The connections between the pericruciate cortex and the medullary reticulospinal (RS) neurons were studied in anesthetized cat. Intracellular recordings were made from reticulospinal neurons and the effects of stimulating different areas of the pericruciate cortex were compared. (1) EPSPs were elicited in all the 93 neurons studied which were antidromically activated by spinal stimulation and had an IS-SD notch on the ascending limb of their antidromic spikes. According to the conduction velocity (c.v.) of the axon and the minimal EPSP latency to cortical stimulation, the neurons could be divided into two groups, i.e. fast-conducting RS neurons (FRS neurons, c.v. > 45 m/s) and slow-conducting RS neurons (SRS neurons, c.v. < 45 m/s). The minimal latencies of FRS neurons were equal to or shorter than 2 ms whereas those of SRS neurons were longer than 2 ms. (2) EPSPs with short latency (< 2 ms) could be evoked in FRS neurons by stimulating a relatively wide cortical area including the major part of precruciate area 4 and area 6, with a central area of strongest excitatory effect located in area 4 slighthly medial to the tip of the cruciate sulcus. Stimulation of the postcruciate area 4 only produced long latency EPSPs. (3) By extrapolation from the cortical and peduncular latencies and the conducting distances it was revealed that the earliest part of the minimal latency EPSPs were monosynaptically evoked in FRS neurons and were mediated by fastconducting corticobulbar fibers. (4) FRS neurons could be excited by stimuli applied to both ipsilateral and contralateral pericruciate cortex. The influence from the contralateral cortex was slightly stronger.
Motor cortexReticulospinal neuronCorticobulbar pathwaysCat