Modulation of cerebral somatosensory evoked potentials arising from tibial and sural nerve stimulation during rhythmic active and passive movements of the human lower limb

The magnitudes of cerebral somatosensory evoked potentials (SEPs), following stimulation of cutaneous or muscle afferents in the upper limb, are reduced during active and passive movements of the fingers. The generalizability of such a movement effect was tested for lower limb events. We measured SEP magnitudes following activation of cutaneous (sural) and mixed (tibial) nerves during the flexion phase of active and passive rhythmic movements of the human lower limb. In eight volunteers, 150 SEPs per condition were recorded from Cz' referenced to Fpz'. Compared to stationary controls, both active and passive movements significantly depressed the early SEP components (P1-N1) [mean values, to 12.8%, 9.9% respectively for tibial nerve and to 29.6%, 25.6% for sural nerve stimulation, p < 0.05]. The attenuation was still observed when only one leg was moved and with stimulation at an earlier point in the flexion phase of movement. Visual fixation did not significantly affect P1-N1 amplitudes, compared to eyes closed. As previously shown, soleus H reflexes with stable M waves were significantly depressed during the movements (p < 0.05). The general construct may be that centripetal flow initiated from somatosensory receptors during limb movement leads to modulation of both spinal and cortical responses following large diameter cutaneous or muscle afferent activation.     

Morphometric ultrastructural evaluation of the axonal endings in the neuromuscular junctions of pigeons after long lasting limitation of movement

To analyze the discharge patterns of the reticulospinal (R-S) neurons associated with four-limb movement, we recorded the unit spikes of 108 R-S neurons in 18 thalamic cats. (1) Unit spikes of R-S neurons exhibited alternating firings during leg movements, not only stepping on the treadmill but also upon passive flexion and extension movement by the experimenter's hand. (2) R-S neurons manifested firing patterns associated with diagonal, reciprocal and quadrupedal leg movements. About half of the neurons showed reciprocal patterns upon bilateral forelimb movements; spikes were increased when the ipsilateral forelimb was in a backward position; they were decreased when that leg was in a forward position. In contrast, the spikes were increased when the contralateral forelimb was placed forward and decreased when it was backward. About 15% of the R-S neurons showed discharge patterns correlated with quadrupedal leg movements. Firing increased when the left forelimb and right hindlimb were placed backward and the left hindlimb and right forelimb were forward. In contrast, when the position of all 4 limbs was reversed, firing rates decreased. (3) When brief touch stimulation was applied to the skin around the leg, bursting spikes were obtained; these were suppressed upon touching the skin of the contralateral limb. Even after transection of the muscle nerves, alternating firings were observed. (4) Local anesthesia to the shoulder joint resulted in a marked reduction of spontaneous discharges and alternating firings. (5) Our results indicate that afferents of joints and of cutaneous origins in individual limbs ascend to the brainstem reticular formation, that integrative action is organized as pattern generation in that region, and that this patterned information is sent to the spinal cord via the reticulospinal tracts.     

Development of a scoring system to assess disability in an Italian elderly population. Evaluation of a disability questionnaire

Patients with idiopathic and symptomatic restless legs syndrome (RLS) suffer from "dyskinesia while awake" or "daytime myoclonus" when at rest preceded by sensory symptoms. In order to characterise the RLS either as reflex movement or as voluntary movement we measured movement-related cortical potentials in 5 idiopathic and 8 uraemic RLS patients. Movements from both legs were polygraphically recorded concomitantly with cortical activity 2000 msec before to 500 msec after onset of EMG activity. These data were compared with a voluntary simulation of each patient's movement pattern and with 5 age-matched controls performing dorsiflexion of the right, left and both feet. Cortical activity preceding daytime myoclonus was absent in RLS patients whereas self-initiated leg movements in patients elicited onset times (1180-1380 msec) and amplitudes of Bereitschaftspotential (readiness potential) not significantly different from readiness potentials in control subjects (P > 0.05). Lack of movement-related potentials in myoclonus and/or dyskinesias during daytime in RLS patients is compatible with an involuntary mechanism of induction and points towards a subcortical or spinal origin of RLS.     

