Neck Muscle Afferents Research Articles

Neck muscle afferents influence oromotor and cardiorespiratory brainstem neural circuits

Sensory information arising from the upper neck is important in the reflex control of posture and eye position. It has also been linked to the autonomic control of the cardiovascular and respiratory systems. Whiplash associated disorders (WAD) and cervical dystonia, which involve disturbance to the neck region, can often present with abnormalities to the oromotor, respiratory and cardiovascular systems. We investigated the potential neural pathways underlying such symptoms. Simulating neck afferent activity by electrical stimulation of the second cervical nerve in a working heart brainstem preparation (WHBP) altered the pattern of central respiratory drive and increased perfusion pressure. Tracing central targets of these sensory afferents revealed projections to the intermedius nucleus of the medulla (InM). These anterogradely labelled afferents co-localised with parvalbumin and vesicular glutamate transporter 1 indicating that they are proprioceptive. Anterograde tracing from the InM identified projections to brain regions involved in respiratory, cardiovascular, postural and oro-facial behaviours—the neighbouring hypoglossal nucleus, facial and motor trigeminal nuclei, parabrachial nuclei, rostral and caudal ventrolateral medulla and nucleus ambiguus. In brain slices, electrical stimulation of afferent fibre tracts lateral to the cuneate nucleus monosynaptically excited InM neurones. Direct stimulation of the InM in the WHBP mimicked the response of second cervical nerve stimulation. These results provide evidence of pathways linking upper cervical sensory afferents with CNS areas involved in autonomic and oromotor control, via the InM. Disruption of these neuronal pathways could, therefore, explain the dysphagic and cardiorespiratory abnormalities which may accompany cervical dystonia and WAD.

The ventrolateral upper cervical cell group in cat projects to all rostrocaudal levels of the periaqueductal gray matter

The lateral ventral horn of the upper cervical (C 1-3vl) cord in rat, cat and monkey contains many cells that project to the periaqueductal gray matter (PAG). Until now it was assumed that these cells only project to the ventrolateral part of the caudal PAG. Because the ventrolateral caudal PAG is involved in quiescence and hypotension, it was hypothesized that the C 1-3vl-PAG projecting cells play a role in immobility behavior, possibly activated by neck muscle afferents. However, in the present anterograde and retrograde study in cat we showed that C 1-3vl cells do not only target the caudal PAG, but terminate even more abundantly in the intermediate and rostral parts of the PAG. There, projections target the ventrolateral column, like in the caudal PAG, but also terminate in the lateral and dorsomedial columns. This finding, combined with the current, albeit limited, physiological data on C 1-3vl-PAG and C 1-3vl-thalamic projecting cells, sheds a new light on the possible functions of C 1-3vl cells. It might be that the C 1-3vl cells with complex response properties and large receptive fields have a relay function similar to cells in the dorsal column nuclei, lateral cervical or central cervical nucleus. Other C 1-3vl cells might receive somatic or visceral input that was never tested before, or perhaps other input, like vestibular information. It might also be that these cells project to the more caudal cord to modulate visceral input, with ascending collaterals to more rostrally located structures, including mesencephalon and thalamus.

Influence of vestibular activation on respiration in humans.

The purpose of this study was to determine the effects of the semicircular canals and otolith organs on respiration in humans. On the basis of animal studies, we hypothesized that vestibular activation would elicit a vestibulorespiratory reflex. To test this hypothesis, respiratory measures, arterial blood pressure, and heart rate were measured during engagement of semicircular canals and/or otolith organs. Dynamic upright pitch and roll (15 cycles/min), which activate the otolith organs and semicircular canals, increased respiratory rate (Delta2 +/- 1 and Delta3 +/- 1 breaths/min, respectively; P < 0.05). Dynamic yaw and lateral pitch (15 cycles/min), which activate the semicircular canals, increased respiration similarly (Delta3 +/- 1 and Delta2 +/- 1, respectively; P < 0.05). Dynamic chair rotation (15 cycles/min), which mimics dynamic yaw but eliminates neck muscle afferent, increased respiration (Delta3 +/- 1; P < 0.05) comparable to dynamic yaw (15 cycles/min). Increases in respiratory rate were graded as greater responses occurred during upright (Delta5 +/- 2 breaths/min) and lateral pitch (Delta4 +/- 1) and roll (Delta5 +/- 1) performed at 30 cycles/min. Increases in breathing frequency resulted in increases in minute ventilation during most interventions. Static head-down rotation, which activates otolith organs, did not alter respiratory rate (Delta1 +/- 1 breaths/min). Collectively, these data indicate that semicircular canals, but not otolith organs or neck muscle afferents, mediate increased ventilation in humans and support the concept that vestibular activation alters respiration in humans.

