Labyrinthine and neck reflexes recorded from spinal single motoneurons in the cat

Static and dynamic changes in body orientation modulate spinal reflex excitability in humans

In the present study, we investigated the modulation pattern of the soleus H reflex in healthy subjects in response to imposed static and dynamic changes in body angle, referenced to the vertical plane. Soleus H reflexes were recorded using conventional methods with subjects either supine or while they were erect. Changes in body angle were initiated with subjects lying supine on a tilt table. Table position was controlled via a motor and could move from the horizontal to the upright position and beyond. Elastic bands around the trunk (upper and lower part) and around the thigh and shank secured subjects' position. In the vertical position, the soleus H reflex exhibited a strong depression in all subjects tested, reaching amplitudes as low as 40+/-8.1% of the control reflex (Ho). With subjects supine, positioning the body at 10 degrees, 20 degrees, 40 degrees, 60 degrees, 90 degrees, -50 degrees and -20 degrees all resulted in a significant facilitation of the soleus H reflex. The reflex magnitude at these angles ranged from 140+/-17.2% to 180+/-10.9% of the Ho. Reflex facilitation was also observed following dynamic tilt of the body in the sagittal plane (at 1.8 degrees /s) with the H reflex reaching amplitudes as high as 300+/-18.3% of Ho. Our findings indicate that changes in body orientation induced a significant facilitation of the H reflex magnitude in soleus motoneurones that were essentially independent of angular change in body orientation or of movement direction. In addition, they highlight the potent modulatory effects that natural stimulation of the vestibular system can have on reflex excitability. The implications of our findings are discussed in relation to the maintenance of body posture.

Posture-related changes of soleus H-reflex excitability

We investigated whether the modulatory effects of segmental and descending inputs on the soleus H reflex are modified by postural conditions. Fourteen healthy volunteers received a transcranial magnetic stimulus (TMS) or percutaneous electrical stimulation of the posterior tibial nerve (PTN), preceding by 0 to 400 ms the elicitation of the soleus H reflex in supine, sitting, and standing positions. In all positions, TMS induced an early period of facilitation at interstimulus intervals (ISIs) ranging between 5 and 35 ms. In supine and sitting positions, there was a second period of facilitation at ISIs between 60 and 90 ms, which was absent or significantly reduced in the standing position. PTN induced a strong inhibition of the H reflex in all positions up to 125 ms. In supine and sitting positions, inhibition continued up to 400 ms, whereas it was significantly reduced or completely absent beyond 125 ms in the standing position. These results demonstrate posture-related differences in the modulatory effects of descending and segmental inputs on the excitability of the H-reflex circuit.

Modulation of soleus H reflex by lateral tilting in man

Static vestibular influences on extensor tone of the lower limbs in man were studied by analyzing the changes in right soleus H-reflex (RSHR) area in relation to lateral tiltings. Eight normal adult volunteers were tested in an experimental situation designed to minimize all afferent inputs, except the vestibular ones. Each subject was seated on a chair which could be tilted laterally from the vertical to both sides. Lateral tiltings were applied at a random order from the vertical (0 degree, control position) to 4 degrees, 8 degrees, 12 degrees, 16 degrees, and 20 degrees of both sides (test positions). The results showed inhibition in SHR area of the leg ipsilateral to the tilting and facilitation of the contralateral SHR. These data indicate that, in man, as in the decerebrate cat, tonic labyrinth reflexes act asymmetrically and that, in static condition, the vestibular system modulates muscle tone of the lower limbs adequately to counteract lateral perturbation of upright position.

Interaction of tonic labyrinth and neck reflexes in man

Interaction of tonic labyrinth and neck reflexes was studied in 3 healthy volunteers by analyzing changes in Soleus H-Reflex (SHR) area in relation to both lateral tiltings and neck rotations. By using a Kermath chair each subject was tilted laterally from the vertical to the left and to the right up 15 degrees in steps of 5 degrees and at the same time the longitudinal body axis, keeping the head fixed, was rotated to the right and to the left up to 15 degrees in steps of 5 degrees. All combinations of lateral tiltings and neck rotations were tested. Each test position was followed by a return to 0 degree for both rotation and tilting (control position). Twelve H-reflexes of right soleus muscle were recorded in each test and control position and the changes in RSHR area were expressed as percentage variations from the mean value absorbed in the pretest and post-test control position. Our data indicate that in man, as in animals, labyrinth and neck reflexes act in the opposite direction, and that in the static condition their contribution to postural stabilization is equal.

