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Responses of cat C1 spinal cord dorsal and ventral horn neurons to noxious and non-noxious stimulation of the head and face
181
Brain Research, 555 (1991) 181-192 © 1991 Elsevier Science Publishers B.V. 0006-8993/91/$03.50 ADONIS 000689939116647Q BRES 16647
Research Reports
Responses of c a t C 1 spinal cord dorsal and ventral horn neurons to noxious and non-noxious stimulation of the head and face Eric H. Chudler 1, Warren E.
Foote 2
and Charles E.
Poletti 1
Laboratory of Pain Research, Departments of 1Neurosurgery and 2psychiatry, Massachusetts General Hospital, Boston, MA 02114 (U.S.A.) (Accepted 8 January 1991)
Key words: Cervical spinal cord; Trigeminal; Pain; Electrophysiology Previous anatomical studies have shown that trigeminal and cervical afferent nerve fibers project to the upper cervical segments of the spinal cord. To determine the response properties of neurons in the upper cervical spinal cord, we studied the response of C 1 dorsal and ventral horn cells to electrical and graded mechanical stimulation of the face, head and neck in anesthetized cats. Neurons were classified as low-threshold-mechanoreceptive (LTM), wide-dynamic-range (WDR), nociceptive-specific (NS) or unresponsive, based on their responsiveness to graded mechanical stimulation. Extracellular single unit recordings were obtained from 118 neurons excited by cervical (24), trigeminal (39) or both cervical and trigeminal (55) stimulation and from 24 neurons unresponsive to peripheral stimulation. Based on neuronal mechanical response properties, 52.2% of the responsive neurons were classified as LTM, 35.9% as WDR and 11.9% as NS. WDR neurons exhibited more convergence and had larger receptive fields than either NS or LTM neurons. WDR and NS neurons had longer first spike latencies than LTM neurons at all tested sites. Only WDR neurons were found to project to the contralateral caudal thalamus. Within C1, LTM neurons were located primarily in laminae III and IV, WDR neurons in lamina V and NS neurons in laminae VII and VIII. These data suggest that some neurons in the first cervical segment of the spinal cord receive convergent input fore trigeminal and cervical pathways and may be involved in mediating orofacial and cranial pain. INTRODUCTION T h e u p p e r cervical spinal cord (segments C1, C 2 and C3) is a region where information transmitted by the trigeminal and cervical nerves may converge. A n a t o m i c a l studies using various techniques have d e m o n s t r a t e d that trigeminal p r i m a r y afferent fibers from a n u m b e r of nerves p r o j e c t as far caudal as the second (C2) and third (C3) cervical segments of the spinal cord 5'28'a3-35'4°" 50,54-s6. Transganglionic transport of horseradish peroxidase ( H R P ) from C 2 cutaneous nerves supplying the pinna and skin of the occiput results in terminal labelling in the dorsal horn as far rostral as the spinomedullary junction 3. M o r e o v e r , terminal labelling after injections of H R P into cervical dorsal r o o t ganglia and degeneration p a t t e r n s after section o f the u p p e r cervical nerves overlap labelling seen in the spinal cord after H R P injection into the trigeminal ganglion 1s,29,45. M o s t electrophysiological studies investigating the neural mechanisms subserving orofacial pain and headache have focused on the trigeminal brainstem sensory nuclear complex rostral to the spinomedullary junction. Since subnucleus caudalis has similar anatomical and functional characteristics as the spinal dorsal horn, it has
been referred to as the m e d u l l a r y dorsal horn 2a,~. Single unit recordings in subnucleus caudalis have identified neurons that can be classified as low-threshold-mechanoreceptive (LTM), wide-dynamic-range ( W D R ) and nociceptive-specific (NS) b a s e d on their cutaneous receptive field p r o p e r t i e s 4'16'26'52. C o n v e r g e n t afferent input from trigeminal, cervical and hypoglossal nerves onto single neurons in subnucleus caudalis was f o u n d to be m o r e c o m m o n for W D R and NS neurons than for LTM neurons 24,26,52. Evidence of trigeminal and cervical convergence onto single neurons in dorsal and ventral horns of the u p p e r cervical spinal cord is sparse. A b r a h a m s et al. 1 r e p o r t e d that about 40% of the neurons r e c o r d e d in the dorsal and ventral horns of the u p p e r cervical spinal cord could be excited by electrical stimulation of b o t h the infraorbital nerve (trigeminal nerve) and neck muscle afferent nerves (cervical nerve). A d d i t i o n a l l y , l a m i n a V I I neurons in the C 1 segment of the spinal cord r e s p o n d e d to electrical stimulation of the C 3 nerve and t o o t h pulp and had cutaneous receptive fields including the cornea and pinna 62. N e u r o n s located in the dorsal and ventral horns of C1 and C 2 have also b e e n shown to r e s p o n d to electrical stimulation o f u p p e r cervical nerve roots and
Correspondence: E.H. Chudler, Laboratory of Pain Research, Department of Neurosurgery, MGH-east; Bldg. 149, 13th Street, Charlestown, MA 02129, U.S.A.
182 the t r i g e m i n a l g a n g l i o n 37. H o w e v e r , d a t a c o n c e r n i n g the f r e q u e n c y o f c o n v e r g e n t input, the c u t a n e o u s m e c h a n i c a l response
properties
and
receptive
field locations
of
n e u r o n s l o c a t e d in the u p p e r cervical c o r d h a v e n o t b e e n adequately described. The
present
study
investigated
the
electrical
and
m e c h a n i c a l r e s p o n s e p r o p e r t i e s o f n e u r o n s l o c a t e d in the dorsal and v e n t r a l h o r n s of the C1 s e g m e n t of the spinal cord. T h e skin i n n e r v a t e d by the t r i g e m i n a l and u p p e r cervical n e r v e s was electrically s t i m u l a t e d to identify single n e u r o n s in the C 1 spinal cord. G r a d e d m e c h a n i c a l s t i m u l a t i o n was used to m a p the c u t a n e o u s r e c e p t i v e fields o f e a c h r e s p o n s i v e n e u r o n and to classify e a c h n e u r o n as L T M , W D R o r N S type. T h e s e n e u r o n s w e r e f u r t h e r classified as p r o j e c t i o n o r n o n - p r o j e c t i o n types if t h e y r e s p o n d e d to electrical s t i m u l a t i o n of the contralateral c a u d a l t h a l a m u s . T h e s e d a t a d e m o n s t r a t e that the C1 s e g m e n t o f t h e spinal c o r d is an i m p o r t a n t r e g i o n s u b s e r v i n g o r o f a c i a l a n d cranial pain and m a y play a role in r e f e r r e d p a i n f r o m t h e t r i g e m i n a l and u p p e r cervical dermatomes.
MATERIALS AND METHODS
Animal preparation Experiments were performed on 14 adult male or female cats weighing between 2.3 and 5.0 kg (mean = 3.7 kg). Cats were pretreated with atropine sulfate (0.4 rag, i.m.) and initially anesthetized with ketamine hydrochloride (30 mg/kg, i.m.). The femoral vein and artery of the left leg were cannulated for infusing drugs and fluids and for recording blood pressure, respectively. A tracheostomy was performed to allow artificial respiration. Anesthesia was subsequently maintained with periodic injections of sodium pentobarbital (5 mg/kg, i.v.). During recording sessions, cats were paralyzed with small doses (3-4 mg/kg) of gallamine triethiodide. The flexion withdrawal reflex of each cat was tested during periods of recovery from paralysis. If a flexion reflex was elicited, additional doses of pentobarbital sodium were administered. Spontaneous motor activity never occurred. Moreover, anesthesia was sufficient such that increases in blood pressure did not occur in response to noxious stimuli. Positive-pressure ventilation was employed to keep the end-tidal CO2 concentration between 3.5 and 4.5%. Rectal temperature was monitored and maintained between 37 and 39 °C by a circulating water heating pad. Experiments were terminated if physiological conditions could not be maintained within normal ranges. Cats were placed in a stereotaxic apparatus and a vertebral clamp was attached to a spinous process at the mid-thoracic level. A bilateral pneumothorax was performed to stabilize the recording preparation. A craniectomy was performed in each cat to permit recording and subsequent stimulation of the caudal thalamus. An array of 3 monopolar, epoxy-coated, stainless steel electrodes (tip diameter: 0.1 mm; interelectrode distance: 1.0 ram) was stereotaxically placed in the caudal thalamus (AP: 6.5-8.0 mm; LR: 3.5-6.5 mm) 6'57. Evoked potentials elicited by electrical stimulation of the face and head were used to position thalamic electrodes at sites producing the largest responses. Removal of the occipital bone and a cervical laminectomy (C~ and C2) were performed to expose the caudal medulla and upper cervical spinal cord. The dura mater was cut and retracted for several millimeters rostrai and caudal to the C 1 nerve rootlets. Once the dura was retracted, the spinal cord was kept moist by periodic
application of warm saline or mineral oil.
Stimulation procedures Platinum needle electrodes were inserted into the skin of the cheek, scalp, vibrissae and ear and the gingiva lateral to the mandibular canine tooth. Ipsilateral electrical stimuli (0.2 ms duration, 0.2-10.0 mA, 0.5 Hz, square waves) delivered through these electrodes were used to search for single neurons in C 1 spinal cord. Insertion of stimulating electrodes was made in accordance with trigeminal and cervical dermatomes that have been delimited in other studies 2"~4'22. Therefore, electrodes in the ginglva, cheek and vibrissae corresponded to the trigeminal dermatomes, while those in the ear and scalp were related to the C~ and C 2 dermatomes. In 3 cats, an additional pair of silver disk electrodes (electrode diameter: 0.45 ram) was placed on the dura overlying the superior sagittal sinus near the junction with the transverse sinus. Thalamic stimulation also consisted of 0.2 ms constant current square-waves between 0.05 and 2.0 mA. Criteria for antidromic activation of spinothalamic tract neurons included constant spike latency, ability to follow high frequency stimulation (333 Hz) and collision of the antidromic and orthodromic spikes. Mechanical stimuli were delivered to determine each neuron's receptive field after the responsiveness of each neuron to electrical stimuli was characterized. Mechanical stimuli consisted of air puffs, brushing with a hair brush, light tapping with a wooden dowel, non-noxious pressure applied with a large arterial clip and noxious pinch applied with a small arterial clip. Noxious pinch applied to the skin of the investigators with the small arterial clip was always judged to be painful. The extent of each receptive field was mapped and drawn on a figure of the cat's head, face and neck. Neurons were classified as responding to stimulation within only the trigeminal dermatome, only the cervical dermatomes, or to both the cervical and trigeminal dermatomes. Neurons were further classified into three categories based on their responsiveness to the graded mechanical stimuli. LTM neurons responded to air puffs, brushing, light tapping or pressure, but did not increase their discharge frequency to stimuli in the noxious range. WDR cells were also excited by brushing, light tapping or pressure, but were maximally responsive to noxious mechanical stimuli (pinch). NS neurons were activated only by noxious mechanical stimuli. Classification of cells in this manner was similar to procedures used in the spinal and medullary dorsal horns 9'17'20'23'24'31'53.
