[Motor cortex control of fast and slow motoneurons innervating forelimb muscles of the cat]

Intracellular potentials of forelimb motoneurons of pyramidal cats were recorded by glass microelectrodes following repetitive stimulation of the contralateral pericruciate cortex, and patterns of the postsynaptic potentials (PSPs) evoked thereby were analysed. The motoneurons tested were identified by antidromic invasion from either of the following nerves: long (lo. T), lateral (la. T) and medial (med. T) heads of triceps, anconeus (Anc), biceps (Bi), deep radial (dR), median (M) and ulnar (U). The motoneurons were classified into fast and slow motoneurons on the basis of the duration of afterhyperpolarization (AHP) and the axonal conduction velocity. All of Anc and 65% of med. T motoneurons were classified into slow motoneurons. On the other hand, 35% of med. T, 84% of lo. T and la. T motoneurons were classified into fast motoneurons. About 70% of motoneurons innervating Bi and wrist controlling muscles (dR, M and U) were classified into fast motoneurons and the rest into slow motoneurons. Repetitive stimulation of the pericruciate cortex produced predominantly inhibitory PSPs (I-type) or mixed PSPs (M-type) in 67% of Anc and 56% of slow triceps motoneurons, while it evoked predominantly excitatory PSPs (E-type) in 97% of fast triceps motoneurons. Thus, patterns of PSPs in elbow extensor (Anc and triceps) motoneurons produced by cortical stimulation were in general excitatory to fast and inhibitory to slow motoneurons, similarly to the case in hindlimb motoneurons. In dR, M and U motoneurons, cortical stimulation produced E-type (84%) or M-type (16%) PSPs in fast motoneurons, while it evoked M-type (55%) or E-type (45%) PSPs in slow motoneurons. Patterns of PSPs in these motoneurons produced by cortical stimulation had a tendency similar to the case in triceps motoneurons; excitatory components of PSPs were predominant in fast motoneurons and inhibitory components were mainly observed in slow motoneurons. In most fast and slow Bi motoneurons, E-type or M-type PSPs were observed when the precruciate area was stimulated. With post-cruciate stimulation, however, excitatory components became smaller and inhibitory components were marked and changes in the pattern of PSPs were observed in 37% of motoneurons, as compared with precruciate stimulation. Thus, in Bi motoneurons, the qualitative difference of patterns of PSPs evoked by cortical stimulation in fast and slow motoneurons, as observed definitely in triceps motoneurons described avobe, was not clearly recognized.