This chapter discusses the transmitter metabolism of the spinal motoneuron, representing a clear-cut cholinergic neuron. It aims to locate both synthesizing and inactivating systems of acetylcholin (ACh) within various parts of this particular neuron. Synaptic transmission is thought to be mediated by a jet-like extrusion of ACh from the presynaptic terminal, ACh being bound to receptor moieties on the postsynaptic membrane (according to the classical theory of neurochemical transmission); alternatively, ACh may exert two subsequent dual intracellular actions, first in the presynaptic and then in the postsynaptic cell, without ever traversing the synaptic cleft. Under the electron microscope, acetylcholinesterase (AChE) reaction in the motoneurons is located within the lumina of the granular endoplasmic reticulum, concentrated in parallel arrays making up Nissl bodies. Mitochondria within Nissl granules, or the lamellae of the Golgi system located as a rule among Nissl granules, are devoid of any enzyme reaction. Accordingly, it appears that both AChE and choline acetylase (ChA) are transported from the perikaryon via the axon to the terminal, supposing that sites of choline transport (actually shown by the autoradiographic localization of 14CHC-3) are closely related to those of ACh synthesis. Enzymes can be located by autoradiographically demonstrating their specific isotope-labelled inhibitors injected in advance into the animals in vivo.
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