Spinal monoaminergic systems: an aspect of somatic motor function

A synopsis is made of the kinds of monoaminergic terminals that are found in the ventral horn of the spinal cord, and of the physiological effects these monoamines exert on interneurons (INs), alpha motoneurons (alpha-MNs), and gamma motoneurons (gamma-MNs). It is suggested that in addition to the usual transmitter mechanism involving the activation of fast ionic conductances, monoamines in the ventral horn may function as neuromodulators and as part of the neuronal control for a limb. Experiments are then described in which it is shown that the monoamines can exert selective effects on INs, alpha-MNs, and gamma-MNs. The activation of gamma-MNs in the spinal cord partly depleted of 5-hydroxytryptamine (5-HT) is a striking example of that selectivity. The neurochemical basis of the physiological effects of monoamines on the cord is briefly reviewed. It is suggested that after the administration of dihydroxyphenylalanine (DOPA), all three major monoamines--dopamine (DA), norepinephrine (NE), and 5-HT--could be involved in mediating the physiological effects of DOPA. The evidence for the presence of dopaminergic nerves in the cord is briefly described, and their possible physiological significance is discussed.