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Chapter 13 Integration of vestibular and neck afferent signals in the central cervical nucleus
0. Pompetano and J.H.J. Allum (Eds.) P r u g r m Brain Rrrerrnh, Vol. 16 @ 1988 El\cvier Science Publishers B.V. (Biomedical Division)
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CHAPTER 13
Integration of vestibular and neck afferent signals in the central cervical nucleus T. Hongo2, T. Kitama' and K. Yoshida' 'Department of Physiology, Institute of Basic Medical Sciences, University of Tsukuba, Tsukuba. Ibaraki-ken 305, and 2Department of Neurophysiology, Institute of Brain Research, School of Medicine, University of Tokyo, Hongo 7-3-1,Bunkyo-ku, Tokyo I 13, Japan
The responses of spinocerebellar tract neurons in the CCN to natural stimulation of vestibular and neck muscle receptors were investigated in decerebrate cats. The spike activity of single CCN neurons was recorded extracellularly with a floating microelectrode in the C2 segment of the spinal cord. CCN neurons exhibited clear modulation of firing activity in response to sinusoidal rotation of the head in the vertical planes but not to rotation in the horizontal semicircular canal plane. The response to vertical rotation was attributed to stimulation of the vertical semicircular canals, since selective stimulation of otolith organs by static tilt of the head in the same plane had no effect. The response was approximately in phase with the head angular velocity at 0.05-2.0 Hz. Vector analysis suggested that the optimal orientation of vertical rotation varied considerably among neurons. The CCN neurons increased or decreased their firing rate in response to stretch of dorsal neck muscles. The time course of the responses suggested that muscle spindle primaries were most likely to be the receptors responsible for both excitatory and inhibitory responses. The input was highly muscle specific in that most CCN neurons received excitation from one muscle or two synergists, and that a group of neurons was specifically inhibited from biventer cervicis and complexus. The vestibular and neck muscle afferent signals converged with specific patterns of combination on individual CCN neurons. It is suggested that each CCN neuron integrates signals from the labyrinth and the neck and provides the cerebellum with information concerning the head movement in a certain vertical direction.
Introduction The CCN existing in the CILC4 segments [23] is now known to receive inputs from the neck and the labyrinth, and to project to the cerebellum as a niajor spinocerebellar tract of the upper cervical cord. Since Matsushita and Ikeda [16] and Wiksten [29] first demonstrated its spinocerebellar nature using the technique of retrograde transport of HRP, the knowledge of the input and output organizations of the CCN neurons has accumulated in the last decade. The mode of projection of CCN neurons to the cerebellum, including the main termination in labules 1-11, has been well documented anatomically [14-18, 30, 31, 351 and electrophysiologically [lo] in the cat. Dense projection to the CCN of cervical dorsal root fibres has long been recognized [13, 22, 27, 321. Subsequent anatomical [ 3 , 12, 281 and physiological [ 1, 121 studies demonstrated that the CCN receives afferent input from the dorsal neck muscles. Another major source of sensory input to the CCN has been shown to originate from the vestibular labyrinth. In a previous study we have shown that CCN neurons are excited by electrical stimulation of the contralateral vestibular nerve and inhibited from the ipsilateral vestibular nerve [I 11. Stimulation of individual ampullary nerves further indicated that the semicircular canals
Abbreviurions: BCC, biventer cervicis and complex; CCN, central cervical nucleus; HRP, horseradish peroxidase; OCC, obliques capitis caudalis; RCD, rectus capitis dorsalis; Spl, splenius.
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constitute a source from which the vestibular effect is evoked. In the present study we have attempted to investigate the dynamic properties of the responses of CCN neurons to natural stimulation of vestibular and neck muscle receptors in order to elucidate the functional role of the CCN spinocerebellar tract. We will describe the responses of single CCN neurons to sinusoidal rotation and static tilt of the head and also to muscle stretch. In addition, some preliminary data, suggesting a highly specific pattern of convergence from vestibular and neck muscle afferents, will be presented. Experiments were performed on decerebrate cats prepared under halothane anaesthesia. The animal was immobilized with pancuronium bromide and maintained under artificial respiration. Blood pressure in the femoral artery was monitored and maintained above 100 mmHg, and rectal temperature was kept at 37-38°C throughout the experiment. Bipolar silver-ball electrodes were placed on the round and oval windows for electrical stimulation of the vestibular nerves. Ensembles of needle electrodes were inserted into the white matter of the cerebellar anterior lobe for stimulation of spinocerebellar axons. Various neck muscles were dissected with the innervation kept intact and prepared for natural stimulation. After laminectomy from C1 to C4, the spinal cord was transected between the C3 and C4 segments. This was done to eliminate a possible sensory inflow from the body and limbs during rotation and tilt for vestibular stimulation. The spike activity of single CCN neurons was recorded extracellularly with a floating microelectrode in the C2 segment of the left side. CCN neurons were identified by antidromic activation from the anterior lobe vermis, and only those which were activated orthodromically following electrical stimulation of the contralateral (right) vestibular nerve were selected for further analysis. Responses to input from the vestibular labyrinth Responses of single CCN neurons to whole-body rotation were examined. With the head tilted nose-
down by 27" the animal was rotated sinusoidally in three orthogonal planes which were approximately coplanar with canal planes. For stimulation of the horizontal canal pair, the animal was rotated in the earth-horizontal plane (HC mode). For stimulation of the right anterior-left posterior canal pair (r-AC mode) and the right posterior-left anterior canal pair (r-PC mode), the animal was rotated in the vertical plane orienting 45" and 135" from the sagittal plane (clockwise as viewed from above), respectively. Thus the r-AC and the r-PC mode rotations should stimulate both vertical canals and otolith organs. T o evaluate the contribution of otolith inputs, responses to static tilts which should provide pure otolith stimuli were also examined. Fig. 1A exemplifies responses of a left CCN neuron to sinusoidal rotation in the r-AC plane at 0.5 Hz with an amplitude of 13.5"/s. The firing rate was modulated in an approximately sinusoidal manner around the resting level (45 spikes/s, see record in Fig. lB, middle traces). Fig. 1B shows the steady-state activity of the same neuron at the standard position (middle traces) and during 10" static tilt in the same r-AC plane in both directions (left and right traces). Although the angular displacement during the tilt was more than twice as large as the positional change that occurred during sinusoidal stimulus, the tilt stimuli elicited no appreciable change in the firing rate. Thus the clear modulation of firing rate during rotation shown in Fig. 1A was attributable to stimulation of vertical canals. The vast majority of CCN neurons exhibited clear modulations of firing rate in response to rotation in the vertical plane of either r-AC or r-PC mode or both, but not to HC mode rotation. In contrast, static tilts in these planes had no effect in any of the CCN cells examined. In a few cells, weak modulations were observed during H C mode rotation. Such weak modulations could, however, be well accounted for as caused by stimulation of the vertical canals, because the vertical canals cannot be exactly perpendicular to the plane of rotation due to variations of the canal orientations [ 5 ] . In fact, stimulation of the vertical canals evoked
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A
ROTATION 0.5Hz
I-CCN
5s
8 TILT I-CCN -
FIRING RATE 10' NOSE DOWN
0'
10' NOSE UP
Fig. 1. Responses of a left CCN cell to sinusoidal rotation at 0.5 Hz (A) and to static tilt (B) in r-AC plane. In the head velocity curve in A, upward deflexion indicates right side-nose down direction. See text for further explanations.
