Pattern of segmental monosynaptic input to cat dorsal neck motoneurons.

Abstract

Monosynaptic input from muscle nerves to mononeurons of splenius and biventer cervicis, muscles that are subdivided by tendinous inscriptions and innervated by 3-4 spinal segments, was studied. The organization of the motor innervation in splenius was studied. Recording EMG [electromyogram] with a roving electrode while stimulating the distal end of cut muscle nerves shows that the motor innervation of splenius is compartmentalized. Each segmental nerve innervates a separate area of muscle mainly corresponding to zones between tendinous inscriptions. Intracellular recording from splenius and biventer motoneurons shows that monosynaptic excitatory postsynaptic potentials (EPSP) are produced by stimulation of the muscle nerve containing the motoneuron''s axon (homonymous nerve) and often by stimulation of more caudal nerves to the same muscle. Frequency of EPSP occurrence and average EPSP amplitude decrease as nerves in more distant segments are stimulated. Stimulation of nerves rostral to the motoneurons is much less effective. This rostrocaudal asymmetry is more pronounced in splenius than in biventer motoneurons. Monosynaptic EPSP evoked by stimulation of the homonymous and other nerves have thresholds at or near muscle nerve threshold. They are presumably due mainly to stimulation of Ia-afferents from spindle primary receptors. Since motoneurons often receive monosynaptic input from several muscle nerves innervating different compartments, afferent input from spindle primary receptors is not strictly compartmentalized. Changes seen as successively more distant nerves are stimulated emphasize a role for topography in determining monosynaptic connectivity.