe damage is apparent in conditions such as Wallenberg’s syndrome, in which localized damage to the nucleus ambiguus causes disruption to motor fibres leading to the glossopharyngeal (IX), vagus (X) and accessory (XI) nerves, affecting muscles in the palate and the larynx, as well as fast beat-to-beat control of heart rate. The nucleus tractus solitarius is predominantly located in the medulla. It is a sensory nucleus, which receives input from many visceral receptors including stretch receptors in the lungs and the carotid sinus. The nucleus tractus solitarius is responsible for
Brainstem The brainstem is the most densely packed region of the brain with vital function. It has little redundancy of function in the event of damage, which is likely, owing to the proximity of the aqueduct of Sylvius (cerebral aqueduct), which is susceptible to occlusion. The brainstem is not bilaterally paired in function, each side has a bias towards differing autonomic function. A specific example of this is the right vagus (X) nerve, which originates in the medulla oblongata and appears to direct chronotropic (beat to beat, as
ANAESTHESIA AND INTENSIVE CARE MEDICINE 6:5
152
© 2005 The Medicine Publishing Company Ltd
PHYSIOLOGY
integrating the sensory information arriving for the baroreflex; the inferior nucleus tractus solitarius is responsible for encoding sensory information from the heart, lungs, blood vessels and gut. Vagally mediated spontaneous and evoked motor activity in the gut declines during anaesthesia, as does respiratory sinus arrhythmia. The superior nucleus tractus solitarius is also responsible for taste on the anterior 66% of the tongue from the glossopharyngeal (IX) nerve, and taste buds in the pharynx from the vagus (X) nerve. The nucleus tractus solitarius connects to the dorsal vagal motor nucleus and the nucleus ambiguus, both of which innervate the heart and modulate heart rate variability. The dorsal vagal motor nucleus also innervates secretory glands in the pharynx, lungs and gut, along with peripheral ganglia modulating smooth muscle tone in blood vessels, lung and gut. Cerebellum: the cerebellum comprises 50% of the neurons in the brain, even though it occupies only 10% of the volume. The cerebellum adjusts the output of the major descending systems of the brain, acting as a comparator with information received from the spinal cord, vestibular apparatus and nuclei, medulla, pons, reticular formation and cerebral cortex. Cerebellar efferent pathways project to the brainstem and thalamus to modulate a variety of motor and other functions. The cerebellum comprises three regions: • vestibulocerebellum, receiving input from the semicircular canals and otolith organ and encoding the position and movement of the head • spinocerebellum, regulating body and limb movement • cerebrocerebellum, evaluating sensory information to evaluate the planning of movement. The cerebellum is made up of three distinct layers: molecular layer, Purkinje cell layer and granular layer. Purkinje neurons in the cerebellum have large cell bodies and complex dendritic fields, and form many copies of the same basic neural circuit. The Purkinje cells are the only efferent axons leaving the cerebellum. Afferent climbing fibres pass through the granular layer and terminate at the granular layer at dendrites of the Purkinje cells, climbing through the dendritic tree. Afferent mossy fibres end in the granular layer at granule cells. Granule cells produce two collaterals (parallel fibres) in a T shape; collaterals viewed in cross-section explain the dot-like nature of the molecular layer. Climbing fibres synapse directly on the Purkinje neurons, whereas mossy fibres connect to the Purkinje cells via interneurons (granule and basket neurons). Lesions in the cerebellum result in hypotonia, ataxia and action- or intention-tremor. Lesions in the vestibulocerebellum are characterized by impaired eye movement, difficulty in maintaining balance and irregular movement, which becomes normal when supine. The spinocerebellum receives pathways from the spinal cord via precerebellar nuclei in the brainstem reticular formation. Different versions of the changing state of the subject and the environment are compared.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 6:5
153
© 2005 The Medicine Publishing Company Ltd
The brain: functional divisions Chris J D Pomfrett
Anaesthesia is not the same as natural sleep. The basis of contemporary balanced anaesthesia is the use of the CNS as a target, specific regions of which yield a component of good anaesthesia. Hypnosis (elimination of consciousness) and amnesia are achieved by disrupting the normal function of the cerebral cortex and underlying brain structures. Progressively greater depths of anaesthesia cause progressive reduction in the global metabolic rate of the brain. The adult brain (Figure 1) weighs 1250–1450 g within a volume of 1400 ml. It comprises the: • telencephalon (cerebral cortex, basal ganglia and olfactory bulb) • diencephalon (thalamus, hypothalamus and habenula) • mesencephalon or midbrain (structures around the cerebral aqueduct, superior and inferior colliculi and the cerebral peduncles)
The adult brain Olfactory bulb
Cerebral cortex Corpus callosum Diencephalon Midbrain Cerebellum Pons Medulla Spinal cord
1
Chris J D Pomfrett is Non-clinical Lecturer in Neurophysiology Applied to Anaesthesia at the University of Manchester, UK. He qualified from Queen Mary College, and The Medical College of St Bartholomew’s Hospital, London. He has conducted research into determining the depth of anaesthesia and diagnosing prion disease using measurements taken from the brainstem.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 6:5