strong effects on these cells, and when the head position was changed by 10" from the standard 27" pitch to 17" pitch, the modulation due to horizontal rotation changed just as expected if the vertical canals were responsible. In regard to the receptor origin of the vestibular input, therefore, it can be concluded that the CCN cells so far analysed received input only from the vertical canals. The gain and phase with respect to the head velocity were computed from the average of responses to 10-100 successive stimulus cycles. Within the range of stimulus amplitudes employed (2.3 to 13.5"/s, at 0.5 Hz), the response amplitudes varied with the stimulus amplitudes in an approximately linear manner, i.e. the gain was constant. The phase of response was essentially independent of the stimulus amplitude. For example, the cell shown in Fig. 2 exhibited response amplitudes of 4.3, 8.6, and 11.7 (spikes/s/("/s) for stimulus velocities of 4.5, 9.0 and 13.5"/s, respectively. The phase was about O", and varied by no more than 4" for these three stimulus amplitudes.
In most CCN cells tested, the phase of response was approximately in phase with the head velocity that should excite the contralateral canal, ranging from + 30" (lead) to - 30" (lag) at 0.5 Hz. The phase was roughly constant over frequencies from 0.05 to 2.0 Hz. These phase properties were similar to those of vestibular nucleus neurons [21, 261. In some cells, the response to r-PC mode rotation was about 180" out of phase, i.e. firing rate decreased during contralateral posterior canal activation. Since no excitatory effects on CCN cells were found after electrical stimulation of the ipsilateral vestibular nerve [ 1 11, these cells were considered to be inhibited from the contralateral (right) posterior canal. Gain of responses to r-AC mode and r-PC mode rotations varied considerably from neuron to neuron. While some neurons responded almost exclusively to either r-AC or r-PC mode rotation, others were responsive to both (Fig. 2A). To obtain a quantitative measure of the total canal input to each cell, the orientation of the vertical plane at which rotation would evoke a maximum response
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timal orientation varied from 9" to 157". The wide range of optimal orientation indicates that a given rotation influences the activity of individual cells differentially. The available data, though limited, AC. MODE appear to indicate that CCN cells consist of subpo_____---pulations having different directional sensitivities ROLL MODE for vertical rotations (see Fig. 5).
B
A 25
0
25
/
0.5
PITCH MOD1
~. 0.5
HEAD VELOCITY
1
0.5Hz
P.C. MODE
W.I.
Fig. 2. Responses of a left CCN cell to sinusoidal rotation in different vertical planes. (A) Modulation of firing rate to r-AC mode (top) and r-PC mode (middle) rotations at 0.5 Hz. Averaged from 30 cycles. (B) Solid vectors represent the gain of the response to r-AC mode, r-PC mode, pitch mode and roll mode rotations. The dotted vector indicates the predicted response in the optimal plane. See text for further explanations.
(optimal plane), and the gain in the optimal plane were calculated in individual cells. These were obtained from the gain and phase of r-AC and r-PC mode responses at 0.5 Hz, assuming a linear summation of effects from different canals. Fig. 2 illustrates for one cell the actual r-AC and r-PC mode responses and the predicted response at the optimal plane. This cell exhibited clear modulations in response to both r-AC and r-PC mode rotations (Fig. 2A). The gain and phase of the r-AC mode response were 0.82 (spikes/s)/("/s) and - 20.1", and those of the r-PC mode response were 0.63 (spikes/ s)/("/s) and + 12.6", respectively. In Fig. 2B each response is represented in vector form where the vector length is proportional to the gain and the polar angle shows the orientation of the rotation plane. Vectorial summation of the r-AC and r-PC mode responses predicted the optimal plane at 81" (clockwise from the sagittal plane) and the maximum gain of 0.99 (spikes/s)/("/s) (dotted vector). The actual responses to pitch and roll mode rotations were in good agreement with the prediction (Fig. 2B), indicating the validity of the assumption of linear summation. Similar analyses in 10 CCN cells so far studied showed that the gain of the optimal response ranged from 0.32 to 2.18 (spikes/s)/("/s) and the op-
Responses to input from the neck muscles
The responses of CCN neurons to electrical stimulation of the dorsal root of cervical segments and of various peripheral nerves in the neck and the forelimb have been studied by Hirai et al. [9, 121 and Abrahams et al. [I]. These studies have shown that monosynaptic excitatory effects are exerted by afferents from neck muscles and are conveyed via the dorsal roots of the upper cervical segments. No detectable excitation with a monosynaptic latency was elicited from the forelimb nerves, though nerves from shoulder muscles were not tested. We were unsuccessful in finding effects clearly ascribable to afferents from vertebral joints or ligaments, which have been reported to cause neck-limb [20] or neck-ocular reflex actions [8]. Detailed analyses were therefore focused on the effects from the neck muscle afferents in the previous study [12]. The main findings obtained by using electrical stimulation of the neck muscle nerves were as follows. (1) Monosynaptic excitation was evoked in CCN neurons after stimulation of nerves to dorsal neck muscles such as Spl, BCC, OCC and RCD. (2) Thresholds for the monosynaptic excitation were near the threshold of the nerve, suggesting that it was caused by group I afferents. (3) The excitatory input was muscle specific in that individual neurons received excitation from one muscle, or from synergists (e.g. BCC-RCD, SpI-OCC). There were some cells in which convergence from BCC and Spl occurred, but then the excitatory effect from one muscle overwhelmed that from the other. (4) Inhibition could also be evoked at short latencies after stimulation of BCC but not Spl.
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sp./s
n
-
2s
Fig. 3. Response of a CCN cell to stretch of BCC muscle. Upper trace: extracellular spikes of single CCN cells recorded through a high-pass filter. Lower trace: firing frequency of the larger unit in the upper trace. The solid lines in the bottom indicate the periods of muscle stretch.
Based on these results, we attempted to investigate effects of natural stimulation of the muscle (stretch or local pressure) on the firing activity of single CCN neurons recorded extracellularly. A floating microelectrode was useful for this test. Fig.