Modulation of flexor carpi radialis H reflex by lateral tilts in man

Static vestibular influences on upper limb flexor tone were studied in man by analyzing the changes in flexor carpi radialis H reflex area with lateral tilting of the longitudinal body axis. Ten healthy volunteers and 2 labyrinthine defective patients were tested in an experimental situation designed to minimize all afferent inputs except vestibular ones. Each subject was seated on a chair which could be tilted laterally to the left or the right from the vertical. Head and trunk were fixed upright, upper and lower limbs in half-flexed position and forearm in an intermediate position between supination and pronation. Lateral tilting was applied at random from the vertical (0 degrees control position) to left and right (4 degrees, 8 degrees, 12 degrees, 16 degrees, 20 degrees test positions). Each test position was followed by a return to 0 degrees and in each control and test position 20 consecutive H reflexes were recorded. The data observed in the normal subjects showed flexor tone inhibition in the arm which was tilted downwards and facilitation in the contralateral arm. These findings suggest that in man, like in animals, labyrinth reflexes act asymmetrically and in the opposite direction to neck reflexes.

Convergent effects from vestibulospinal tract and primary cutaneous afferent fibers on motoneurons to ankle extensor and flexor muscles in humans

The latency and magnitude of cutaneomuscular reflexes, evoked by stimulation of the sural nerve in the contralateral and ipsilateral tibialis anterior and soleus, were investigated in normal human subjects during static tilts in the pitch axis. For all subjects the test reflex consisted of oscillating sequences of excitation and inhibition, each muscle exhibiting a characteristic pattern defined by its latency and sign of the initial phase. The latency of the various components and the sign of the initial phase did not vary with angle of tilt. However, the results of an ANOVA demonstrate a highly significant tilt-dependent modulation of the amplitude of the test reflex for the initial inhibitory phase of the contralateral tibialis anterior. We propose that this tilt-dependent effect on the crossed cutaneomuscular reflex originates from activity in the otolith organ receptors.

Effects of stimulation of vestibular and neck receptors on Deiters neurons projecting to the lumbosacral cord

The activity of lateral vestibular nucleus (LVN) neurons, antidromically identified by stimulation of the spinal cord at T12 and L1, thus projecting to the lumbosacral segments of the spinal cord (IVS neurons), was recorded in precollicular decerebrate cats during rotation about the longitudinal axis either of the whole animal (labyrinth input) or of the body only while the head was kept stationary (neck input). Among the IVS neurons tested for vestibular stimulation, 76 of 129 units (i.e. 58.9%) responded to roll tilt of the animal at the standard parameters of 0.026 Hz, +/- 10 degrees. The gain and the sensitivity of the first harmonic responses corresponded on the average to 0.47 +/- 0.44, SD, impulses X s-1 X deg-1 and 3.24 +/- 3.15, SD, %/deg, respectively. As to the response patterns, 51 of 76 units (i.e. 67.1%) were excited during side-down and depressed during side-up tilt, whereas 15 (i.e. 19.7%) showed the opposite behavior. In both instances the peak of the responses occurred with an average phase lead of about +21.0 +/- 27.2, SD, deg with respect to the extreme side-down or side-up position of the animal. Moreover, the former group of units showed almost a twofold larger gain with respect to the latter group (t-test, p less than 0.05). Among the IVS neurons tested for neck stimulation, 75 of 109 units (68.8%) responded to neck rotation at the standard parameters. The gain and the sensitivity of the first harmonic responses corresponded on the average to 0.49 +/- 0.40, SD, impulses X s-1 X deg-1 and 3.30 +/- 3.42, SD, %/deg, respectively, thus being similar to the values obtained for the labyrinth responses. However, 59 of 75 units (i.e. 78.6%) were excited during side-up neck rotation and depressed during side-down neck rotation, while 8 of 75 units (i.e. 10.7%) showed the opposite pattern. In both instances the peak of the responses occurred with an average phase lead of +52.0 +/- 18.3, SD, deg for the extreme side-up or side-down neck displacements. Further, the former group of units showed a larger gain than the latter group. Histological controls indicated that 102 of 129 (i.e. 79.0%) IVS neurons tested for labyrinth stimulation and 86 of 109 (i.e. 78.9%) IVS neurons tested for neck stimulation were located in the dorsocaudal part of LVN, the remaining IVS neurons being located in the rostroventral part of LVN.(ABSTRACT TRUNCATED AT 400 WORDS)

