Stimulation-induced setting of postural muscle tone in the decerebrate rat

These studies demonstrate the presence of pontomedullary areas in the rat brainstem which, when stimulated electrically, serve to set postural muscle tone in the hindlimbs. Low amplitude stimulation of the dorsal tegmental field (DTF) was found to inhibit postural muscle tone and, in some rats, was found to decrease mean arterial pressure. Low amplitude stimulation of the ventral tegmental field (VTF) was found to increase postural muscle tone and, in all cases tested, was found to increase mean arterial pressure.     

Discharges of neurons in the midpontine dorsal tegmentum of mesencephalic cat during locomotion

Discharges of neurons in the midpontine dorsal tegmental field (DTF neurons) were recorded and analyzed during locomotion and were compared with those of reticulospinal neurons (RS neurons) located lateral to the DTF area. The conduction velocity of the descending axon of the DTF neurons was significantly smaller than that of the RS neurons. During locomotion, the DTF neurons showed a tonic increase in the discharge rate. In contrast, the discharge rate of the RS neurons showed cyclic modulation in step with locomotion.     

Depression of diaphragmatic and external intercostal muscle activities elicited by stimulation of midpontine dorsal tegmentum in decerebrate cats

This paper describes the inhibitory influences on external intercostal muscle activity and diaphragmatic activity, evoked by stimulation of the dorsal tegmental field (DTF) of the pons in decerebrate cats. Stimulation of the DTF along the midline decreased both the diaphragmatic and the external intercostal activities. However, the inhibitory influences on the above two kinds of respiratory muscle activities were different in nature. Diaphragmatic activity, once suppressed by stimulation, recovered in spite of the continuation of stimulation. In contrast, DTF stimulation depressed tonic discharges of external intercostal muscle, and the depressed or abolished tonic discharges did not resume even after termination of stimulation. Rhythmic external intercostal muscle activity, synchronized with diaphragmatic activity, was also suppressed by DTF stimulation and the suppressed rhythmic activity seemed difficult to recover during stimulation, compared with the recovery process of the diaphragmatic activity.     

Locomotion elicited by pinna stimulation in the acute precollicular-post- mammillary decerebrate cat

In the acute precollicular-postmammillary decerebrate cat, pinna stimulation was found to elicit four-legged locomotion on the treadmill. The adequate stimuli for eliciting locomotion were of the nature of deep pressure with the fingertips. The receptive field of the pinna were distributed along the edge of the scapha. When the existing postural tonus after decerebration were highly exaggerated or depressed, the same stimulation failed to elicit locomotion.     

Units in tegmental nuclei responding to stimulation of gastric vagal and greater splanchnic fibers in the cat

The response of neurons in the ventral and dorsal tegmental nuclei during electrical stimulation of the gastric vagal fibers which serve the proximal stomach and the left greater splanchnic fibers were evaluated in chloralose-anesthetized cats. The mean latency of 181 gastric vagally evoked unitary responses recorded in the tegmental nuclei was 352.2 ms, whereas the latency of the left greater splanchnic-evoked tegmental response was significantly less (63.2 ms). The unitary responses to the gastric vagal and greater splanchnic fibers stimulation were bilaterally distributed in the ventral and dorsal tegmental nuclei. Convergence of the gastric vagal input from the proximal stomach and the left greater splanchnic input was observed in 151 units (83 percent). Stimulation of the greater splanchnic nerve usually resulted in a short latency excitation followed by an inhibitory effect on gastric vagally evoked responses. The results suggested that some convergent splanchnic inhibition of gastric vagally evoked responses was mediated via an interneuron. Projections from the nucleus tractus solitarius and the parabrachial nucleus to the tegmental nuclei were also identified electrophysiologically by direct microstimulation of the two former areas. The significant number of gastric vagal and splanchnic evoked unitary responses recorded in the ventral and dorsal tegmental nuclei suggested that they may serve as an important pontine site for processing of visceral information between the nucleus tractus solitarius and forebrain sites.     