Recording procedures Extracellular recordings from the spinal cord were made with epoxy-coated tungsten microelectrodes (1-10 Mr2). Large receptive fields and the inability of C~ spinal cord neurons to follow high frequency (500 Hz) electrical stimulation of the skin distinguished spinal cord neurons from primary afferent nerve fibers. Additionally, C 1 spinal cord recordings had negative spikes and most had clear initial segment-somatodendritic breaks, while axonal recordings had initially positive-going action potentials. Microelectrode penetrations into the spinal cord ranged from a point midway between the most rostral C 2 nerve rootlet and most caudal C~ nerve rootlet to the level of the most rostral C 1 nerve rootlet. Electrode tracks were made perpendicular to the spinal cord surface. Parallel electrode penetrations, separated by 250-500/zm, were made in medial-lateral rows at 500-/~m intervals along the rostral-caudal extent of C r Neuronal recordings were amplified, filtered (300 Hz-3 kHz), and recorded on magnetic tape for off-line data analysis.
Data analysis Signals were led to a digitizing oscilloscope for measurement of the latency from the start of each cutaneous electrical stimulus to the peak of the action potential. Three trials consisting of suprathreshold stimuli for each effective stimulus site were used to obtain the minimum latency of each spike. The minimum current intensity (current threshold) necessary to elicit an action potential was also determined for each neuron and stimulation site by decreasing the
183 stimulus intensity until only 50% of the stimuli were effective. Differences in the latencies and current thresholds of LTM, WDR and NS neurons for each stimulus location were analyzed using one-way Analysis of Variance (ANOVA) procedures followed by paired-comparisons using the Newman-Keuls test. Brainwave Discovery software was used to discriminate individual spikes and to construct peristimulus time histograms of neuronal responses elicited by mechanical stimuli. A 10-s sample of spontaneous activity was obtained from these histograms before receptive fields were mapped.
Histological procedures In selected cases, the location of the recording electrode was marked with an electrolytic lesion (cathodal, 50-100 #A, 15-30 s) placed at the recording site. No more than 3 lesions were produced in one animal. Lesions were spaced widely to avoid confusion between recording loci. An additional lesion (cathodal, 200 #A, 20 s) was produced at the thalamic stimulating site. After completion of data collection, cats were overdosed with pentobarbital sodium. Some cats were then perfused through the heart with 10% formalin. The caudal medulla, upper cervical spinal cord and brain of all cats were removed and post-fixed in 10% formalin, then placed into phosphate-buffered 30% sucrose. Serial frozen sections (50 gm
thickness) were made through the thalamus and upper cervical spinal cord. Tissue sections containing lesions were mounted on subbed slides, dehydrated and stained with thionin. Lesions were reconstructed using a microprojector and a microscope with a drawing tube attachment. The laminar organization of the upper cervical spinal cord was distinguished using criteria established by Rexed ~'49. The stereotaxic atlases of Berman and Jones 6 and Snider and Niemer 57 were used to identify structures in the caudal thalamus.
RESULTS
Extracellular recordings were obtained from 142 neurons in the dorsal and ventral horns of the C 1 spinal cord. Of these neurons, 25 neurons exhibited spontaneous activity that varied in frequency from 0.7 to 25.9 Hz (mean + S.E.M. = 8.5 + 1.2 Hz). The remainder of the neurons showed no spontaneous activity and were identified using electrical stimulation of the skin or gingiva.
A
B
5 ms
C
D 200.
j
Brush • vv
~
•
150. Air P u f f
0
,,,.,
@
i
Pinch
m
100. Pressure
@ U)
50"
0
10
20
30
40
1ram
Time ( s e e ) Fig. 1. Electrical and mechanical response properties of a C 1 LTM type neuron. A: neuronal response to electrical stimulation of the dorsal aspect of the ear. Minimum first spike latency is at 12.9 ms. Arrow indicates the electrical stimulus onset. B: receptive field of neuron to mechanical stimulation of the ear. Shaded area represents the region that when stimulated excited the neuron. C: peristimulus time histogram of the neuronal response to graded mechanical stimulation applied to the ear. Air puffs and brushes elicited the highest discharge frequency. D: location of the recording site was in lamina IV of the C1 spinal cord.
184
A
B
mO
It
C
D
Pinch
120"
0 m
60-
Pressure
o 0 Brush
30-
0
10
20 Time
30
40 1ram
(sec)
Fig. 2. Electrical and mechanical response properties of a C 1 WDR type neuron. A: neuronal response to electrical stimulation of the cheek (a), vibrissae (b), gingiva (c) and ear (d). Arrow indicates the stimulus onset. B: cutaneous receptive field of neuron to mechanical stimulation of the head and face. Shaded area represents the region that when stimulated excited the neuron. C: peristimulus time histogram of the neuronal response to graded mechanical stimulation applied to the skin adjacent (lateral) to the eye. While non-noxious brushes and pressure did activate this neuron, the maximum response was to noxious pinch. D: location of the recording site was in the reticulated area of lamina V of the C 1 spinal cord. T h e n u m b e r s o f C1 n e u r o n s that r e s p o n d e d to electri-
t h r e s h o l d stimulus intensities. I n c r e a s i n g electrical stim-
cal s t i m u l a t i o n o f orofacial and cranial areas are listed in
ulus intensity p r o d u c e d m u l t i p l e spikes a n d r e d u c e d first
T a b l e I. C 1 n e u r o n s r e s p o n d e d with 1 or 2 spikes to
spike latencies. E l e c t r i c a l s t i m u l a t i o n o f t h e e a r o r c h e e k excited m o r e t h a n 5 0 % of t h e t e s t e d n e u r o n s . Stimulation of t h e s u p e r i o r sagittal sinus (SSS) a c t i v a t e d o n l y 7.1%
TABLE I Mean latencies, mean current thresholds for activation and the number o f C 1 neurons responsive to electrical stimulation o f various sites on the head and face
of the t e s t e d
neurons
and
resulted
in l o n g e r
m i n i m u m first spike latencies t h a n s t i m u l a t i o n o f s o m a t i c structures (Table I). T h e m e a n electrical t h r e s h o l d s n e c e s s a r y to a c t i v a t e C 1 n e u r o n s did n o t differ significantly with stimulus l o c a t i o n (F5,157 = 0.71; P > 0.05).
Site
no. responsive/ no. tested
Mean latency (ms + S.D.)
Mean threshold (mA + S.D.)
Cheek Scalp Ear Gingiva Vibrissae SSS
72/126 30/103 69/126 43/126 38/98 2/28
8.6 + 11.1 + 10.1 + 10.0 + 9.3 + 17.9 +
2.9 + 3.3 + 2.7 + 3.1 + 3.7 + 2.8 +
3.6 6.6 4.7 3.7 4.7 7.8
2.2 2.4 2.3 2.3 2.3 2.8
R e s p o n s e p r o p e r t i e s o f C 1 s p i n a l c o r d n e u r o n s to m e c h a n ical s t i m u l a t i o n B a s e d on t h e n e u r o n a l r e s p o n s e p r o p e r t i e s to m e c h a n -
ical s t i m u l a t i o n , C 1 n e u r o n s w e r e classified as L T M , W D R o r N S n e u r o n s . T h e r e s p o n s e s of d i f f e r e n t classes of C 1 spinal c o r d n e u r o n s to electrical and m e c h a n i c a l
185
B
A
5 ms
C
D
20-
Pinch
15. 0
@
® 10@ .1¢ CL Brush G) 5-
Prissure
t~
V
0
V
V
II
V
'
1'0
' Time
I
,i !
20
3' 0
(see)
Fig. 3. Electrical and mechanical response properties of a C~ nociceptive-specific (NS) type neuron. A: neuronal response to electrical stimulation of the vibrissae. Arrow indicates the stimulus onset. B: cutaneous receptive field of the neuron to mechanical stimulation of the face. Shaded area represents the region that when stimulated activated the neuron. C: peristimulus time histogram of the neuronal response to graded mechanical stimulation applied to the skin ventral to the eye. Only noxious pinch resulted in activation of this neuron. D: location of the recording site was in lamina VII.
stimulation are illustrated in Figs. 1, 2 and 3. The neuron depicted in Fig. 1 was classified as an LTM neuron. Electrical stimulation applied to the ear excited this neuron with a latency of 12.9 ms (Fig. 1A). This neuron responded to innocuous mechanical stimulation (air puffs/brush) applied to the dorsal and ventral surfaces of the ear (Fig. 1B). Increasing stimulus intensity into the noxious range did not produce further increases in neuronal discharge (Fig. 1C). All LTM neurons responded to mechanical stimuli with a short latency. Two types of LTM neurons were observed: one type was excited by hair movement and had a rapidly adapting response and the other type responded maximally to pressure. Neurons responding best to pressure continued to discharge during stimulus application. W D R neurons responded to innocuous stimulation, but w e r e maximally responsive to noxious mechanical
30
• Cervical only [ ] Trigeminal only [ ] Cervical and Trigeminal
25
2 20 Z
..121
E
10
Z
5
LTM
WDR
NS
Fig. 4. Number of neurons classified as LTM, WDR and NS neurons that received cutaneous input from the trigeminal, cervical or trigeminal and cervical dermatomes. There were significant differences between the number of LTM, WDR and NS neurons that received input from these dermatomal divisions (Z2 = 17.89, P < 0.01).
