Control of the tonic vibration reflex by the brain stem reticular formation in the cat

The tonic vibration reflex (TVR) has been used to investigate the areas in the brain stem which influence tone in flexor and extensor muscles in the hind limb of the cat.Tonic contraction of triceps surae and tibialis anterior was inhibited by stimulation of the medial medullary reticular formation and facilitated by lateral medullary stimulation. Between the inhibitory and facilitatory areas, a mixed area was found which produced facilitation on direct stimulation which was altered to inhibition when the contralateral motor cortex or internal capsule was stimulated simultaneously.A small area in the caudal medulla reciprocally enhanced and suppressed the TVR in flexors and extensors respectively.     

Discharge pattern of single motor units in the tonic vibration reflex of human triceps surae.

Using a single fibre EMG electrode the firing pattern of 46 motor units in the triceps surae has been studied during vibration of the Achilles tendon at frequencies of 25--200 Hz. Potentials activated in the tonic vibration reflex (TVR) were phase-locked to the vibration cycle but tended to become somewhat less so with continued vibration. The firing pattern of voluntarily activated motor units became locked to the waveform by the application of the vibrator. The discharges of 21 motor units were studied during low threshold (sub-M wave) tetanic stimulation of the tibial nerve at 25--100 Hz. No evidence was found of synchronization of potentials activated in the resulting tonic contraction. During weak voluntary contractions, stimulation also failed to regularize voluntarily activated motor units. The findings can be reconciled by postulating that, in normal man, vibration activates monosynaptic and polysynaptic pathways, the latter circuit being adequate to generate reflex contraction, while the former merely affects the temporal patterning of the motor outflow.     

Reflex responses from the sural nerve to tibialis anterior muscle in hemiplegic patients: the relation between the responses and Babinski sign]

Stimulation of the sural nerve of healthy subjects induced short latency inhibition in the ipsilateral tibialis anterior muscle and facilitation in peroneal muscle. We examined lower limb muscle responses after stimulation of the sural nerve in 19 patients with hemiplegia caused by cerebro-vascular disease and compared them with the control responses. The sural nerve was stimulated electrically (3 or 5 square wave pulses of 0.5 ms repeated at 250 Hz) during weak tonic contraction. Stimulation was triggered to average the rectified surface electromyography (EMG) of the test muscle. Usually 100 - 200 sweeps were averaged. After stimulation, the tibialis anterior muscle on the affected side of the hemiplegic patients showed the patterns of inhibition, facilitation, and no response, whereas all responses on the unaffected side, except those of one patient, were inhibition. The peroneal muscle on both sides showed only facilitation as in the controls. Abnormal responses of the tibialis anterior muscle on the affected side were present in many patients who had the Babinski sign. Abnormal responses in the tibialis anterior muscle of the affected side may have been due to contributions by disinhibition of the flexor reflex, late-recruited motor units or both.     

Excitation and inhibition processes in the neurons of the thalamic motor nuclei of normal cats and following an N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced lesion of the nigrostriatal dopaminergic system

Peculiarities of excitation and inhibition evoked in motor thalamic nuclei (VA-VL) neurons by electrical stimulation of red nucleus were studied on intact cats and after injection of N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP; 5 mg/kg i.m., p.d. during five days). Two days after the last injection as much as 48% of nigral neurons were destroyed and the content of dopamine in the caudate nucleus fell to 30% as compared to intact animals. Before acute experiments all cats were anaesthetized with ketalar and immobilized with myorelaxine. It was found that MPTP injections caused a decrease of the inhibition duration and effectiveness in relay and nonrelay VA-VL neurons. The inhibition deficiency was accompanied by shortening of latencies of orthodromic responses evoked by red nucleus stimulation and facilitation of antidromic spikes invasion into somata of relay neurons after motor cortex stimulation. It was suggested that the reduction of GABAergic nigro-thalamic influences modulated by dopamine underlay the developing deficiency of inhibition.     