3 shows an example of responses of a single CCN neuron to stretching the BCC muscle. Note that the firing rate greatly increased transiently at the early phase of stretch, maintained an elevated level during stretch, and abruptly decreased even to pause when the muscle was released. The firing pattern was very similar to that of muscle spindle primaries in possessing both dynamic and static responses [19, 241, suggesting that the excitatory effect was of group Ia fibre origin. The interpretation is in good agreement with the observation that fibres identified to originate from spindle primaries distribute terminals in the CCN as revealed by their intra-axonal staining with HRP (Fig. 4 [12]). The excitatory responses of CCN neurons to muscle stretch were usually as in Fig. 3 , and were seen in about 70% of CCN neurons examined. Decrease of the firing rate was also produced by muscle stretch in some CCN cells. Since electrical stimulation of the nerve to the muscle stretched also
B
I Fig. 4. Trajectory (A) and terminals (B) of HRP-stained axons identified as low-threshold neck muscle afferents. Five collaterals are superimposed at the C2 segment. (From [12]).
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caused suppression, the effect was interpreted as due to inhibition caused by stretch-evoked impulses in the nerve from that muscle and not due to unloading of other muscles that supplied excitatory input to the CCN cell. The inhibition was probably also of group Ta fibre origin, since it occurred with a small stretch and persisted throughout the period of stretch. The receptive field for the excitation and inhibition caused by muscle stretch was explored in individual CCN neurons. The input was found to be muscle specific, in general agreement with the results obtained by electrical stimulation of the muscle nerves [12]. The excitatory input, when present, originated from single or synergic muscles of either the BCC-RCD group or the Spl-OCC group, while inhibition came from BCC and not from any other muscles examined. These results indicated that CCN neurons consist of subgroups, each of which receive specific spatial pattern of excitatory and/or inhibitory inputs from muscle spindle afferents of different neck muscles. According to the patterns of muscle input, the following four subgroups of CCN neurons have so far been observed: (1) those excited from BCC; ( 2 ) those inhibited from BCC; (3) those excited from Spl-OCC; (4) those excited from SplOCC and inhibited from BCC. Convergence of inputs from the semicircular canal and the neck muscles
The majority of CCN cells examined received inputs from both the labyrinth and the neck muscles, in agreement with the previous results obtained by electrical stimulation [9, 1 I]. We therefore attempted to disclose the pattern of convergence from the two sources onto the same CCN neurons. Despite technical difficulties we could complete, in a limited number of cells, most of the necessary tests using rotation and muscle stretch without losing the cell recorded from. The results obtained in 6 cells are shown in Fig. 5, where the origin of muscle for excitation and inhibition by stretch (A) and the vector (gain and direction) of the response at the optimal plane as defined above (B) are given for each
LEFT-CCN UNITS
A
MUSCLE INPUT
0
EXCITATION
-
WHlElTlON NOEFFECT
0
CANALINPUT ROSTRAL
RIGHT
CAUDAL
Fig. 5. Patterns of convergence of neck muscle (A) and vestibular (B) inputs to 6 CCN neurons. In B the response gain and orientation of optimal plane are shown in vector form, as in Fig. 2. The number attached to each vector refers to the unit in A. See text for further explanations.
cell. The diagram in B shows that the directions of the optimal plane are anterior in units 1 and 2, lateral in units 3 and 4, and posterior in units 5 and 6. It is interesting to note that the differences in the canal input are well correlated with the pattern of input from neck muscles. Cells with anterior vectors were inhibited from BCC without effects from Spl; cells with lateral vectors were excited from Spl and inhibited from BCC; and cells with posterior vectors were excited from BCC with no effects from Spl. Such highly specific patterns of convergence suggest that each CCN neuron integrates sensory signals related to the head movement in a particular direction, with respect to both body and space. Discussion
Our results demonstrate that the CCN neurons receive afferent signals arising from both the vestibular and neck muscle receptors and provide the cerebellum with integrated information about the head movement with respect to the trunk and space. One of the striking features of the vestibular input is that CCN neurons receive input only from the ver-
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tical canals. The absence of the horizontal canal input is in agreement with the previous results obtained by electrical stimulation of the individual ampullary nerves [I 11. The previous study [I 11 has also revealed various patterns of convergence from different vertical canals on single CCN neurons. The present results extend this previous finding by showing that each cell has its own optimal orientation of vertical rotation which varied widely among neurons. Similar convergence from different canal pairs has been observed in some second-order vestibular neurons [2].It remains to be studied whether the convergence takes place at the vestibular nuclei or at the CCN. As for the contribution of the otolith input, selective stimulation of the otolith organs by static tilt was shown to have negligible effect. In this regard it should be pointed out that the functional organization of vestibular inputs to CCN neurons is quite different from that to dorsal neck motoneurons, in which both the horizontal canal and the otolith organs also make a substantial contribution to the vestibular induced activity [2, 4, 6, 7, 25, 341. The properties of the response of CCN neurons to muscle stretch are in general accordance with the previous electrophysiological and anatomical findings [12], in showing that the input is of muscle spindle origin and the receptive field of each cell is limited to one muscle or close synergists. A close correlation was found between the optimal direction of the head rotation for vestibular stimulation and the pattern of muscle input in each cell (see Fig. 5). Given the complex geometry of neck muscles and their actions, it is difficult to give a simple explanation for the highly specific pattern of convergence of the two inputs. Nevertheless it would be of interest to consider the manner of interaction of the two signals when the head is passively rotated with respect to a stationary trunk. The result in Fig. 5 suggests that if a CCN neuron is excited from the labyrinth by a passive head rotation toward its preferred direction, effects from neck muscles are either inhibition from stretched muscles or disfacilitation from released muscles. If the head is moved to the opposite direction, the canal input
