Identifiable neurons controlling penile eversion in the leech.

1. This paper describes the neuroanatomy and electrophysiology of motor neurons causing penile eversion in the leech. 2. The male organ is innervated by ganglia 5 and 6 of the 34 ganglia in the leech brain through special sex nerves deriving from anterior roots. These sex ganglia have at least 200 more neurons than the other midbody ganglia. Many of the extra neurons are involved in reproductive behavior. 3. Two pairs of motor neurons on the ventral side of ganglion 6, named rostral and lateral neurons, are the only ones that elicit full penile eversion. Evidence that the lateral and rostral neurons are, in fact, motor neurons comes from HRP and electrophysiological studies. HRP injections reveal that each neuron's single primary axon grows into the sex nerve. Electrophysiological evidence is twofold: a) action potentials of lateral and rostral cells can still contract the genitalia after the neurons are deafferented from chemical synaptic input in the ganglion by high Mg2+, b) their action potentials are followed by junction potentials in male organ muscle fibers.