150
© 2005 The Medicine Publishing Company Ltd
PHYSIOLOGY
Limbic system The limbic system (Figure 2) is responsible for the recall and expression of emotions such as fear, anger, pleasure and contentment. It is a prime target for anaesthesia, because elimination of emotion and the recall of unpleasant events is the basis of adequate hypnosis. Damage to different regions of the limbic system results in a characteristic loss of a component of emotion or memory. The limbic system is also a vital component of consciousness. Consciousness is not located in one particular region of the brain, though localized lesions in the solitary nucleus of the medulla oblongata (brainstem) or the anterior cingulate gyrus (a component of the limbic system) lead to coma. Consciousness appears to be the result of coordinated firing of large assemblages of neurons, whereas subconscious behaviour can be initiated and sustained by groups of relatively few neurons. Fast binding of relevant sensory inputs with large assemblage activity is probably required for the expression of consciousness and components of the limbic system are probably involved. Two types of consciousness have been suggested: core consciousness controls homeostasis whereas extended consciousness controls social interaction. Clinical sedation progressively abolishes extended consciousness, and general anaesthesia abolishes core consciousness.
• metencephalon (pons and cerebellum) • myelinocephalon (including the medulla oblongata). The most prevalent excitatory neurotransmitter in the brain is glutamate. γ-aminobutyric acid (GABA) receptors are the most important inhibitory receptors in the brain, apart from the brainstem, where glycine is also an important inhibitory neurotransmitter. There are many subtypes of receptor protein. This means that although a specific brain region may have a particular type of receptor, there is no guarantee that the receptor performs in the same manner as a slightly differently configured subtype of the same receptor in another region. For example, the potency of propofol and volatile anaesthesia at GABAA and glycine receptors is conferred by the protein receptor configuration directed by two amino acids; if those amino acids are substituted, the receptor is no longer sensitive to hypnotic anaesthetic agents. Cerebral cortex The cerebral cortex comprises millions of neurons. One of the most common is the pyramidal cell (Betz cell, see page 149). Axons from single pyramidal cells may terminate many centimetres from the cell body, against targets such as other pyramidal cells or, in the case of the corticospinal tract, even spinal interneurons linked to motor neurons. Graded potentials from thousands of pyramidal cells near recording electrodes are the basis for the electroencephalogram (EEG). Single neuron recordings from the sensory regions of the cerebral cortex, such as the primary visual cortex, have been performed since the 1960s and have revealed much about sensory processing. Histology of the cerebral cortex reveals six distinct layers. Layers I–IV receive input from other regions of the cerebral cortex and brain, mainly from the thalamus (layer IV). Layers V and VI house the cell bodies of cortical efferent interneurons. In the most heavily studied region, the primary visual cortex (V1 or Brodmann’s area 17), much initial processing is performed by ‘simple cells’ (i.e. neurons sensitive to visual stimuli such as a line moving at a particular orientation of movement). ‘Complex cells’ receive input from simple cells along with parallel direct input from the lateral geniculate nucleus, and are selective to movement in a particular direction (not just orientation) and spatial frequency; they are actively inhibited by movement at other orientations. Complex cells are often ‘end stopped’ (hypercomplex) whereby only stimuli of a specific length, orientation and spatial frequency are encoded into action potentials. Subtle changes in action potential frequency, and hence the sensitivity of these neurons to different characteristics of visual stimuli, have been observed with increasing depths of volatile anaesthesia. Visual neurons in the primary visual cortex are arranged in columns of similar orientation selectivity. These columns have been directly visualized by the use of voltage-sensitive dyes on the surface of the exposed cerebral cortex. Complex cells are also often sensitive to textured visual stimuli (e.g. wood grain pattern on a desk, or a ‘hidden eye’ visual illusion), to which simple cells are largely insensitive. Some have suggested that this is evidence for parallel processing of information in the primary visual cortex, rather than a strict hierarchy of function proposed by earlier researchers. It is likely that other sensory regions of the cerebral cortex have a similar organization, and most regions have been topographically mapped to the environmental spatial or frequency position of stimuli.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 6:5