Discharge activity of spindle afferents from the gastrocnemius-soleus muscle during head rotation in the decerebrate cat

The activity of spindle afferents originating from both primary and secondary endings of the isometrically extended (6-8 mm) gastrocnemius-soleus (GS) muscle was recorded in precollicular decerebrate cats during sinusoidal head rotation about the longitudinal axis above a stationary body. In the first group of experiments to test the influence of vestibular volleys on fusimotor neurons, an acute bilateral neck deafferentation at C1-C3 was performed to eliminate possible influences arising from neck receptors; head rotation (0.026 Hz, +/- 15 degrees) induced a weak periodic rate modulation in 6/38 (15.8%) of the tested spindle afferents; the average response gain was 0.18 +/- 0.12, SD imp./s/deg (mean firing rate, 18.9 +/- 2.8 imp./s), and the average phase angle was -43.2 +/- 47.0 degrees, SD lag with respect to ipsilateral side-down displacement of the head (alpha-response pattern). In a second group of experiments head rotation studied after acute bilateral section of VIII cranial nerve, thereby stimulating only neck receptors, failed to influence in a reliable manner the firing rate of 38 additional spindle afferents. In a third group of experiments in which both VIII nerves and cervical dorsal roots were left intact, head rotation induced a response in 7/45 (15.6%) of the tested spindle afferents similar to that observed after cervical deafferentiation and thus depended on stimulation of labyrinth receptors alone. Over the examined frequency range of head rotation from 0.015 to 0.325 Hz (+/- 15 degrees), the response gain of spindle afferents was relatively stable during sinusoidal labyrinth stimulation. For most of the spindle afferents the phase angle of the response elicited at the lower frequencies was related to the direction of head orientation towards the ipsilateral sidedown, thus being attributed to labyrinth volleys originating from macular receptors; at 0325 Hz the stimulus was less effective and some units showed a phase advance relative to head position which was attributed to costimulation of canal receptors. Displacement of the muscle under study obtained by either rotation of the whole animal or body alone beneath a stationary head elicited a periodic modulation of spindle afferent discharge, independent of head orientation or type of preparation, in 51/73 (70%) of the muscle spindles tested; the average response gain was 0.20 +/- 0.19, SD imp./s/deg, and an average phase lead of +14.1 +/- 20.5 degrees, SD with respect to the peak of the ipsilateral side-down displacement of the body or of the animal was observed.(ABSTRACT TRUNCATED AT 400 WORDS)

The controversy over the influence of static tilt on soleus motoneuron excitability in man

The influence of static fore-aft body tilts on the excitability of the human soleus motoneuronal pool had been studied by mapping the H reflex amplitude as a function of head positions by two groups of investigators with different results. In 11 of 13 subjects studied, we (C.W.Y. Chan and R.E. Kearney, 1982, Neurosci. Lett. 33: 333-338) found that the H reflex was a minimum when the head was near the vertical position, and often increased as a sinusoidal function of tilt to a maximum of 60 degrees forward or backward. In contrast, I.G. Aiello, G. Rosati, G. Serra, V. Jugnoli, and M. Manca (1983, Exp. Neurol. 79: 18-26) reported a linear decrease in the H reflex amplitude in all their subjects when the body was tilted from the vertical to the horizontal supine position. The differences in these findings were discussed from the point of view of methodology and in light of recent animal and human studies.

