Conduction block silences parts of a chemical synapse in the leech central nervous system.

1. The pressure (P) sensory neurones innervating the ventral skin of the medicinal leech have receptive fields comprising a central region of skin innervated by two thicker axons and two neighbouring regions innervated by two thinner axons. Impulses originating in the thinner axons may fail to propagate through the central ganglion, apparently blocked at the branch point of large and small axons. 2. The P neurone excites the longitudinal (L) motoneurone, and blocked impulses originating in the anterior fine axon produce e.p.s.p.s that are less than one‐half normal amplitude. Blocked impulses in the posterior fine axon are typically ineffective. 3. The branches of P and L neurones, marked with intracellularly injected horseradish peroxidase or with Lucifer Yellow, make synaptic contact at up to sixty‐six sites within the neuropile. Of P neurone branches emerging from two fine axons, those from the posterior axon make fewer contacts, usually one or two at most, while branches from the anterior axon represent no more than half the total contacts. From cell to cell there is some variation in the total number of contacts, the distribution of branches, and the strength of transmission. 4. The locations of contacts measured morphologically correlate well with their distributions as predicted from reductions in e.p.s.p. amplitude during conduction block.