would be inhibitory while changes in muscle length would produce either disinhibition or facilitation. It therefore appears that effects from the vestibular and neck receptors are of opposite polarity and may more or less cancel each other. It is interesting to note that similar opposite effects of vestibular and neck afferent actions have been shown in interneurons of the lower cervical cord, though mainly otolith inputs appear to contribute in these neurons [331. References Abrahams, V.C., Anstee, G. and Richmond, F.J.R. (1979) Neck muscle and trigeminal input to the upper cervical cord and lower medulla of the cat. Can. J . Physiol. Phurrnacol., 57: 642-651. Baker, J., Goldberg, J. and Peterson, B. (1985) Spatial and temporal properties of the vestibulocollic reflex in decerebrate cats. J. Neurophysiol., 54: 735-756. Bakker, D.A., Richmond, F.J.R. and Abrahams, V.C. (1984) Central projections from cat suboccipital muscles: A study using transganglionic transport of horseradish peroxidase. J. Comp. Neurol., 288: 409421. Berthoz, A. and Anderson, J.H. (1971) Frequency analysis of vestibular influence on extensor motoneurons. 11. Relationship between neck and forelimb extensors. Bruin Res., 34: 376380. Estes, M.S., Blanks, R.H.I. and Markham, C.H. (1975) Physiologic characteristics of vestibular first-order canal neurons in the cat. I. Response plane determination and resting discharge characteristics. J . Neurophysiol., 38: 12321249. Ezure, K. and Sasaki, S. (1978) Frequency-response analysis of vestibular induced neck reflex in cat. I. Characteristics of neural transmission from horizontal semicircular canal to neck motoneurons. J . Neurophysiol., 41: 445458. Ezure, K., Sasaki, S., Uchino, Y . and Wilson, V.J. (1978) Frequency-response analysis of vestibular-induced neck reflex in cat. 11. Functional significance of cervical afferents and polysynaptic descending pathways. J . Neurophysiol., 41: 459471. Hikosaka, 0. and Maeda, M. (1973) Cervical effects on abducens motoneurons and their interaction with vestibuloocular reflex. Exp. Brain Res., 18: 512-530. Hirai, N., Hongo, T. and Sasaki, S. (1978) Cerebellar projection and input organizations of the spinocerebellar tract arising from the central cervical nucleus in the cat. Brain Res., 157: 341-345. Hirai, N., Hongo, T. and Sasaki, S. (1984) A physiological study of identification, axonal course and cerebellar projec-
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tion of spinocerebellar tract cells in the central cervical nucleus of the cat. Exp. Brain Res., 55: 272-285. [Ill Hirai, N., Hongo, T., Sasaki, S. and Yoshida, K. (1979) The neck and labyrinthine influences on cervical spinocerebellar tract neurons of the central cervical nucleus in the cat. In: R. Granit and 0. Pompeiano (Eds.), Reflex Control qf Posture and Movement, Progress in Brain Research, Vol. 50, Elsevier, Amsterdam, pp. 529-536. [I21 Hirai, N., Hongo, T., Sasaki, S., Yamashita, M. andYoshida, K. (1984) Neck muscle afferent input to spinocerebellar tract cells of the central cervical nucleus in the cat. Exp. Brain Res., 55: 286-300. [13] Imai, Y. and Kusama, T. (1969) Distribution of the dorsal root fibers in the cat. An experimental study with the Nauta method. Brain Res., 13: 338-359. [14] Matsushita, M. and Hosoya, Y. (1982) Spinocerebellar projections to lobules 111 to V of the anterior lobe in the cat, as studied by retrograde transport of horseradish peroxidase. J . Cornp. Neurol., 208: 127-143. [I51 Matsushita, M., Hosoya, Y . and Ikeda, M. (1979)Anatomical organization of the spinocerebellar system in the cat, as studied by retrograde transport of horseradish peroxidase. J . Comp. Neurol., 184: 81-106. [16] Matsushita, M. and Ikeda, M. (1975) The central cervical nucleus as cell origin of a spinocerebellar tract arising from the cervical cord: a study in the cat using horseradish peroxidase. Brain Res., 100: 412417. [17] Matsushita, M. and Okado, N. (1981) Spinocerebellar projections to lobules I and I1 of the anterior lobe in the cat, as studied by retrograde transport of horseradish peroxidase. J . Comp. Neurol., 197: 411424. [I81 Matsushita, M., Tanami, T. and Yaginuma, H. (1984) Differential distribution of spinocerebellar fiber terminals within the lobules of the cerebellar anterior lobe in the cat: an anterograde WGA-HRP study. Bruin Res., 305: 157161. [I91 Matthews, P.B.C. (1972) Mammalian Muscle Receptors and their Central Actions, Arnold, London, pp. 20-194. [20] McCouch, G.P., Deering, I.D. and Ling, T.H. (1951) Location of receptors for tonic neck reflexes. J . Neurophysiol., 14: 191-195. [21] Melvill Jones, G. and Milsum, J.H. (1970) Characteristics of neural transmission from the semicircular canal to the vestibular nuclei of cats. J . Physiol. London, 209: 295-3 16. [22] Ranson, S.W., Davenport, H.K. and Doles, E.A. (1932) Intramedullary course of the dorsal root fibers of the first
three cervical nerves. J . Comp. Neurol., 54: 1-12. [23] Rexed, B. (1954) A cytoarchitectonic atlas of the spinal cord of the cat. J . Comp. Neurol., 100: 297-379. [24] Richmond, F.J.R. and Abrahams, V.C. (1979) Physiological properties of muscle spindles in dorsal neck muscles of the cat. J . Neurophysiol., 42: 604-617. [25] Schor, R.H. and Miller, A.D. (1981) Vestibular reflexes in neck and forelimb muscles evoked by roll tilt. J . Neurophysiol., 46: 167-178. [26] Shinoda, Y. and Yoshida, K. (1974) Dynamic characteristics of responses to horizontal head angular acceleration in vestibuloocular pathway in the cat. J . Neurophysiol., 37: 653-673. [27] Shriver, M.E., Stein, B.M. and Carpenter, M.B. (1968) Central projections of spinal dorsal roots in the monkeys. I. Cervical and upper thoracic dorsal roots. Am. J . Anat., 123: 27-74. [28] Takahashi, O., Takeuchi, Y. and Matsushima, R. (1985) Direct connections of primary afferent fibers with central cervical nucleus neurons projecting to the cerebellum in the cat. Brain Res., 328: 390-395. [29] Wiksten, B. (1975) The central cervical nucleus - a source of spinocerebellar fibres, demonstrated by retrograde transport of horseradish peroxidase. Neurosci. Lett., 1: 81-84. [30] Wiksten, B. (1979) The central cervical nucleus in the cat. 11. The cerebellar connections studied with retrograde transport of horseradish peroxidase. Exp. Bruin Res., 36: 155-1 73. [31] Wiksten, B. (1979) The central cervical nucleus in the cat. 111. The cerebellar connections studied with anterograde transport of 3H-leucine. Exp. Brain Res., 36: 175-189. [32] Wiksten, B. and Grant, G. (1983) The central cervical nucleus in the cat. IV. Afferent fiber connections. An experimental anatomical study. Exp. Brain Res., 51: 405412. [33] Wilson, V.J., Ezure, K. and Timerick, S.J.B. (1984) Tonic neck reflex of the decerebrate cat: Response of spinal interneurons to natural stimulation of neck and vestibular receptors. J . Neurophysiol., 51: 567-517. [34] Wilson, V.J. and Maeda, M. (1974) Connections between semicircular canals and neck motoneurones in the cat. J. Neurophysiol., 37: 34C357. [35] Yaginuma, H. and Matsushita, M. (1986) Spinocerebellar projection fields in the horizontal plane of lobules of the cerebellar anterior lobe in the cat: an anterograde wheat germ agglutinin-horseradish peroxidase study. Brain Res., 365: 345-349.