Vestibular input to the caudate nucleus

Unilateral stimulation of peripheral vestibular nerve branches in encephale isolé cats evoked potentials of 3- to 7-msec latency in the banks of the contralateral anterior suprasylvian sulcus and 4- to 15-msec latency in the head of the caudate nucleus bilaterally. The responses were distributed in the dorsomedial region of the ipsilateral caudate and dorsolateral region of the contralateral caudate. Stimulation of peripheral cochlear nerve branches also evoked responses in areas of caudate partially overlapping those responsive to vestibular stimulation. Stimulation of the vestibular projection area of cortex evoked a response in caudate but did not alter the response to vestibular nerve stimulation. Ablation of the vestibular projection area of cortex did not alter the vestibular responses in caudate. However, caudate and cortical responses to vestibular nerve stimulation were severely decreased by lesions of the magnocellular portion of medial geniculate body. We conclude that there are vestibular projections to the caudate nuclei which require passage through the medial geniculate body and probably other thalamic nuclei, but not through the vestibular projection area of cortex.     

Crossed and uncrossed central effects of muscle spindle afferents from the lateral pterygoid muscle of the guinea pig

Physiological evidence is presented for the presence of stretch reflexes in the lateral pterygoid (Pt) muscle of the guinea pig. The central reflex effects of excitation of Pt stretch reflex afferents were also investigated. Passive lateral jaw displacement, which resulted in stretch of the Pt muscle on the side of jaw movement and stretch of the zygomatico-mandibularis (Z) muscle on the side contralateral to the movement, evoked increased EMG activity in these muscles. Stimulation of the trigeminal mesencephalic nucleus (mes V) evoked monosynaptic reflexes in both the Pt and Z nerves. Tonic stretch of the Pt muscle facilitated the monosynaptic reflex in the Pt nerve evoked by stimulation of mes V. Tonic vibration of the Pt muscle facilitated the mes V evoked monosynaptic reflex in the nerves to the ipsilateral Pt and contralateral Z muscles. Conversely, tonic vibration of the Z muscle facilitated the monosynaptic reflex evolved by mes V stimulation in the contralateral Pt and ipsilateral Z nerve. The results support the view that muscle spindles exist in the Pt and Z muscles and that there is a monosynaptic stretch reflex for both the Pt and Z muscles with cell bodies located in the mes V nucleus. It was also shown that the ipsilateral Pt muscle and the contralateral Z muscle act as synergists in the production of lateral jaw movements and that the organization of the stretch reflexes originating from the Pt and Z muscles support their synergistic action.     

Effect of vibration of the ankle stretch reflex in man

Vibration at frequencies above 50 Hz applied to the tendon of the extensor muscles of the ankle joint produce the tonic vibration reflex (TVR) which increases when the vibration frequency is increased. The TVR affects a joint's mechanical response to sinusoidal and random oscillations in a manner similar to that seen with tonic voluntary contraction. Although the myotatic reflex is suppressed by vibration, repeated stretches of sinusoidal oscillation produce an average EMG response which is not different in magnitude from the no vibration case. Either polysynaptic mechanisms at the spinal cord level of mechanisms involving higher centers (and possibly both) are able to overcome the inhibitory mechanisms at the Ia-alpha motoneuron level in producing a stretch evoked resonance near 6 Hz. The degree of inhibition of the myotatic component of the stretch reflex is proportional to the vibration frequency. This is in contrast to the facilitation of the myotatic reflex produced by tonic voluntary contraction. Vibration does not seem to influence the post-myotatic component (> 100 msec) of the stretch reflex. These results indicate that the post-myotatic responses to limb perturbation are not only different in their latency but also in their functional dependence upon peripheral influences.     

Mechanisms mediating ipsilateral limb hyperflexion after cerebellar paravermal cortical ablation or cooling