Descending inhibitory pathway responsible for simultaneous suppression of postural tone and respiration in decerebrate cats

The present study was performed in order to elucidate whether the suppressive effects on postural tone and respiration evoked by stimulation of the dorsal tegmental field (DTF) of the pons are relayed through the neurons in the ponto-medullary reticular formation. First, the DTF was functionally identified, and then a microelectrode was inserted into the caudal pontine and medullary reticular formation to investigate the distribution of the neurons monosynaptically activated by stimulation of the DTF. The monosynaptically activated neurons were distributed within the nucleus reticularis gigantocellularis (NRGc) in the caudal pons and medulla. The spinal cord (L1) was stimulated to study whether such monosynaptically activated neurons project to the spinal cord. Almost all the neurons monosynaptically activated by DTF stimulation were antidromically activated by spinal stimulation. This result indicates that the neurons in the NRGc activated monosynaptically by DTF stimulation send axons to the spinal cord. Tonic micro-stimulation was then delivered to the site in the NRGc, from which the monosynaptically driven units were recorded by DTF stimulation. The stimulation resulted in the parallel suppression of postural tone and respiration, similar to the suppressive effects elicited by DTF stimulation. The present study suggests the possible existence of a descending inhibitory pathway through the reticulospinal neurons in the NRGc responsible for parallel suppression of postural tone and respiration.     

Muscle tone suppression and stepping produced by stimulation of midbrain and rostral pontine reticular formation

Stimulation of the midbrain retrorubral (RRN), ventral paralemniscal tegmental field (vFTP), reticular tegmental (TRN), and pedunculopontine tegmental (PPN) nuclei was found to produce bilateral suppression of muscle tone in the unanesthetized, decerebrate animal. The RRN is the most rostral area shown to produce such suppression. This muscle tone suppression was frequency- and intensity-dependent. At low stimulus intensities, bilateral suppression was produced at these sites. At higher current and frequency levels, 2 types of muscle responses were found, excitation in PPN and RRN and initial suppression followed by excitation in TRN and vFTP. The mean latency to muscle tone suppression was not significantly different in TRN (36.8 msec) and RRN (36.5 msec). However, muscle tone suppression latency was significantly shorter in vFTP (31 msec) and PPN (27.1 msec). In addition to muscle tone suppression, stepping-like activity could be elicited at the same points by consecutive train stimulations in PPN and single train stimulation in TRN and vFTP. Thus, systems producing atonia are colocalized with those producing locomotion. We hypothesize that the midbrain atonia regions control more caudal regions producing muscle tone suppression in REM sleep, and that the locomotor and atonia eliciting regions are normally coactivated during REM sleep.     

Morphine suppression of bradycardia induced by electrical stimulation of gigantocellular reticular nucleus in the cat

In chloralose-urethane-anesthetized cats, intravertebral injection of morphine (1,2 and 4 mg/kg) promoted a drastic suppression of the bradycardia elicited by stimulating the medullary gigantocelular reticular nucleus (GRN). The degree of blockadge of GRN-induced cardioinhibition was directly dependent upon the dose of morphine and inversely related to the reticular stimulus train intensity and pulse frequency. The possibility that the GRM may be a site of action for morphine in its production of hypotension and bradycardia was discussed.     

The effect of pyramidal stimulation upon tail muscle motoneurons in the decerebrate cat

This study aimed to determine the effects of the corticospinal tract (CST) on the motoneurons innervating the tail muscles in cats. The stimulation of the pyramidal tract predominantly evoked excitatory postsynaptic potentials (EPSPs; 48/90 motoneurons: 53%). Single-pulse stimulation produced EPSPs in 18 of 48 motoneurons, but double shocks evoked postsynaptic potentials in most of the remaining cells (26/48). Monosynaptic excitatory connections between pyramidal tract fibers and tail motoneurons were confirmed in 4 motoneurons. Inhibitory postsynaptic potentials (IPSPs) were recorded from motoneurons innervating long tendinous tail muscles (7/90: 8%) and the shortest neuronal pathways of IPSPs were shown to be disynaptic pathways. Interactions between the CST and reflex pathways from low-threshold muscle and cutaneous afferents innervating the tail and hindlimbs were observed.     