Influence of head-down and lateral decubitus neck flexion on heart rate variability.

The purpose of this study was to examine the response of heart rate variability (HRV), a noninvasive index of autonomic control, to head-down neck flexion (HDNF), which engages both otoliths and neck muscle afferents, and to lateral decubitus neck flexion (LNF), in which neck afferents are activated, whereas otolith afferent input is not. HRV and forearm blood flow were evaluated in participants lying prone, during HDNF, lying in the lateral decubitus position, and during LNF. Compared with the prone position, HDNF resulted in lower high-frequency (46.9 +/- 7.1 vs. 62.3 +/- 6.2) and higher low-frequency (53.1 +/- 7.1 vs. 37.7 +/- 6.2) power, expressed as normalized units, along with higher low-frequency-to-high-frequency ratio (1.65 +/- 0.3 vs. 0.78 +/- 0.2), whereas LNF resulted in no alterations in HRV indexes. Furthermore, there were no significant differences in forearm blood flow or vascular resistance among any of the positions. Our data suggest that otolith organs influence autonomic modulation of the heart, supporting previous studies reporting that HDNF elicits increased sympathetic outflow. These data further suggest that HDNF results in a parasympathetic withdrawal from the heart in addition to sympathetic activation.

Neck afferents and muscle sympathetic activity in humans: implications for the vestibulosympathetic reflex.

We have shown previously that head-down neck flexion (HDNF) in humans elicits increases in muscle sympathetic nerve activity (MSNA). The purpose of this study was to determine the effect of neck muscle afferents on MSNA. We studied this question by measuring MSNA before and after head rotation that would activate neck muscle afferents but not the vestibular system (i.e., no stimulation of the otolith organs or semicircular canals). After a 3-min baseline period with the head in the normal erect position, subjects rotated their head to the side (approximately 90%) and maintained this position for 3 min. Head rotation was performed by the subjects in both the prone (n = 5) and sitting (n = 6) positions. Head rotation did not elicit changes in MSNA. Average MSNA, expressed as burst frequency and total activity, was 13 +/- 1 and 13 +/- 1 bursts/min and 146 +/-34 and 132 +/- 27 units/min during baseline and head rotation, respectively. There were no significant changes in calf blood flow (2.6 +/- 0.3 to 2.5 +/- 0.3 ml.100 ml-1.min-1, n = 8) and calf vascular resistance (39 +/- 4 to 41 +/- 4 units; n = 8). Heart rate (64 +/- 3 to 66 +/- 3 beats/min; P = 0.058) and mean arterial pressure (90 +/- 3 to 93 +/- 3; P < 0.05) increased slightly during head rotation. Additional neck flexion studies were performed with subjects lying on their side (n = 5), MSNA, heart rate, and mean arterial pressure were unchanged during this maneuver, which also does not engage the vestibular system. HDNF was tested in 9 of the 13 subjects. MSNA was significantly increased by 79 +/- 12% (P < 0.001) during HDNF. These findings indicate that neck afferents activated by horizontal neck rotation or flexion in the absence of significant force development do not elicit changes in MSNA. These findings support the concept that HDNF increases MSNA by the activation of the vestibular system.