The relationship between stature, growth, and short-term changes in height and weight in normal prepubertal children

Electrical stimulation of the superior laryngeal nerve (SLN) can elicit reflex responses in the cricothyroid (CT) and thyroarytenoid (TA) muscles. We made bilateral recordings of the responses evoked in these muscles in piglets by the stimulation of either the right or the left superior laryngeal nerve (SLN). The stimulus intensity was gradually increased to study the "persistence" of the responses. We observed a direct, ipsilateral response in the CT muscle, and reflex, ipsilateral and crossed responses in both CT and TA muscles. The ipsilateral or contralateral responses obtained in TA muscles, following stimulation of the left SLN, were significantly delayed in comparison with those evoked by stimulation of the right SLN. This delay cannot be explained by the difference in length between the right and the left recurrent laryngeal nerves, but rather by an asymmetry in the sensory afferent pathway. The functional significance of this observation remains to be determined.     

Innervation territories of single sympathetic C fibers in human skin

Microneurography techniques were used to record action potentials from unmyelinated nerve fibers (C fibers) in the cutaneous fascicles of the peroneal nerve in healthy volunteers. C units were identified by their long latency responses to electrical stimulation of their terminals in the skin. Their responsiveness to mechanical or heat stimuli applied to the skin or to sympathetic reflex provocation tests was determined by transient slowing of conduction velocity following activation (marking technique). In a sample of 381 C units, 59 were unresponsive to mechanical and thermal stimulation of their endings, but responded to sympathetic reflex provocation tests, e.g., arousal or deep inspiration. They were classified as sympathetic efferent units. On average, conduction velocities of sympathetic units were lower (0.78 +/- 0.12 m/s, mean +/- SD) than those of mechano-heat (CMH) or mechanoresponsive (CM) afferent C units (0.91 +/- 0.14 m/s). Endings of most of the sympathetic units were located in the skin of toes or in the foot dorsum. Innervation territories of 16 sympathetic units were mapped by means of conditioning transcutaneous electrical stimuli. Twelve units had one continuous skin territory, whereas two units had two and two other units had three and five separate territories, respectively. The mean innervated area was 128 mm2 (range: 24-350 mm2). Innervation territories of sympathetic units were of approximately the same size in different skin regions on the lower leg, foot, or toes. Based on responses to whole body cooling and warming, two units were tentatively classified as vasoconstrictor and sudomotor units, respectively. Eleven units were tested for responsiveness to iontophoresis of acetylcholine in their innervation territories. In five of them, activity was induced that was not due to central reflex activity but instead due to antidromic activation from the peripheral terminals. Iontophoresis of saline or histamine was ineffective. These findings confirm the existence of excitatory cholinergic receptors in the terminal membrane of some sympathetic units, possibly sudomotors.     

Distribution and latency of muscle responses to transcranial magnetic stimulation of motor cortex after spinal cord injury in humans

Noninvasive transcranial magnetic stimulation (TMS) of the motor cortex was used to evoke electromyographic (EMG) responses in persons with spinal cord injury (n = 97) and able-bodied subjects (n = 20, for comparative data). Our goal was to evaluate, for different levels and severity of spinal cord injury, potential differences in the distribution and latency of motor responses in a large sample of muscles affected by the injury. The spinal cord injury (SCI) population was divided into subgroups based upon injury location (cervical, thoracic, and thoracolumbar) and clinical status (motor-complete versus motor-incomplete). Cortical stimuli were delivered while subjects attempted to contract individual muscles, in order to both maximize the probability of a response to TMS and minimize the response latency. Subjects with motor-incomplete injuries to the cervical or thoracic spinal cord were more likely to demonstrate volitional and TMS-evoked contractions in muscles controlling their foot and ankle (i.e., distal lower limb muscles) compared to muscles of the thigh (i.e., proximal lower limb muscles). When TMS did evoke responses in muscles innervated at levels caudal to the spinal cord lesion, response latencies of muscles in the lower limbs were delayed equally for persons with injury to the cervical or thoracic spinal cord, suggesting normal central motor conduction velocity in motor axons caudal to the lesion. In fact, motor response distribution and latencies were essentially indistinguishable for injuries to the cervical or thoracic (at or rostral to T10) levels of the spine. In contrast, motor-incomplete SCI subjects with injuries at the thoracolumbar level showed a higher probability of preserved volitional movements and TMS-evoked contractions in proximal muscles of the lower limb, and absent responses in distal muscles. When responses to TMS were seen in this group, the latencies were not significantly longer than those of able-bodied (AB) subjects, strongly suggestive of "root sparing" as a basis for motor function in subjects with injury at or caudal to the T11 vertebral body. Both the distribution and latency of TMS-evoked responses are consistent with highly focal lesions to the spinal cord in the subjects examined. The pattern of preserved responsiveness predominating in the distal leg muscles is consistent with a greater role of corticospinal tract innervation of these muscles compared to more proximal muscles of the thigh and hip.     

