SQUID GIANT-AXONS - MODEL FOR NEURON SOMA

Insertion of electrically floating wires along the axis of a squid [Loligo pealii] giant axon produces an apparent increase in diameter in the region where the wire surface was treated to give it a low resistance. The shape of action potentials propagating into this region depend upon the surface resistance (and length) of the wire. As this segment''s internal resistance is lowered by reducing the wire''s surface resistance, the characteristic sequence of changes in the action potential seen at the transition region is the duration increases; 2 peaks develop, the 1st generated in the normal axon region and the 2nd one generated later in the axial wire region (b), and blockage occurs (for a very low resistance wire). Action potentials recorded at the membrane region near the tip of the axial wire in (b) resemble those recorded at the initial segment of neurons upon antidromic invasions. Squid axon action potentials propagated from a normal region into that containing the low resistance wire resemble antidromic invasions recorded in neuron somas. Hyperpolarizing current pulses applied through the wire act as if the wire surface resistance was momentarily reduced. For example, the 2 components of the action potential recorded at the axial wire membrane region noted in (b) can be sequentially blocked by the application of increasing hyperpolarizing current through the wire. Similar effects are seen when hyperpolarizing currents are injected into motoneuron somas. The geometrical properties of the junction of a neuron axon with its soma may be in themselves sufficient to determine the shape of the action potentials usually recorded by microelectrodes.