Motor neurons are large, reflecting the length of their axons. Each controls a scattered group of individual muscle fibres called a motor unit: a unit may comprise as few as a dozen fibres in some of the muscles of the middle ear and in eye muscles, or as many as 1500 in large and crude muscles such as gluteus maximus. In the cat’s leg, a single unit is capable of lifting some 10 g. Force is controlled partly by changes in the firing frequency of individual motor neurons, and partly by the process of recruitment, discussed in Chapter 2 (p. 38), in which more and more units are brought into play as the required muscle tension increases. Normally it is the smallest neurons, innervating the most ‘tonic’ and least fatigable muscle fibres, that are the first to be recruited. Although individual motor neurons may sometimes fire at very low frequencies, this does not normally cause discrete twitching of the muscles because different units fire out of synchrony with one another. One of the functions of the Renshaw cells, mentioned in Chapter 3, is probably to provide a kind of lateral inhibition between motor neurons that discourages synchronization of this kind.

Motor neurons have an orderly and systematic arrangement within the cord, in clumps not quite well enough defined to be called nuclei, which reflects the topology of the muscles they serve. Medial neurons innervate the muscles of the trunk, the most distal parts of limbs are governed by the most lateral neurons, and flexors and extensors tend to be under the control of the more dorsal and ventral groups, respectively. Because these cells represent the ultimate funnel through which all nervous excitation must pass whenever a motor act is made, whatever its source, together they form what is sometimes called the final common path. On these cells terminate all the afferents from interneurons, and in some cases receptors, that serve the various spinal reflexes and responses, as well as some of the descending paths from higher levels. The quantity of information that thus converges on a single ventral horn cell is enormous, and is reflected in the huge size of its dendritic tree, which may often extend over a large part of the grey matter of the cord.

Sensory afferents entering the dorsal roots can give rise to spinal reflexes, by projecting – either directly or via interneurons – on to motor neurons (p. 55). It is never easy to define exactly what is meant by a reflex. Any response
that can be elicited from a spinal animal is certainly a spinal reflex. But we have already seen that although a response may be essentially a spinal one, in the sense that its neural circuitry lies entirely within the spinal cord, yet one may not be able to elicit it in the spinal preparation because the usual facilitating, permissive, influences descending from the brain are absent. It is not quite enough to describe a reflex as an automatic, reproducible, response that is independent of the will, as one may readily influence one’s own spinal reflexes by willed inhibition or facilitation from above. For example, there is a mechanism in the cord called the withdrawal or flexion reflex that causes a rapid flexion when the skin is touched by a hot object or other noxious stimulus: yet we all know from personal experience that in cases of necessity – when carrying a plate that proves to be hotter than we thought when we picked it up – we can (up to a point) inhibit the reflex completely. We shall see later on that what at first sight appears the simplest and most automatic spinal reflex of all, the monosynaptic stretch reflex, is actually under almost total control from other, mainly descending, influences. In fact when we look at the mode of termination of the tracts descending from the brain, we find that the great majority of them end not on the motor neurons themselves, but rather on the interneurons that form part of these reflex arcs. Descending control is not so much of muscles as of actions, amounting to a selection from the cord’s repertoire: the brain plays on the spinal cord not as one plays a piano, but rather as one selects a disc from a juke box.

Five important tracts descend from brain to spinal cord; four of these come from closely neighbouring parts of the brain, in the brainstem and medulla. These are the reticular formation, the vestibular nuclei, the red nucleus, and the tectum; the fifth source of descending fibres is the cerebral cortex.