Ultrastructural Analysis of C-Type Synapses in Thoracic Motoneurones of the Cat

Abstract

The C-type synapse is one of five main morphological ‘types’ contacting cat thoracic alpha motoneurones. Its name derives from a morphologically distinctive 10–15 nm subsynaptic cistern which underlies its major length [5], and this is topographically associated with a subjacent contiguous postsynaptic Nissl body. Indirect evidence attributes the C-type to intrasegmental short-axon propriospinal pathways [6, 21, 22], a source directly proven in cat cervical and opossum trigeminal nuclei [11, 20]. C-type synapses characterise alpha motoneurones of the oculomotor [31], hypoglossal [4], phrenic [9], cervical [20], thoracic [24] and lumbosacral nuclei [5]. They do not occur on cat lumbosacral Renshaw cells [17] or thoracic gamma motoneurones [12], and recent preliminary studies of HRP-labelled cat alpha motoneurones innervating cat sphincter ani muscle have also failed to identify C-types despite their presence on neighbouring unlabelled large diameter motoneurones (Pullen, unpublished observations).