186
LTM
WDR
NS
LTM
(~ 20 E
£ Z
.~
5
z
o
E
1T
~
1C
2T
1T+C
3T
2T+C 3T+C
3T
2T+C 3T+C
WDR
10
Z
~
s
10[ E
z
0
1T
1C
2T
1T+C
NS
o= Z
Fig. 5. Typical receptive field locations LTM, W D R and NS neurons. Five examples of each type of neuron are shown in 3 columns. Shaded area drawn on lateral and dorsal views of the cat's head and face represent the region that when mechanically stimulated activated each neuron.
stimulation. An example of a WDR neuron is illustrated in Fig. 2. Electrical stimulation of the cheek (a), vibrissae (b), gingiva (c) and ear (d) all activated this neuron (Fig. 2A). Additionally, innocuous brushing or pressure applied to the entire ipsilateral face, ear, tongue, gingiva and scalp excited this neuron (Fig. 2B). However, as shown in Fig. 2C, a noxious pinch applied within the cutaneous receptive field resulted in an additional increase in discharge frequency. Spontaneous activity of 2 WDR neurons in the C 1 spinal cord was inhibited by mechanical stimulation outside the neuron's excitatory receptive fields. In these cases, application of a noxious pinch applied to the contralateral pinna inhibited spontaneous activity for the duration of the pinch. When the pinch stimulus was terminated, spontaneous activity
T A B L E II
Representation of the ophthalmic, maxillary and mandibular cutaneous receptive fields of CI spinal cord neurons Each value represents the n u m b e r of times each division of the trigeminal nerve was incorporated into the cutaneous receptive field of LTM, W D R and NS neurons.
Cell type
LTM WDR NS
Number of cells Ophthalmic
Maxillary
Mandibular
8 18 5
21 20 7
28 25 5
r~
_E Z
0
1T
1C
2T
1T+C
3T 2T+C
3T+C
Fig. 6. N u m b e r of n e u r o n s classified as LTM, W D R and NS that received cutaneous input from trigeminal and cervical d e r m a t o m e s . I T = one division of the trigeminal nerve; 1C = C1/C 2 d e r m a t o m e only; 2T = two divisions of the trigeminal nerve; IT + C = one division of the trigeminal nerve and the C1/C 2 d e r m a t o m e ; 2T + C = two divisions of the trigeminal nerve and the C1/C 2 d e r m a t o m e ; 3T = three divisions of the trigeminal nerve; 3T + C = three divisions of the trigeminal nerve and the C~/C 2 d e r m a t o m e .
rapidly increased back to prestimulus levels. NS neurons did not respond to innocuous stimuli, but were activated only by noxious pinch stimuli. The NS neuron depicted in Fig. 3 responded to electrical stimulation of the vibrissae with a latency of 20.8 ms (Fig. 3A) and had a receptive field including the ophthalmic and maxillary divisions of the trigeminal nerve (Fig. 3B). While brushing or pressure applied to the receptive field did not excite this neuron, a noxious pinch produced a significant increase in discharge frequency (Fig. 3C). The numbers of neurons classified as LTM, WDR and NS are illustrated in Fig. 4. LTM neurons accounted for 52.2% of the driven cells recorded in C 1. WDR (35.9%) and NS (11.9%) neurons were encountered less frequently. Neurons having receptive fields encompassing only the ipsilateral trigeminal dermatome were usually excited by cutaneous stimulation of the cheek or ventral aspect of the pinna. Receptive fields incorporating the C 1 and C 2 dermatomes included the skin or hair of the dorsal aspect of the ear, neck, or scalp at or caudal to vertex. Typical examples of the receptive field locations of LTM, WDR and NS neurons are shown in Fig. 5. Of the 92
187
A
5
ms
D
C a
a
b
b
C C I
A
&
0.1
mV 10
ms
Fig. 7. Antidromic responses of a C 1 WDR neuron to stimulation of the caudal thalamus. A: three overlapping traces of the response elicited by thalamic stimulation (600/~A). The latency of the response was constant at 3.8 ms. Arrow indicates the stimulus onset. B: antidromic response of the neuron to high frequency thalamic stimulation. Arrows indicate the thalamic stimulus onset, arrowheads indicate the antidromic spikes. C,D: collision of orthodromic spikes elicited by electrical stimulation of the ear (C) and cheek (D) with antidromic spikes elicited by thalamic stimulation. Top and middle traces show the neuronal response to electrical stimulation of the skin and thalamus, respectively. Bottom traces illustrate the absence of the antidromic spike when preceded by orthodromie spikes. Solid arrows, orthodromie stimulus onset; open arrows, antidromic stimulus onset; solid arrowheads, location of antidromic spike; open arrowheads, expected location of antidromic spikes during collision.
neurons classified as LTM, W D R
or NS types, 40
TABLE III
received input from both the cervical and trigeminal
Mean latencies (ms + S.D.) o f trigeminal and cervical afferent inputs to LTM, W D R and NS neurons in the C 1 spinal cord
d er m at o m es. H o w e v e r , W D R neurons were m o r e likely
Input
LTM
WDR
NS
Scalp Ear Gingiva Vibrissae Cheek SSS
5.9+2.1 6.5+2.8 7.0 + 2.2 5.3 + 1.9 6.5 + 2.3 -
13.3+8.7 13.1+3.0 10.5 + 2.5 11.3 + 3.7 9.9 + 3.2 17.9 + 7.8
14.0+2.1 16.6+4.9 11.5 ___3.3 13.9 ___5.6 12.7 + 3.8 -
to receive convergent input from both trigeminal and cervical d e r m a t o m e s than w e r e either NS or LTM neurons. As illustrated in Fig. 4, 26 of 33 (78.8%) W D R neurons received input from trigeminal and cervical d e r m a t o m e s , while only 2 of 11 (18.2%) NS cells and 12 of 48 (25.0%) LTM cells r e c e i v e d c o n v e r g e n t information. Th e differences in the n u m b e r o f L T M , W D R and NS neurons receiving trigeminal and cervical information
188
A7.9 Fig. 8. Locations of effective thalamic stimulation sites marked with electrolytic lesions. Each solid circle represents a site in the caudal thalamus that was effective in antidromically activating C 1 spinal cord neurons. Thalamic sections are drawn at two different anterior-posterior planes: AP +6.4 and AP +7.9. Nomenclature of thalamic nuclei is based on Berman and Jones6. BIC, brachium of the inferior colliculus; CP, cerebral peduncle; LGN, lateral geniculate nucleus; i G m , magnocellular medial geniculate; MGp; principal medial geniculate; ML, medial lemniscus; MM, medial mammillary nucleus; POt, posterior complex, intermediate region; POI., posterior complex, lateral re#on; POre, posterior complex, medial region; OT, optic tract; RE, reticular complex; SGN, suprageniculate nucleus; SUB, subthalamic nucleus; VBA, ventrobasal complex, arcuate nucleus; VMB, basal ventromedial nucleus.
were statistically significant (X2 = 17.89, P < 0.01). The receptive fields of LTM and NS neurons encompassed only one division of the trigeminal nerve or the C1 and C 2 d e r m a t o m e s in 25 of 48 cells (52.1%) and 5 of 11 cells (45.5%), respectively (Fig. 6). In contrast, only 5 of 33 (15.2%) W D R neurons had receptive fields incorporating just 1 division of the trigeminal nerve or a cervical nerve, while 18 of 33 (54.5%) W D R neurons had receptive fields encompassing 2 or 3 divisions of the trigeminal nerve and the C t and C2 d e r m a t o m e s . W h e n the trigeminal d e r m a t o m e was included in the cutaneous receptive field, there was no a p p a r e n t bias for any division of the trigeminal nerve: each division of the trigeminal nerve was r e p r e s e n t e d equally (Table II; X2 = 5.02, P > 0.05). Bilateral receptive fields, extending across the midline to the contralaterai ear or side of the scalp, were found in 6 of 33 W D R neurons. N o n e of the LTM or NS neurons r e s p o n d e d to stimulation on the contralateral side of the body. Both of the neurons that were activated by SSS stimulation were classified as W D R neurons and received trigeminai and cervical convergent somatic input.
.......
.o."
Fig. 9. Locations of lesion marks used to determine LTM, WDR and NS recording sites in the C1 spinal cord. Solid triangles, LTM neurons; solid circles, WDR neurons; solid squares, NS neurons. Dotted line indicates the reticulated region within the C1 spinal cord.
189 Neurons with mechanoreceptive fields involving the mandibular or maxillary divisions of the trigeminal nerve were found more frequently in the medial half of lamina III and IV, while those having receptive fields including the mandibular division or the C 1 and C2 dermatomes, were found more laterally. Additionally, neurons in rostral sections of the C 1 spinal cord had a higher incidence of receptive fields incorporating the gingiva and vibrissae compared to caudal sections. The somatotopic organization of neurons in lamina V was difficult to assess due to their large receptive fields.
