Dendroaxonic synapses in the substantia gelatinosa glomeruli of the spinal trigeminal nucleus of the cat.

The glomeruli in the substantia gelatinosa layer of the spinal trigeminal nucleus of the cat contain three kinds of dendritic processes. One of these, the type 2 dendrite, contains large synaptic vesicles in its spine heads and in its shafts. The type 2 dendrite receivers axodendritic synapses from primary trigeminal afferent (C) axons and an occasional axodendritic synapse from small axonal (P) endings with small synaptic vesicles. The type 2 dendrites in turn form dendroaxonic synapses on the C endings. The dendroaxonic synapse and the axodendritic synapse of the C ending typically occur in reciprocal pairs. The axodendritic synapse usually lies in the depths of scalloped depressions in the surface of the C ending while the dendroaxonic synapse is found on the rim of the depression. Type 1 spines, i.e., dendritic spines receiving axodendritic synapses from the primary ending and lacking synaptic vesicles, also receive dendrodentritic synapses from type 2 dendrites. The type 2 dendrite with its large, rounded synaptic vesicles is considered to be excitatory at its dendroaxonic and dendrodendritic synapses. The type 2 dendrites course from glomerulus to glomerulus receiving their excitatory input through the axodendritic synapses of C axons. A type 2 dendrite, in response to C axon excitation would activate type 1 spines directly through their dendrodendritic synapses (C leads to 2 leads to 1) and indirectly by increasing transmitter release at the axodendritic synapses of the C axonal endings through their dendroaxonic synapses (2 leads to C leads to 1). The type 2 dendrites could serve two functions. First, they may prolong transmitter release from the axodendritic synapses of C axonal endings beyond the time of arrival of incoming potentials because of the reciprocal pairing of dendroaxonic and axodendritic synapses (C in equilibrium 2). Second, they may extend the spatial range of the excitatory output of active primary afferent axons to type 1 spines of glomeruli whose primary afferent axons may be inactive (C leads to 2 leads to 1).