The whole-body shortening reflex of the medicinal leech: motor pattern, sensory basis, and interneuronal pathways

Abstract

The leech whole-body shortening reflex consist of a rapid contraction of the body elicited by a mechanical stimulus to the anterior of the animal. We used a variety of reduced preparations - semi-intact, body wall, and isolated nerve cord - to begin to elucidate the neural basis of this reflex in the medicinal leech Hirudo medicinalis. The motor pattern of the reflex involved an activation of excitatory motor neurons innervating dorsal and ventral longitudinal muscles (dorsal excitors and ventral excitors respectively), as well as the L cell, a motor neuron innervating both dorsal and ventral longitudinal muscles. The sensory input for the reflex was provided primarily by the T (touch) and P (pressure) types of identified mechanosensory neuron. The S cell network, a set of electrically-coupled interneurons which makes up a 'fast conducting pathway' in the leech nerve cord, was active during shortening and accounted for the shortest-latency excitation of the L cells. Other, parallel, interneuronal pathways contributed to shortening as well. The whole-body shortening reflex was shown to be distinct from the previously described local shortening behavior of the leech in its sensory threshold, motor pattern, and (at least partially) in its interneuronal basis.

Detection and selective avoidance of near ultraviolet radiation by an aquatic annelid: the medicinal leech

“…A similar behavior has been well documented in leeches when stimulation is applied to the anterior body (Magni and Pelligrino, 1978). When tactile stimuli or electric shocks are applied to the anterior portion of the leech, it shortens rapidly (Kristan et al, 1982;Shaw and Kristan, 1995). Although the S-cell of the fastconducting pathway is activated during shortening (Shaw and Kristan, 1995), and is required for sensitization of the response (Sahley et al, 1994), it is neither necessary nor sufficient for expression of the behavior (Shaw and Kristan, 1999;Arisi et al, 2001).…”

Multiplexed modulation of behavioral choice

“…The leech is a useful animal for elucidating the cellular mechanisms underlying behavioral choice because leech behaviors such as swimming (Brodfuehrer et al, 1995), crawling (Eisenhart et al, 2000), shortening (Shaw and Kristan, 1995) and local bending (Lockery and Kristan, 1990a;Lockery and Kristan, 1990b) are well characterized and have been utilized to study decision-making at the neuronal circuit level (Briggman et al, 2005;Kristan and Gillette, 2007;Gaudry and Kristan, 2009).…”

A 3-Synapse Positive Feedback Loop Regulates the Excitability of an Interneuron Critical for Sensitization in the Leech

“…Like the tail-withdrawal reflex in Aplysia, the whole body shortening reflex is mediated in part by a monosynaptic reflex arc in which sensory neurons terminate directly onto motor neurons and by multisegmental interneurons (Shaw and Kristan, 1995). The same sensory neurons also excite the S interneuron, which provides additional excitatory input to the motor neurons, but the S cell cannot, on its own, shorten the animal.…”