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Integration of vestibular and neck afferent signals in the central cervical nucleus T. Hongo2, T. Kitama' and K. Yoshida' 'Department of Physiology, Institute of Basic Medical Sciences, University of Tsukuba, Tsukuba. Ibaraki-ken 305, and 2Department of Neurophysiology, Institute of Brain Research, School of Medicine, University of Tokyo, Hongo 7-3-1,Bunkyo-ku, Tokyo I 13, Japan
The responses of spinocerebellar tract neurons in the CCN to natural stimulation of vestibular and neck muscle receptors were investigated in decerebrate cats. The spike activity of single CCN neurons was recorded extracellularly with a floating microelectrode in the C2 segment of the spinal cord. CCN neurons exhibited clear modulation of firing activity in response to sinusoidal rotation of the head in the vertical planes but not to rotation in the horizontal semicircular canal plane. The response to vertical rotation was attributed to stimulation of the vertical semicircular canals, since selective stimulation of otolith organs by static tilt of the head in the same plane had no effect. The response was approximately in phase with the head angular velocity at 0.05-2.0 Hz. Vector analysis suggested that the optimal orientation of vertical rotation varied considerably among neurons. The CCN neurons increased or decreased their firing rate in response to stretch of dorsal neck muscles. The time course of the responses suggested that muscle spindle primaries were most likely to be the receptors responsible for both excitatory and inhibitory responses. The input was highly muscle specific in that most CCN neurons received excitation from one muscle or two synergists, and that a group of neurons was specifically inhibited from biventer cervicis and complexus. The vestibular and neck muscle afferent signals converged with specific patterns of combination on individual CCN neurons. It is suggested that each CCN neuron integrates signals from the labyrinth and the neck and provides the cerebellum with information concerning the head movement in a certain vertical direction.
Introduction The CCN existing in the CILC4 segments [23] is now known to receive inputs from the neck and the labyrinth, and to project to the cerebellum as a niajor spinocerebellar tract of the upper cervical cord. Since Matsushita and Ikeda [16] and Wiksten [29] first demonstrated its spinocerebellar nature using the technique of retrograde transport of HRP, the knowledge of the input and output organizations of the CCN neurons has accumulated in the last decade. The mode of projection of CCN neurons to the cerebellum, including the main termination in labules 1-11, has been well documented anatomically [14-18, 30, 31, 351 and electrophysiologically [lo] in the cat. Dense projection to the CCN of cervical dorsal root fibres has long been recognized [13, 22, 27, 321. Subsequent anatomical [ 3 , 12, 281 and physiological [ 1, 121 studies demonstrated that the CCN receives afferent input from the dorsal neck muscles. Another major source of sensory input to the CCN has been shown to originate from the vestibular labyrinth. In a previous study we have shown that CCN neurons are excited by electrical stimulation of the contralateral vestibular nerve and inhibited from the ipsilateral vestibular nerve [I 11. Stimulation of individual ampullary nerves further indicated that the semicircular canals
Abbreviurions: BCC, biventer cervicis and complex; CCN, central cervical nucleus; HRP, horseradish peroxidase; OCC, obliques capitis caudalis; RCD, rectus capitis dorsalis; Spl, splenius.
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constitute a source from which the vestibular effect is evoked. In the present study we have attempted to investigate the dynamic properties of the responses of CCN neurons to natural stimulation of vestibular and neck muscle receptors in order to elucidate the functional role of the CCN spinocerebellar tract. We will describe the responses of single CCN neurons to sinusoidal rotation and static tilt of the head and also to muscle stretch. In addition, some preliminary data, suggesting a highly specific pattern of convergence from vestibular and neck muscle afferents, will be presented. Experiments were performed on decerebrate cats prepared under halothane anaesthesia. The animal was immobilized with pancuronium bromide and maintained under artificial respiration. Blood pressure in the femoral artery was monitored and maintained above 100 mmHg, and rectal temperature was kept at 37-38°C throughout the experiment. Bipolar silver-ball electrodes were placed on the round and oval windows for electrical stimulation of the vestibular nerves. Ensembles of needle electrodes were inserted into the white matter of the cerebellar anterior lobe for stimulation of spinocerebellar axons. Various neck muscles were dissected with the innervation kept intact and prepared for natural stimulation. After laminectomy from C1 to C4, the spinal cord was transected between the C3 and C4 segments. This was done to eliminate a possible sensory inflow from the body and limbs during rotation and tilt for vestibular stimulation. The spike activity of single CCN neurons was recorded extracellularly with a floating microelectrode in the C2 segment of the left side. CCN neurons were identified by antidromic activation from the anterior lobe vermis, and only those which were activated orthodromically following electrical stimulation of the contralateral (right) vestibular nerve were selected for further analysis. Responses to input from the vestibular labyrinth Responses of single CCN neurons to whole-body rotation were examined. With the head tilted nose-
down by 27" the animal was rotated sinusoidally in three orthogonal planes which were approximately coplanar with canal planes. For stimulation of the horizontal canal pair, the animal was rotated in the earth-horizontal plane (HC mode). For stimulation of the right anterior-left posterior canal pair (r-AC mode) and the right posterior-left anterior canal pair (r-PC mode), the animal was rotated in the vertical plane orienting 45" and 135" from the sagittal plane (clockwise as viewed from above), respectively. Thus the r-AC and the r-PC mode rotations should stimulate both vertical canals and otolith organs. T o evaluate the contribution of otolith inputs, responses to static tilts which should provide pure otolith stimuli were also examined. Fig. 1A exemplifies responses of a left CCN neuron to sinusoidal rotation in the r-AC plane at 0.5 Hz with an amplitude of 13.5"/s. The firing rate was modulated in an approximately sinusoidal manner around the resting level (45 spikes/s, see record in Fig. lB, middle traces). Fig. 1B shows the steady-state activity of the same neuron at the standard position (middle traces) and during 10" static tilt in the same r-AC plane in both directions (left and right traces). Although the angular displacement during the tilt was more than twice as large as the positional change that occurred during sinusoidal stimulus, the tilt stimuli elicited no appreciable change in the firing rate. Thus the clear modulation of firing rate during rotation shown in Fig. 1A was attributable to stimulation of vertical canals. The vast majority of CCN neurons exhibited clear modulations of firing rate in response to rotation in the vertical plane of either r-AC or r-PC mode or both, but not to HC mode rotation. In contrast, static tilts in these planes had no effect in any of the CCN cells examined. In a few cells, weak modulations were observed during H C mode rotation. Such weak modulations could, however, be well accounted for as caused by stimulation of the vertical canals, because the vertical canals cannot be exactly perpendicular to the plane of rotation due to variations of the canal orientations [ 5 ] . In fact, stimulation of the vertical canals evoked
157
A
ROTATION 0.5Hz
I-CCN
5s
8 TILT I-CCN -
FIRING RATE 10' NOSE DOWN
0'
10' NOSE UP
Fig. 1. Responses of a left CCN cell to sinusoidal rotation at 0.5 Hz (A) and to static tilt (B) in r-AC plane. In the head velocity curve in A, upward deflexion indicates right side-nose down direction. See text for further explanations.
