Integration in descending motor pathways controlling the forelimb in the cat. 13. Corticospinal effects in shoulder, elbow, wrist, and digit motoneurones.

The effect of corticospinal volleys evoked by stimulation of the contralateral pyramid was investigated using intracellular recordings from alpha-motoneurones to forelimb muscles. Confirming and extending previous observations (Illert et al. 1977, Illert and Wiedemann 1984), short latency EPSPs within a disynaptic range were evoked by a train of pyramidal volleys in all varieties of shoulder, elbow, wrist and digit motoneurones. The amplitude of pyramidal EPSPs was sensitive to the stimulus repetition rate. Maximal amplitudes were observed around 2-4 Hz, while at 10 Hz the early EPSP was markedly reduced and the long latency EPSP abolished. The persistence of disynaptic EPSPs after a corticospinal transection in C5/C6 suggested that, for all types of forelimb motor nuclei, disynaptic EPSPs are relayed by C3-C4 propiospinal neurones (PNs) (c.f. Illert et al. 1977). The transection, however, caused a clear reduction in the EPSP of all motoneurone types. After a ventral lesion of the lateral funicle in C5/C6 interrupting the axons of the C3-C4 PNs, disynaptic (and possibly trisynaptic) EPSPs were evoked by a short train of pyramidal volleys. It is postulated that intercalated neurones in a disynaptic cortico-motoneuronal pathway also exist in the forelimb segments. Disynaptic pyramidal IPSPs were observed in most types of forelimb motor nuclei both before and after a corticospinal transection in C5/C6. At all joints, pyramidal excitation dominated in motoneurones to physiological flexors, while in extensor motoneurones mixed excitation and inhibition or dominant inhibition was common. Comparison of pyramidal effects in slow motoneurones (classified according to the after-hyperpolarization duration) to the long head of the triceps and anconeus revealed dominant excitation in the former and inhibition in the latter. It is suggested that the slow motor units in these muscles differ in their function although both muscles are elbow extensors.