The pathway from cerebellar paravermal cortex, nucleus interpositus, red nucleus to rubrospinal tract is essential for manifestations of tonic flexion and exaggerated flexor responses after paravermal cortical ablation or cooling. Activity of nucleus interpositus and red nucleus was studied electrophysiologically in acute preparations with cooling of paravermal cortex under Nembutal or chloralose anesthesia. Recordings with a pair of macroelectrodes showed increased spontaneous activity and evoked response by peripheral stimulation in nucleus interpositus and red nucleus after cooling of paravermal cortex. In nucleus interpositus, extracellular studies with microelectrode showed that spontaneous activity increased in 17 of 29 units, had no change in eight units and decreased in four units and evoked response by peripheral stimulation increased in 23 units and had no change in six units after cooling of paravermal cortex. The increased response to peripheral stimulation changed from facilitation or inhibition or no response before cooling to prolonged facilitation or was masked by high spontaneous activity after cooling. This study suggests that paravermal cortical cooling or ablation abolishes cortical inhibition of the nucleus interpositus which in turn excites the red nucleus, rubrospinal tract, and spinal flexor motoneurons to produce ipsilateral limb hyperflexion.     

Amygdala and masseteric reflex. I. Facilitation, inhibition and diphasic modifications of the reflex, induced by localized amygdaloid stimulation.

The changes in amplitude of the monosynaptic masseteric reflex (MR), induced by stimulation of the amygdaloid area for the defence reaction (N. basalis, pars magnocellularis) and in other subdivisions of the amygdaloid complex, were studied in cats with spinal section maintained under Flaxedil. Simultaneously, the the effects of stimulation on the tonic activity of the masseteric nerve were observed. A maintained facilitation of the MR was elicited by stimulation of the lateral nucleus, the parvocellular portion of the basal nucleus and the cortical nucleus, while the reflex was inhibited during stimulation of the medial-most portion of the posterior amygdala. Diphasic changes of the MR amplitude (initial facilitation followed by delayed inhibition) were regularly observed when stimulating the magnocellular portion of the basal nucleus. These diphasic changes were closely correlated with the previously described diphasic resporatory and cardiac responses elicited from the same are (Bonvallet and Gary Bobo 1972). The initial facilitation probably corresponds to the "alerting" stage of the defence reaction and the delayed inhibition, associated with cortical, respiratory and cardiac activation, to the "defensive" stage of the reaction. Stimulation of the same area also provokes tonic or rhythmical discharges of the masseteric motoneurons which frequently occur during the delayed inhibition of the MR. The main efferent pathway mediating these motor effects is probably the ansa lenticularis.     

Tonic vibration reflex in spasticity, Parkinson''s disease, and normal subjects

The tonic vibration reflex (TVR) has been studied in the quadriceps and triceps surae muscles of 34 spastic, 15 Parkinsonism, and 10 normal subjects. The TVR of spasticity develops rapidly, reaching a plateau level within 2-4 sec of the onset of vibration. The tonic contraction was often preceded by a phasic spike which appeared to be a vibration-induced equivalent of the tendon jerk. The initial phasic spike was usually followed by a silent period, and induced clonus in some patients. No correlation was found between the shape of the TVR and the site of the lesion in the central nervous system. The TVR of normal subjects and patients with Parkinsonism developed slowly, starting some seconds after the onset of vibration, and reaching a plateau level in 20-60 sec. A phasic spike was recorded occasionally in these subjects, but the subsequent tonic contraction followed the usual time course. Muscle stretch increased the quadriceps TVR of all subjects, including those with spasticity in whom the quadriceps stretch reflex decreased with increasing stretch. It is suggested that this difference between the tonic vibration reflex and the tonic stretch reflex arises from the selective activation of spindle primary endings by vibration, while both the primary and the secondary endings are responsive to muscle stretch. The TVR could be potentiated by reinforcement in some subjects. Potentiation outlasted the reinforcing manoeuvre, and was most apparent at short muscle lengths. As muscle stretch increased, thus producing a larger TVR, the degree of potentiation decreased. It is therefore suggested that the effects of reinforcement result at least partially from the activation of the fusimotor system. Since reinforcement potentiated the TVR of patients with spinal spasticity in whom a prominent clasp-knife phenomenon could be demonstrated, it is suggested that the effects of reinforcement are mediated by a descending pathway that traverses the anterior quadrant of the spinal cord.     

