Inhibition in motoneurons

If the contraction of a particular limb muscle is to be effective in producing movement, it is essential that the muscles which oppose this action (the antagonists) should be relaxed. In the monosynaptic stretch reflex this is brought about by inhibition of the motoneurons of the antagonistic muscles. Fig. 8.4 shows the arrangement of the neurons involved. We have seen that group Ia fibres from the stretch receptors in a particular muscle synapse with motoneurons innervating that muscle. They also synapse with small interneurons which themselves innervate the motorneurons of antagonistic muscles. It is these interneurons which exert the inhibitory action on the motoneurons.

This inhibitory action can be examined by inserting a micro-electrode into a motoneuron and stimulating the group Ia fibres from an antagonistic muscle. Fig. 8.5 shows the results of such an experiment. The responses consist of small hyperpolarizing potentials known as inhibitory postsynaptic potentials, or IPSPs.

Fig. 8.4. The direct inhibitory pathway. The diagram is much simplified in that there are many afferent, inhibitory and efferent neurons at each stage; each inhibitory interneuron is innervated by several afferents, and itself innervates several motoneurons.

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Fig. 8.4. The direct inhibitory pathway. The diagram is much simplified in that there are many afferent, inhibitory and efferent neurons at each stage; each inhibitory interneuron is innervated by several afferents, and itself innervates several motoneurons.

Fig. 8.5. Inhibitory postsynaptic potentials (IPSPs) in a cat spinal motoneuron, produced by stimulating the group Ia fibres from the antagonistic muscle. The stimulus intensity was higher for the lower trace than for the upper, so that more group Ia fibres were excited. From Coombs, Eccles and Fatt (1955b).

The form of the IPSP is very similar to that of the EPSP, apart from the fact that it is normally hyperpolarizing. Displacement of the motoneuron membrane potential produces more or less linear changes in the size of the IPSP, with a reversal potential at about —80 mV. This is near to the equilibrium potentials of both chloride and potassium ions. Injection of chloride ions into the soma causes an immediate reduction in the reversal potential, so that the IPSP becomes a depolarizing response at the normal membrane potential. This suggests very strongly that an increase in the chloride ion conductance of the postsynaptic membrane is involved in the production of the

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IPSP.

We can conclude that the IPSP in a spinal motoneuron is produced in a way very similar to that of the EPSP and the end-plate potential. An action potential arriving at the presynaptic terminal causes release of the transmitter substance (glycine in this case) into the synaptic cleft. The glycine combines with glycine receptors whose intrinsic ion channels then open to allow chloride ions to flow into the postsynaptic cell, so producing the IPSP.

In the brain most of the inhibitory responses are produced not by glycine but by gamma-aminobutyric acid (GABA). GABA receptors are of two types: the GABAa receptors have intrinsic ion channels, whereas the GABAb receptors do not. The structure and properties of the GABAA receptors are very similar to those of glycine receptors.

The subunits of the GABAA and glycine receptors have very similar sequences, and show some identity with those of the nicotinic acetylcholine receptor; the three receptors form a gene family with, we presume, a common evolutionary origin. It is likely that each receptor consists of five subunits surrounding a central pore, just as in the nicotinic acetylcholine receptor.

IPSPs show spatial and temporal summation just as EPSPs do.

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