MAPPING MOTOR-NEURON ACTIVITY TO OVERT BEHAVIOR IN THE LEECH - INTERNAL-PRESSURE PRODUCED DURING LOCOMOTION

Several behaviour patterns have been studied in the leech at both the kinematic and neuronal levels, However, very little is known about how patterns of motor neurone activity map to actual movements. Internal pressure is an essential biomechanical property in this process, being responsible for producing the rigidity and posture that allow the dir ected delivery of forces produced by muscle contraction, To obtain a b etter understanding of the biomechanical processes involved in movemen t of the leech, we have measured the internal pressure of the animal b y placing catheters through the body wall and into the gut of intact a nimals showing normal patterns of behaviour. Each type of behaviour ha d a characteristic pressure waveform. The elongation phase of crawling produced a rapid increase in pressure that peaked when midbody segmen ts were maximally elongated, The pressure produced during the contract ion phase of crawling depended on the type of crawl, only inchworm cra wling producing a second peak, Whole-body shortening in response to a head poke also produced a pressure peak, but it had a faster rise time . Swimming produced the largest pressure, which was marked by a large sustained increase that fluctuated phasically with undulations of the body. Dual pressure recordings using two catheters demonstrated that p ressure was not uniform along the length of the leech, indicating that the body cavity is functionally compartmentalised. Injecting fluid in to the gut via a recording catheter allowed us to determine the effect s of increasing internal volume on pressure. In line with previous pre dictions made using an abstract biomechanical model of the leech hydro skeleton, we found that an increase in the volume caused a reduction i n the pressure, We are in the process of constructing a more realistic biomechanical model of the leech, based on actual data reported elsew here. The results in this paper will provide key tests for refining th ese models.