Muneo Shimamura

Neural mechanisms of the corneal blinking reflex in cats.

Neural mechanisms of the blinking reflex elicited by corneal stimulation were analyzed with electrophysiological techniques in the encéphale isolé cat. (1) Mechanical and electrical stimulation elicited two successive responses of the electromyogram of the orbicularis oculi. Neuromuscular unit studies revealed that the same unit was excited twice and that the latencies of both responses corresponded well with the two EMG responses. (2) The late response was easily affected by anoxia and pentobarbital administration, and was also abolished with the slow-wave sleep stage. (3) Both responses were abolished by ipsilateral transection between the inferior colliculus and genu of the facial nerve. (4) Compared with the latencies of the EMG, the sum of the conduction times through the sensory trigeminal nucleus and the facial nucleus corresponded with the latency of the early response. The sum of the conduction times through the reticular formation, added to the former reflex arc, corresponded to the latency of the late response. (5) The reflex pathway of the early response is consistent with a three-neuron arc passing through the sensory trigeminal nucleus and the facial nucleus. The late response may employ a multisynaptic arc passing through the brain stem medial reticular formation between the sensory trigeminal nucleus and the facial nucleus.

Reticular neuron activities associated with locomotion in thalamic cats

Unit discharges of the reticular neurons and EMGs of extensors and flexors were recorded in the 4 limbs of thalamic cats during stepping on a motor-driven treadmill to study the functional role of the spino-bulbo-spinal (SBS) reflex in locomotion. (1) Reticular neurons showed alterations of increased or decreased discharge rate depending on the phase of the step cycle; they were divided into input (ascending), output (reticulospinal, R-S) and internuncial neurons underlying the SBS reflex. (2) R-S neurons showed increased spikes from the end o the stance to the beginning of the swing phase; in this period, flexor muscle EMGS were obtained. The spikes decreased or disappeared when extensor EMGs were observed. To analyze the relationship between activity in R-S neurons and flexor muscles, R-S neuron spikes were used for triggering and post-spike EMGs were averaged. Averaged EMGs of the correlated flexor revealed a 12 and 6 ms latency for hind- and forelimbs, respectively. Most R-S neurons elicited EMGs of flexor in muscles of one ipsilateral leg, some elicited flexor EMGs of both the ipsilateral fore-and hindlimbs. (3) Input neurons of the reticular formation from spinal ascending tracts fired spikes upon touching of the skin over the correlated limb. During stepping, they showed two types of response patterns: a burst discharge appeared for the leg which was swinging and touching the belt, the other pattern was of continuous discharges at the end of stance and at the beginning of swing phases. (4) Internuncial neurons which relay impulses between the input and output neurons also showed alternate firings depending on the phase of the step cycle. Spikes could be elicited by touching the skin over the 4 limbs. (5) The relationship among the activities of the 3 groups of reticular neurons was analyzed by the post- or pre-spike averaging technique and appropriate latency responses were obtained. (6) The results indicate that flexor EMGs were excited by cutaneous afferents via the SBS reflex in cats stepping on the treadmill. The role of the SBS reflex in locomotion is discussed.

Discharge patterns of reticulospinal neurons corresponding with quadrupedal leg movements in thalamic cats

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 experimenters 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.

Reticulospinal tracts involved in the spino-bulbo-spinal reflex in cats.

Abstract Twenty-three chloralosed cats were used to examine the spinal descending pathways of the spino-bulbo-spinal (SBS) reflex. Transection of the ventrolateral funiculus in the spinal cord at the thoracic level abolished the SBS reflex of caudal spinal segments ipsilateral to the transection, but did not abolish the ascending propriospinal reflex and SBS reflex of rostral segments. Unit discharges elicited in axons of the ventrolateral funiculus at L3 by sural nerve stimulation had the appropriate latency for mediating the SBS reflex. These axons originated in the medial bulbar reticular formation, since the responses had a consistent short latency and followed repetitive stimulation of the bulbar reticular formation (up to a rate of 300 Hz). Conduction velocities of reticulospinal axons ranged widely from 20 to 120 m/sec. There were two peaks, fast (95 m/sec) and slow (35 m/sec). The fast conducting fibers showed oligo-spikes, high amplitude and wide distribution of latency. Almost 90% of the axons with slow conduction exhibited multi-spikes, low amplitude and narrow distribution. Unitary responses of the ventral rootlet at L7 were elicited by sural nerve stimulation and correlated with the latency of the SBS reflex. The unitary response was also evoked by train pulse stimulation of the ventrolateral funiculus in the spinal cord. Conduction velocities of descending spinal tracts ranged from 20 to 60 (mean±S.D.,35 ± 8) m/sec. We may conclude that the descending spinal pathway of the SBS reflex is the slowly conducting reticulospinal tract which originates in the medial bulbar reticular formation and passes through the ventrolateral funiculus of the spinal cord.

