Stretch receptors and body wall muscle in leeches.

1. A new kind of muscle receptor has been described in the medicinal leech, the first stretch receptor in a soft-bodied invertebrate to be identified and characterized. 2. The sensory responses to stretch in these receptors are different from those of other more well known muscle receptors such as vertebrate spindles or crustacean stretch receptors. 3. The leech receptors provide the sensory innervation for the tubular muscle layers of the body wall, and they respond to stretch of the muscle with hyperpolarizing potentials that are conducted passively to the CNS. 4. This review summarizes their unusual morphology, membrane properties and sensory responses to stretch; their association with particular muscle fibre types in leech body wall; their synaptic connections; and their role in a well characterized rhythmic behaviour in the leech-swimming.