Patterns of GFAP-immunoreactivity parallel the tonotopic axis in the developing dorsal cochlear nucleus

The gain of the H reflex attenuates during passive stepping and pedalling movements of the leg. We hypothesized that the kinematics of the movement indirectly reflect the receptor origin of this attenuation. In the first experiment, H reflexes were evoked in soleus at 26 points in the cycle of slow, passive pedalling movement of the leg and at 13 points with the leg static (the ankle was always immobilized). Maximum inhibition occurred as the leg moved through its most flexed position (P < 0.05). Inhibition observed in the static leg was also strongest at this position (P < 0.05). The increase in inhibition was gradual during flexion movement, with rapid reversal of this increase during extension. In the second experiment, the length of stretch of the vasti muscles was modelled. Variable pedal crank lengths and revolutions per minute (rpm) altered leg joint displacements and angular velocities. Equivalent rates of stretch of the vasti, achieved through different combinations of joint displacements and velocities, elicited equivalent attenuations of mean reflex magnitudes in the flexed leg. Reflex gain exponentially related to rate of stretch (R2 = 0.98 P < 0.01). The results imply that gain attenuation of this spinal sensorimotor path arises from spindle discharge in heteronymous extensor muscles of knee and/or hip, concomitant with movement.     

Functional organisation of the facial motor system in man

To investigate human corticobulbar projections, electromyographic responses from orbicularis oculi and orbicularis oris muscles were recorded in 11 healthy subjects after transcranial magnetic stimulation. Selective activation of lower facial motoneurones of one hemisphere was reached with the round coil 4 cm lateral to the vertex on a line to the external auditory meatus with stimulus intensities from 45 to 55% (100% = 1.5 T). The mean latency of the OR muscle was 11.5 +/- 1.77 ms contralaterally. Ipsilateral cortical responses were observed in 5 subjects (45%) at a mean latency of 13.88 +/- 2.17 ms. Corticobulbar innervation may have affected bilateral responses in the lower facial muscles as those persisted even after lidocaine blockade of both supraorbital nerves. The functional importance of ipsilateral projections to the lower facial muscles in man is lower than that of the contralateral projections, as evidenced by the fact that they cannot be observed in all subjects or in all motor units. The influence of the trigeminal sensory afferents was excluded from the study after blockade of both supraorbital nerves.     

The expression, origin and function of tenascin during peripheral nerve formation in the chick

During development of the peripheral nervous system, the extracellular matrix molecule tenascin has been found to be closely associated with growing axons. However, its origin and function in peripheral nerve formation are far from clear. In this study, we examined the expression of tenascin during outgrowth of sensory, motor and sympathetic preganglionic axons, and assessed its origin and function in peripheral nerve formation. During outgrowth of sensory and motor axons, a high concentration of tenascin and its mRNA was found to surround sensory and motor axons in the newly formed spinal nerves. The source of this tenascin was examined through a series of surgical manipulations. Neural crest removals did not alter the distribution of tenascin protein or its mRNA surrounding the spinal nerves. Transplantation of quail somites into chick embryos showed that, similar to the distribution of tenascin, there is a high concentration of somitic cells surrounding the spinal nerves. Moreover, somite removals resulted in a reduction of the tenascin and tenascin mRNA surrounding the spinal nerves. Taken together, these results suggest that the majority of the tenascin surrounding the spinal nerves is of somitic origin. Possible functions of tenascin associated with peripheral nerve formation were examined through injections of tenascin or its antiserum into individual somites prior to or during axon outgrowth. Injections of tenascin or its antiserum did not alter the trajectory of peripheral axons in the anterior half of the somite, nor produce gross abnormalities in the morphology of peripheral nerves, suggesting that tenascin does not play a crucial role in the early formation of peripheral nerves.     

