Abstract
Synaptic transmission and dynamic aspects of signal processing were studied in the leech central nervous system (CNS). Specific questions addressed were: (1) how are synapses distributed between touch sensory cells, (2) can conduction block in presynaptic pressure sensory neurons affect synaptic transmission to different postsynaptic targets, and (3) how are sensory synaptic signals integrated in motoneurons?Chapter I shows that functional electrical synapses between touch (T) sensory neurons are distributed in the ganglion. During conduction block, selective activation of branches of the anterior axon of the presynaptic T cell, for example, produced a synaptic potential 37% of control amplitude, which was consistent with counts of 40% of contacts made by these branches, a distribution also confirmed by laser axotomy. Conduction block can thus modulate electrical synapses within the ganglion by silencing particular contacts between cells.Chapter II shows that naturally occurring conduction block in another sensory neuron, the medial pressure (mP) cell, differentially affects synaptic transmission to three different postsynaptic neurons: the anterior pagoda (AP) cell, the longitudinal (L) motoneuron and the annulus erector (AE) motoneuron. The mP cell was found to have one major receptive field, innervated by a large axon, with contiguous anterior and posterior minor receptive fields, each innervated by a fine axon, all on ipsilateral dorsolateral skin. Exciting only the anterior part of the mP cell during conduction block at the anterior axon's central branch point reduced transmission to the AE cell much more than to the AP or L cells, while exciting only the posterior part of the mP cell during conduction block at the posterior axon's central branch point had the reverse effect. Laser microsurgery and dye-marking of cell pairs confirmed that the anterior branches synapse with the AP and L cells while the posterior branches synapse with the AE cell. Thus, conduction block can act as a switch, so that the mP cell may transmit to only one group of neurons or another, depending upon how it is stimulated.The monosynaptic connection between the mP cell and AE motoneuron mediates reflexive erection of annuli when the leech's dorsal skin is pressed. Chapter III describes, based on laser microsurgery and combined dye staining, how functional synaptic inputs to the AE from each mP sensory neuron are distributed bilaterally. Synaptic potentials sum at the primary axon fork, as confirmed by measurements of impulse timing. The results explain the concerted action of the AE cells to erect complete annuli.