strong effects on these cells, and when the head position was changed by 10" from the standard 27" pitch to 17" pitch, the modulation due to horizontal rotation changed just as expected if the vertical canals were responsible. In regard to the receptor origin of the vestibular input, therefore, it can be concluded that the CCN cells so far analysed received input only from the vertical canals. The gain and phase with respect to the head velocity were computed from the average of responses to 10-100 successive stimulus cycles. Within the range of stimulus amplitudes employed (2.3 to 13.5"/s, at 0.5 Hz), the response amplitudes varied with the stimulus amplitudes in an approximately linear manner, i.e. the gain was constant. The phase of response was essentially independent of the stimulus amplitude. For example, the cell shown in Fig. 2 exhibited response amplitudes of 4.3, 8.6, and 11.7 (spikes/s/("/s) for stimulus velocities of 4.5, 9.0 and 13.5"/s, respectively. The phase was about O", and varied by no more than 4" for these three stimulus amplitudes.
In most CCN cells tested, the phase of response was approximately in phase with the head velocity that should excite the contralateral canal, ranging from + 30" (lead) to - 30" (lag) at 0.5 Hz. The phase was roughly constant over frequencies from 0.05 to 2.0 Hz. These phase properties were similar to those of vestibular nucleus neurons [21, 261. In some cells, the response to r-PC mode rotation was about 180" out of phase, i.e. firing rate decreased during contralateral posterior canal activation. Since no excitatory effects on CCN cells were found after electrical stimulation of the ipsilateral vestibular nerve [ 1 11, these cells were considered to be inhibited from the contralateral (right) posterior canal. Gain of responses to r-AC mode and r-PC mode rotations varied considerably from neuron to neuron. While some neurons responded almost exclusively to either r-AC or r-PC mode rotation, others were responsive to both (Fig. 2A). To obtain a quantitative measure of the total canal input to each cell, the orientation of the vertical plane at which rotation would evoke a maximum response
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timal orientation varied from 9" to 157". The wide range of optimal orientation indicates that a given rotation influences the activity of individual cells differentially. The available data, though limited, AC. MODE appear to indicate that CCN cells consist of subpo_____---pulations having different directional sensitivities ROLL MODE for vertical rotations (see Fig. 5).
B
A 25
0
25
/
0.5
PITCH MOD1
~. 0.5
HEAD VELOCITY
1
0.5Hz
P.C. MODE
W.I.
Fig. 2. Responses of a left CCN cell to sinusoidal rotation in different vertical planes. (A) Modulation of firing rate to r-AC mode (top) and r-PC mode (middle) rotations at 0.5 Hz. Averaged from 30 cycles. (B) Solid vectors represent the gain of the response to r-AC mode, r-PC mode, pitch mode and roll mode rotations. The dotted vector indicates the predicted response in the optimal plane. See text for further explanations.
(optimal plane), and the gain in the optimal plane were calculated in individual cells. These were obtained from the gain and phase of r-AC and r-PC mode responses at 0.5 Hz, assuming a linear summation of effects from different canals. Fig. 2 illustrates for one cell the actual r-AC and r-PC mode responses and the predicted response at the optimal plane. This cell exhibited clear modulations in response to both r-AC and r-PC mode rotations (Fig. 2A). The gain and phase of the r-AC mode response were 0.82 (spikes/s)/("/s) and - 20.1", and those of the r-PC mode response were 0.63 (spikes/ s)/("/s) and + 12.6", respectively. In Fig. 2B each response is represented in vector form where the vector length is proportional to the gain and the polar angle shows the orientation of the rotation plane. Vectorial summation of the r-AC and r-PC mode responses predicted the optimal plane at 81" (clockwise from the sagittal plane) and the maximum gain of 0.99 (spikes/s)/("/s) (dotted vector). The actual responses to pitch and roll mode rotations were in good agreement with the prediction (Fig. 2B), indicating the validity of the assumption of linear summation. Similar analyses in 10 CCN cells so far studied showed that the gain of the optimal response ranged from 0.32 to 2.18 (spikes/s)/("/s) and the op-
Responses to input from the neck muscles
The responses of CCN neurons to electrical stimulation of the dorsal root of cervical segments and of various peripheral nerves in the neck and the forelimb have been studied by Hirai et al. [9, 121 and Abrahams et al. [I]. These studies have shown that monosynaptic excitatory effects are exerted by afferents from neck muscles and are conveyed via the dorsal roots of the upper cervical segments. No detectable excitation with a monosynaptic latency was elicited from the forelimb nerves, though nerves from shoulder muscles were not tested. We were unsuccessful in finding effects clearly ascribable to afferents from vertebral joints or ligaments, which have been reported to cause neck-limb [20] or neck-ocular reflex actions [8]. Detailed analyses were therefore focused on the effects from the neck muscle afferents in the previous study [12]. The main findings obtained by using electrical stimulation of the neck muscle nerves were as follows. (1) Monosynaptic excitation was evoked in CCN neurons after stimulation of nerves to dorsal neck muscles such as Spl, BCC, OCC and RCD. (2) Thresholds for the monosynaptic excitation were near the threshold of the nerve, suggesting that it was caused by group I afferents. (3) The excitatory input was muscle specific in that individual neurons received excitation from one muscle, or from synergists (e.g. BCC-RCD, SpI-OCC). There were some cells in which convergence from BCC and Spl occurred, but then the excitatory effect from one muscle overwhelmed that from the other. (4) Inhibition could also be evoked at short latencies after stimulation of BCC but not Spl.
159
sp./s
n
-
2s
Fig. 3. Response of a CCN cell to stretch of BCC muscle. Upper trace: extracellular spikes of single CCN cells recorded through a high-pass filter. Lower trace: firing frequency of the larger unit in the upper trace. The solid lines in the bottom indicate the periods of muscle stretch.
Based on these results, we attempted to investigate effects of natural stimulation of the muscle (stretch or local pressure) on the firing activity of single CCN neurons recorded extracellularly. A floating microelectrode was useful for this test. Fig.
