Current excitation threshold in sensory neurons of leech central nervous system

1. Sensory cells in segmental ganglia of the leech (Hirudo medicinalis L.), N (noxious), P (pressure), and T (touch) cells, were stimulated by linearly rising currents. The electrical response to intracellular stimulation with prolonged square-wave currents, the shape of the action potential, and the degree of repetitive activity were also examined. 2. The thresholds for first production of an action potential by linearly rising currents (expressed as multiples of rheobase, I/IO), plotted as a function of the action-potential latency, provide a measure of accomodation. These T-L curves show that the P and T cells accommodate more rapidly than the N cells. 3. An index of accommodation, the accommodation coefficient (I/Io)s/2, was defined as the current required to elicit a first action potential 0.5 s after the beginning of a linearly rising stimulus current. These accommodation coefficients also reflect the property that P and T cells accommodate more rapidly, on the average, than N cells. 4. Threshold depolarization is higher with linearly rising currents than with square-wave currents. 5. Comparison of the accommodation rates of these sensory somata with the published data for other somata and axons, on the basis of accommodation coefficient, minimal current gradient, and/or threshold-latency curve, shows that a) N cells, at the slow end of the leech-soma accommodation range, accommodate at about the same rate as most vertebrate neurons; and b) some vertebrate neurons have accomodation rates as high as or higher than those of the T and P cells. The accommodation rates of most vertebrate neurons are related in a characteristic way to undershoot shape, repetitive-firing properties, and adaptation. Comparable relationships are found in the leech if the N and T cells alone are considered, but the P cells (with extremely rapid accomodation) depart notably from this pattern--they adapt slowly, fire repetitively in response to a maintained stimulus, and have an undershoot of intermediate amplitude and duration.