Modulation of hindlimb reflexes by tonic neck positions in cats

Inhibition of midbrain-evoked tonic and rhythmic motor activity by cutaneous stimulation in decerebrate cats

The effect of mechanical and electrical stimulation of cervical cutaneous afferents was analysed on both the centrally induced tonic and rhythmic activities in hindlimb antagonist muscle nerves of 16 decerebrate paralysed cats. Electrical stimulation of dorsal midbrain evoked in the nerve to the tibialis anterior muscle (TAn) either rhythmic discharges (n=14), associated with tonic discharges in ten cats, or only tonic discharges (n=4). Centrally induced activity in the ipsilateral nerve to gastrocnemius medialis (GMn) occurred in fewer cats (n=12) and displayed similar patterns as in TAn. Manual traction of the scruff of the neck reduced the TAn tonic and rhythmic discharges (n=6) by 73% (P<0.05) and 71% (P<0.05), respectively, and reduced only the tonic component of GMn discharges (by 41%, n=3). Electrical stimulation (impulses 0.1-0.5 ms, 50 Hz) of cervical nerves belonging to C5 or C6 dermatomes, the intensity (0.4-4 mA) of which induced minimal inhibition of both TAn and GMn discharges, reduced significantly the tonic component of TAn discharges (by 39%, n=4). At higher intensities of electrical cervical nerve stimulation (2-6 mA) inducing maximal inhibitory effect, both tonic and rhythmic activities in TAn and GMn were both significantly reduced by, respectively, 81% and 94% in TAn (n=7), and by 49% and 43% in GMn (n=7). Electrical cervical nerve stimulation consistently reduced the isolated tonic discharge in TAn by 66% (n=4, P<0.05) and in GMn by 23% (n=3) when present. Thus the tonic component was more sensitive to inhibition than the rhythmic component of hindlimb muscle nerve activity.

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.

Responses of locus coeruleus and subcoeruleus neurons to sinusoidal neck rotation in decerebrate cat

The electrical activity of 99 neurons located in the locus coeruleus-complex, namely in the dorsal (n = 26) and the ventral part of the locus coeruleus (n = 46) as well as the locus subcoeruleus (n = 27), has been recorded in precollicular decerebrate cats during sinusoidal displacement of the neck. This was achieved by rotation of the body about the longitudinal axis of the animal, while maintaining the head stationary. A proportion of these neurons showed some of the main physiological characteristics attributed to the noradrenergic locus coeruleus neurons, i.e. (i) a slow and regular resting discharge, and (ii) a typical biphasic response to fore and hindpaw compression consisting of short bursts of impulses followed by a period of quiescence, due at least in part to recurrent or lateral inhibition of the corresponding neurons. Moreover, 14 out of the 99 neurons were activated antidromically by stimulation of the spinal cord at T12 and L1, thus being considered as coeruleo- or subcoeruleospinal neurons. Among these locus coeruleus-complex neurons tested, 73 out of 99 (i.e. 73.7%) responded to neck rotation at the standard frequency of 0.15 Hz and at the peak amplitude of displacement of 10 degrees. In particular 40 of 73 units (i.e. 54.8%) were excited during side-down neck rotation and depressed during side-up rotation, while 18 of 73 units (i.e. 24.7%) showed the opposite pattern. In both instances the peak of the responses occurred with an average phase lead of +34.2 degrees for the extreme side-down or side-up neck displacement; however, the response gain (impulses/s per deg) was on the average more than two-fold higher in the former than in the latter group of units. The remaining 15 units (i.e. 20.5%) showed phase angle values which were intermediate between those of the two main populations. As to the coeruleo or subcoeruleospinal neurons, 11 of 14 units (78.6%) responded to the neck input, the majority (nine of 11 units, i.e. 81.8%) being excited during side-down neck rotation. Within the explored region, the proportion of responsive units was higher in the locus subcoeruleus (85.2%) than in the locus coeruleus, both dorsal and ventral (69.4%). Moreover, units located in the former structure showed on the average a response gain higher than that found in the latter structures. Similar results were also obtained from the population of locus subcoeruleus-complex neurons which fired at a low rate (less than or equal to 5.0 impulses/s).(ABSTRACT TRUNCATED AT 400 WORDS)

Influences of neck receptors on soleus motoneuron excitability in man

We studied the influence of the asymmetric tonic neck reflexes on the excitability of the human soleus motoneuronal pool by mapping the H amplitude as a function of rotation of the body relative to the fixed head. Eight normal adult volunteers were tested. On each subject 15 consecutive H reflexes were recorded from the right soleus muscle, for each of the following test position, 4 degrees, 8 degrees, 12 degrees, 16 degrees, as well as at the control position (0 degrees), both before and after each change in body position. Our results showed that the H reflex amplitude was progressively facilitated for contralateral rotation in respect to the recording side and conversely inhibited for ipsilateral rotation. The results indicate that neck receptors of one side enhance the excitability of the contralateral soleus motoneurons and depress the ipsilateral ones.