Effect of stimulation of the spinal cord on phasic and tonic reflexes in patients with central motor neuron damage

Spinal cord stimulation (SCS) is a method enabling the control of increased muscle tonus to be achieved in various spinal cord injuries. Polyelectromyographic (PEMG) methods were used for neurophysiological assessment of the degree of cord damage and persistent spinal reflexes as well as supramedullary influences. The analysed material comprised 40 PEMG records in 19 patients with spastic paraparesis or paraplegia after cord injury, cord tumour or multiple sclerosis. In 15 cases tentative epidural cord stimulation was done and 11 patients received implantation of a system for long-term stimulation. In most cases the epidural electrodes were implanted below the damaged segment, usually in the thoracic part of the cord. Before and after SCS beginning PEMG was done with a 16-channel Mingograph Siemens Elema with simultaneous recording of the responses from the symmetric muscles: quadriceps, semitendinous, adductor femoris, anterior tibialis and triceps surae. The effect of SCS was analysed on exteroceptive and proprioceptive reactions during testing of knee and ankle reflexes, and on the response of the muscles to vibration. In most patients a reduction was observed of the intensity of tendon reflexes, particularly the spread of the reflex to the contralateral extremity was no longer seen. The vibration reflex had a tonic character persisting in 48% of the studied muscles, even in patients with clinically complete transsection of the cord. The change of the character of monosynaptic reflexes and the presence of the vibration reflex suggest that SCS modifies the proprioceptive segmental spinal reactions.     

Motor cortical control of cardiovascular bulbar neurones projecting to spinal autonomic areas

There is evidence that the motor cortex is involved in cardiovascular adjustments associated with somatic motor activity, as it has functional connections with the ventrolateral medulla, a brainstem region critically involved in the control of blood pressure and the regulation of plasma catecholamine levels. The ventrolateral medulla sends projections to the spinal intermediolateral nucleus, where preganglionic neurones controlling heart and blood vessels (T2 segment) and adrenal medulla (T8 segment) are found. The aim of the present study was to determine whether electrical stimulation of the rat motor cortex induces cardiovascular responses and Fos expression in ventrolateral medulla neurones projecting to the T2 and T8 segments. After a set of experiments designed to record cardiovascular parameters (blood pressure and plasma catecholamine levels), injections of retrograde tracer (Fluorogold) were performed in the intermediolateral nucleus of two groups of rats, at the T2 or at the T8 segmental levels. Five days later, the motor cortex was stimulated in order to induce Fos expression in the ventrolateral medulla. Stimulation of the motor cortex induced: (1). hypotension and a significant decrease in plasma noradrenaline levels, and (2). a significant increase in the number of the double-labelled neurones in the rostral ventrolateral medulla projecting to T2. These data demonstrate that cardiovascular adjustments, preparatory to, or concomitant with, motor activity may be initiated in the motor cortex and transmitted to cardiac and vasomotor spinal preganglionic neurones, via the ventrolateral medulla.     

Labyrinth and neck reflex modification of the tonic vibration reflex in the decerebrate cat

The interaction between tonic labyrinth or neck reflexes and the tonic vibration reflex acting on the medial head of triceps in the decerebrate cat is described. Medial triceps was isotonically loaded and reflex actions were measured as changes in muscle length. Natural stimulation of the receptors giving rise to tonic labyrinth or neck reflexes can either enhance or diminish the size of a pre-existing tonic vibration reflex. It is also shown that descending activity from either the labyrinth or neck reflex systems can completely suppress the tonic vibration reflex, whereas the tonic vibration reflex was never observed to suppress an established labyrinth or neck reflex.     

Ia presynaptic inhibition after muscle twitch in the arm

Contraction of upper limb muscles in healthy subjects was used to investigate presynaptic inhibition at spinal level. The H reflex recorded in the forearm flexor muscles in response to median nerve stimulation was depressed in amplitude from 400 ms to 1 s after a muscle twitch induced by transcranial stimulation, root stimulation, direct biceps stimulation, and triceps tendon tap. Stimulation of the cutaneous branch of musculocutaneous nerve, ipsilateral triceps and contralateral biceps, and biceps tendon tap did not alter H-reflex size. Forearm flexor H-reflex amplitude is therefore related to changes in proprioceptive inflow secondary to the biceps muscle twitch. Root and direct muscle stimulation both failed to reduce the size of the motor evoked potential (MEP) after transcranial magnetic stimulation, suggesting that the inhibition acts at presynaptic level. Attenuation of H-reflex amplitude was related to the size of the muscle twitch and was less pronounced during an isometric twitch than during free joint movement. Our results suggest that the biceps muscle twitch produces long-lasting inhibition of the Ia afferents from forearm flexor muscles. This is an important and a simple mechanism for suppressing proprioceptive input during movement.     