Anatomical studies on the cuneocerebellar neurons in the gerbil by using HRP method

The projection from the external cuneate nucleus (ECN) to the cerebellum was studied in the gerbil following the retrograde transport of minute injections (0.05-0.1 microliter, 30% solution) or implantations of horseradish peroxidase (HRP) in various folia of the cerebellar cortex and deep nuclei. The morphology of the labelled neurons, as well as their quantitative distribution, was also examined. The projections were in general bilateral but predominantly ipsilateral. A remarkable finding, however, was the predominant contralateral projection from the medial cell column of ECN representing the paw region. Topographically, the nuclear areas receiving the thoracic, shoulder and neck muscle afferents projected bilaterally to vermis lobules I-V and VIII-IX, lobulus simplex, crura Ia and Ib, paraflocculus and flocculus, and contralaterally to crus II and nucleus interpositus; the nuclear regions receiving the forelimb muscle afferents sent fibers bilaterally to vermis lobules V-VIII, paramedian lobule and copula pyramidis, and ipsilaterally to crus II and nucleus interpositus. Based on their somal size, three classes of neurons were distinguished: small, medium and large cells. The small and medium cells constituted 79 and 20% of the population of the labelled cells respectively, whereas the large cells were only occasionally identified. In the cell columns of the caudal ECN representing the forelimb (Lan et al. 1994), most labelled neurons were medium cells which made up 65% of the total labelled medium cells. In contrast, the majority (90%) of the labelled cells in the rostral ECN were of the small-sized variety. The functional significance of the segregation, both in the projection patterns and sizes of somata of the cuneocerebellar neurons is discussed.

Integration in trigeminal premotor interneurones in the cat

The profile of integration in a sample of 183 interneurones localized in the subnucleus-gamma of the oral nucleus of the spinal trigeminal tract (NVspo-gamma) has been analyzed. 134 neurones were tested for inputs from primary afferents of the trigeminal, facial and cervical nerves as well as for inputs from the midbrain and from the cervical spinal cord. The remaining 49 neurones were tested for inputs from the primary afferents and for descending convergence from defined sites within the oro-facial primary projections of the cerebral cortex. It was found that the interneurones, mainly recorded in the dorsal and dorsomedial aspect of the NVspo-gamma, receive short latency inputs from the low threshold oral and perioral afferents and longer latency inputs from the high threshold jaw and neck muscle afferents. There was evidence for convergence from the cervical segmental level (29%) and some of the neurones had axon terminals in the superior colliculus. However, the interneurones did not receive a descending tectal input. About 80% of the NVspo-gamma interneurones were activated from the orofacial primary projection fields within cytoarchitectonic areas 3a and 3b of the coronal gyrus. This input was topographically organized and the neurones were activated from the same oral and perioral region of the periphery as the cortical region from which the descending projections themselves originated. Minimum latencies indicated a monosynaptic connection. The convergence profile onto the NVspo-gamma interneurones appeared unique as compared with interneurones located in the intertrigeminal area. Aspects of the possible functional roles of the NVspo-gamma neurones are discussed in relation to ongoing oro-facial ("masticatory") movements. The properties of a selected sample of NVspo-gamma interneurones, which were antidromically activated from the digastric subnucleus of the trigeminal motor nucleus, are reported in a companion paper (Olsson and Westberg 1991).

The cat cervical dorsal root ganglia: general cell-size characteristics and comparative study of neck muscle, neck cutaneous and phrenic afferents

Sizes of neuronal somata in the cat cervical dorsal root ganglia were determined at different levels (C1–C8). The average value and class distribution of mean cell diameter were analyzed. The ganglia from C1 to C5 could be clearly distinguished from those at levels of brachial plexus afferents (C6–C8) with respect to cell size range, distribution and average. The size distribution, most often limited to 70 μm from C1 to C5, skewed to more than 90 μm from C6 to C8. Cells in the 35–50 μm range of diameter constituted the main portion of the cell population (49–52%) at the C1–C5 levels, whereas from C6 to C8 51–77% of the ganglion cell bodies were more than 50 μm in diameter. The cell size distribution of afferents projecting from C1 to C5 and supplying different muscle or cutaneous targets was studied following retrograde labeling with horseradish peroxidase conjugated to wheatgerm agglutinin. Sizes of cell bodies of biventer cervicis (postural muscle), phrenic (purely respiratory muscle) and cutaneous afferents were all similar. The labelled cell bodies were in the majority (51–64%) less than 35 μm in diameter and ranged towards smaller diameters than counterstained cells in the corresponding ganglia. In spite of similarities in cell size distribution it was estimated from the fiber caliber spectra of the labelled afferents that both unmyelinated and myelinated cutaneous afferents originate from larger cell bodies than muscle afferents in the same diameter range.

