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Sunday, August 2, 2009

When i is silent, stimulation of e causes excitation of s But where i is stimulated, i releases GABA(see above,

. Now the efferent nerve fiber is stimulated and the muscle Mp contracts. AF also gives collateral ac, which synapses with a different nerve cell soma. The axon of this soma is the motor neuron to the antagonist muscle Ma, When the AF is stimulated, the collateral is also stimulated but the result of the collateral stimulation is development of IPSP at the termination of the collateral, (due to liberation of proper chemical transmitter at this synapse) so that the nerve fiber supplying the antagonistic muscle is inhibited (so that the antagonist muscle Ma is relaxed). The phenomenon is also called, 'reciprocal inervation' (of Sherrington). The teleologic principle is obvious. When a group of muscles, say, the flexors of the elbow contract the opposing (antagonist j muscles, (extensars of the elbow in this example), must relax to ensure flexion. Renshaw cell inhibition From the big sized anterior horn cells of the spinal cord, emerge Aa motoneurons which end in the skeletal muscles. Now, upper motor neuron or cortico spinal (pyramidal) Tract fibers impinge on these Aa moloneurons. Therefore, when the corticospinal tract fires. Aa motoneurons are stimulated Fig. 1QA.3.6 Collaterals from the Aa motoneurons emerge and impinge upon cells, called Renshaw cells (fig 10A.3.7) When the Aa fibers are stimulated, the Renshaw cells, therefore, are also stimulated. The axon of the Renshaw cell now inhibit the nerve cell soma of the Aa neurons. This phenomenon is called Renshaw cell inhibition (after Renshaw, who discovered it in 1946).

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