MAPPING MOTOR-NEURON ACTIVITY TO OVERT BEHAVIOR IN THE LEECH .1. PASSIVE BIOMECHANICAL PROPERTIES OF THE BODY-WALL

As an initial step in constructing a quantitative biomechanical model of the medicinal leech (Hirudo medicinalis), we determined the passive properties of its body wall over the physiological range of dimension s. The major results of this study were. 1. The ellipsoidal cross sect ion of resting leeches is maintained by tonic muscle activation as wel l as forces inherent in the structure of the body wall (i.e., residual stress). 2. The forces required for longitudinal and circumferential stretch to maximum physiological dimensions were similar in magnitude. Cutting out pieces of body wall did not affect the passive longitudin al or circumferential properties of body wall away from the edges of t he cut. 3. The strain (i.e., the percentage change in dimension) of di fferent body segments when subject to the same force was identical, de spite differences in muscle cross-sections. 4. Serotonin, a known modu lator of tension in leech muscles, affected passive forces at all phys iological muscle lengths. This suggests that the longitudinal muscle i s responsible for at least part of the passive tension of the body wal l. 5. We propose a simple viscoelastic model of the body wall. This mo del captures the dynamics of the passive responses of the leech body w all to imposed step changes in length. Using steady-state passive tens ions predicted by the viscoelastic model we estimate the forces requir ed to maintain the leech at any given length over the physiological ra nge.