Central projections from cat suboccipital muscles: a study using transganglionic transport of horseradish peroxidase

Central projections of suboccipital muscle nerves were examined following exposure of cut peripheral nerves to the tracer horseradish peroxidase. Labelled fibers entered the C1 and C2 dorsal roots and accumulated in the dorsolateral part of the dorsal funiculus. Many labelled fibers entered the grey matter of C1 to C3 in ventrally directed bundles which passed medially to the base of the dorsal horn. No terminal labelling was apparent in superficial layers of the dorsal horn. However, labelled fibers ramified extensively throughout medial parts of the intermediate laminae, in and around the central cervical nucleus. Labelled fibers also projected toward the ventral horn. In cats subjected to ventral root section at the time of peripheral nerve exposure, a modest distribution of reaction product was observed deep in the ventral horn. In cats which did not undergo ventral root section, anterograde projections in the ventral horn were obscured by the simultaneous retrograde filling of motoneurons both in the ventromedial nucleus and on the medial and lateral borders of the gray matter. Labelled axons also coursed rostrally into the medulla where they formed a circumscribed bundle between the main cuneate nucleus and the spinal nucleus of V. Three consistent regions of HRP deposition could be identified at medullary levels. Dense accumulations of reaction product were present in circumscribed regions of the external cuneate nucleus (ECN) throughout its rostrocaudal extent. A second zone of dense labelling occurred in the intermediate nucleus of Cajal, where it appeared to form a continuing column rostral to the central cervical nucleus in C1-C3. Sparse labelling was restricted to a third zone in the ventrolateral part of the main cuneate nucleus.

Responses of medullary reticulospinal neurons to sinusoidal rotation of neck in the decerebrate cat

The electrical activity of 132 neurons located in the inhibitory area of the medullary reticular formation, namely, in the medial aspects of the nucleus reticularis gigantocellularis, magnocellularis and ventralis has been recorded in precollicular decerebrate cats during sinusoidal displacement of the neck. This was achieved by rotation of the body about the longitudinal axis of the animal, while maintaining the head stationary. In particular, 85 neurons were activated antidromically by stimulation of the spinal cord at T12 and L1, the remaining 47 units were not activated antidromically. Among these reticular neurons tested, 66 out of 85 (i.e. 77.6%) of the neurons that were, and 31 out of 47 (i.e. 66.0%) of the neurons that were not antidromically activated responded to slow neck rotation at the frequency of 0.026 Hz and at the peak amplitude of displacement of 10 degrees. The units influenced by neck rotation showed a periodic modulation of the firing rate in response to sinusoidal stimulation of neck receptors. In particular, 70 of 97 units (i.e. 72.2%) were excited during side-down neck rotation and depressed during side-up rotation, while 19 of 97 units (i.e. 19.6%) showed the opposite pattern. In both instances, the peak of the responses occurred with an average phase lead of +41 degrees for the extreme side-up or side-down neck displacement. The remaining 8 units (i.e. 8.2%) showed a prominent phase shift of the peak of their response relative to neck position. The proportion of units excited during side-down neck rotation were almost equally distributed throughout the whole rostro-caudal extent of the reticular structures explored. Responses to neck rotation were detectable at 0.25 degrees of peak displacement. The gain (imp./s/deg.) and the sensitivity (%/deg., i.e. percentage change of the mean firing rate per degree of displacement) in responses of reticulospinal neurons decreased by increasing the peak amplitude of neck rotation from 1 to 10 degrees at a frequency of 0.026 Hz. Therefore, the system did not behave linearly with respect to amplitude of stimulation. By increasing the frequency of stimulation from 0.008 to 0.32 Hz at the fixed amplitude of 10 degrees, the gain, sensitivity and phase lead of responses increased for frequencies of neck rotation above 0.051 Hz. Reticulospinal neurons may thus monitor changes in neck position as well as in velocity of neck rotation. Responses of reticulospinal neurons to neck rotation are discussed in relation to the responses to the same stimulus recently described of vestibulospinal neurons originating from the lateral vestibular nucleus.(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)

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...

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.