1. The effect of electrical stimulation of the brain stem on reflex transmission has been investigated in decerebrate cats after partial transection of the spinal cord.2. Brain stem stimuli that do not evoke inhibitory post-synaptic potentials (IPSPs) in motoneurones or primary afferent depolarization may still effectively depress the excitatory and inhibitory synaptic actions evoked from the flexor reflex afferents (FRA) and from Ib afferents. There is no effect on post-synaptic potentials from Ia afferents or on Renshaw IPSPs. The depression is not associated with any measurable change in conductance over the motoneuronal membrane.3. There is also inhibition from the brain stem of transmission from the FRA (but not from Ia and Ib afferents) to primary afferent terminals and to ascending spinal pathways.4. It is concluded that this inhibition from the brain stem is exerted at an interneuronal level in spinal reflex paths.5. The inhibitory action is evoked from the region of Magoun's inhibitory centres in the brain stem and is mediated by axons with a conduction velocity of at least 20 m/sec. The axons are distributed in the dorsal part of the lateral funicle.6. The pathway mediating the inhibition from the brain stem is named the dorsal reticulospinal system. Its possible role in maintaining the decerebrate control of reflexes is discussed and related to the problem of a selective control of some paths from a primary afferent system.

Animals↗

PubMed1968

Reticulospinal inhibition of interneurones.

1. The effect of electrical stimulation of the brain stem on interneurones in the dorsal horn and intermediary region has been investigated in decerebrate cats after partial transection of the spinal cord.2. Stimuli that effectively depress reflex transmission without giving a primary afferent depolarization inhibit the discharge evoked from the flexor reflex afferents in interneurones.3. Brain stem stimulation did not give post-synaptic potentials in the great majority of interneurones but effectively depressed the excitatory post-synaptic potentials (EPSPs) and inhibitory post-synaptic potentials (IPSPs) evoked from the flexor reflex afferents in these interneurones.4. IPSPs were, however, evoked in five of seventy-eight intracellularly recorded interneurones. These five interneurones were monosynaptically activated from primary afferents.5. It is tentatively postulated that a dorsal reticulospinal system inhibits reflex transmission by giving post-synaptic inhibition in first order interneurones. The results are also discussed in relation to effects on interneurones from other descending pathways.

Animals↗

PubMed1967

Fusimotor stimulation and the dynamic sensitivity of the secondary ending of the muscle spindle.

The response of secondary endings of de-efferented muscle spindles in the soleus muscle of the anaesthetized cat was recorded during stretching of the muscle at velocities of 5-30 mm/sec. Stimulation of single fusimotor fibres, most and probably all of which were static fusimotor fibres, produced no appreciable change in the normally small dynamic sensitivity of these endings.

Animals↗

PubMed1967

The relative sensitivity to vibration of muscle receptors of the cat.

1. Longitudinal vibration was applied to the de-efferented soleus muscle of anaesthetized cats while recording the discharge of single afferent fibres from the proprioceptors within the muscle. Conditions were defined under which vibration can be used to excite selectively the primary endings of muscle spindles without exciting the secondary endings of muscle spindles or Golgi tendon organs.2. Frequencies of vibration of 100-500 c/s were used. The maximum amplitude of vibration which the vibrator could produce fell with increasing frequency; it was 250 mu (peak to peak) for 100 c/s and 20 mu for 500 c/s.3. Primary endings of muscle spindles were very sensitive to vibration. Most could be ;driven' to discharge one impulse for each cycle of vibration over the whole of the above range of frequencies, provided the initial tension was moderate (20-200 g wt.). The amplitude of vibration required to produce driving usually varied by less than a factor of two over the whole range of frequencies. The most sensitive endings could be driven by vibrations of below 10 mu amplitude.4. Stimulation of single fusimotor fibres, whether static or dynamic fusimotor fibres, increased the sensitivity of primary endings to vibration. Contraction of the main muscle, produced by stimulating alpha motor fibres, reduced the sensitivity of primary endings even when fusimotor fibres were also being stimulated.5. The secondary endings were very insensitive to longitudinal vibration and with the amplitudes available not one of twenty-five endings could be driven at 150 c/s or above; one ending could be driven at 100 c/s by vibration of 250 mu amplitude. Stimulation of single fusimotor fibres, probably all of which were static fusimotor fibres, made them slightly more sensitive to vibration but none of them approached the sensitivity of the primary endings.6. The Golgi tendon organs were as insensitive as the secondary endings when the muscle was not contracting and none could be driven at any frequency in spite of quite high tensions in the muscle. However, when the muscle was made to contract by stimulating alpha fibres in ventral root filaments the tendon organs became appreciably more sensitive, the degree of sensitization increasing approximately with the strength of the contraction. They never became as sensitive as the primary endings, and with the amplitudes of vibration available none was driven at frequencies of over 250 c/s.7. When the amplitude of vibration was somewhat below that required to produce driving of an ending it still produced some increase in its mean frequency of discharge. However, amplitudes of vibration of 25-50 mu applied to a non-contracting muscle, whether with or without fusimotor stimulation, produced driving of nearly all primary endings without any significant increase in the mean frequency of firing of secondary endings or Golgi tendon organs. Such vibration can therefore be used as a specific stimulus for the primary endings in order to investigate the central effects or repetitive discharge of the Ia afferent fibres from them.8. Experiments on endings in the peroneus longus muscle showed that these behaved similarly to those in soleus.

Animals↗

PubMed1966

The inhibitory action of noradrenaline and other monoamines on spinal neurones.

1. L-Noradrenaline (NA), 5-hydroxytryptamine (5-HT) and acetylcholine (ACh) were administered micro-electrophoretically to feline lumbar neurones while recording their spike potentials extracellularly.2. There was no evidence to suggest that NA acts as an excitatory transmitter in the spinal cord.3. NA had potent inhibitory effects on some interneurones as revealed by a depression of spontaneous and synaptic firing and on the firing to a local application of an excitant amino acid. The effects on Renshaw cells and motoneurones were less marked.4. The depressant actions of 5-HT were less marked than those of NA. ACh and carbamylcholine had depressant effects on some NA-sensitive interneurones but were invariably far less potent and on other NA-sensitive cells were completely inactive.5. NA had no detectable effect on the normal spike amplitude but when the action potentials were reduced by excessive depolarization then both NA and synaptic inhibition increased the spike amplitude; this effect could be due to a hyperpolarization of the cell membrane.6. There was a correlation between the distribution of NA-sensitive cells and the relative densities of NA-containing terminals in various layers of the grey matter.7. It was postulated that NA acts as an inhibitory transmitter released from the terminals of descending pathways in the spinal cord. Other possible mechanisms were discussed but lacked experimental support.

Journal Article↗

PubMed1966