Synaptic input from identified muscle afferents to neurones of the dorsal spinocerebellar tract in the cat.

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Abstract
Single identified group Ia and Ib muscle afferent fibers were injected with horseradish peroxidase in the lumbar dorsal columns of anesthetized cats. The morphological details of the axons collaterals and terminal boutons of these muscle afferents within Clarke''s column were subsequently reconstructed. The rostro-caudal extent of synaptic terminals from a single afferent fiber within Clarke''s column was found to be restricted to less than 1 mm. In the same experiments, dorsal spinocerebellar tract (d.s.c.t.) neurons were retrogradely labeled by injection of horseradish peroxidase into the cerebellum. Synaptic contacts between labeled group Ia and Ib afferent fibers and the soma and proximal dendrites of d.s.c.t. neurons were found. The synaptic contacts from both Ia and Ib fibers varied greatly in size, from 1 .times. 1 .mu.m up to giant synapses of 20 .times. 3 .mu.m. Excitatory post-synaptic potentials (e.p.s.p.) were evoked in d.s.c.t. neurons by impulses in single group I muscle afferent fibers. The fluctuations in peak amplitude of each e.p.s.p. were determined from e.o.s.p. and noise recordings, using a numerical deconvolution procedure. In general, these single-fiber e.p.s.p. fluctuated between discrete amplitudes separated by an incremental amplitude which was approximately constant. This incremental amplitude did not depend on the average peak amplitude of the particular e.p.s.p. examined. Anatomical observations of giant boutons arising from Ia and Ib afferent fibers contacting d.s.c.t. neurones raises the possibility of multiple transmitter release sites within an individual synaptic bouton. Synaptic transmission between group I muscle afferents and d.s.c.t. neurons may occur with discrete all-or-nothing e.p.s.p. associated with transmitter release sites.