Turning off the central contribution to contractions evoked by neuromuscular electrical stimulation

Neuromuscular electrical stimulation can generate contractions through both peripheral and central mechanisms. The peripheral mechanism involves the direct activation of motor axons, while the central mechanism involves the activation of sensory axons that recruit spinal neurons through a reflex pathway. For use in functional electrical stimulation. One must have control over turning the central mechanism on and off. We investigated whether inhibition developed through antagonist muscle (tibialis anterior, TA) contractions elicited by electrical stimulation or by volition can turn off the central mechanism in triceps surae. Both electrical stimulation and voluntary contractions of TA reduced or eliminated plantar flexion torque produced by the central mechanism, indicating that inhibition induced via these contractions can effectively turn off the central contribution to force. These findings suggest that patterns of electrical stimulation may be able to generate periodic muscle contractions by turning the central contribution to muscular contractions on and off.     

Effects of lateralized reaching and cerebellar stimulation on unit activity of motor cortex and caudate nucleus in rats

Rats trained to reach into a narrow tubular feeder for food were implanted with bipolar stimulation electrodes in the dentate nucleus (DNT) ipsilateral to the preferred forepaw. Unit activity was recorded in freely moving animals (N = 15) with capillary microelectrodes inserted with a head-mounted microdrive into the contralateral motor cortex and caudate nucleus. Single-pulse stimulation of the DNT, subthreshold for eliciting overt movement and for interfering with reaching, evoked in both structures short-lasting excitatory responses sometimes preceded by brief inhibition. Reach-triggered stimulation changed the perireach histograms of the motor cortex neurons (N = 46) not only by interference with the movement-related and stimulation-elicited activities in the poststimulus interval but also considerably prolonged the prereach excitation. The perireach histograms of caudate neurons (N = 50) were not changed by the reach-triggered DNT stimulation in the prereach or in the poststimulus interval, probably because the movement-related activity overrode the electrical perturbation. The results indicated that the impairment of reaching caused by DNT stimulation was due to disorganization of the neuronal activity patterns in the motor cortex rather than in the caudate nucleus. The earlier onset of movement-related excitation in the motor cortex during stimulus-triggering reaching reflects attempts to cope with the disturbance by predictive reorganization of the motor discharge.     

Cerebellar and cerebral inputs to physiologically identified efferent cell groups in the red nucleus of the cat

Cells of the red nucleus of the cat have been classified into 4 projection sets, consisting of cells which could be fired antidromically from (1) only the contralateral spinal cord, (2) only the ipsilateral medulla, (3) both the contralateral spinal cord and the ipsilateral medulla, and (4) both the contralateral spinal cord and brachium conjunctivum. Each of these projection sets receives excitatory synaptic input from axons leaving the cerebellum in the superior cerebellar peduncle, crossing to the red nucleus of the opposite side, and sending axon collaterals — in at least some cases — to the thalamus. Most cells in each projection set are also excited following stimulation of the ipsilateral precruciate cerebral cortex. Rubral cells with medullary, cerebellar, and spinal-projecting axons are found dispersed throughout the rostrocaudal extent of the nucleus, although caudal cells are primarily rubrospinal and rubral cells sending axons to the ipsilateral medulla are predominantly located in the rostral portions of the red nucleus.     

Neurophysiological evaluation of areflexia in Holmes-Adie syndrome.