Natural cutaneous stimulation induces late and long-lasting facilitation of extensor motoneurons in the cat

An investigation was made of the effects of physiological cutaneous stimulation on the excitability of extensor motoneurons in spinal unanesthetized cats. The time course of changes in the monosynaptic reflex (MSR) amplitude of the soleus (Sol) and gastrocnemius medialis (GM) and lateralis (GL) was studied after conditioning stimulation with air jets (delivered to different regions of the skin of the ipsilateral hind limb), pinpricks, or stretching of the skin of the heel induced by passive rotation of the tibio-tarsal joint. Low-intensity electrical stimulation of the sural or saphenous nerves was also employed in order to condition the MSRs of the triceps surae muscles. Hair bending, skin indentation or stretching, as well as electrical nerve stimulation, can induce a similar biphasic excitability cycle of the extensor MSRs, characterized by an early inhibition followed by a late facilitatory period (LFP). The LFP started approximately 20 ms after the arrival of the cutaneous afferent volley, and lasted about 80 ms. Conditioned MSRs could attain values corresponding to 200% or more of controls. The receptive field of the LFP evoked by the air jet proved to be as large as the whole leg and foot skin surface. No significant differences were found in the extent of the late facilitation in the MSRs of Sol, GM and GL, conditioned by electrical stimulation. The LFP was also present, after conditioning stimulation of the same types as above, in intact (and spinal) chloralose-anesthetized cats.     

Membrane potential and input resistance of cat spinal motoneurons in wakefulness and sleep

An approximation of the membrane resistance - input resistance - was measured with double-barrelled micropipettes in hindlimb motoneurons during wakefulness and the states of sleep in chronically implanted, restrained cats. During postural atonia in rapid-eye-movement (REM) sleep, motoneurons had a sustained hyperpolarization of 5–8 mV and a 30% decrease in input resistance. There was little difference in membrane potential or input resistance between wakefulness and the other sleep states. The results directly support the hypothesis that motoneuron deactivation in REM sleep arises from active inhibition.     

Effects of ethmoidal nerve stimulation on respiration-related neurones in the dorsal medulla of the cat

Stimulation of the nasal mucosa produces a number of respiratory reflexes the afferent limb of which is provided by the ethmoidal nerve, a branch of the trigeminal nerve. In the cat this nerve terminates within the trigeminal nucleus. It has no direct projection to brainstem respiratory centres. This study examines the response of respiratory-related neurones in the nucleus of the solitary tract (NTS) to ethmoidal stimulation. It demonstrates that these neurones show both excitatory and inhibitory responses to ethmoidal stimulation. Thus, the NTS appears to be involved in respiratory reflexes initiated by stimulation of the nasal mucosa.     

Modulation of transmission in flexion reflex pathways by knee joint afferent discharge in the decerebrate cat

In the intact hindlimb, flexion reflexes are most easily obtained when the knee is extended. Movement of the knee joint after the muscles have been tenotomized and held at fixed lengths still modulates flexion reflex excitability. After anaesthesia of the joint capsule movement of the limb no longer affects flexion reflex excitability.     

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.     

Release of heat-loss responses in paradoxical sleep by thermal loads and by pontine tegmental lesions in cats

Thermoregulatory heat-loss responses at high ambient temperatures were studied in intact cats and those with bilateral electrolytic lesions in the pontine tegmentum during wakefulness (W), slow-wave sleep (SWS), paradoxical sleep (PS) and PS without atonia induced by the lesions. Panting (respiratory rate 90/min) was present W, SWS, and in some cases, during PS. The percentage of the PS episodes with panting was directly related ambient temperature. In intact cats at 30 °C, panting occurred in 8% of the PS episodes; at 35 °C, in 52%, and at 40 °C, in 77%. The percentage of PS episodes with panting higher in the pontine-lesioned cats (90% at 35 °C), probably another indication of the altered thermoregulation of such animals. Thermoregulatory responses to heat load, and thermoregulation in general, have previously been shown to be suppressed in PS. Because hypothalamic thermosensitive neurons lack thermal responses during PS, the partial activation of heat-loss responses observed here may depend upon the function of extrahypothalamic brainstem areas.     

