Synaptic excitation in motoneurons

Motoneurons are the nerve cells which directly innervate skeletal muscle fibres. Their cell bodies lie in the ventral horn of the spinal cord, and their axons pass out to the peripheral nerves via the ventral roots. The cell body, or soma, is about 70 fxm across, and extends into a number of fine branching processes, the dendrites, which may be up to 1 mm long. The surface of the soma and dendrites is covered with small presynaptic nerve terminals, and these regions of contact show the typical features of chemically transmitting synapses: a synaptic cleft and synaptic vesicles in the presynaptic cell.

Fig. 8.2. Anatomical organization of the monosynaptic stretch reflex system (a). This diagram is much simplified: there are in fact very many stretch receptors and afferent and efferent neurons associated with each muscle. Diagram (b) indicates how the afferent fibres branch to synapse with different members of the motoneuronal pool.

Fig. 8.2. Anatomical organization of the monosynaptic stretch reflex system (a). This diagram is much simplified: there are in fact very many stretch receptors and afferent and efferent neurons associated with each muscle. Diagram (b) indicates how the afferent fibres branch to synapse with different members of the motoneuronal pool.

Intracellular recording shows that motoneurons have a resting potential of about —70 mV. Depolarization of the membrane by about 10 mV results in the production of an action potential which propagates along the axon to the nerve terminals. Experiments involving the injection of various ions into motoneurons indicate that the ionic basis of their resting and action potentials is much the same as in squid axons. That is to say, the resting potential is slightly less than the potassium equilibrium potential, the action potential is caused primarily by a regenerative increase in sodium permeability, and the ionic gradients necessary for these potentials are dependent upon an active extrusion of sodium ions.

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