Response properties of C 2 spinal cord neurons to electrical stimulation The mean minimum latencies to electrical stimulation of the head and face for LTM, W D R and NS neurons are shown in Table III. Electrical stimulation resulted in significantly different minimum latencies for LTM, W D R and NS neurons (F2,198 = 59.10; P < 0.01). Paired comparisons revealed that W D R and NS neurons had significantly (P < 0.05) longer latencies than LTM neurons at all stimulation sites. However, the mean current thresholds of nociceptive neurons were not statistically different from that of non-nociceptive neurons (F2,139 = 0.45, P > 0.05). Thalamic projection of C1 spinal cord neurons Electrical stimulation of the caudal thalamus activated 11 of 33 W D R neurons. The mean antidromic peak latency of the 11 W D R neurons was 3.8 ms (+0.6 ms S.D.). None of the tested LTM (n = 42) or NS (n = 9) neurons responded to thalamic stimulation. Fig. 7 illustrates the response of a W D R neuron that was antidromically activated by electrical stimulation of the contralateral caudal thalamus. Fig. 7A shows the constant latency (3.8 ms) of the antidromically activated action potential. This neuron also followed high frequency thalamic stimulation successfully (Fig. 7B). Collision of antidromic spikes with orthodromic spikes elicited by electrical stimulation of the gingiva and ear is illustrated in Fig. 7C and D. Thalamic sites from which C 1 neurons could be activated antidromically are illustrated in Fig. 8. Most thalamic stimulating sites were located in and around the medial part of the thalamic posterior group (POre). The averaged threshold value necessary to activate C 1 spinal cord neurons with thalamic stimulation was 600/~A (+400 g A , S.D.). Location of C 1 spinal cord recording sites Histological reconstruction of recording sites as determined by the location of lesion marks was possible for 24 cells. Fig. 9 illustrates the locations of lesions made for LTM, W D R and NS neurons. Most LTM neurons were
located in laminae III and IV, while W D R neurons were encountered most often in lamina V, especially in the lateral reticulated area. Four W D R neurons were also found in the ventral horn in laminae VII and VIII. All cells classified as thalamic projection neurons were located in lamina V. Lesion marks made for 3 NS neurons were located in laminae VII and VIII. No lesion marks were found in lamina I or II. DISCUSSION This study has confirmed earlier results t'37 demonstrating convergence of cervical and trigeminal information in the upper cervical spinal cord. These findings have been extended in the present study to include data concerning cutaneous convergence between different classes (i.e. LTM, W D R and NS) of C 1 neurons. Regardless of neuronal class, convergence between trigeminal and cervical cutaneous input was found to occur in 50% of the classified C 1 neurons. This value is comparable to that found for cervical and trigeminal afferent convergence onto neurons in nucleus caudalis 53 and in the C 2 dorsal and ventral horns 1. The present study has demonstrated that the degree of convergence in the C~ spinal cord is related to the classification (i.e. LTM, W D R or NS) of each neuron. W D R neurons were more likely to receive convergent input than were LTM or NS neurons. The receptive field sizes of W D R neurons incorporated more divisions of the trigeminal nerve than did those of LTM or NS neurons. Larger receptive fields and more extensive convergence of afferent input onto W D R neurons compared to LTM or NS neurons is also a general feature found in more caudal segments of the spinal cord 17'42 and in nucleus caudalis 4'23'41'53. However, Ferrington et al. 19 reported that cervical (C7 segment) spinothalamic tract neurons in alpha-chloralose anesthetized cats had small receptive fields regardless of their classification. The large receptive fields of W D R neurons in C 1 suggests that these cells may be more involved in arousal or attentional mechanisms of pain rather than in localizing painful stimuli. Most C 1 neurons receiving input from the cervical dermatomes responded to mechanical stimulation of the dorsal aspect of the pinna or the scalp at or caudal to vertex. Feline trigeminal ganglion cells have not been shown to respond to stimulation of these areas 14'36'sl. Conversely, studies examining the cutaneous receptive fields of the upper cervical nerves have demonstrated that stimulation of the dorsal pinna and scalp can activate fibers in the C1, C 2 or C 3 rami 2,22. At present, the contribution of each upper cervical nerve to the convergence pattern observed in the current study is not known. Previous anatomical studies have indicated that the
190 mandibular, maxillary and ophthalmic divisions of the trigeminal nerve project to the upper cervical segments of the spinal cord 4°'5.-56. The auriculotemporalis nerve (mandibular division of the trigeminal nerve) innervates the skin on the ventral aspect of the ear and terminates in laminae I through V of the C1, C 2 and C 3 segments of the spinal cord 54. The high incidence of C 1 spinal cord neurons responding to stimulation of the ventral part of the pinna physiologically confirms these data. Therefore, these data demonstrate that the mandibular division of the trigeminal nerve is well represented in the C~ segment of the spinal cord. While studies using anterograde degeneration have shown that the maxillary division of the trigeminal nerve projects to the same level as the mandibular division 34, experiments using horseradish peroxidase show that the mandibular division projects more densely to C1 than does the maxillary division 54. In the present study, 51.1% and 38.8% of the C 1 neurons responded to stimulation of the cheek and vibrissae (maxillary nerve), respectively. These data, as well as the receptive field locations of all C 1 neurons, are in agreement with Shigenaga et al. 54, demonstrating that more caudal parts of the face and head are represented more strongly in the upper cervical spinal cord. The mean first spike latencies to electrical stimulation of the skin were significantly longer in C1 nociceptive neurons compared to non-nociceptive neurons. This is in agreement with previous reported latencies of LTM, W D R and NS neurons in nucleus caudalis 24'53. However, other investigations have failed to observe latency differences between LTM and W D R neurons 4'15. The short latency responses of LTM neurons suggest the involvement of the myelinated A-beta nerve fibers. Smaller diameter afferents (A-delta nerve fibers) may be responsible for the longer latency responses of nociceptive neurons. Alternatively, a polysynaptic pathway from the periphery to the recorded C~ spinal cord neuron which would also result in longer first spike latencies cannot be precluded. Since unmyelinated C-nerve fibers have conduction velocities below 2 m/s, the earliest first spike latency recorded in C 1 would occur near 50 ms. The absence of latencies over 50 ms precludes the possibility that C-nerve fibers were responsible for any of the responses observed in C~. The lack of C-nerve fiber activation may have been due to the relatively short duration (0.2 ms) stimuli used to search for neurons. Additionally, the anesthetic used may have depressed the C-fiber evoked response of C~ spinal cord neurons. The number of C 1 spinal cord neurons projecting to the contralateral caudal thalamus was relatively small. This is surprising given anatomical data indicating that the upper cervical spinal cord contains many spinothalamic tract neurons 7'8'1°. Previous studies have reported
that LTM, W D R and NS neurons in nucleus caudalis all project to the contralateral thalamus 4'21'53. While no LTM or NS neurons were found to project directly to the thalamus in the present study, 33.3% of the W D R neurons were antidromically activated by electrical stimulation of the thalamus. As in the present study, Ferrington et al. 19 found that stimulation of the medial part of the thalamic posterior region (POm) activated many neurons in the cervical spinal cord. These data confirm previous anatomical data demonstrating a projection from the upper cervical spinal cord to the caudal thalamus 7'8'1°. However, it must be noted that stimulus current spread to areas adjacent t o P O m (e.g. ventrobasal nuclei) and to fibers of passage to more rostral thalamic nuclei may also be responsible for activation of upper cervical spinal cord neurons. The distribution of LTM, W D R and NS neurons in the upper cervical spinal cord is similar to that found in more caudal segments of the cervical spinal cord 19 and in nucleus caudalis 25'26. LTM neurons were located primarily in laminae III and IV, W D R neurons in laminae V and NS neurons in laminae V, VII and VIII. Some W D R neurons with large receptive fields were also found in the ventral horn of C 1. Molinari 43 and Yokota and Koyama 62 also indicated that some ventral horn neurons respond to nociceptive stimuli and have large bilateral cutaneous receptive fields. Many nociceptive neurons in the medullary reticular formation (subnucleus reticularis dorsalis and subnucleus ventralis) also possess large receptive fields including the head and face 59'60'62'63. While previous studies have shown that NS neurons are often located in laminae I and 1124'27'47'53, n o NS cells were recorded in the superficial laminae of the C 1 spinal cord in the present study. The percentage of NS neurons found in C~ in the current study is comparable to that reported in some studies of nucleus caudalis 25'53, but is less than that of other studies 4. It is possible that NS cells were not adequately sampled in the present study for several reasons. First, the tungsten metal electrodes may have been biased against recording from NS cells. Since many NS cells are located in laminae I and II of the medullary 24,27,47'53 and spinal dorsal horn 11'46, inadequate NS cell sampling may have occurred due to difficulty in maintaining stable recordings in these superficial laminae. Additionally, the microelectrodes may not have been able to discriminate small neurons in the superficial laminae sufficiently 19. Secondly, substantial 'dimpling' of the spinal cord surface caused by the microelectrodes may have prevented adequate sampling of the superficial laminae. Third, electrical current thresholds of many NS cells may have been higher than the stimulus intensities used to search for C1 neurons. Another area of the spinal cord that may receive
191 convergent input from the cervical and trigeminal systems is the lateral cervical nucleus (LCN). T h e L C N , located v e n t r o l a t e r a l to the dorsal horn in the C 1 and C2 segments of the spinal cord, receives input from second o r d e r cells within laminae I I I - V of the dorsal horn. M a n y neurons in the L C N are characterized by large receptive fields that occasionally include most of the b o d y 12'3°. While it has also been suggested that L C N neurons receive input from the t o o t h pulp and trigeminal vascular structures 38' 39,44, receptive fields incorporating the h e a d and face are rare 3°. T h e absence of a significant population of u p p e r cervical dorsal horn neurons responsive to superior sagittal sinus stimulation has also been r e p o r t e d previously 38. Thus, neurons in the dorsal and ventral horns of C 1 a p p e a r to be m o r e i m p o r t a n t for receiving convergent cutaneous input from trigeminal and u p p e r cervical d e r m a t o m e s . M a n y clinical observations suggest the involvement of the u p p e r cervical nerves in trigeminal dysesthesias.
K e r r 32 r e p o r t e d that stimulation of the C 1 nerve evokes pain in the orbit, frontal a r e a and vertex. Disease involving C 2 and C 3 nerves can also result in facial pain 13"51'58'61. A n e s t h e t i c blocks o r section of the u p p e r cervical nerves can often alleviate facial pain 61. A d d i tionally, 70% of patients with t e m p o r o m a n d i b u l a r joint disease complain of pain in the occipital region ( A . K . Bouckoms and D. Keith, p e r s o n a l communication). The extensive convergence of trigeminal and cervical input onto C1 spinal cord neurons m a y account for m a n y of these observations. F u r t h e r m o r e , convergence of trigeminal and cervical somatic information in the u p p e r cervical dorsal and ventral horns m a y m e d i a t e pain referred to orofacial and cranial structures.