Monocular movement of contralateral eye following cortical stimulation in the coronal sulcus of cat

Monocular movements of the contralateral eye were demonstrated following the stimulation of the internal kneelike portion of the cortex in the coronal sulcus of the cat. The stimulation of this cortical area in the coronal sulcus yielded rapid medial or slightly oblique ventromedial movement of the contralateral eye. While ablation of the same cortical area produced contralateral eye shift toward the dorsolateral side and the shifted eye became uncontrollable. The results suggested that the control mechanism of eye movement involves unilateral innervation system to the contralateral side as seen in the extremities.     

Dorsal horn cells in the cat responding to stimulation of the plantar cushion

Dorsal horn cells (DHC) in the spinal cord of cats responding to low threshold mechanical stimulation of the plantar cushion (PC), labelled with intracellular horseradish peroxidase, displayed a wide variety of anatomical and physiological characteristics, some of which were quite consistent within a given lamina. These DHC had cell bodies located medially in spinal segment L7, in laminae III, IV and VI. Dendritic fields varied greatly, for the most part conforming to patterns corresponding to their locations in the dorsal horn. Axon collaterals of most DHC ramified near their cell bodies, chiefly projecting ventral ward within the same lamina as the cell body or into more ventral laminae. Cells with the shortest central delays had cell bodies in laminae III and IV. Response latencies of DHC to PC stimulation were only slightly greater than the latencies of primary afferent fibers. Lamina IV cells had mechanical thresholds comparable to those of the primary afferents, significantly lower than the thresholds of cells of laminae III or VI. The receptive fields (RF) of the lamina IV cells were significantly larger than those of the primary afferent fibers or of the lamina VI cells. Relationships among RF area, mediolateral locations of cell bodies and dendritic spread were investigated.     

Convergence of muscle spindle afferents on single neurons of the cat dorsal spino-cerebellar tract and their synaptic efficacy

By means of tungsten microelectrodes, action potentials from axons within the dorsal spino-cerebellar tract (DSCT) and from muscle spindle afferents were recorded. A quantitative study was performed in monomuscular DSCT neurons which were excited predominantly by Ia fibers originating in the gastrocnemius muscles. In some experiments single Ia fibers were stimulated electrically while the impulse sequence of a DSCT neuron postsynaptic to the respective afferent fiber was recorded. The gastrocnemius DSCT neurons receive excitatory inputs from 10-18 Ia muscle spindle afferents. The efficacy of each of these inputs is very similar. Thus the neuronal activation decreased approximately linearly with the number of the excitatory afferents cut. Cross-correlograms between the impulse sequence of a Ia gastrocnemius muscle spindle afferent and a DSCT neuron postsynaptic to it exhibited an increased discharge probability of the DSCT neuron from 3-4 ms to 10 ms after the Ia action potential. With increasing impulse rates of the Ia afferent fibers, the excitatory efficacy of the single action potential decreased, but the overall excitation increased with the presynaptic discharge frequency, according to a hyperbolic function. This effect was tested by electrical stimulation of a single Ia axon exciting the DSCT neuron recorded. Interval histograms computed from DSCT neuron impulse trains at steady stretch conditions were predominantly monomodal. They can be well approximated by a Gaussian distribution. The coefficient of variation was independent of the mean activity. At impulse rates above 25 imp X s-1 a negative correlation between successive intervals was observed in first order joint interval diagrams. With an increasing mean discharge rate this correlation (expressed as the serial linear correlation coefficient of the first order r1,2) became stronger up to--0.62 at 90 imp X s-1. Only in a few neurons did the higher order linear correlation coefficients deviate significantly from zero. In 15% of the observed histograms double discharging (mean interval 3-5 ms) produced bimodal distributions. Under steady-state conditions the response of Ia-activated DSCT cells are linearly related to muscle stretch within a middle range of extensions. The differences between Ia impulse pattern and DSCT neuron impulse pattern at steady stretch are discussed. The number of large dendrites of the principal cells in the nucleus dorsalis (Clarke's column) corresponds to the number of excitatory afferent muscle fibers. It is assumed that each excitatory Ia axon sends one axon collateral to the DSCT neuron, forming a climbing type terminal mainly on one of the large dendrites of a DSCT cell.