3 shows an example of responses of a single CCN neuron to stretching the BCC muscle. Note that the firing rate greatly increased transiently at the early phase of stretch, maintained an elevated level during stretch, and abruptly decreased even to pause when the muscle was released. The firing pattern was very similar to that of muscle spindle primaries in possessing both dynamic and static responses [19, 241, suggesting that the excitatory effect was of group Ia fibre origin. The interpretation is in good agreement with the observation that fibres identified to originate from spindle primaries distribute terminals in the CCN as revealed by their intra-axonal staining with HRP (Fig. 4 [12]). The excitatory responses of CCN neurons to muscle stretch were usually as in Fig. 3 , and were seen in about 70% of CCN neurons examined. Decrease of the firing rate was also produced by muscle stretch in some CCN cells. Since electrical stimulation of the nerve to the muscle stretched also
B
I Fig. 4. Trajectory (A) and terminals (B) of HRP-stained axons identified as low-threshold neck muscle afferents. Five collaterals are superimposed at the C2 segment. (From [12]).
160
caused suppression, the effect was interpreted as due to inhibition caused by stretch-evoked impulses in the nerve from that muscle and not due to unloading of other muscles that supplied excitatory input to the CCN cell. The inhibition was probably also of group Ta fibre origin, since it occurred with a small stretch and persisted throughout the period of stretch. The receptive field for the excitation and inhibition caused by muscle stretch was explored in individual CCN neurons. The input was found to be muscle specific, in general agreement with the results obtained by electrical stimulation of the muscle nerves [12]. The excitatory input, when present, originated from single or synergic muscles of either the BCC-RCD group or the Spl-OCC group, while inhibition came from BCC and not from any other muscles examined. These results indicated that CCN neurons consist of subgroups, each of which receive specific spatial pattern of excitatory and/or inhibitory inputs from muscle spindle afferents of different neck muscles. According to the patterns of muscle input, the following four subgroups of CCN neurons have so far been observed: (1) those excited from BCC; ( 2 ) those inhibited from BCC; (3) those excited from Spl-OCC; (4) those excited from SplOCC and inhibited from BCC. Convergence of inputs from the semicircular canal and the neck muscles
The majority of CCN cells examined received inputs from both the labyrinth and the neck muscles, in agreement with the previous results obtained by electrical stimulation [9, 1 I]. We therefore attempted to disclose the pattern of convergence from the two sources onto the same CCN neurons. Despite technical difficulties we could complete, in a limited number of cells, most of the necessary tests using rotation and muscle stretch without losing the cell recorded from. The results obtained in 6 cells are shown in Fig. 5, where the origin of muscle for excitation and inhibition by stretch (A) and the vector (gain and direction) of the response at the optimal plane as defined above (B) are given for each
LEFT-CCN UNITS
A
MUSCLE INPUT
0
EXCITATION
-
WHlElTlON NOEFFECT
0
CANALINPUT ROSTRAL
RIGHT
CAUDAL
Fig. 5. Patterns of convergence of neck muscle (A) and vestibular (B) inputs to 6 CCN neurons. In B the response gain and orientation of optimal plane are shown in vector form, as in Fig. 2. The number attached to each vector refers to the unit in A. See text for further explanations.
cell. The diagram in B shows that the directions of the optimal plane are anterior in units 1 and 2, lateral in units 3 and 4, and posterior in units 5 and 6. It is interesting to note that the differences in the canal input are well correlated with the pattern of input from neck muscles. Cells with anterior vectors were inhibited from BCC without effects from Spl; cells with lateral vectors were excited from Spl and inhibited from BCC; and cells with posterior vectors were excited from BCC with no effects from Spl. Such highly specific patterns of convergence suggest that each CCN neuron integrates sensory signals related to the head movement in a particular direction, with respect to both body and space. Discussion
Our results demonstrate that the CCN neurons receive afferent signals arising from both the vestibular and neck muscle receptors and provide the cerebellum with integrated information about the head movement with respect to the trunk and space. One of the striking features of the vestibular input is that CCN neurons receive input only from the ver-
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tical canals. The absence of the horizontal canal input is in agreement with the previous results obtained by electrical stimulation of the individual ampullary nerves [I 11. The previous study [I 11 has also revealed various patterns of convergence from different vertical canals on single CCN neurons. The present results extend this previous finding by showing that each cell has its own optimal orientation of vertical rotation which varied widely among neurons. Similar convergence from different canal pairs has been observed in some second-order vestibular neurons [2].It remains to be studied whether the convergence takes place at the vestibular nuclei or at the CCN. As for the contribution of the otolith input, selective stimulation of the otolith organs by static tilt was shown to have negligible effect. In this regard it should be pointed out that the functional organization of vestibular inputs to CCN neurons is quite different from that to dorsal neck motoneurons, in which both the horizontal canal and the otolith organs also make a substantial contribution to the vestibular induced activity [2, 4, 6, 7, 25, 341. The properties of the response of CCN neurons to muscle stretch are in general accordance with the previous electrophysiological and anatomical findings [12], in showing that the input is of muscle spindle origin and the receptive field of each cell is limited to one muscle or close synergists. A close correlation was found between the optimal direction of the head rotation for vestibular stimulation and the pattern of muscle input in each cell (see Fig. 5). Given the complex geometry of neck muscles and their actions, it is difficult to give a simple explanation for the highly specific pattern of convergence of the two inputs. Nevertheless it would be of interest to consider the manner of interaction of the two signals when the head is passively rotated with respect to a stationary trunk. The result in Fig. 5 suggests that if a CCN neuron is excited from the labyrinth by a passive head rotation toward its preferred direction, effects from neck muscles are either inhibition from stretched muscles or disfacilitation from released muscles. If the head is moved to the opposite direction, the canal input
