CABLE PROPERTIES OF A STRAIGHT NEURITE OF A LEECH NEURON PROBED BY A VOLTAGE-SENSITIVE DYE

We measured a time-resolved map of electrical activity in a thin straight neurite (1.5 mu m thick, 500 mu m long) at a resolution of 8 mu m and 0.4 ms. The neurite was obtained by guided outgrowth of an identified neuron of the leech on lanes of extracellular matrix protein. The electrical signals were detected by a fluorescent voltage-sensitive dye. We observed the voltage that was caused by an action potential elicited at the soma and by a Gaussian hyperpolarization induced at the soma, respectively. We compared the data with numerical solutions of the cable equation using the Hodgkin-Huxley parametrization. We could attribute the experimental results of depolarization and of hyperpolarization to the propagation of an action potential along an ''active'' cable and to the spread along a ''passive'' cable, respectively, if we assigned rather high specific resistances to the cytoplasm (R(I) = 250 Ohm.cm) and to the membrane (R(M) = 22 k Ohm.cm(2)). This assignment explained the slow velocity of 150 mu m/ms of a pulse by active propagation and the limited range of 200 fun of a pulse by passive spread.