Structure of the intraspinal projections of single, identified muscle spindle afferents from neck muscles of the cat

The morphology and frequency of collaterals originating from single afferents supplying primary endings of muscle spindles in dorsal neck muscles have been examined using intra-axonal injections of HRP. Within the segment in which the afferent entered the spinal cord, one collateral was found for every 3.3 mm of stained axon. In contrast, afferents--one of more segments rostral to the segment in which they entered the spinal cord--had fewer collaterals: One collateral was found for every 6.3 mm of stained axon. The branching structure and terminal distribution of the collaterals were generally similar regardless of the muscle from which the afferent originated and the segment in which the collateral was found. Boutons were found in 2 zones: One of these was located in the intermediate zone, within and around the central cervical nucleus, and the other was found in laminae VIII and IX, including the motoneuron nuclei. The ventral termination zone of collaterals in the same segment as their parent axon entered the spinal cord was larger and had more boutons than the same projection of collaterals whose parent axon entered the spinal cord 1 or 2 segments caudal to the segment in which the collateral was found. These results indicate that afferents supplying primary endings of neck muscle spindles are more likely to contact neurons in the same segment in which the afferent enters the spinal cord than in more rostral segments. However, even within the same segment in which the afferent enters the spinal cord, the projection of neck muscle afferents to the ventral horn is less dense than the corresponding projection of hindlimb muscle spindle afferents in the lumbosacral spinal cord.

Projection of high threshold dorsal neck muscle afferents onto the upper cervical cord

Projection of high threshold dorsal neck muscle afferents onto the upper cervical cord

Neck muscle afferent projections to the brainstem of the monkey: implications for the neural control of gaze.

Brainstem projections of first-order afferent neurons that innervate the suboccipital muscles of the monkey have been determined by using the technique of transganglionic transport of wheat-germ-agglutinin-conjugated horseradish peroxidase (WGA/HRP) and HRP. Neck muscle afferents distribute to several distinct regions located within the caudal brainstem and rostral spinal cord. Terminal labeling was heaviest within the ventral portion of the ipsilateral lateral cuneate nucleus. Muscle afferent terminals also distributed to ventrolateral portions of the pars triangularis division of the cuneate nucleus. Projections were consistent with the known somatotopic (i.e., both place and modality) organization of the cuneate nucleus. Moreover, neck muscle projections to the cuneate nucleus were, in part, coincident with those previously demonstrated for the extraocular muscles (Porter: J. Comp. Neurol. 247:133-143, '86). Sparse terminal projections were noted in the central cervical nucleus. In addition, light terminal labeling was present in group x of the vestibular complex and in an ill-defined region along the lateral margin of the brainstem. Present observations, which provide the first complete description of the central distribution of neck muscle afferent neurons in the primate, may contribute to the known substrate for eye/head coordination.

Neck muscle afferent input to spinocerebellar tract cells of the central cervical nucleus in the cat.