Influence of static tilt on soleus motoneuron excitability in man

The influence of whole head-body rotations with respect to gravity on the excitability of the human soleus motoneuron pool, was studied by mapping the H reflex amplitude as a function of static head displacements. In most subjects, the H reflex reached a minimum when the head was near the vertical position (90 degrees), and often increased as a sinusoidal function of tilt in the sagittal plane. These changes were attributed to the tonic labyrinthine reflexes which, unlike some findings in decerebrate cats, generally increase the extensor tone of the human lower limb when the head is displaced from the upright position.

Canal-neck interaction in vestibular nuclear neurons of the cat

The convergence and interaction of horizontal semicircular canal and neck proprioceptive inputs were studied in neurons of the caudal two thirds of the vestibular nuclear complex. Extracellular neuron activity was recorded under muscle relaxation and slight anesthesia in chronically prepared cats. The following stimulations were applied: horizontal rotations of (a) the whole body (labyrinth stimulation), (b) the trunk vs. the stationary head (neck stimulation), and (c) the head vs. the stationary trunk (combined labyrinth and neck stimulation). Of 152 neurons investigated, 83 (55%) showed convergence of the two inputs. In about half of these neurons, the neck input was very weak and hardly affected the labyrinthine response during head rotation. Judged from the response pattern, several of these neurons presumably were related to vestibulo-oculomotor function (i.e., vestibular nystagmus). In the other half (i.e., 27% of all neurons), sensitivity of the two inputs was similar. Both labyrinthine and neck responses contained a dynamic ("velocity") component; neck responses of more than half of these neurons had, in addition, a static ("position") component. The dynamic components were either "antagonistic" or "synergistic" as to their convergence during head rotation. When applying this combined stimulation, the dynamic components summed linearly, yielding subtration in case of antagonistic convergence and addition in case of synergistic convergence. In contrast, the static components of the neck responses remained largely unchanged during head rotation. However, the static head-to-trunk deflection determined the tonic discharge level in such neurons and thus facilitated or disfacilitated the dynamic responses to superimposed labyrinth stimulation. We suggest that the two patterns of labyrinthine neck interaction observed in vestibular nuclear neurons, i.e., subtration and addition, may be involved in the postural control of the trunk and head, respectively. In contrast, interference of the neck input with vestibulo-oculomotor function appears to be almost negligible in the intact cat.

The horizontal and vertical cervico-ocular reflexes of the rabbit

Horizontal and vertical cervico-ocular reflexes of the rabbit (HCOR, VCOR) were evoked by sinusoidal oscillation of the body about the vertical and longitudinal axes while the head was fixed. These reflexes were studied over a frequency range of 0.005-0.800 Hz and at stimulus amplitudes of +/- 10 degrees. When the body of the rabbit was rotated horizontally clockwise around the fixed head, clockwise conjugate eye movements were evoked. When the body was rotated about the longitudinal axis onto the right side, the right eye rotated down and the left eye rotated up. The mean gain of the HCOR (eye velocity/body velocity) rose from 0.21 and 0.005 Hz to 0.27 at 0.020 Hz and then declined to 0.06 at 0.3Hz. The gain of the VCOR was less than the gain of the HCOR by a factor of 2-3. The HCOR was measured separately and in combination with the horizontal vestibulo-ocular reflex (HVOR). These reflexes combine linearly. The relative movements of the first 3 cervical vertebrae during stimulation of the HCOR and VCOR were measured. For the HCOR, the largest angular displacement (74%) occurs between C1 and C2. For the VCOR, the largest relative angular displacement (45%) occurs between C2 and C3. Step horizontal clockwise rotation of the head and body (HVOR) evoked low velocity counterclockwise eye movements followed by fast clockwise (resetting) eye movements. Step horizontal clockwise rotation of the body about the fixed head (HCOR) evoked low velocity clockwise eye movements which were followed by fast clockwise eye movements. Step horizontal clockwise rotation of the head about the fixed body (HCOR + HVOR) evoked low velocity counterclockwise eye movements which were not interrupted by fast clockwise eye movements. These data provide further evidence for a linear combination of independent HCOR and HVOR signals.

