Effects of activity in single sensory fibres on the discharge patterns of dorsal spinocerebellar tract cells.

1. Electrical activity of dorsal spinocerebellar tract (DSCT) was recorded extracellularly and intracellularly in Clarke's column nucleus. 2.Trains of impluses were elicited in DSCT cells by static stretch of the hind limb muscles, and the distributions of intervals between the impulses were computed. These distributions were essentially similar before and after impalement of the cells with the micropipette, suggesting that cell injury caused by penetration was insignificant. 3. Patterns of synaptic activity revealed that the DSCT cells examined were innervated by a limited number of muscle afferents. 4. In some DSCT neurons, all excitatory post-synaptic potentials (e.p.s.p.s) elicited by a single fibre were suprathreshold, and e.p.s.p.s elicited by other afferents were usually subthreshold. These neurons discharge very regularly during muscle stretch, and the spike initiation followed nearly one-to-one relation in response to activity of the sensory fibre eliciting suprathreshold e.p.s.p.s. 5. The discharge pattern was irregular in other DSCT cells. In these cases, single sensory fibres evoked e.p.s.p.s. which did not always reach the firing threshold because of random fluctuation in their amplitude. 6. DSCT neurons often show low frequency discharges (about 10/sec) in the absence of sensory activation. In these conditions only small e.p.s.p.s. were detected (average, 0.53 mV) suggesting that the average membrane potential is very close (1-i mV) to the firing threshold. 7. It is concluded that the statistical properties of firing patterns in DSCT neurons activated by muscle afferents depend on (a) the large amplitude of e.p.s.p.s., (b) their firing threshold and (c) random fluctuations in the e.p.s.p.s. amplitude.