Extracellular and intracellular recordings were made from spinocerebellar tract neurones of the central cervical nucleus (CCN) in C1-C3 segments of the anaesthetized cat. These neurones were identified by antidromic activation from the cerebellar peduncle. Stimulation of the ipsilateral dorsal root elicited extracellular spikes or EPSPs with a monosynaptic latency in almost all CCN neurones in the same segment (segmental input). Late excitatory effects were also observed in about one third of CCN neurones. The monosynaptic EPSP was occasionally followed by an IPSP. The excitatory input from the dorsal root to CCN neurones was extended over several segments for some CCN neurons (extrasegmental input). Monosynaptic excitation was evoked in CCN neurones after stimulation of dorsal neck muscle nerves as well; i.e. splenius (SPL), biventer cervicis and complexus (BCC), rectus capitus dorsalis, and obliquus capitus caudalis. Thresholds for this excitation were near the threshold of the nerve, suggesting that it originated from group I fibres. The component of excitation added after strong stimulation of neck muscle nerves would be attributed to group II fibres. When a CCN neurone received excitatory input from the nerve of one muscle, it was generally not affected by stimulation of other nerves in the same segment. Such muscle specificity of segmental input was the principal pattern of connexion of neck muscle afferents with CCN neurones. In some cases, however, excitatory convergence from SPL and BCC nerves onto single CCN neurones or excitation from the SPL nerve and inhibition from the BCC nerve were also observed. Nearly half of the CCN neurones received input from one muscle nerve of the same segment and not from the afferent of the same muscle of different segments, indicating a segment specificity of input. In the remaining CCN neurones, weaker excitatory effects were induced from afferents of different segments as well. In such extrasegmental effects, inputs to CCN neurones from caudal segments predominated in frequency over those from rostral segments. The origin of extrasegmental input was generally confined to the same muscle. Low threshold muscle afferents from the SPL and BCC were intraaxonally stained with HRP. The collaterals of the stained fibre distributed branchlets and terminals to the CCN, laminae VII, VIII, and motor nuclei. Two fibres responding to local muscle prodding or stretch showed a similar morphology. The findings indicated that muscle spindle afferents from primary endings projected to the CCN.

Reflex effects from high threshold neck muscle afferents on hind limb extensor gamma motoneurones in the cat.

The supra-segmental control of hind limb gamma motoneurones from neck muscle receptors has been studied in decerebrated and in spinal cats. Stretch of individual dorsal neck muscles was not an adequate stimulus for evoking long spinal reflexes to gamma motoneurones of gastrocnemius-soleus (GS) muscles unless the stretch was maximal or excessive. Pressure applied to the neck muscles, or intramuscular injections of KCl solution (0.1 ml, 5%), did affect the discharge of GS gamma motoneurones. Excitation was more evident than inhibition. We conclude that the long spinal reflex effects originate from high threshold mechanoreceptors, or nociceptors, rather than muscle spindles.

Responses of cat mesencephalic reticulospinal neurons to stimulation of superior colliculus, pericruciate cortex, and neck muscle afferents.

Neurons were recorded extracellularly in the mesencephalic reticular formation outside the interstitial nucleus of Cajal in cerebellectomized cats anesthetized with alpha chloralose. Reticulospinal neurones were identified by antidromic stimulation of the upper cervical segments. Stimulation in the deep layers of the ipsilateral superior colliculus evoked firing in 36% of reticulospinal neurons. For many neurons thresholds for activation were high in the intermediate tectal layers and declined as the electrodes entered the underlying tegmentum. However, low threshold points were found above the deep fiber layer within the superior colliculus for some cells. Stimulation of the contralateral superior colliculus excited 10% of neurons and thresholds for activation were high above the deep fiber layer for all neurons. Stimulation of the ipsilateral and contralateral pericruciate cortex excited 39 and 21% of neurons, respectively. The lowest threshold area was found in the frontal eye fields. Sixteen percent of neurons received excitation from neck muscle afferents (C2 biventer-cervicis) bilaterally. Comparison of responses between mesencephalic reticulospinal neurons and interstitiospinal neurons (Fukushima et al. 1981) showed that responses of the two groups of neurons were similar when the pericruciate cortex and neck muscle afferents were stimulated. However, a difference was observed in tectal responses. since low threshold points were rarely observed above the deep fiber layer for interstitiospinal neurons.

Effects of stimulation of frontal cortex, superior colliculus, and neck muscle afferents on interstitiospinal neurons in the cat.