Anterior cingulate gyrus Ablation of the prefrontal cortex or cingulate gyrus eliminates concern over chronic pain, and pain no longer elicits powerful emotions. Functional imaging studies of the anterior cingulate gyrus have implicated it in the presence of conscious awareness during light sedation. Activity in the left anterior cingulate proportionally decreases with increasing depth of anaesthesia and appears to correlate with the level of hypnosis. Hypothalamus and thalamus The hypothalamus and thalamus form the diencephalon. The thalamus is the main relay for ascending information entering the cerebral cortex. The dorsomedial thalamus has to be active during light anaesthesia for explicit recall of events to occur; it is important
Limbic system Prefrontal cortex
Association cortex
Cingulate gyrus
Hippocampal formation
Amygdala
Anterior thalamic nuclei
Mammillary body Hypothalamus
2
151
© 2005 The Medicine Publishing Company Ltd
PHYSIOLOGY
for the formation of explicit memory and its subsequent retrieval. The dorsomedial thalamus is one example of the thalamic regions, each of which demonstrates varied functionality. Unconscious regulation of autonomic function is integrated between the autonomic, somatic and endocrine systems by the hypothalamus, the main function of which is to keep internal conditions constant. The hypothalamus is responsible for homeostasis, especially by directing motivation (e.g. temperature, feeding, thirst). Control of core temperature is an important role performed by the hypothalamus, the posterior region of which compares information from relatively warmer neurons on the anterior hypothalamus with cold receptors in the skin. Internal heat is increased by raising the metabolic rate in combination with a decrease in peripheral blood flow in the skin. When core temperature rises, sweating is promoted and dilation of blood vessels in the skin promotes cooling. Anaesthesia disrupts homeostatic control originating from the hypothalamus, and this is a difference between natural sleep and anaesthesia. During natural sleep, the set point of temperature control falls, but it is still under active control by the hypothalamus. Natural sleep is regulated via the hypothalamus, partly by the release of endogenous sleep-promoting hormones.
opposed to ionotropic or within beat) heart rate variability. Brainstem and cerebral laterality of autonomic function is a fascinating and clinically important topic that is surprisingly poorly reported in the literature. Brainstem sensory nuclei are arranged in columns in a similar manner to sensory regions in the posterior horn of the spinal column. The tract of the solitary nucleus (nucleus tractus solitarius) is large and responsible for many crucial functions. The solitary nucleus lies within the visceral sensory column. The special sensory column and vestibular and auditory nuclei of the vestibulocochlear (VIII) nerve receive input from the vestibular organs and the cochlea. Auditory evoked potentials derived from these regions are largely resistant to the effects of hypnotic anaesthesia, their effects being more noted on the auditory cortex. The somatic sensory column receives input from the vagus (X), facial (VII), glossopharyngeal (IX), and trigeminal (V) nerves. The presence of noxious stimuli from the head, face and neck is transmitted through these nerves and via the trigeminal spinal nucleus, as is fine touch, which also uses the main sensory trigeminal nucleus. Vibration and position are encoded by the mesencephalic nucleus of the trigeminal nerve. Midbrain: the midbrain primarily controls sensory and motor function, particularly the control of eye movement and the coordination of visual and auditory reflexes. The midbrain provides much of the processing necessary to maintain posture, walking, positioning and jumping behaviours. However, the midbrain does not initiate these behaviours. An animal with an intact brainstem, including the midbrain but no higher brain, exhibits righting reflexes (i.e. it can restore its normal posture from an unusual starting point) and normal muscle tone (though visual righting reflexes are eliminated in an animal with no higher brain). Without the midbrain, the righting reflexes disappear and the animal exhibits high muscle tonus (decerebrate rigidity) but can still remain upright because the inputs from labyrinth and neck proprioceptors are still intact.
Hippocampus The hippocampus is involved in the acquisition of memory from experience, and is important for the recall of spatial position. Functional imaging has suggested that people with enhanced mapping skills have greater activity in the hippocampus. Lesions to specific regions of the hippocampus have resulted in subjects who are unable to recall any events that occurred after the lesion, even though memory of events up to the trauma is secure. Such subjects have proved that long-term memory is not necessary for consciousness; as long as subjects still have a functional working (real-time) memory they exhibit normal consciousness. Amygdala The amygdala comprises five nuclei in three groups: the central nucleus; the cortical and medial nuclei; the basal and lateral nuclei. There is extensive connectivity between the amygdala and the telencephalon, diencephalon, midbrain and brainstem (solitary nucleus). The amygdala integrates information from the senses and from within the body (visceral afferents) and associates it with the memory of past experience. Amygdala-driven aversion is particularly profound. Patients with amygdala lesions can see no wrong in people or places that have caused them particular emotional distress in the past. Paradoxically, such behaviour is considered to be mature by many cultures. Recent research suggests that memories mediated by the amygdala are disrupted profoundly during propofol anaesthesia.