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Brain Research, 555 (1991) 181-192 © 1991 Elsevier Science Publishers B.V. 0006-8993/91/$03.50 ADONIS 000689939116647Q BRES 16647
Research Reports
Responses of c a t C 1 spinal cord dorsal and ventral horn neurons to noxious and non-noxious stimulation of the head and face Eric H. Chudler 1, Warren E.
Foote 2
and Charles E.
Poletti 1
Laboratory of Pain Research, Departments of 1Neurosurgery and 2psychiatry, Massachusetts General Hospital, Boston, MA 02114 (U.S.A.) (Accepted 8 January 1991)
Key words: Cervical spinal cord; Trigeminal; Pain; Electrophysiology Previous anatomical studies have shown that trigeminal and cervical afferent nerve fibers project to the upper cervical segments of the spinal cord. To determine the response properties of neurons in the upper cervical spinal cord, we studied the response of C 1 dorsal and ventral horn cells to electrical and graded mechanical stimulation of the face, head and neck in anesthetized cats. Neurons were classified as low-threshold-mechanoreceptive (LTM), wide-dynamic-range (WDR), nociceptive-specific (NS) or unresponsive, based on their responsiveness to graded mechanical stimulation. Extracellular single unit recordings were obtained from 118 neurons excited by cervical (24), trigeminal (39) or both cervical and trigeminal (55) stimulation and from 24 neurons unresponsive to peripheral stimulation. Based on neuronal mechanical response properties, 52.2% of the responsive neurons were classified as LTM, 35.9% as WDR and 11.9% as NS. WDR neurons exhibited more convergence and had larger receptive fields than either NS or LTM neurons. WDR and NS neurons had longer first spike latencies than LTM neurons at all tested sites. Only WDR neurons were found to project to the contralateral caudal thalamus. Within C1, LTM neurons were located primarily in laminae III and IV, WDR neurons in lamina V and NS neurons in laminae VII and VIII. These data suggest that some neurons in the first cervical segment of the spinal cord receive convergent input fore trigeminal and cervical pathways and may be involved in mediating orofacial and cranial pain. INTRODUCTION T h e u p p e r cervical spinal cord (segments C1, C 2 and C3) is a region where information transmitted by the trigeminal and cervical nerves may converge. A n a t o m i c a l studies using various techniques have d e m o n s t r a t e d that trigeminal p r i m a r y afferent fibers from a n u m b e r of nerves p r o j e c t as far caudal as the second (C2) and third (C3) cervical segments of the spinal cord 5'28'a3-35'4°" 50,54-s6. Transganglionic transport of horseradish peroxidase ( H R P ) from C 2 cutaneous nerves supplying the pinna and skin of the occiput results in terminal labelling in the dorsal horn as far rostral as the spinomedullary junction 3. M o r e o v e r , terminal labelling after injections of H R P into cervical dorsal r o o t ganglia and degeneration p a t t e r n s after section o f the u p p e r cervical nerves overlap labelling seen in the spinal cord after H R P injection into the trigeminal ganglion 1s,29,45. M o s t electrophysiological studies investigating the neural mechanisms subserving orofacial pain and headache have focused on the trigeminal brainstem sensory nuclear complex rostral to the spinomedullary junction. Since subnucleus caudalis has similar anatomical and functional characteristics as the spinal dorsal horn, it has
been referred to as the m e d u l l a r y dorsal horn 2a,~. Single unit recordings in subnucleus caudalis have identified neurons that can be classified as low-threshold-mechanoreceptive (LTM), wide-dynamic-range ( W D R ) and nociceptive-specific (NS) b a s e d on their cutaneous receptive field p r o p e r t i e s 4'16'26'52. C o n v e r g e n t afferent input from trigeminal, cervical and hypoglossal nerves onto single neurons in subnucleus caudalis was f o u n d to be m o r e c o m m o n for W D R and NS neurons than for LTM neurons 24,26,52. Evidence of trigeminal and cervical convergence onto single neurons in dorsal and ventral horns of the u p p e r cervical spinal cord is sparse. A b r a h a m s et al. 1 r e p o r t e d that about 40% of the neurons r e c o r d e d in the dorsal and ventral horns of the u p p e r cervical spinal cord could be excited by electrical stimulation of b o t h the infraorbital nerve (trigeminal nerve) and neck muscle afferent nerves (cervical nerve). A d d i t i o n a l l y , l a m i n a V I I neurons in the C 1 segment of the spinal cord r e s p o n d e d to electrical stimulation of the C 3 nerve and t o o t h pulp and had cutaneous receptive fields including the cornea and pinna 62. N e u r o n s located in the dorsal and ventral horns of C1 and C 2 have also b e e n shown to r e s p o n d to electrical stimulation o f u p p e r cervical nerve roots and
Correspondence: E.H. Chudler, Laboratory of Pain Research, Department of Neurosurgery, MGH-east; Bldg. 149, 13th Street, Charlestown, MA 02129, U.S.A.
182 the t r i g e m i n a l g a n g l i o n 37. H o w e v e r , d a t a c o n c e r n i n g the f r e q u e n c y o f c o n v e r g e n t input, the c u t a n e o u s m e c h a n i c a l response
properties
and
receptive
field locations
of
n e u r o n s l o c a t e d in the u p p e r cervical c o r d h a v e n o t b e e n adequately described. The
present
study
investigated
the
electrical
and
m e c h a n i c a l r e s p o n s e p r o p e r t i e s o f n e u r o n s l o c a t e d in the dorsal and v e n t r a l h o r n s of the C1 s e g m e n t of the spinal cord. T h e skin i n n e r v a t e d by the t r i g e m i n a l and u p p e r cervical n e r v e s was electrically s t i m u l a t e d to identify single n e u r o n s in the C 1 spinal cord. G r a d e d m e c h a n i c a l s t i m u l a t i o n was used to m a p the c u t a n e o u s r e c e p t i v e fields o f e a c h r e s p o n s i v e n e u r o n and to classify e a c h n e u r o n as L T M , W D R o r N S type. T h e s e n e u r o n s w e r e f u r t h e r classified as p r o j e c t i o n o r n o n - p r o j e c t i o n types if t h e y r e s p o n d e d to electrical s t i m u l a t i o n of the contralateral c a u d a l t h a l a m u s . T h e s e d a t a d e m o n s t r a t e that the C1 s e g m e n t o f t h e spinal c o r d is an i m p o r t a n t r e g i o n s u b s e r v i n g o r o f a c i a l a n d cranial pain and m a y play a role in r e f e r r e d p a i n f r o m t h e t r i g e m i n a l and u p p e r cervical dermatomes.
MATERIALS AND METHODS
Animal preparation Experiments were performed on 14 adult male or female cats weighing between 2.3 and 5.0 kg (mean = 3.7 kg). Cats were pretreated with atropine sulfate (0.4 rag, i.m.) and initially anesthetized with ketamine hydrochloride (30 mg/kg, i.m.). The femoral vein and artery of the left leg were cannulated for infusing drugs and fluids and for recording blood pressure, respectively. A tracheostomy was performed to allow artificial respiration. Anesthesia was subsequently maintained with periodic injections of sodium pentobarbital (5 mg/kg, i.v.). During recording sessions, cats were paralyzed with small doses (3-4 mg/kg) of gallamine triethiodide. The flexion withdrawal reflex of each cat was tested during periods of recovery from paralysis. If a flexion reflex was elicited, additional doses of pentobarbital sodium were administered. Spontaneous motor activity never occurred. Moreover, anesthesia was sufficient such that increases in blood pressure did not occur in response to noxious stimuli. Positive-pressure ventilation was employed to keep the end-tidal CO2 concentration between 3.5 and 4.5%. Rectal temperature was monitored and maintained between 37 and 39 °C by a circulating water heating pad. Experiments were terminated if physiological conditions could not be maintained within normal ranges. Cats were placed in a stereotaxic apparatus and a vertebral clamp was attached to a spinous process at the mid-thoracic level. A bilateral pneumothorax was performed to stabilize the recording preparation. A craniectomy was performed in each cat to permit recording and subsequent stimulation of the caudal thalamus. An array of 3 monopolar, epoxy-coated, stainless steel electrodes (tip diameter: 0.1 mm; interelectrode distance: 1.0 ram) was stereotaxically placed in the caudal thalamus (AP: 6.5-8.0 mm; LR: 3.5-6.5 mm) 6'57. Evoked potentials elicited by electrical stimulation of the face and head were used to position thalamic electrodes at sites producing the largest responses. Removal of the occipital bone and a cervical laminectomy (C~ and C2) were performed to expose the caudal medulla and upper cervical spinal cord. The dura mater was cut and retracted for several millimeters rostrai and caudal to the C 1 nerve rootlets. Once the dura was retracted, the spinal cord was kept moist by periodic
application of warm saline or mineral oil.
Stimulation procedures Platinum needle electrodes were inserted into the skin of the cheek, scalp, vibrissae and ear and the gingiva lateral to the mandibular canine tooth. Ipsilateral electrical stimuli (0.2 ms duration, 0.2-10.0 mA, 0.5 Hz, square waves) delivered through these electrodes were used to search for single neurons in C 1 spinal cord. Insertion of stimulating electrodes was made in accordance with trigeminal and cervical dermatomes that have been delimited in other studies 2"~4'22. Therefore, electrodes in the ginglva, cheek and vibrissae corresponded to the trigeminal dermatomes, while those in the ear and scalp were related to the C~ and C 2 dermatomes. In 3 cats, an additional pair of silver disk electrodes (electrode diameter: 0.45 ram) was placed on the dura overlying the superior sagittal sinus near the junction with the transverse sinus. Thalamic stimulation also consisted of 0.2 ms constant current square-waves between 0.05 and 2.0 mA. Criteria for antidromic activation of spinothalamic tract neurons included constant spike latency, ability to follow high frequency stimulation (333 Hz) and collision of the antidromic and orthodromic spikes. Mechanical stimuli were delivered to determine each neuron's receptive field after the responsiveness of each neuron to electrical stimuli was characterized. Mechanical stimuli consisted of air puffs, brushing with a hair brush, light tapping with a wooden dowel, non-noxious pressure applied with a large arterial clip and noxious pinch applied with a small arterial clip. Noxious pinch applied to the skin of the investigators with the small arterial clip was always judged to be painful. The extent of each receptive field was mapped and drawn on a figure of the cat's head, face and neck. Neurons were classified as responding to stimulation within only the trigeminal dermatome, only the cervical dermatomes, or to both the cervical and trigeminal dermatomes. Neurons were further classified into three categories based on their responsiveness to the graded mechanical stimuli. LTM neurons responded to air puffs, brushing, light tapping or pressure, but did not increase their discharge frequency to stimuli in the noxious range. WDR cells were also excited by brushing, light tapping or pressure, but were maximally responsive to noxious mechanical stimuli (pinch). NS neurons were activated only by noxious mechanical stimuli. Classification of cells in this manner was similar to procedures used in the spinal and medullary dorsal horns 9'17'20'23'24'31'53.