Responses of vestibulospinal neurons to neck and macular vestibular inputs in the presence or absence of the paleocerebellum

1. The role of the paleocerebellum in determining the responses of lateral vestibular nucleus (LVN) neurons either to independent or combined stimulation of macular vestibular and neck receptors has been investigated in decerebrate cats. Sinusoidal rotation around the longitudinal axis at 0.026 Hz, 5-10 degrees, represented the constant input parameters. Among the tested neurons, 100 and 131 units were recorded in animals with intact cerebellum and following partial cerebellectomy, respectively. The units were classified according to their anatomical location in either the rostroventral (cLVN) or dorsocaudal (ILVN) part of the LVN; units also were activated antidromically from the spinal cord. 2. The majority of units responded to stimulation of macular receptors both in preparations with intact cerebellum (75.0%) or with partial cerebellectomy (71.8%), the response being primarily in phase with the direction of animal orientation. The proportion of responding units and their response sensitivity were greater in the cLVN than ILVN in each preparation; no significant differences in mean firing rate and response sensitivity were observed between the two preparations for each subdivision of the LVN. In animals with cerebellum intact, the majority of cLVN and ILVN units were excited during side-down tilt; following partial cerebellectomy, this predominant response pattern still was present in cLVN but was reversed in ILVN. 3. About one-half of the units responded to sinusoidal stimulation of neck receptors in both preparations, the response being mainly in phase with the direction of neck orientation. In the intact cerebellum preparations, the proportion of cLVN units responding to neck rotation was greater than that of ILVN units, but no difference in response sensitivity was observed between these units. Following partial cerebellectomy, the proportion of cLVN units responsive to the neck input was reduced but that of ILVN units was not; however, the average response sensitivity was halved for both cLVN and ILVN units. In preparations with cerebellum intact, most of the cLVN units were excited during side-down neck rotation, whereas ILVN units were excited mainly by rotation in the opposite sense; following partial cerebellectomy, the majority of units were excited during side-up neck rotation, not only in ILVN but also in cLVN. 4. Units receiving a convergent input from both receptors were more numerous in cLVN (72.7%) than ILVN (41.8%) in preparations with intact cerebellum; following partial cerebellectomy, this disproportion of responsive units in the two divisions (45.3% and 44.9%, respectively) disappeared, due to a reduced number of cLVN units responding to the neck input. In both preparations, the macular input had a relatively greater influence on the cLVN whereas the neck input was more effective on the ILVN. 5...

Reciprocal responses to tilt of neurones within both fore- and hindlimb regions of Deiters' nucleus

In decerebrate cats with cerebellum intact the frequency response of 102 neurons located within the lateral vestibular nucleus (LVN) to sinusoidal stimulation of vestibular receptors was analyzed. Positional sensitive units, showing a reciprocal pattern of response to lateral tilting, characterized by an excitation during ipsilateral and a depression during contralateral tilt, were equally found in the rostroventral (forelimb) and dorsocaudal (hindlimb) divisions of the LVN. No unit was found to be excited during both ipsilateral and contralateral tilts. A comparison between these findings and those reported in cerebellectomized preparations indicates that the reciprocal pattern of response to tilt of neurons, particularly located in the hindlimb region of the LVN, depends upon the anatomical integrity of the cerebellum.

Responses of Purkinje cells of the cerebellar vermis to neck and macular vestibular inputs

1. The dynamic analysis of the control exerted by neck and macular vestibular receptors on the cerebellar cortex has been investigated in precollicular decerebrate cats submitted to sinusoidal rotation along the longitudinal axis of the animal at the frequency of 0.026 Hz and at peak amplitudes up to 10 degrees for the neck input and 15 degrees for the macular input. 2. Purkinje (P) cells located in the vermal cortex of the cerebellar anterior lobe, particularly in the longitudinal parasagittal zone which projects to the ipsilateral lateral vestibular nucleus (LVN), showed a sinusoidal modulation of the firing rate in response to sinusoidal stimulation of the neck receptors or the vestibular receptors, the phase of the responses being in most units related to the extreme neck or head position. Mossy fiber (MF) and/or climbing fiber (CF) responses of the same or different P-cells to the two inputs were observed. 3. The sensitivity of the MF-response of the P-cells to the neck input, elicited by sinusoidal rotation of the neck and expressed in per cent of the average firing rate per degree of neck rotation, corresponded on the average to 2.71 +/- 1.67, S.D. This value was significantly higher than that of the MF-response of the P-cells to the macular input elicited by sinusoidal tilt along the longitudinal axis of the whole animal, which correspond to 1.71 +/- 1.01, S.D. 4. Most of the MF-responses of the P-cells to the neck input were characterized by an excitation during side-down rotation of the neck and by an inhibition during side-up rotation, whereas most of the MF-responses of the P-cells to the macular input showed just the opposite behavior, being inhibited by side-down tilt of the animal and excited by side-up tilt. 5. Units which received a convergent input from both neck and macular receptors and showed an antagonistic pattern of response to the two inputs were tested during rotation of the head alone, in order to excite simultaneously the two kinds of receptors. Due to the higher sensitivity of the neck over the macular response, the magnitude of the combined response tended to be similar to the difference between the individual ones. Moreover, the phase of the resulting response was always modified with respect to that of the response to the neck input alone, and became in some instances related to velocity of neck rotation rather than to neck position. 6. These findings indicate that opposite responses to neck and macular inputs occur at corticocerebellar level. However, a final integration of the two inputs, leading to suppression of the conflicting responses, may occur either at medullary (LVN) or at spinal cord level.

