Reflex organization of cat masticatory muscles

On the pharmacology of ascending, decending and recurrent postsynaptic inhibition of the cuneo-thalamic relay cells in the cat

1. In cats decerebrated or anaesthetized with pentobarbitone, cells of the middle third of the cuneate nucleus that were excited by tactile stimulation of the ipsilateral forelimb (responding to displacement of hairs, skin or joints) and inhibited by electrical stimulation of the contralateral pyramid, were invariably inhibited by electrical stimulation of the ipsilateral forepaw and the contralateral forelimb nerves.2. In 50% of the cats, the cells were more fully identified by placing electrodes stereotaxically in the contralateral medial lemniscus. Recurrent inhibition was always a concomitant of the antidromic action potential.3. The intensity and the duration of inhibition evoked by all of these pathways was totally resistant to iontophoretic and intravenous strychnine in doses at least 5 times that required to block completely the response of the same cells to iontophoretic glycine and was extremely sensitive to either iontophoretic bicuculline or picrotoxin.4. Although the inhibition was invariably sensitive to intravenous picrotoxin, no significant change occurred in the duration or intensity of the inhibition when bicuculline was administered intravenously (5 or 6 times) as repeated doses of 0.2 mg/kg.5. Postsynaptic inhibition in the cuneate may be mediated by gamma-aminobutyric acid released from the nerve terminals of a common pool of interneurones shared by ascending, descending and recurrent pathways. Since the receptors involved in this pathway are resistant to intravenous bicuculline, they may well be distinct from those responsible for changes in the primary afferent terminal excitability, usually believed to be associated with presynaptic inhibition.

Effect of voluntary contraction of the masseter and other muscles upon the masseteric reflex in man

Experiments were carried out on seven adult subjects in order to establish the relationship between the magnitude of the masseteric reflex and the amount of voluntary activity present in the muscle at the time the reflex was evoked. At the same time, an effort was made to determine whether the magnitude of the reflex could be enhanced by the simultaneous voluntary contraction of muscles other than that being tested (the Jendrassik manoeuvre). The reflex was evoked by applying controlled downward thrusts to the mandible so as to produce a constant displacement in each case, and the response of the masseter muscle was recorded by means of small bipolar surface electrodes attached to the skin over the muscle. These responses were averaged by a computer in the presence of various static loads supported by the mandible. It was found that in all subjects the amplitude of the masseteric reflex appeared to increase as the weight supported by the mandible increased, and that in the majority of subjects it was possible to demonstrate Jendrassik facilitation by simultaneous contraction of the muscles of the upper limbs. The results of the experiments suggest that the enhancement of the masseteric reflex by voluntary contraction of the jaw-closing muscles may be due to autogenetic factors, synergistic factors, or both, and that at least two processes contribute to the amplitude of the masseteric reflex evoked by tooth contact during mastication-namely, the stimulation of muscle spindles by the impact of opposing teeth and facilitation caused by voluntary activity in the jaw-closing muscles before tooth contact.

Evidence for central timing of rhythmical mastication

1. The origin of the co-ordination and rhythm of mastication provoked by electrical stimulation of the putamen and corticobulbar pathways was studied in the rabbit.2. Bursts of activity were recorded from the mandibular and hypoglossal nerves and the hypoglossal nucleus, which were in phase with the observed masticatory movements. Discharges occurred alternately in nerves to jaw-opening and jaw-closing muscles.3. The rate of burst discharges was not altered by paralysis, or by large variations in the stimulation frequency.4. Regularly recurring bursts of activity continued to occur in the hypoglossal nucleus in response to random frequency stimulation after severing branchial nerves, cervical nerves and the spinal cord of paralysed rabbits.5. Mechanical deformation of the brain of vascular or respiratory origin was discounted as the origin of the rhythm.6. It is concluded that mastication is controlled by a brain-stem pattern generator which can be activated by adequate inputs from certain higher centres and, as concluded in other studies, from the oral cavity itself.

Spatial synaptic distribution of recurrent and group Ia inhibitory systems in cat spinal motoneurones

1. The reversal potentials of several types of inhibitory post-synaptic potentials (IPSPs) have been studied in cat spinal motoneurones with and without modification of intracellular chloride ion (Cl(-)) concentration. Single barrel intracellular micropipette electrodes have been used.2. When studied with potassium citrate filled micropipettes, the reversal potential for IPSPs evoked by stimulation of antagonist group Ia afferents is the same as that for recurrent IPSPs evoked by antidromic stimulation of motoneurone axon collaterals, confirming earlier observations (Araki, Ito & Oscarsson, 1961; Coombs, Eccles & Fatt, 1955).3. Studied with potassium chloride filled micropipettes. the reversal potential for the group Ia IPSP was found to be different from that for the recurrent IPSP when the amount of Cl(-) diffusing or iontophoretically injected into the motoneurone was small. The amount of difference in reversal potential varied from cell to cell but when present the group Ia IPSP reversed to a depolarizing potential more readily than the recurrent IPSP in all cases.4. Interaction between recurrent IPSPs and monosynaptic excitatory post-synaptic potentials (EPSPs) was also studied and the amount of non-linearity of potential summation was measured.5. The results are consistent with the hypothesis that the terminations of Renshaw cells responsible for the recurrent IPSP are located largely on the proximal dendrites of motoneurones, while the terminations of the interneurones generating the group Ia IPSP appear to be closer to or on the cell somata.