The somatosensory system of the neck and its effects on the central nervous system

BACKGROUND: There is some evidence to suggest that dysfunction in the sensory system of the neck may result in a gamut of signs and symptoms. However, a sound understanding of the somatosensory system in the neck and its normal influence on the central nervous system is essential before signs and symptoms can be identified as representations of ill health or disease arising from the neck. OBJECTIVE: To briefly review current knowledge of the somatosensory system of the neck and to consider its connections and influence on the central nervous system. DATA SOURCES: Information was obtained from peer-reviewed scientific journals and proceedings of scientific meetings that have investigated or considered anatomical and physiological aspects of the sensory system in the necks of human and nonhuman vertebrates. CONCLUSION: Studies involving human and nonhuman vertebrates have provided considerable information about the anatomy of the sensory receptors located in the neck and about where information from these receptors is relayed in the spinal cord and brain. Physiological experiments involving electrical and natural stimulation of the head and neck regions have identified a role for some of these receptors in neck-evoked reflexes. It is clear that in addition to signaling nociception, the somatosensory system of the neck may influence the motor control of the neck, eyes, limbs, respiratory muscles and possibly the activity of some preganglionic sympathetic nerves.     

Generalisability of sensory gating during passive movement of the legs

Movement-related gating of somatosensory evoked potentials in the upper limb is restricted mainly to nerve stimulation supplying the moved limb segment. In the lower limb, this principle may not be followed. Tibial nerve (stimulation at the knee) somatosensory evoked potentials (SEPs) and soleus H reflexes exhibit quite similar patterns of modulation during movement. We hypothesised that movement-related gating of initial SEPs in the leg would be generalised from ipsilateral to contralateral leg movement and that such sensory gating would not be generalised to modalities with no functional relevance to the movement. Somatosensory, visual, and auditory evoked potentials (SEPs, VEPs, and AEPs) were recorded from scalp electrodes during unilateral passive movement. Short-latency tibial nerve SEPs, representing the first cortical components, and soleus H reflexes in both the moved leg and the stationary leg were attenuated compared to non-movement controls (p<0.05). Neither VEPs nor middle latency AEPs were modulated (p>0.05). We conclude that sensory gating occurs during contralateral movement. This gating is absent in other sensory modalities with no apparent functional relationship to the imposed movement.     

Abnormal sensory nerve conduction in multifocal demyelinating neuropathy with persistent conduction block

Two patients exhibited chronic, slightly asymmetric weakness and wasting with fasciculations of the upper limb and hand muscles. Motor nerve conduction studies showed features of multifocal conduction block in nerve segments other than those usually involved in entrapment syndromes. The F wave was markedly delayed in the median and ulnar nerves. Transcranial cortical and cervical root magnetic stimulation showed bilaterally delayed thenar responses with normal central conduction time. Needle electromyography demonstrated a chronic denervation pattern with large polyphasic motor units in several muscles of the upper limbs. Sensory symptoms were mild and limited to paresthesias in the fingertips. Sensory nerve conduction velocity and sensory nerve action potential amplitudes were normal in elbow-to-wrist and wrist-to-finger segments of the median and ulnar nerves, but there was a delayed cortical response and unrecognizable Erb's point and cervical responses in the somatosensory evoked potentials to median nerve electrical stimulation. Electrophysiologic examination was normal in most nerves of the lower limbs. These two patients, meeting clinical and electrophysiologic criteria of multifocal neuropathy with conduction block, demonstrate that sensory fibers may also be involved in this syndrome.     

Sensori-sensory afferent conditioning with leg movement: gain control in spinal reflex and ascending paths

Studies are reviewed, predominantly involving healthy humans, on gain changes in spinal reflexes and supraspinal ascending paths during passive and active leg movement. The passive movement research shows that the pathways of H reflexes of the leg and foot are down-regulated as a consequence of movement-elicited discharge from somatosensory receptors, likely muscle spindle primary endings, both ipsi- and contralaterally. Discharge from the conditioning receptors in extensor muscles of the knee and hip appears to lead to presynaptic inhibition evoked over a spinal path, and to long-lasting attenuation when movement stops. The ipsilateral modulation is similar in phase to that seen with active movement. The contralateral conditioning does not phase modulate with passive movement and modulates to the phase of active ipsilateral movement. There are also centrifugal effects onto these pathways during movement. The pathways of the cutaneous reflexes of the human leg also are gain-modulated during active movement. The review summarizes the effects across muscles, across nociceptive and non-nociceptive stimuli and over time elapsed after the stimulus. Some of the gain changes in such reflexes have been associated with central pattern generators. However, the centripetal effect of movement-induced proprioceptive drive awaits exploration in these pathways. Scalp-recorded evoked potentials from rapidly conducting pathways that ascend to the human somatosensory cortex from stimulation sites in the leg also are gain-attenuated in relation to passive movement-elicited discharge of the extensor muscle spindle primary endings. Centrifugal influences due to a requirement for accurate active movement can partially lift the attenuation on the ascending path, both during and before movement. We suggest that a significant role for muscle spindle discharge is to control the gain in Ia pathways from the legs, consequent or prior to their movement. This control can reduce the strength of synaptic input onto target neurons from these kinesthetic receptors, which are powerfully activated by the movement, perhaps to retain the opportunity for target neuron modulation from other control sources.     

