Neuroanatomy

Here the emphasis is different. We look at a motor neuron as a part of the motor subsystem of the nervous system. From the number of axon terminals on its receptive surface (around 50,000, each signifying a synapse), we see that this large cell, final in the chain of communication from sensory periphery to muscles, has a vast range of inputs. This includes PNS primary afferents, spinal and brain stem connections, feedback and feedforward circuits from other motor neurons, self-directed inputs from recurrent axon collaterals of its own and neighboring local interneurons, and direct, express lines from the cerebral cortex 1 m upstream from this motor neuron. We recall that each input releases a chemical messenger eliciting excitation, inhibition, or modulation of the target cell, depending on the nature of receptor molecules and second messenger systems in it and other complex factors. We know that each input has a specified place on the soma, dendrites, or axonal initial segment of the motor neuron and that this orderly display favors input identification and efficacy in control of this harried cell.

Moving peripherally, e.g., in the sciatic nerve, we find that the axon terminals in the muscular system end on muscle fibers in the gastrocnemius. With methods for tracing nerve fibers, we find that sensory axons supplying stretch receptors (muscle spindles) in this muscle lead back into the CNS over the dorsal roots of spinal nerves to synapse on our motor neuron and all its homonymous neighbors, thereby closing the external arc of a spinal neuromuscular control loop: the monosynaptic stretch reflex. We now see the place and role of the motor neuron in the human nervous system. Its inputs include large, well-myelinated, fast-conducting axons that excite it monosynaptically when the gastrocnemius is stretched. Its output to gastrocne-mius fibers enable it, with the help of other motor neurons, to maintain our erect posture.

Our virtual excursion teaches us that neuroanatomy is less interested in cytological detail and more interested in the identity, interconnections, and interactions of neurons, in ''wiring diagrams,'' in chemical colorations, and in the functional roles of neurons in networks.

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