Interstitiospinal neurons were activated by antidromic stimulation of the spinal cord ventromedial funiculus at C1 and C4 in cerebellectomized cats under chloralose anesthesia. Neurons responding only to C1 were classified as N cells and those responding both to C1 and C4 were classified as D cells, as in previous experiments (Fukushima et al. 1980a). Vestibular branching interstitiospinal and reticulospinal neurons were also identified as in the previous experiments. Stimulation of the ipsilateral pericruciate cortex evoked firing in 31% of N cells, 41% of D cells and 35% of the vestibular branching neurons, while stimulation of the contralateral cortex excited 6% of N cells, 29% of D cells and 14% of vestibular branching neurons. Response latencies ranged from 2 to 15 ms after the effective pulse. By measuring the thresholds of activation of these neurons while changing the depth of the stimulating electrodes, and by mapping the cortical areas, it was shown that the lowest threshold areas were in the frontal eye fields and the anterior sigmoid gyrus near the presylvian sulcus (Area 6). Stimulation of the latter area often evoked neck or shoulder muscles contraction. Stimulation in the deep layers of the ipsilateral superior colliculus evoked firing in about 20% of interstitiospinal neurons and about 42% of vestibular branching neurons, with typical latencies 2-3 ms after the effective pulse, while stimulation of the contralateral superior colliculus was rarely effective. N cells and D cells responded similarly. Thresholds for activation were high in the intermediate tectal layers and declined as the electrodes entered the underlying tegmentum. This suggests that the superior colliculus is not the main source of synaptic inputs to these neurons. Low threshold points were found above the deep fiber layer when stimulating electrodes were inserted into the pretectum. Stimulation of the C2 biventer cervicis nerve excited about 8% of N cells, 18% of D cells, and 15% of vestibular branching neurons bilaterally with typical latencies around 10 ms. Similar results were obtained when C2 splenius nerves were stimulated. The fibers responsible for such excitation are probably group II, since stimuli stronger than 1.8 times threshold of the lowest threshold fibers were needed to evoke excitation. Response decrement was often observed when stimuli were repeated at 1/s, while no such decrement was observed at the rate of 1/3 s. When the convergence of cortical and labyrinthine excitatory inputs was studied, 36% of interstitiospinal neurons received single inputs either from the pericruciate cortex or from the labyrinth, 22% of neurons received convergent excitation from both and the remaining 42% did not respond to either stimulus. Although vestibular branching neurons rarely received labyrinthine inputs, they frequently showed convergence of excitation to stimulation of the frontal cortex, superior colliculus and vestibular nuclei.

Excitatory and inhibitory interactions of extraocular and dorsal neck muscle afferents in the cat frontal cortex

Excitatory and inhibitory responses of 318 neurons to extraocular and dorsal neck muscle afferent stimulation were recorded extracellularly in the frontal cortex of 34 chloralose-anesthetized cats. Electrical stimulation of the oculomotor nerve to the ipsilateral and contralateral superior rectus muscle of the eye elicited single-cell responses in 33 and 40% of more than 160 cells tested, respectively, and stimulation of the abducens nerve to the lateral rectus muscle elicited responses in 47% of 161 cells tested, at latencies of 7 to 50 ms. Responses to a branch of the ipsilateral and contralateral biventer cervicis/complexus nerve of the dorsal neck were observed in 42 and 64% of more than 150 cells tested, respectively, and 32% of 165 cells responded to the contralateral rectus capitis dorsalis major/obliquus capitis caudalis nerve, at latencies of 5 to 47 ms. Inhibitory effects were evaluated extracellularly by observing the effect of electrical nerve stimulation against a background of glutamate-induced excitation of cells in the frontal cortex. Stimulation of extraocular muscle nerves inhibited activity in 20 of 93 cells, and dorsal neck muscle afferent fibers had inhibitory effects on 20 of 64 cells tested. Excitatory and inhibitory convergence between afferent fibers from one extraocular and one or more dorsal neck muscles was observed in 60% of 234 cells tested, and was higher than the degree of convergence between some pairs of extraocular muscle nerves, or between pairs of dorsal neck muscle nerves. The highest degree of convergence was between extraocular muscle afferent fibers and those from longitudinally extended dorsal neck muscles, which cause large head deviations upon contraction. The interacting effects of dorsal neck and extraocular muscle afferent fibers at comparable latencies in the cat frontal cortex suggest an involvement of these regions in coordinated eye and head movement.