The syndrome of painful legs and moving toes--a case report

The syndrome of painful legs and moving toes is an uncommon and distressing condition with pain in the feet or legs and involuntary movements of the toes. It can follow spinal cord or cauda equina trauma, lumbar radiculopathy, injury to the feet, peripheral neuropathy or without any preceding causes. Ephaptic transmission in damaged nerve roots or peripheral nerves with central reorganisation may be the underlying mechanism of the syndrome. Treatment is difficult. We report a case of this syndrome following peripheral neuropathy, with a good early response to the GABA agonists baclofen and clonazepam. The role of different GABA agonists in the treatment of this condition needs to be better defined.     

Locomotor-like movements evoked by leg muscle vibration in humans

We attempted to elicit automatic stepping in healthy humans using appropriate afferent stimulation. It was found that continuous leg muscle vibration produced rhythmic locomotor-like stepping movements of the suspended leg, persisting up to the end of stimulation and sometimes outlasting it by a few cycles. Air-stepping elicited by vibration did not differ from the intentional stepping under the same conditions, and involved movements in hip and knee joints with reciprocal electromyogram (EMG) bursts in corresponding flexor and extensor muscles. The phase shift between evoked hip and knee movements could be positive or negative, corresponding to 'backward' or 'forward' locomotion. Such an essential feature of natural human locomotion as alternating movements of two legs, was also present in vibratory-evoked leg movements under appropriate conditions. It is suggested that vibration evokes locomotor-like movements because vibratory-induced afferent input sets into active state the central structures responsible for stepping generation.     

Trigeminal disynaptic circuit mediating corneal afferent input to M. depressor palpebrae inferioris motoneurons in the pigeon (Columba livia)

Corneal afferent projections to the trigeminal brainstem nuclear complex (TBNC) and associated structures, as determined by transganglionic transport of various tracers, were found to be predominantly concentrated in two distinct patches in the dorsolateral medulla at periobex levels. One was in the external cuneate nucleus, and the other was in the ventralmost part of the ophthalmic division of the TBNC. The projections of putative second-order neurons in these regions, as determined by injections of wheat germ agglutinin conjugated to horseradish peroxidase into the dorsolateral medulla, were found to include the dorsal trigeminal motor nucleus (Vd), which innervates the M. depressor palpebrae inferioris. Electrical stimulation of Vd, which elicited lower eyelid movements, was then used to guide injections of tracer into Vd, which retrogradely labeled clusters of neurons in the corneal afferent recipient regions of the dorsolateral medulla. The lower eyelid of pigeons, unlike the nictitating membrane and upper lid, does not appear to be appreciably involved in either reflex blinking in response to relatively mild stimulation of the cornea (e.g., air puff), or in eye closure during the saccade-like head movements associated with walking, or in eye closure during pecking; but in response to a stimulus that makes corneal contact, an upward movement of the lower lid follows descent of the nictitating membrane and upper lid as part of a defensive eye-closing mechanism. The anatomical results thus appear to define a dedicated disynaptic trigeminal sensorimotor circuit for the control of lower eyelid motility in response to mechanical or noxious stimuli of the cornea. Injections of tracers into the lower and upper eyelids labeled palpebral sensory afferents that terminated predominantly in maxillary and ophthalmic portions, respectively, of the dorsal horn of upper cervical spinal segments. These terminal fields were therefore largely separate from those of corneal afferents. There were no specific corneal afferent projections upon accessory abducens motoneurons that innervate the two muscles controlling the nictitating membrane.     

Central generation of grooming motor patterns and interlimb coordination in locusts

Coordinated bursts of leg motoneuron activity were evoked in locusts with deefferented legs by tactile stimulation of sites that evoke grooming behavior. This suggests that insect thoracic ganglia contain central pattern generators for directed leg movements. Motoneuron recordings were made from metathoracic and mesothoracic nerves, after eliminating all leg motor innervation, as well as all input from the brain, subesophageal ganglion, and prothoracic ganglion. Strong, brief trochanteral levator motoneuron bursts occurred, together with silence of the slow and fast trochanteral depressor motoneurons and activation of the common inhibitor motoneuron. The metathoracic slow tibial extensor motoneuron was active in a pattern distinct from its activity during walking or during rhythms evoked by the muscarinic agonist pilocarpine. Preparations in which the metathoracic ganglion was isolated from all other ganglia could still produce fictive motor patterns in response to tactile stimulation of metathoracic locations. Bursts of trochanteral levator and depressor motoneurons were clearly coordinated between the left and right metathoracic hemiganglia and also between the mesothoracic and the ipsilateral metathoracic ganglia. These data provide clear evidence for centrally generated interlimb coordination in an insect.     