Forelimb reflexes modulated by tonic neck positions in cats

The modulation of monosynaptic forelimb reflexes by tonic neck positions was investigated in cats with the head fixed. Lateral flexion of the body in a horizontal plane markedly facilitates reflexes of the deep radial nerve (DR) in the ipsilateral forelimb, while the antagonistic ulnar nerve (ULN) reflexes are strongly inhibited. Opposite effects are seen after contralaternal body movement. Dorsiflexion of the body clearly increases DR-reflexes and exerts a reciprocal although more pronounced inhibition on ULN reflexes. Opposite effects appear after ventriflexion. The reflex modulation starts with head-body displacements of approximately 5 degrees and increases with increasing angles. Furthermore reflex modulation does not depend on the intact cerebrum and cerebellum. The comparison of forelimb and hindlimb reflexes shows a decrease of the neck influences along the spinal cord.

Vestibular and somatosensory interaction in the cat vestibular nuclei

The vestibular nuclei of cats were explored extracellulary with micropipettes to locate units with a resting discharge rate which responded to rotation in the horizontal plane. These units were examined for somatosensory input from neck and limbs. Fewer than half responded to somatosensory stimulation. The neck region was the body area most effective in influencing unitary activity. The response pattern most often noted was an increase and decrease in discharge frequency when the body was moved towards and away from the recording electrode respectively. Change in discharge rate was observed to be primarily dependant upon neck velocity and not upon absolute neck position. Half of the somatosensory units received input from either the forelimbs or the hindlimbs, while the remaining half responded to both.

Modulation of hindlimb reflexes by tonic neck positions in cats

In cats with the head fixed in a stereotactic frame a slight facilitation of the gastrocnemiussoleus (GS) monosynaptic reflexes in the ipsilateral hindlimb, accompanied by an inhibition of the monosynaptic reflex of the antagnostic deep peroneal nerve (DP), occurs following lateral flexion and rotation around the body axis at an angle of 25 degrees. Following dorsiflexion of the body a moderate inhibition of the extensor reflexes (GS) up to 10% and a reciprocal slight excitation in the same range of the monosynaptic reflexes of flexor muscles (DP) is recorded. On the other hand after ventriflexion of the body there is a marked inhibition in the range of 50% of all the monosynaptic extensor and flexor reflexes for all the stimulus intensities used. This inhibition can already be demonstrated in ventriflexion of 10 degrees-15 degrees. All these patterns of reflex modulation are similar in cortically intact, ischemically decorticated and precollicularly decerebrated cats. Furthermore these patterns are not essentially influenced by cerebellectomy, although all reflex amplitudes are reduced. However, the spinal reflexes can not be modulated by these body movements after acute spinalization.