Pons: the pons is the site of origin for the facial motor cranial nerves and their nuclei (e.g. trigeminal (V) nerve and its nucleus). The frontalis electromyogram (fEMG) has been studied extensively because it declines with increased anaesthetic depth, especially at relatively light levels of anaesthesia; the fEMG reflects activity in the pons. The pons falls within the territory of the medial branch of the basilar artery. Medulla oblongata: the medulla is a region of the brainstem responsible for autonomic control and falls within the territory of the anterior spinal artery. Damage to the anterior medulla results in ‘locked-in’ syndrome in which a patient is unable to move or communicate but is otherwise conscious. Damage to the posterior medulla often results in profound coma. Less severe damage is apparent in conditions such as Wallenberg’s syndrome, in which localized damage to the nucleus ambiguus causes disruption to motor fibres leading to the glossopharyngeal (IX), vagus (X) and accessory (XI) nerves, affecting muscles in the palate and the larynx, as well as fast beat-to-beat control of heart rate. The nucleus tractus solitarius is predominantly located in the medulla. It is a sensory nucleus, which receives input from many visceral receptors including stretch receptors in the lungs and the carotid sinus. The nucleus tractus solitarius is responsible for
Brainstem The brainstem is the most densely packed region of the brain with vital function. It has little redundancy of function in the event of damage, which is likely, owing to the proximity of the aqueduct of Sylvius (cerebral aqueduct), which is susceptible to occlusion. The brainstem is not bilaterally paired in function, each side has a bias towards differing autonomic function. A specific example of this is the right vagus (X) nerve, which originates in the medulla oblongata and appears to direct chronotropic (beat to beat, as
ANAESTHESIA AND INTENSIVE CARE MEDICINE 6:5
152
© 2005 The Medicine Publishing Company Ltd
PHYSIOLOGY
integrating the sensory information arriving for the baroreflex; the inferior nucleus tractus solitarius is responsible for encoding sensory information from the heart, lungs, blood vessels and gut. Vagally mediated spontaneous and evoked motor activity in the gut declines during anaesthesia, as does respiratory sinus arrhythmia. The superior nucleus tractus solitarius is also responsible for taste on the anterior 66% of the tongue from the glossopharyngeal (IX) nerve, and taste buds in the pharynx from the vagus (X) nerve. The nucleus tractus solitarius connects to the dorsal vagal motor nucleus and the nucleus ambiguus, both of which innervate the heart and modulate heart rate variability. The dorsal vagal motor nucleus also innervates secretory glands in the pharynx, lungs and gut, along with peripheral ganglia modulating smooth muscle tone in blood vessels, lung and gut. Cerebellum: the cerebellum comprises 50% of the neurons in the brain, even though it occupies only 10% of the volume. The cerebellum adjusts the output of the major descending systems of the brain, acting as a comparator with information received from the spinal cord, vestibular apparatus and nuclei, medulla, pons, reticular formation and cerebral cortex. Cerebellar efferent pathways project to the brainstem and thalamus to modulate a variety of motor and other functions. The cerebellum comprises three regions: • vestibulocerebellum, receiving input from the semicircular canals and otolith organ and encoding the position and movement of the head • spinocerebellum, regulating body and limb movement • cerebrocerebellum, evaluating sensory information to evaluate the planning of movement. The cerebellum is made up of three distinct layers: molecular layer, Purkinje cell layer and granular layer. Purkinje neurons in the cerebellum have large cell bodies and complex dendritic fields, and form many copies of the same basic neural circuit. The Purkinje cells are the only efferent axons leaving the cerebellum. Afferent climbing fibres pass through the granular layer and terminate at the granular layer at dendrites of the Purkinje cells, climbing through the dendritic tree. Afferent mossy fibres end in the granular layer at granule cells. Granule cells produce two collaterals (parallel fibres) in a T shape; collaterals viewed in cross-section explain the dot-like nature of the molecular layer. Climbing fibres synapse directly on the Purkinje neurons, whereas mossy fibres connect to the Purkinje cells via interneurons (granule and basket neurons). Lesions in the cerebellum result in hypotonia, ataxia and action- or intention-tremor. Lesions in the vestibulocerebellum are characterized by impaired eye movement, difficulty in maintaining balance and irregular movement, which becomes normal when supine. The spinocerebellum receives pathways from the spinal cord via precerebellar nuclei in the brainstem reticular formation. Different versions of the changing state of the subject and the environment are compared.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 6:5
153
© 2005 The Medicine Publishing Company Ltd