Effects of spino-bulbo-spinal reflex volleys on flexor motoneurons of hindlimb in the cat

Summary o (1) Experiments were carried out on cats with chloralose anesthesia or decerebration to extend observations on the spino-bulbo-spinal (SBS) reflex and to analyze the effects of SBS reflex volleys on flexor motoneurons. (2) Stimulation of cutaneous afferents yielded late (SBS) reflex responses from the spinal ventral root 9 , as well as late responses in the primary afferents of flexor muscular and cutaneous origin. (3) A single volley in a cutaneous nerve yielded augmentation, early and late, and prolonged influences on the flexor mono- and polysynaptic reflexes (MSR and PSR). Early augmentation corresponded to facilitation at the spinal segment by direct volleys in cutaneous afferents. Late augmentation and other prolonged influences may be due to the SBS reflex system: (a) spinal transection at the C 1 level eliminated both; (b) they appeared under the same conditions, i.e. with similar spontaneous fluctuations, similarly blocked by pentobarbital anesthesia and asphyxia; and (c) the latency of late augmentation was the same as that of the SBS reflex response in the same ventral root. (4) Late augmentation seems to be due to the postsynaptic excitation, since there were EPSPs in flexor motoneurons at the same time as the SBS reflex. (5) The prolonged influences on the flexor reflexes included two different mechanisms: diminution of PSR and augmentation of MSR. The prolonged diminution of PSR and augmentation of MSR may be due to complex mechanisms underlying the supraspinal processes: presynaptic inhibition, postsynaptic inhibition, defacilitation and disinhibition of the motoneurons and spinal interneurons since (a) a late peak of the depolarization affecting the SBS reflex was obtained in afferent fibers of cutaneous and muscle origin following stimulation of the sural nerve, and (b) prolonged hyperpolarization and depolarization seen in flexor motoneurons involved two different processes, IPSP and defacilitation, and EPSP and disinhibition.

Role of joint afferents in relation to the initiation of forelimb stepping in thalamic cats

To analyze the roles of joint afferents in relation to initiation of forelimb stepping in thalamic cats, we recorded the unit spikes of the cervical dorsal roots, stimulated the joint afferents, and applied local anesthesia to the joint capsule. Almost all of the joint afferents of the shoulder, elbow, wrist and finger adapted slowly and exhibited alternating firing during forelimb stepping. About 45% of the afferents of each joint showed firings as the limb moved from forward to backward. About 44% of the afferents exhibited discharges as the limb moved from backward to forward. The remaining afferents showed firings as the limb moved in both directions. The application of local anesthesia to joints of the shoulder, elbow or wrist resulted in a marked reduction of forelimb stepping. Forelimb stepping was evoked by electric stimulation of the joint capsule, when excitabilities of flexor motoneurons were increased due to muscle stretching. Impulses originating in the joint afferents of the forelimb entered the spinal cord and ascended to the dorsolateral funiculus of the cervical cord, since forelimb stepping was abolished after bilateral transection of this part. Our results indicate that joint afferents may play an important role in the initiation of forelimb stepping in thalamic cats walking on a motor-driven treadmill.

The midbrain reticular formation as an integration center for the ‘near reflex’ in the cat

This report describes an experimental study on the localization of converging organization of the near-reflex triad in the chloralose-anesthetized encéphale-isolé cat, in which electromyographic (EMG) recordings were used to elicit responses from the intrinsic and extrinsic eye muscles. Electrical stimulation to several subdivisional areas in the oculomotor nuclear complex evoked EMGs in both the iris sphincter and ciliary muscles. Conduction time from the caudal Edinger-Westphal nucleus to the postganglionic ciliary nerve was about 1.8 ms, whereas that to the iris sphincter muscle was about 6.5 ms. Conduction time from the anteromedian nucleus to the muscle was about 4.5 ms; however, that from the postganglionic short ciliary nerve to the muscle was about 6.7 ms. A direct pathway without synapse in the ciliary ganglion is suggested. Excitatory responses were elicited in the effectors of the near-reflex triad by electrical stimulation of the midbrain reticular formation of the dorsomedial division adjacent to the magnocellular red nucleus (MRFdmMRN). Converging movements in electro-oculography (EOG) were also observed. Conduction time from the MRF to the iris sphincter muscle was about 5.6 ms, whereas that to the postganglionic short ciliary nerve was 5.1 ms. The neural connection between the MRF and the muscle is thought to be mediated by the anteromedian subnucleus. Electrical stimulation of the posteromedial division of the Clare-Bishop (C-B) area evoked a discharge on the MRF and EMGs of all effectors of the triad. The sum of the conduction time from the C-B area to the MRF and that from the MRF to EMGs corresponds well to the latency of EMGs evoked by C-B area stimulation. We conclude that the MRFdmMRN is the supranuclear organization which converges the sensory-motor cortical activities on the precise linkage of the near-reflex triad and becomes an integration center for each nucleus in the oculomotor nuclear complex.