Dynamic relations between natural vestibular inputs and activity of forelimb extensor muscles in the decerebrate cat. II. motor output during rotations in the horizontal plane

Decerebrate cats with the spinal cord sectioned at low thoracic levels were submitted to rotations in the horizontal plane. The position of the animal with respect to the axis of rotation was such that horizontal canal afferents were activated either alone or in combination with macular afferents. The EMG activity from the triceps brachii muscles of both forelimbs was recorded. The main findings were as follows. (1) The motor output to each forelimb extensor is increased by an increase in the activity of the horizontal canal afferents from the contralateral labyrinth. The phase of the motor output with respect to that of the vestibular afferents shows a lag which increases with frequency, reaching about 85 degrees at 1.0 Hz. (2) The macular and horizontal canal inputs are independently processed in the central nervous system and the motor output in response to both inputs applied simultaneously is a linear summation of the outputs expected for each of the inputs.

Asymmetric tonic labyrinth reflexes and their interaction with neck reflexes in the decerebrate cat

1. Tonic labyrinth and neck reflexes were studied separately and in combination in the decerebrate cat with C1 and C2 spinal roots cut. Reflex effects were observed as changes in length of the isotonically loaded medial head of triceps. 2. The tonic labyrinth reflexes acted asymmetrically on the medial head of triceps. Side-down rotation of the head produced shortening in medial triceps, whereas side-up rotations of the head resulted in a lengthening. 3. The tonic neck reflexes acted asymmetrically on the medial head of triceps. Side-down rotations of the neck produced a lengthening of medial triceps, whereas side-up rotations of the neck resulted in shortening. 4. Labyrinth and neck reflexes produce opposite effects on the same limb extensor muscle so that, if the neck innervation is intact, head tilting produces no change in muscle length. 5. It is suggested that the interaction between the labyrinth and neck reflexes contributes to the stability of the trunk, allowing the head to move freely on the body without affecting this stability. Labyrinth and neck reflexes need therefore to be considered together as a single system.

Macular input to neurons of the spinoreticulocerebellar pathway

The present report summarizes the results obtained in previous experiments in which the responses to lateral tilt of both reticulocerebellar neurons located in the lateral reticular nucleus (NRL) and ascending spinoreticular tract axons, located in the lateral funiculi of the cervical segments C2-C3, had been recorded in precollicular decerebrate cats. Steady changes in unit activity were evoked by 15-20 degrees tilts which lasted as long as the position of the cat was maintained. Most of the units responsive to tilting showed an increase in discharge rate during tilt in one direction and a decrease in discharge rate during tilt in the opposite direction. Moreover the magnitude of the unitary responses was clearly related to the degree of tilt. The responses of the NRL neurons to tilt were attributed in part to direct efferent vestibuloreticular projections from the Deiters' nucleus, in part to some indirect influence of the lateral vestibulospinal tract on neurons of the cervical ascending spinoreticular pathway. Since most of these ascending units affected by tilt were influenced by stimulation of both ipsilateral and contralateral forelimb nerves, and some of them also by stimulation of high threshold neck muscle afferents, it was postulated that these neurons projected to the forelimb and the neck region of the cerebellar cortex of the anterior lobe. This corticocerebellar region may therefore integrate from time to time not only the signal input originating from macular receptors but also the somatosensory input which originates from neck receptors as a result of the actual displacement in head position induced by the vestibulospinal reflex.

Bilateral effects of vestibular nerve stimulation on activity in the lumbar spinal cord

These experiments were designed to study the effects of vestibular nerve stimulation on the activity of hindlimb motoneuron pools. Two techniques were used to stimulate the vestibular nerves of precollicularly decerebrated cats. In one set of experiments the individual semicircular canals were stimulated via bipolar electrodes placed near the canal nerves. In the second series of experiments the whole vestibular nerve was stimulated with an electrode placed on the intradural nerve. Activity in the hindlimb motoneuron pools was ascertained by evoking monosynaptic reflexes in the various hindlimb nerves. Stimulation of the individual semicircular canals produced response patterns that varied with both the vestibular branch being stimulated and the hindlimb nerve being conditioned. Intradural stimulation of the vestibular nerve, on the other hand, evoked similar response patterns in the antagonist ankle flexor and extensor nerves. The most common pattern consisted of facilitation followed by a period of inhibition. Lesions of descending fiber tracts produced results which suggest the presence of a diffuse pathway, involving the recitular formation, which mediates the observed responses. It is suggested that the biphasic response pattern is analogous to the startle response and the function of the responses is discussed in that context.