PURPOSE: To evaluate ankle areflexia in Holmes-Adie syndrome (HAS). PATIENTS AND METHODS: Hoffmann (H) and Tendon (T) soleus reflexes, tonic vibration reflex (TVR), and polysynaptic extension reflex of soleus muscle (PERS) were evaluated in eight patients with idiopathic HAS. Motor (MNCV) and sensory (SNCV) nerve conduction velocities, compound motor-action potential (CMAP), and sensory action potential (SAP) were also determined in upper and lower limbs. RESULTS: Soleus T reflex was obtained in one out of eight patients, and H-reflex was found in none of the patients. TVR was recorded in four out of eight patients, and PERS in all of the patients. MNCV, SNCV, CMAP and SAP showed normal values in all patients. In six out of the eight patients a late response following the tibial nerve stimulation showed constant latency, amplitude and morphology, with no recovery cycle or vibration inhibition. CONCLUSION: In this study, the neurophysiological spinal reflex circuitry evaluations support the view that HAS ankles areflexia is due to a selective impairement of monosynaptic connections of Ia afferents. A normal nuclear excitability is suggested by polysynaptic activation of the soleus motor nucleus.     

Effects of acute stimulation through contacts placed on the motor cortex for chronic stimulation.

OBJECTIVES: We tried to determine which neural elements were activated in awake subjects by stimulation through contacts placed chronically on the motor cortex. METHODS: We recorded the motor effects of stimulation through 4 disc contacts placed in the subdural space over the motor cortex in 9 patients undergoing chronic stimulation for the control of pain or for the control of the rigidity of multiple system atrophy. RESULTS: Single stimuli could elicit short latency motor evoked potentials or facilitate active motoneurons in the contralateral limbs. The responsible neural elements had a short chronaxie (the pulse duration necessary to reach threshold with a stimulus intensity twice that required to reach threshold at the longest pulse duration used) and refractory period implying that they were myelinated axons. The facilitation was larger with cathodal than with anodal monopolar stimulation. The short latency facilitation in response to the second of two stimuli was greater at condition test intervals of 2-5 ms. This enhancement could be demonstrated with conditioning stimuli subthreshold for the excitation of active motoneurons suggesting that it arose, in part, at the level of the cortex. Single cortical stimuli could result in inhibition of voluntarily activated motoneurons. The inhibition was larger with cathodal than anodal monopolar stimulation. The responsible neural elements also had a short chronaxie and refractory period. CONCLUSIONS: Stimulation in awake subjects through contacts placed chronically over the motor cortex appears to activate axons in the cortex, which excite both corticospinal neurons and inhibitory neurons.     

Split medulla preparation in the cat: arterial chemoreceptor reflex and respiratory modulation of the renal sympathetic nerve activity

The study was undertaken in order to assess the changes in sympathetic output in a split medulla preparation of the cat which, as shown earlier, has impaired respiratory rhythm generation. The effects of medullary midsagittal sections on renal sympathetic nerve firing were investigated in chloralose anesthetized, paralyzed and artificially ventilated cats. Recordings of phrenic and recurrent laryngeal nerve activity served as indices of central respiratory rhythm generation. Sections, 5 mm deep from the dorsal medullary surface and extending 6 mm rostrally and 3 mm caudally to the obex, did not produce any significant changes in heart rate, blood pressure or tonic renal sympathetic nerve firing levels. They decreased or abolished, however, the respiratory rhythmicity in renal sympathetic nerve which paralleled the reduction of inspiratory discharges in phrenic and recurrent laryngeal nerves, and abolished the carotid body chemoreceptor-sympathetic reflex. The inspiratory activity remaining after the sections could still be enhanced by chemoreceptor stimulation. The inhibitory baroreceptor and pulmonary stretch receptor sympathetic reflexes, and the central excitatory effect of CO2 on renal sympathetic nerve firing were not altered. The effects of electrical stimulation within the midsagittal plane of the medulla have shown that descending pathways from the medullary inspiratory neurons (or their medullary collaterals) do not participate in the facilitation of spinal preganglionic neurons during inspiration and in relaying the pulmonary stretch receptor inhibitory sympathetic reflex. A region located close to the obex was identified from which excitatory responses in renal sympathetic nerves, compatible with the response obtained by carotid sinus nerve stimulation, could be evoked. It is concluded that a lesion in the midline of the lower medulla at the level of the obex selectively destroys cells or pathways which relay the carotid body chemoreceptor-sympathetic reflex.