Burst discharges of pontine reticular neurons in relation to forelimb stepping of thalamic and high spinal cats

The paralemniscal pontine reticular neurons (PLRF) exhibited burst discharges when thalamic cats stepped on the moving treadmill, during stepping induced by mesencephalic locomotor region (MLR) stimulation, fictive locomotion by MLR stimulation after immobilization and during MLR stimulation after transection at the C2 level. These results imply that MLR stimulation can evoke burst discharge within the brainstem which may obtain alternating limb movements via descending tract to the cervical cord.

Three types of reticular neurons involved in the spinobulbo-spinal reflex of cats

Reticular neuron activity was recorded in 28 chloralosed cats in order to analyze the reflex arc of the spino-bulbo-spinal (SBS) reflex. Three types of reticular neurons, types I (input), II(output) and III (relay), were identified by unit discharges in response to stimulation of the sural nerve. (1) Type I (input) neurons received spinal ascending volleys monosynaptically and responded to stimulation of the sural nerve with spikes of low amplitude and short latency. Unit spikes, however, were not produced by stimulation of the superficial radial nerve and the sensorimotor cortex. These input neurons were located in the dorsocaudal part of the medial bulbar reticular formation. (2) Type II (output) neurons were part of the reticulospinal tract, which sends axons to the spinal cord, since these neurons exhibited antidromic spikes following stimulation of the ventrolateral funiculus of the spinal cord. Unit spikes were evoked by stimulation either to the sural or superficial radial nerves. These neurons were located in the ventrocaudal part of the medial bulbar reticular formation. (3) Type III neurons included relay neurons. Unit spikes were evoked by stimulation of the sural nerve, superficial radial nerve and sensorimotor cortex. However, unit discharges were not obtained by antidromic stimulation to the reticulospinal tract. These neurons were distributed widely in the brain stem, both in the bulb and pons. (4) Latency difference of unit discharges between input and output neurons was 3.5--5 msec, indicating the presence of interneurons (relays) between input and output neurons. Spikes of output neurons with 3.8--4.2 msec latency were observed following stimulation of the region where input neuron activity was found. We may conclude that three kinds of reticular neurons, input, relay and output, were involved in pathways of the SBS reflex.

The role of the paralemniscal pontine reticular formation in forelimb stepping of thalamic cats

Summary We recorded the neuronal activities from, stimulated, and produced lesions in, various regions in the brainstem of thalamic cats. Neuronal activities were recorded during stepping on a motor-driven treadmill, and were averaged by using the flexor EMG as the trigger. In wide regions of the brainstem, an averaged response was obtained with similar latency as the ipsilateral flexor EMG; however, in the paralemniscal pontine reticular formation (PLRF) the averaged response preceded the flexor EMG. When repetitive electrical stimulation was applied to the PLRF on one side, coordinated movements of bilateral forelimbs were elicited. The ipsilateral forelimb was flexed and the contralateral forelimb extended. EMG recordings revealed that on the side ipsilateral to the stimulation, flexor EMGs appeared and increased gradually in amplitude, and the extensor EMG was suppressed, while the extensor EMG was increased and no flexor EMG was obtained on the side contralateral to the stimulation. When pulse train stimulation was alternatingly applied to the PLRF on bilateral sides, alternating forelimb movement, as in stepping, was evoked, depending on each stimulation. A small, unilateral lesion in the PLRF reduced forelimb stepping on a motor-driven treadmill, particularly the flexor muscle contraction in the forelimb ipsilateral to the lesion. Bilateral PLRF lesions abolished forelimb stepping on both sides. Unilateral transection of the ventral quadrant at the cervical cord abolished PLRF-elicited flexor EMGs and associated suppression of extensor EMG ipsilateral to the transection; however, the side contralateral to the transection remained intact. These results suggest that the PLRF exerts a patterned control of the bilateral forelimb muscles and plays a role in initiating and maintaining forelimb stepping.