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 strai ght 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 identif ied neuron of the leech on lanes of extracellular matrix protein. The electrical signals were detected by a fluorescent voltage-sensitive dy e. We observed the voltage that was caused by an action potential elic ited at the soma and by a Gaussian hyperpolarization induced at the so ma, respectively. We compared the data with numerical solutions of the cable equation using the Hodgkin-Huxley parametrization. We could att ribute the experimental results of depolarization and of hyperpolariza tion to the propagation of an action potential along an ''active'' cab le and to the spread along a ''passive'' cable, respectively, if we as signed rather high specific resistances to the cytoplasm (R(I) = 250 O hm.cm) and to the membrane (R(M) = 22 k Ohm.cm(2)). This assignment ex plained the slow velocity of 150 mu m/ms of a pulse by active propagat ion and the limited range of 200 fun of a pulse by passive spread.