would be inhibitory while changes in muscle length would produce either disinhibition or facilitation. It therefore appears that effects from the vestibular and neck receptors are of opposite polarity and may more or less cancel each other. It is interesting to note that similar opposite effects of vestibular and neck afferent actions have been shown in interneurons of the lower cervical cord, though mainly otolith inputs appear to contribute in these neurons [331. References Abrahams, V.C., Anstee, G. and Richmond, F.J.R. (1979) Neck muscle and trigeminal input to the upper cervical cord and lower medulla of the cat. Can. J . Physiol. Phurrnacol., 57: 642-651. Baker, J., Goldberg, J. and Peterson, B. (1985) Spatial and temporal properties of the vestibulocollic reflex in decerebrate cats. J. Neurophysiol., 54: 735-756. Bakker, D.A., Richmond, F.J.R. and Abrahams, V.C. (1984) Central projections from cat suboccipital muscles: A study using transganglionic transport of horseradish peroxidase. J. Comp. Neurol., 288: 409421. Berthoz, A. and Anderson, J.H. (1971) Frequency analysis of vestibular influence on extensor motoneurons. 11. Relationship between neck and forelimb extensors. Bruin Res., 34: 376380. Estes, M.S., Blanks, R.H.I. and Markham, C.H. (1975) Physiologic characteristics of vestibular first-order canal neurons in the cat. I. Response plane determination and resting discharge characteristics. J . Neurophysiol., 38: 12321249. Ezure, K. and Sasaki, S. (1978) Frequency-response analysis of vestibular induced neck reflex in cat. I. Characteristics of neural transmission from horizontal semicircular canal to neck motoneurons. J . Neurophysiol., 41: 445458. Ezure, K., Sasaki, S., Uchino, Y . and Wilson, V.J. (1978) Frequency-response analysis of vestibular-induced neck reflex in cat. 11. Functional significance of cervical afferents and polysynaptic descending pathways. J . Neurophysiol., 41: 459471. Hikosaka, 0. and Maeda, M. (1973) Cervical effects on abducens motoneurons and their interaction with vestibuloocular reflex. Exp. Brain Res., 18: 512-530. Hirai, N., Hongo, T. and Sasaki, S. (1978) Cerebellar projection and input organizations of the spinocerebellar tract arising from the central cervical nucleus in the cat. Brain Res., 157: 341-345. Hirai, N., Hongo, T. and Sasaki, S. (1984) A physiological study of identification, axonal course and cerebellar projec-
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tion of spinocerebellar tract cells in the central cervical nucleus of the cat. Exp. Brain Res., 55: 272-285. [Ill Hirai, N., Hongo, T., Sasaki, S. and Yoshida, K. (1979) The neck and labyrinthine influences on cervical spinocerebellar tract neurons of the central cervical nucleus in the cat. In: R. Granit and 0. Pompeiano (Eds.), Reflex Control qf Posture and Movement, Progress in Brain Research, Vol. 50, Elsevier, Amsterdam, pp. 529-536. [I21 Hirai, N., Hongo, T., Sasaki, S., Yamashita, M. andYoshida, K. (1984) Neck muscle afferent input to spinocerebellar tract cells of the central cervical nucleus in the cat. Exp. Brain Res., 55: 286-300. [13] Imai, Y. and Kusama, T. (1969) Distribution of the dorsal root fibers in the cat. An experimental study with the Nauta method. Brain Res., 13: 338-359. [14] Matsushita, M. and Hosoya, Y. (1982) Spinocerebellar projections to lobules 111 to V of the anterior lobe in the cat, as studied by retrograde transport of horseradish peroxidase. J . Cornp. Neurol., 208: 127-143. [I51 Matsushita, M., Hosoya, Y . and Ikeda, M. (1979)Anatomical organization of the spinocerebellar system in the cat, as studied by retrograde transport of horseradish peroxidase. J . Comp. Neurol., 184: 81-106. [16] Matsushita, M. and Ikeda, M. (1975) The central cervical nucleus as cell origin of a spinocerebellar tract arising from the cervical cord: a study in the cat using horseradish peroxidase. Brain Res., 100: 412417. [17] Matsushita, M. and Okado, N. (1981) Spinocerebellar projections to lobules I and I1 of the anterior lobe in the cat, as studied by retrograde transport of horseradish peroxidase. J . Comp. Neurol., 197: 411424. [I81 Matsushita, M., Tanami, T. and Yaginuma, H. (1984) Differential distribution of spinocerebellar fiber terminals within the lobules of the cerebellar anterior lobe in the cat: an anterograde WGA-HRP study. Bruin Res., 305: 157161. [I91 Matthews, P.B.C. (1972) Mammalian Muscle Receptors and their Central Actions, Arnold, London, pp. 20-194. [20] McCouch, G.P., Deering, I.D. and Ling, T.H. (1951) Location of receptors for tonic neck reflexes. J . Neurophysiol., 14: 191-195. [21] Melvill Jones, G. and Milsum, J.H. (1970) Characteristics of neural transmission from the semicircular canal to the vestibular nuclei of cats. J . Physiol. London, 209: 295-3 16. [22] Ranson, S.W., Davenport, H.K. and Doles, E.A. (1932) Intramedullary course of the dorsal root fibers of the first
three cervical nerves. J . Comp. Neurol., 54: 1-12. [23] Rexed, B. (1954) A cytoarchitectonic atlas of the spinal cord of the cat. J . Comp. Neurol., 100: 297-379. [24] Richmond, F.J.R. and Abrahams, V.C. (1979) Physiological properties of muscle spindles in dorsal neck muscles of the cat. J . Neurophysiol., 42: 604-617. [25] Schor, R.H. and Miller, A.D. (1981) Vestibular reflexes in neck and forelimb muscles evoked by roll tilt. J . Neurophysiol., 46: 167-178. [26] Shinoda, Y. and Yoshida, K. (1974) Dynamic characteristics of responses to horizontal head angular acceleration in vestibuloocular pathway in the cat. J . Neurophysiol., 37: 653-673. [27] Shriver, M.E., Stein, B.M. and Carpenter, M.B. (1968) Central projections of spinal dorsal roots in the monkeys. I. Cervical and upper thoracic dorsal roots. Am. J . Anat., 123: 27-74. [28] Takahashi, O., Takeuchi, Y. and Matsushima, R. (1985) Direct connections of primary afferent fibers with central cervical nucleus neurons projecting to the cerebellum in the cat. Brain Res., 328: 390-395. [29] Wiksten, B. (1975) The central cervical nucleus - a source of spinocerebellar fibres, demonstrated by retrograde transport of horseradish peroxidase. Neurosci. Lett., 1: 81-84. [30] Wiksten, B. (1979) The central cervical nucleus in the cat. 11. The cerebellar connections studied with retrograde transport of horseradish peroxidase. Exp. Bruin Res., 36: 155-1 73. [31] Wiksten, B. (1979) The central cervical nucleus in the cat. 111. The cerebellar connections studied with anterograde transport of 3H-leucine. Exp. Brain Res., 36: 175-189. [32] Wiksten, B. and Grant, G. (1983) The central cervical nucleus in the cat. IV. Afferent fiber connections. An experimental anatomical study. Exp. Brain Res., 51: 405412. [33] Wilson, V.J., Ezure, K. and Timerick, S.J.B. (1984) Tonic neck reflex of the decerebrate cat: Response of spinal interneurons to natural stimulation of neck and vestibular receptors. J . Neurophysiol., 51: 567-517. [34] Wilson, V.J. and Maeda, M. (1974) Connections between semicircular canals and neck motoneurones in the cat. J. Neurophysiol., 37: 34C357. [35] Yaginuma, H. and Matsushita, M. (1986) Spinocerebellar projection fields in the horizontal plane of lobules of the cerebellar anterior lobe in the cat: an anterograde wheat germ agglutinin-horseradish peroxidase study. Brain Res., 365: 345-349.