Recording procedures Extracellular recordings from the spinal cord were made with epoxy-coated tungsten microelectrodes (1-10 Mr2). Large receptive fields and the inability of C~ spinal cord neurons to follow high frequency (500 Hz) electrical stimulation of the skin distinguished spinal cord neurons from primary afferent nerve fibers. Additionally, C 1 spinal cord recordings had negative spikes and most had clear initial segment-somatodendritic breaks, while axonal recordings had initially positive-going action potentials. Microelectrode penetrations into the spinal cord ranged from a point midway between the most rostral C 2 nerve rootlet and most caudal C~ nerve rootlet to the level of the most rostral C 1 nerve rootlet. Electrode tracks were made perpendicular to the spinal cord surface. Parallel electrode penetrations, separated by 250-500/zm, were made in medial-lateral rows at 500-/~m intervals along the rostral-caudal extent of C r Neuronal recordings were amplified, filtered (300 Hz-3 kHz), and recorded on magnetic tape for off-line data analysis.
Data analysis Signals were led to a digitizing oscilloscope for measurement of the latency from the start of each cutaneous electrical stimulus to the peak of the action potential. Three trials consisting of suprathreshold stimuli for each effective stimulus site were used to obtain the minimum latency of each spike. The minimum current intensity (current threshold) necessary to elicit an action potential was also determined for each neuron and stimulation site by decreasing the
183 stimulus intensity until only 50% of the stimuli were effective. Differences in the latencies and current thresholds of LTM, WDR and NS neurons for each stimulus location were analyzed using one-way Analysis of Variance (ANOVA) procedures followed by paired-comparisons using the Newman-Keuls test. Brainwave Discovery software was used to discriminate individual spikes and to construct peristimulus time histograms of neuronal responses elicited by mechanical stimuli. A 10-s sample of spontaneous activity was obtained from these histograms before receptive fields were mapped.
Histological procedures In selected cases, the location of the recording electrode was marked with an electrolytic lesion (cathodal, 50-100 #A, 15-30 s) placed at the recording site. No more than 3 lesions were produced in one animal. Lesions were spaced widely to avoid confusion between recording loci. An additional lesion (cathodal, 200 #A, 20 s) was produced at the thalamic stimulating site. After completion of data collection, cats were overdosed with pentobarbital sodium. Some cats were then perfused through the heart with 10% formalin. The caudal medulla, upper cervical spinal cord and brain of all cats were removed and post-fixed in 10% formalin, then placed into phosphate-buffered 30% sucrose. Serial frozen sections (50 gm
thickness) were made through the thalamus and upper cervical spinal cord. Tissue sections containing lesions were mounted on subbed slides, dehydrated and stained with thionin. Lesions were reconstructed using a microprojector and a microscope with a drawing tube attachment. The laminar organization of the upper cervical spinal cord was distinguished using criteria established by Rexed ~'49. The stereotaxic atlases of Berman and Jones 6 and Snider and Niemer 57 were used to identify structures in the caudal thalamus.
RESULTS
Extracellular recordings were obtained from 142 neurons in the dorsal and ventral horns of the C 1 spinal cord. Of these neurons, 25 neurons exhibited spontaneous activity that varied in frequency from 0.7 to 25.9 Hz (mean + S.E.M. = 8.5 + 1.2 Hz). The remainder of the neurons showed no spontaneous activity and were identified using electrical stimulation of the skin or gingiva.
A
B
5 ms
C
D 200.
j
Brush • vv
~
•
150. Air P u f f
0
,,,.,
@
i
Pinch
m
100. Pressure
@ U)
50"
0
10
20
30
40
1ram
Time ( s e e ) Fig. 1. Electrical and mechanical response properties of a C 1 LTM type neuron. A: neuronal response to electrical stimulation of the dorsal aspect of the ear. Minimum first spike latency is at 12.9 ms. Arrow indicates the electrical stimulus onset. B: receptive field of neuron to mechanical stimulation of the ear. Shaded area represents the region that when stimulated excited the neuron. C: peristimulus time histogram of the neuronal response to graded mechanical stimulation applied to the ear. Air puffs and brushes elicited the highest discharge frequency. D: location of the recording site was in lamina IV of the C1 spinal cord.
184
A
B
mO
It
C
D
Pinch
120"
0 m
60-
Pressure
o 0 Brush
30-
0
10
20 Time
30
40 1ram
(sec)
Fig. 2. Electrical and mechanical response properties of a C 1 WDR type neuron. A: neuronal response to electrical stimulation of the cheek (a), vibrissae (b), gingiva (c) and ear (d). Arrow indicates the stimulus onset. B: cutaneous receptive field of neuron to mechanical stimulation of the head and face. Shaded area represents the region that when stimulated excited the neuron. C: peristimulus time histogram of the neuronal response to graded mechanical stimulation applied to the skin adjacent (lateral) to the eye. While non-noxious brushes and pressure did activate this neuron, the maximum response was to noxious pinch. D: location of the recording site was in the reticulated area of lamina V of the C 1 spinal cord. T h e n u m b e r s o f C1 n e u r o n s that r e s p o n d e d to electri-
t h r e s h o l d stimulus intensities. I n c r e a s i n g electrical stim-
cal s t i m u l a t i o n o f orofacial and cranial areas are listed in
ulus intensity p r o d u c e d m u l t i p l e spikes a n d r e d u c e d first
T a b l e I. C 1 n e u r o n s r e s p o n d e d with 1 or 2 spikes to
spike latencies. E l e c t r i c a l s t i m u l a t i o n o f t h e e a r o r c h e e k excited m o r e t h a n 5 0 % of t h e t e s t e d n e u r o n s . Stimulation of t h e s u p e r i o r sagittal sinus (SSS) a c t i v a t e d o n l y 7.1%
TABLE I Mean latencies, mean current thresholds for activation and the number o f C 1 neurons responsive to electrical stimulation o f various sites on the head and face
of the t e s t e d
neurons
and
resulted
in l o n g e r
m i n i m u m first spike latencies t h a n s t i m u l a t i o n o f s o m a t i c structures (Table I). T h e m e a n electrical t h r e s h o l d s n e c e s s a r y to a c t i v a t e C 1 n e u r o n s did n o t differ significantly with stimulus l o c a t i o n (F5,157 = 0.71; P > 0.05).
Site
no. responsive/ no. tested
Mean latency (ms + S.D.)
Mean threshold (mA + S.D.)
Cheek Scalp Ear Gingiva Vibrissae SSS
72/126 30/103 69/126 43/126 38/98 2/28
8.6 + 11.1 + 10.1 + 10.0 + 9.3 + 17.9 +
2.9 + 3.3 + 2.7 + 3.1 + 3.7 + 2.8 +
3.6 6.6 4.7 3.7 4.7 7.8
2.2 2.4 2.3 2.3 2.3 2.8
R e s p o n s e p r o p e r t i e s o f C 1 s p i n a l c o r d n e u r o n s to m e c h a n ical s t i m u l a t i o n B a s e d on t h e n e u r o n a l r e s p o n s e p r o p e r t i e s to m e c h a n -
ical s t i m u l a t i o n , C 1 n e u r o n s w e r e classified as L T M , W D R o r N S n e u r o n s . T h e r e s p o n s e s of d i f f e r e n t classes of C 1 spinal c o r d n e u r o n s to electrical and m e c h a n i c a l
185
B
A
5 ms
C
D
20-
Pinch
15. 0
@
® 10@ .1¢ CL Brush G) 5-
Prissure
t~
V
0
V
V
II
V
'
1'0
' Time
I
,i !
20
3' 0
(see)
Fig. 3. Electrical and mechanical response properties of a C~ nociceptive-specific (NS) type neuron. A: neuronal response to electrical stimulation of the vibrissae. Arrow indicates the stimulus onset. B: cutaneous receptive field of the neuron to mechanical stimulation of the face. Shaded area represents the region that when stimulated activated the neuron. C: peristimulus time histogram of the neuronal response to graded mechanical stimulation applied to the skin ventral to the eye. Only noxious pinch resulted in activation of this neuron. D: location of the recording site was in lamina VII.
stimulation are illustrated in Figs. 1, 2 and 3. The neuron depicted in Fig. 1 was classified as an LTM neuron. Electrical stimulation applied to the ear excited this neuron with a latency of 12.9 ms (Fig. 1A). This neuron responded to innocuous mechanical stimulation (air puffs/brush) applied to the dorsal and ventral surfaces of the ear (Fig. 1B). Increasing stimulus intensity into the noxious range did not produce further increases in neuronal discharge (Fig. 1C). All LTM neurons responded to mechanical stimuli with a short latency. Two types of LTM neurons were observed: one type was excited by hair movement and had a rapidly adapting response and the other type responded maximally to pressure. Neurons responding best to pressure continued to discharge during stimulus application. W D R neurons responded to innocuous stimulation, but w e r e maximally responsive to noxious mechanical
30
• Cervical only [ ] Trigeminal only [ ] Cervical and Trigeminal
25
2 20 Z
..121
E
10
Z
5
LTM
WDR
NS
Fig. 4. Number of neurons classified as LTM, WDR and NS neurons that received cutaneous input from the trigeminal, cervical or trigeminal and cervical dermatomes. There were significant differences between the number of LTM, WDR and NS neurons that received input from these dermatomal divisions (Z2 = 17.89, P < 0.01).