Dynamics of the martial arts high front kick

Fast unloaded movements (i.e. striking, throwing and kicking) are typically performed in a proximo-distal sequence, where initially high proximal segments accelerate while distal segments lag behind, after which proximal segments decelerate while distal segments accelerate. The aims of this study were to examine whether proximal segment deceleration is performed actively by antagonist muscles or is a passive consequence of distal segment movement, and whether distal segment acceleration is enhanced by proximal segment deceleration. Seventeen skilled taekwon-do practitioners were filmed using a high-speed camera while performing a high front kick. During kicking, EMG recordings were obtained from five major lower extremity muscles. Based on the kinematic data, inverse dynamics computations were performed yielding muscle moments and motion-dependent moments. The results indicated that thigh deceleration was caused by motion-dependent moments arising from lower leg motion and not by active deceleration. This was supported by the EMG recordings. Lower leg acceleration was caused partly by a knee extensor muscle moment and partly by a motion-dependent moment arising from thigh angular velocity. Thus, lower leg acceleration was not enhanced by thigh deceleration. On the contrary, thigh deceleration, although not desirable, is unavoidable because of lower leg acceleration.     

Evolution of nerve development in frogs. I. The development of the peripheral nervous system in Discoglossus pictus (Discoglossidae)

The gross anatomical development of the peripheral nervous system (PNS) during embryogenesis and metamorphosis in the frog Discoglossus pictus is described based on whole-mount immunostaining for nerves and muscles. In the head, neurite outgrowth starts with the mandibular ramus of the trigeminal nerve at the tailbud stage. Cranial muscles are innervated as soon as they differentiate, beginning at mid-embryonic stages. During late embryonic stages, the course of the trigeminal and facial nerves becomes greatly distorted and changes again drastically during metamorphosis accompanying the reorganization of the jaw muscles. Two occipital somites and nerves develop transitorily but degenerate at late embryonic stages. The hypoglossal nerve develops by fusion of the first and second spinal nerves and receives a transitory contribution of the third and fourth spinal nerve at embryonic stages. In the trunk, several classes of Rohon-Beard neurites could be identified at embryonic stages, one of which forms intersegmental sensory nerves that prefigure the course of the sensory rami of spinal nerves at later stages. We give detailed schedules of PNS and cranial muscle development which, in comparison with data on other frog species described in a companion paper, will serve as a basis to evaluate heterochronic shift during evolution of PNS development in frogs.     

Evidence that trigeminal brainstem interneurons form subpopulations to produce different forms of mastication in the rabbit

To determine how trigeminal brainstem interneurons pattern different forms of rhythmical jaw movements, four types of motor patterns were induced by electrical stimulation within the cortical masticatory areas of rabbits. After these were recorded, animals were paralyzed and fictive motor output was recorded with an extracellular microelectrode in the trigeminal motor nucleus. A second electrode was used to record from interneurons within the lateral part of the parvocellular reticular formation (Rpc-alpha, n = 28) and gamma- subnucleus of the oral nucleus of the spinal trigeminal tract (NVspo-gamma, n = 68). Both of these areas contain many interneurons projecting to the trigeminal motor nucleus. The basic characteristics of the four movement types evoked before paralysis were similar to those seen after the neuromuscular blockade, although cycle duration was significantly decreased for all patterns. Interneurons showed three types of firing pattern: 54% were inactive, 42% were rhythmically active, and 4% had a tonic firing pattern. Neurons within the first two categories were intermingled in Rpc-alpha and NVspo-gamma: 48% of rhythmic neurons were active during one movement type, 35% were active during two, and 13% were active during three or four patterns. Most units fired during either the middle of the masseter burst or interburst phases during fictive movements evoked from the left caudal cortex. In contrast, there were no tendencies toward a preferred coupling of interneuron activity to any particular phase of the cycle during stimulation of other cortical sites. It was concluded that the premotoneurons that form the final commands to trigeminal motoneurons are organized into subpopulations according to movement pattern.