Neck muscle and trigeminal input to the upper cervical cord and lower medulla of the cat

Experiments on chloralose-anaesthetized cats have shown that low-threshold neck muscle afferents project to laminae IV and V in the dorsal horn of the upper cervical cord, to lamina VI including the region which encompasses the central cervical nucleus, as well as to extensive regions of the ventral horn. At posterior medullary levels projections also exist to laminae IV, V, and VI of the spinal nucleus of V (although those to lamina IV are circumscribed), to the deep layers and lateral margin of the cuneate nucleus, and to the inferior olive. These projections are both from low- and high-threshold afferents. Evidence of a functional relationship between the trigeminal and neck muscle afferent system was found both in the upper cervical cord and lower medulla. About 40% of units in both regions receive a convergent input and when convergence could not be demonstrated, prior stimulation of one modality in some instances affected the responsiveness of the unit to the other modality. A motor role was found for some trigeminal afferent projections to the upper cervical cord. Trigeminal afferents consistently activated antidromically identified motoneurons of splenius, biventer cervicis, and complexus.

Frontal projections of dorsal neck and extraocular muscles

We investigated the projection sites of dorsal neck muscle afferents in cats anesthetized with chloralose urethane. Electrical stimulation of a nerve branch to the biventer cervicis and complexus, and nerve branches of the suboccipital nerve to the rectus capitis dorsalis major and to the obliquus capitis caudalis muscles evoked field and single-cell responses in frontal brain regions corresponding with frontal eye field regions in cats, at latencies of 6 to 15 ms. By recording afferent volleys from dorsal rootlets of the first and second cervical segments, it was shown that neuronal activity in frontal brain regions could be elicited by signals from fibers electrophysiologically characterized as belonging to group I. Electrical stimulation of the nerve of the superior rectus muscle also elicited short-latency (6 to 20 ms) field and single-cell activity in the frontal eye field regions. Moreover, a high degree of convergence was observed at the single-cell level between dorsal neck and extraocular muscle afferents. This convergence was significantly higher than the convergence observed between the rectus capitis dorsalis major and obliquus capitis caudalis (deep muscles) with the biventer cervicis and complexus muscles. The convergence of extraocular and dorsal neck muscle afferents onto frontal eye field regions suggests an involvement of these cortical regions in mechanisms related with coordination of sequential eye-head movements.

Effects of static tilt on cervical spinoreticular tract neurons.

The activity of axons located mainly in the ventral part of the lateral funiculi of the cervical segments C2-C3 were recorded in precollicular decerebrate cats and their responses to lateral tilt of the whole animal studied. Units were identified according to their antidromic and/or transynaptic responses to stimulation of the ipsilateral lateral reticular nucleus (NRL) as well as their responses to ipsilateral and/or contralateral forelimb nerve stimulation. In most respects, those units which could be antidromically identified as cervical ascending tract axons showed properties similar to those described for neurons of the spinoreticular pathway, the bVFRT. Among the 106 recorded units, 42 responded to 15 degrees tilts in the median plane. Steady changes in unit discharge frequency were evoked by tilting, which lasted as long as the position of the cat was maintained. The response of the units to tilting consisted of increased in discharge rate during tilt in one direction, while tilt in the opposite direction resulted in a decrease in discharge rate. The magnitude of the responses was clearly related to the degree of tilt. All the units affected by tilt, except one, received inputs from both ipsilateral and contralateral forelimb nerves and some of them also from high-threshold neck muscle afferents. Furthermore, 12 of these units could be orthodromically excited at short latency from the ipsilateral brain stem, suggesting a possible monosynaptic input perhaps transmitted via the lateral vestibulospinal tract. The responses of the cervical ascending spinal tract units to tilt did not appear to depend on peripheral proprioceptive or cutaneous feedback since a number of precautions were taken to eliminate such influences, while control experiments in animals submitted to partial or complete bilateral VIIth nerve section showed the number of neurons responsive to tilt to be profoundly reduced or eliminated. It appeared also that the average spontaneous discharge rate of the ascending units was much lower following bilateral VIIth nerve section than that obtained from the corresponding units recorded in preparations with the VIIth nerves intact. These findings suggested that the responses of the cervical ascending neurons to tilt as well as their spontaneous background activity depended on influences arising from the macular labyrinthine receptors. The observation that the responses of these ascending neurons to peripheral nerve stimulation are modified by tilt further suggested that the macular system may interact with the somatosensory system originating from the forelimb and the neck musculature, thus being able to produce the fine adjustments that cerebellar and brain stem structures exert in the control of posture and movements.

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.