186
LTM
WDR
NS
LTM
(~ 20 E
£ Z
.~
5
z
o
E
1T
~
1C
2T
1T+C
3T
2T+C 3T+C
3T
2T+C 3T+C
WDR
10
Z
~
s
10[ E
z
0
1T
1C
2T
1T+C
NS
o= Z
Fig. 5. Typical receptive field locations LTM, W D R and NS neurons. Five examples of each type of neuron are shown in 3 columns. Shaded area drawn on lateral and dorsal views of the cat's head and face represent the region that when mechanically stimulated activated each neuron.
stimulation. An example of a WDR neuron is illustrated in Fig. 2. Electrical stimulation of the cheek (a), vibrissae (b), gingiva (c) and ear (d) all activated this neuron (Fig. 2A). Additionally, innocuous brushing or pressure applied to the entire ipsilateral face, ear, tongue, gingiva and scalp excited this neuron (Fig. 2B). However, as shown in Fig. 2C, a noxious pinch applied within the cutaneous receptive field resulted in an additional increase in discharge frequency. Spontaneous activity of 2 WDR neurons in the C 1 spinal cord was inhibited by mechanical stimulation outside the neuron's excitatory receptive fields. In these cases, application of a noxious pinch applied to the contralateral pinna inhibited spontaneous activity for the duration of the pinch. When the pinch stimulus was terminated, spontaneous activity
T A B L E II
Representation of the ophthalmic, maxillary and mandibular cutaneous receptive fields of CI spinal cord neurons Each value represents the n u m b e r of times each division of the trigeminal nerve was incorporated into the cutaneous receptive field of LTM, W D R and NS neurons.
Cell type
LTM WDR NS
Number of cells Ophthalmic
Maxillary
Mandibular
8 18 5
21 20 7
28 25 5
r~
_E Z
0
1T
1C
2T
1T+C
3T 2T+C
3T+C
Fig. 6. N u m b e r of n e u r o n s classified as LTM, W D R and NS that received cutaneous input from trigeminal and cervical d e r m a t o m e s . I T = one division of the trigeminal nerve; 1C = C1/C 2 d e r m a t o m e only; 2T = two divisions of the trigeminal nerve; IT + C = one division of the trigeminal nerve and the C1/C 2 d e r m a t o m e ; 2T + C = two divisions of the trigeminal nerve and the C1/C 2 d e r m a t o m e ; 3T = three divisions of the trigeminal nerve; 3T + C = three divisions of the trigeminal nerve and the C~/C 2 d e r m a t o m e .
rapidly increased back to prestimulus levels. NS neurons did not respond to innocuous stimuli, but were activated only by noxious pinch stimuli. The NS neuron depicted in Fig. 3 responded to electrical stimulation of the vibrissae with a latency of 20.8 ms (Fig. 3A) and had a receptive field including the ophthalmic and maxillary divisions of the trigeminal nerve (Fig. 3B). While brushing or pressure applied to the receptive field did not excite this neuron, a noxious pinch produced a significant increase in discharge frequency (Fig. 3C). The numbers of neurons classified as LTM, WDR and NS are illustrated in Fig. 4. LTM neurons accounted for 52.2% of the driven cells recorded in C 1. WDR (35.9%) and NS (11.9%) neurons were encountered less frequently. Neurons having receptive fields encompassing only the ipsilateral trigeminal dermatome were usually excited by cutaneous stimulation of the cheek or ventral aspect of the pinna. Receptive fields incorporating the C 1 and C 2 dermatomes included the skin or hair of the dorsal aspect of the ear, neck, or scalp at or caudal to vertex. Typical examples of the receptive field locations of LTM, WDR and NS neurons are shown in Fig. 5. Of the 92
187
A
5
ms
D
C a
a
b
b
C C I
A
&
0.1
mV 10
ms
Fig. 7. Antidromic responses of a C 1 WDR neuron to stimulation of the caudal thalamus. A: three overlapping traces of the response elicited by thalamic stimulation (600/~A). The latency of the response was constant at 3.8 ms. Arrow indicates the stimulus onset. B: antidromic response of the neuron to high frequency thalamic stimulation. Arrows indicate the thalamic stimulus onset, arrowheads indicate the antidromic spikes. C,D: collision of orthodromic spikes elicited by electrical stimulation of the ear (C) and cheek (D) with antidromic spikes elicited by thalamic stimulation. Top and middle traces show the neuronal response to electrical stimulation of the skin and thalamus, respectively. Bottom traces illustrate the absence of the antidromic spike when preceded by orthodromie spikes. Solid arrows, orthodromie stimulus onset; open arrows, antidromic stimulus onset; solid arrowheads, location of antidromic spike; open arrowheads, expected location of antidromic spikes during collision.
neurons classified as LTM, W D R
or NS types, 40
TABLE III
received input from both the cervical and trigeminal
Mean latencies (ms + S.D.) o f trigeminal and cervical afferent inputs to LTM, W D R and NS neurons in the C 1 spinal cord
d er m at o m es. H o w e v e r , W D R neurons were m o r e likely
Input
LTM
WDR
NS
Scalp Ear Gingiva Vibrissae Cheek SSS
5.9+2.1 6.5+2.8 7.0 + 2.2 5.3 + 1.9 6.5 + 2.3 -
13.3+8.7 13.1+3.0 10.5 + 2.5 11.3 + 3.7 9.9 + 3.2 17.9 + 7.8
14.0+2.1 16.6+4.9 11.5 ___3.3 13.9 ___5.6 12.7 + 3.8 -
to receive convergent input from both trigeminal and cervical d e r m a t o m e s than w e r e either NS or LTM neurons. As illustrated in Fig. 4, 26 of 33 (78.8%) W D R neurons received input from trigeminal and cervical d e r m a t o m e s , while only 2 of 11 (18.2%) NS cells and 12 of 48 (25.0%) LTM cells r e c e i v e d c o n v e r g e n t information. Th e differences in the n u m b e r o f L T M , W D R and NS neurons receiving trigeminal and cervical information
188
A7.9 Fig. 8. Locations of effective thalamic stimulation sites marked with electrolytic lesions. Each solid circle represents a site in the caudal thalamus that was effective in antidromically activating C 1 spinal cord neurons. Thalamic sections are drawn at two different anterior-posterior planes: AP +6.4 and AP +7.9. Nomenclature of thalamic nuclei is based on Berman and Jones6. BIC, brachium of the inferior colliculus; CP, cerebral peduncle; LGN, lateral geniculate nucleus; i G m , magnocellular medial geniculate; MGp; principal medial geniculate; ML, medial lemniscus; MM, medial mammillary nucleus; POt, posterior complex, intermediate region; POI., posterior complex, lateral re#on; POre, posterior complex, medial region; OT, optic tract; RE, reticular complex; SGN, suprageniculate nucleus; SUB, subthalamic nucleus; VBA, ventrobasal complex, arcuate nucleus; VMB, basal ventromedial nucleus.
were statistically significant (X2 = 17.89, P < 0.01). The receptive fields of LTM and NS neurons encompassed only one division of the trigeminal nerve or the C1 and C 2 d e r m a t o m e s in 25 of 48 cells (52.1%) and 5 of 11 cells (45.5%), respectively (Fig. 6). In contrast, only 5 of 33 (15.2%) W D R neurons had receptive fields incorporating just 1 division of the trigeminal nerve or a cervical nerve, while 18 of 33 (54.5%) W D R neurons had receptive fields encompassing 2 or 3 divisions of the trigeminal nerve and the C t and C2 d e r m a t o m e s . W h e n the trigeminal d e r m a t o m e was included in the cutaneous receptive field, there was no a p p a r e n t bias for any division of the trigeminal nerve: each division of the trigeminal nerve was r e p r e s e n t e d equally (Table II; X2 = 5.02, P > 0.05). Bilateral receptive fields, extending across the midline to the contralaterai ear or side of the scalp, were found in 6 of 33 W D R neurons. N o n e of the LTM or NS neurons r e s p o n d e d to stimulation on the contralateral side of the body. Both of the neurons that were activated by SSS stimulation were classified as W D R neurons and received trigeminai and cervical convergent somatic input.
.......
.o."
Fig. 9. Locations of lesion marks used to determine LTM, WDR and NS recording sites in the C1 spinal cord. Solid triangles, LTM neurons; solid circles, WDR neurons; solid squares, NS neurons. Dotted line indicates the reticulated region within the C1 spinal cord.
189 Neurons with mechanoreceptive fields involving the mandibular or maxillary divisions of the trigeminal nerve were found more frequently in the medial half of lamina III and IV, while those having receptive fields including the mandibular division or the C 1 and C2 dermatomes, were found more laterally. Additionally, neurons in rostral sections of the C 1 spinal cord had a higher incidence of receptive fields incorporating the gingiva and vibrissae compared to caudal sections. The somatotopic organization of neurons in lamina V was difficult to assess due to their large receptive fields.
