THE ROLE OF TOUCH, PRESSURE AND NOCICEPTIVE MECHANORECEPTORS OF THE LEECH IN UNRESTRAINED BEHAVIOR

1. The maximum force exerted against an isometric force transducer by 6 leeches weighing 2.6-3.7 g, as they squeezed through apertures of di fferent widths varied inversely with aperture width. 2. T cells in the leech skin code for velocity of indentation, not pressure or displace ment. The frequency with which T cells fire is best described by two l og functions, one for low, another for fast indentations. T cells resp onded to indentation velocities down to 10 mu m s(-1). 3. The average threshold pressure for 5 P cells was 150 kPa and for 5 N cells was 521 kPa. 4. We conclude from these data that when leeches explore their m echanical environment and initiate contact with external objects, the threshold pressure for N cells is rarely crossed. Of the three classes of mechanoreceptor, T cells are the main modality through which leech es obtain contact information, though P cells may occasionally be recr uited for local pressure peaks.