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

Single identified group I a and I b muscle afferent fibres were injected with horseradish peroxidase in the lumbar dorsal columns of anaesthetized cats. The morphological details of the axon 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 fibre 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.) neurones were retrogradely labelled by injection of horseradish peroxidase into the cerebellum. Synaptic contacts between labelled group Ia and Ib afferent fibres and the soma and proximal dendrites of d.s.c.t. neurones were found. The synaptic contacts from both Ia and Ib fibres varied greatly in size, from 1 X 1 micron up to 'giant' synapses of 20 X 3 micron. Excitatory post-synaptic potentials (e.p.s.p.s) were evoked in d.s.c.t. neurones by impulses in single group I muscle afferent fibres. The fluctuations in peak amplitude of each e.p.s.p. were determined from e.p.s.p. and noise recordings, using a numerical deconvolution procedure. In general, these single-fibre e.p.s.p.s 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. Our anatomical observations of 'giant' boutons arising from Ia and Ib afferent fibres contacting d.s.c.t. neurones raises the possibility of multiple transmitter release sites within an individual synaptic bouton. It is proposed that synaptic transmission between group I muscle afferents and d.s.c.t. neurones occurs with discrete all-or-nothing e.p.s.p.s associated with transmitter release sites.