Response properties of C 2 spinal cord neurons to electrical stimulation The mean minimum latencies to electrical stimulation of the head and face for LTM, W D R and NS neurons are shown in Table III. Electrical stimulation resulted in significantly different minimum latencies for LTM, W D R and NS neurons (F2,198 = 59.10; P < 0.01). Paired comparisons revealed that W D R and NS neurons had significantly (P < 0.05) longer latencies than LTM neurons at all stimulation sites. However, the mean current thresholds of nociceptive neurons were not statistically different from that of non-nociceptive neurons (F2,139 = 0.45, P > 0.05). Thalamic projection of C1 spinal cord neurons Electrical stimulation of the caudal thalamus activated 11 of 33 W D R neurons. The mean antidromic peak latency of the 11 W D R neurons was 3.8 ms (+0.6 ms S.D.). None of the tested LTM (n = 42) or NS (n = 9) neurons responded to thalamic stimulation. Fig. 7 illustrates the response of a W D R neuron that was antidromically activated by electrical stimulation of the contralateral caudal thalamus. Fig. 7A shows the constant latency (3.8 ms) of the antidromically activated action potential. This neuron also followed high frequency thalamic stimulation successfully (Fig. 7B). Collision of antidromic spikes with orthodromic spikes elicited by electrical stimulation of the gingiva and ear is illustrated in Fig. 7C and D. Thalamic sites from which C 1 neurons could be activated antidromically are illustrated in Fig. 8. Most thalamic stimulating sites were located in and around the medial part of the thalamic posterior group (POre). The averaged threshold value necessary to activate C 1 spinal cord neurons with thalamic stimulation was 600/~A (+400 g A , S.D.). Location of C 1 spinal cord recording sites Histological reconstruction of recording sites as determined by the location of lesion marks was possible for 24 cells. Fig. 9 illustrates the locations of lesions made for LTM, W D R and NS neurons. Most LTM neurons were
located in laminae III and IV, while W D R neurons were encountered most often in lamina V, especially in the lateral reticulated area. Four W D R neurons were also found in the ventral horn in laminae VII and VIII. All cells classified as thalamic projection neurons were located in lamina V. Lesion marks made for 3 NS neurons were located in laminae VII and VIII. No lesion marks were found in lamina I or II. DISCUSSION This study has confirmed earlier results t'37 demonstrating convergence of cervical and trigeminal information in the upper cervical spinal cord. These findings have been extended in the present study to include data concerning cutaneous convergence between different classes (i.e. LTM, W D R and NS) of C 1 neurons. Regardless of neuronal class, convergence between trigeminal and cervical cutaneous input was found to occur in 50% of the classified C 1 neurons. This value is comparable to that found for cervical and trigeminal afferent convergence onto neurons in nucleus caudalis 53 and in the C 2 dorsal and ventral horns 1. The present study has demonstrated that the degree of convergence in the C~ spinal cord is related to the classification (i.e. LTM, W D R or NS) of each neuron. W D R neurons were more likely to receive convergent input than were LTM or NS neurons. The receptive field sizes of W D R neurons incorporated more divisions of the trigeminal nerve than did those of LTM or NS neurons. Larger receptive fields and more extensive convergence of afferent input onto W D R neurons compared to LTM or NS neurons is also a general feature found in more caudal segments of the spinal cord 17'42 and in nucleus caudalis 4'23'41'53. However, Ferrington et al. 19 reported that cervical (C7 segment) spinothalamic tract neurons in alpha-chloralose anesthetized cats had small receptive fields regardless of their classification. The large receptive fields of W D R neurons in C 1 suggests that these cells may be more involved in arousal or attentional mechanisms of pain rather than in localizing painful stimuli. Most C 1 neurons receiving input from the cervical dermatomes responded to mechanical stimulation of the dorsal aspect of the pinna or the scalp at or caudal to vertex. Feline trigeminal ganglion cells have not been shown to respond to stimulation of these areas 14'36'sl. Conversely, studies examining the cutaneous receptive fields of the upper cervical nerves have demonstrated that stimulation of the dorsal pinna and scalp can activate fibers in the C1, C 2 or C 3 rami 2,22. At present, the contribution of each upper cervical nerve to the convergence pattern observed in the current study is not known. Previous anatomical studies have indicated that the
190 mandibular, maxillary and ophthalmic divisions of the trigeminal nerve project to the upper cervical segments of the spinal cord 4°'5.-56. The auriculotemporalis nerve (mandibular division of the trigeminal nerve) innervates the skin on the ventral aspect of the ear and terminates in laminae I through V of the C1, C 2 and C 3 segments of the spinal cord 54. The high incidence of C 1 spinal cord neurons responding to stimulation of the ventral part of the pinna physiologically confirms these data. Therefore, these data demonstrate that the mandibular division of the trigeminal nerve is well represented in the C~ segment of the spinal cord. While studies using anterograde degeneration have shown that the maxillary division of the trigeminal nerve projects to the same level as the mandibular division 34, experiments using horseradish peroxidase show that the mandibular division projects more densely to C1 than does the maxillary division 54. In the present study, 51.1% and 38.8% of the C 1 neurons responded to stimulation of the cheek and vibrissae (maxillary nerve), respectively. These data, as well as the receptive field locations of all C 1 neurons, are in agreement with Shigenaga et al. 54, demonstrating that more caudal parts of the face and head are represented more strongly in the upper cervical spinal cord. The mean first spike latencies to electrical stimulation of the skin were significantly longer in C1 nociceptive neurons compared to non-nociceptive neurons. This is in agreement with previous reported latencies of LTM, W D R and NS neurons in nucleus caudalis 24'53. However, other investigations have failed to observe latency differences between LTM and W D R neurons 4'15. The short latency responses of LTM neurons suggest the involvement of the myelinated A-beta nerve fibers. Smaller diameter afferents (A-delta nerve fibers) may be responsible for the longer latency responses of nociceptive neurons. Alternatively, a polysynaptic pathway from the periphery to the recorded C~ spinal cord neuron which would also result in longer first spike latencies cannot be precluded. Since unmyelinated C-nerve fibers have conduction velocities below 2 m/s, the earliest first spike latency recorded in C 1 would occur near 50 ms. The absence of latencies over 50 ms precludes the possibility that C-nerve fibers were responsible for any of the responses observed in C~. The lack of C-nerve fiber activation may have been due to the relatively short duration (0.2 ms) stimuli used to search for neurons. Additionally, the anesthetic used may have depressed the C-fiber evoked response of C~ spinal cord neurons. The number of C 1 spinal cord neurons projecting to the contralateral caudal thalamus was relatively small. This is surprising given anatomical data indicating that the upper cervical spinal cord contains many spinothalamic tract neurons 7'8'1°. Previous studies have reported
that LTM, W D R and NS neurons in nucleus caudalis all project to the contralateral thalamus 4'21'53. While no LTM or NS neurons were found to project directly to the thalamus in the present study, 33.3% of the W D R neurons were antidromically activated by electrical stimulation of the thalamus. As in the present study, Ferrington et al. 19 found that stimulation of the medial part of the thalamic posterior region (POm) activated many neurons in the cervical spinal cord. These data confirm previous anatomical data demonstrating a projection from the upper cervical spinal cord to the caudal thalamus 7'8'1°. However, it must be noted that stimulus current spread to areas adjacent t o P O m (e.g. ventrobasal nuclei) and to fibers of passage to more rostral thalamic nuclei may also be responsible for activation of upper cervical spinal cord neurons. The distribution of LTM, W D R and NS neurons in the upper cervical spinal cord is similar to that found in more caudal segments of the cervical spinal cord 19 and in nucleus caudalis 25'26. LTM neurons were located primarily in laminae III and IV, W D R neurons in laminae V and NS neurons in laminae V, VII and VIII. Some W D R neurons with large receptive fields were also found in the ventral horn of C 1. Molinari 43 and Yokota and Koyama 62 also indicated that some ventral horn neurons respond to nociceptive stimuli and have large bilateral cutaneous receptive fields. Many nociceptive neurons in the medullary reticular formation (subnucleus reticularis dorsalis and subnucleus ventralis) also possess large receptive fields including the head and face 59'60'62'63. While previous studies have shown that NS neurons are often located in laminae I and 1124'27'47'53, n o NS cells were recorded in the superficial laminae of the C 1 spinal cord in the present study. The percentage of NS neurons found in C~ in the current study is comparable to that reported in some studies of nucleus caudalis 25'53, but is less than that of other studies 4. It is possible that NS cells were not adequately sampled in the present study for several reasons. First, the tungsten metal electrodes may have been biased against recording from NS cells. Since many NS cells are located in laminae I and II of the medullary 24,27,47'53 and spinal dorsal horn 11'46, inadequate NS cell sampling may have occurred due to difficulty in maintaining stable recordings in these superficial laminae. Additionally, the microelectrodes may not have been able to discriminate small neurons in the superficial laminae sufficiently 19. Secondly, substantial 'dimpling' of the spinal cord surface caused by the microelectrodes may have prevented adequate sampling of the superficial laminae. Third, electrical current thresholds of many NS cells may have been higher than the stimulus intensities used to search for C1 neurons. Another area of the spinal cord that may receive
191 convergent input from the cervical and trigeminal systems is the lateral cervical nucleus (LCN). T h e L C N , located v e n t r o l a t e r a l to the dorsal horn in the C 1 and C2 segments of the spinal cord, receives input from second o r d e r cells within laminae I I I - V of the dorsal horn. M a n y neurons in the L C N are characterized by large receptive fields that occasionally include most of the b o d y 12'3°. While it has also been suggested that L C N neurons receive input from the t o o t h pulp and trigeminal vascular structures 38' 39,44, receptive fields incorporating the h e a d and face are rare 3°. T h e absence of a significant population of u p p e r cervical dorsal horn neurons responsive to superior sagittal sinus stimulation has also been r e p o r t e d previously 38. Thus, neurons in the dorsal and ventral horns of C 1 a p p e a r to be m o r e i m p o r t a n t for receiving convergent cutaneous input from trigeminal and u p p e r cervical d e r m a t o m e s . M a n y clinical observations suggest the involvement of the u p p e r cervical nerves in trigeminal dysesthesias.
K e r r 32 r e p o r t e d that stimulation of the C 1 nerve evokes pain in the orbit, frontal a r e a and vertex. Disease involving C 2 and C 3 nerves can also result in facial pain 13"51'58'61. A n e s t h e t i c blocks o r section of the u p p e r cervical nerves can often alleviate facial pain 61. A d d i tionally, 70% of patients with t e m p o r o m a n d i b u l a r joint disease complain of pain in the occipital region ( A . K . Bouckoms and D. Keith, p e r s o n a l communication). The extensive convergence of trigeminal and cervical input onto C1 spinal cord neurons m a y account for m a n y of these observations. F u r t h e r m o r e , convergence of trigeminal and cervical somatic information in the u p p e r cervical dorsal and ventral horns m a y m e d i a t e pain referred to orofacial and cranial structures.
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