Depolarizing afterpotentials in myelinated axons of mammalian spinal cord

Microelectrode recordings were made from 5-10 micron dia axons of adult rat spinal cord in vitro. Action potentials in response to electrical stimulation were recorded intracellularly and electrical characteristics of the axons were examined by injecting current pulses through a bridge circuit. All action potentials larger in amplitude than 80 mV were followed by depolarizing afterpotentials, similar to those recorded in peripheral axons [Barrett and Barrett (1982) J. Physiol., Lond. 323, 117-144]. The afterpotential could be described as the sum of three exponential components, the time constants of which (tau 1, tau 2 and tau 3) were 25.2 +/- 5.6, 3.1 +/- 0.8 and 0.8 +/0 0.3 ms, respectively, at 25 degrees C and a membrane potential of -80 mV. The maximal amplitudes of the afterpotential components, obtained by extrapolating to the peak of the action potential, were 3.8 +/- 1.0, 6.4 +/- 5.2 and 21.7 +/- 9.8 mV, for action potential amplitudes of 102 +/- 11 mV. The amplitude of the longest component of the afterpotential decreased with depolarization and increased with hyperpolarization at the recording site. The amplitude decreased markedly with increase of temperature to physiological levels, in conjunction with the expected decrease in action potential duration. Similar afterpotential components were present in the response of the axon to injected hyperpolarizing current pulses. The observations are consistent with the suggestion [Barrett and Barrett (1982) J. Physiol., Lond. 323, 117-144] that the afterpotential results from charging of the axolemmal capacitance by current passing through the myelin sheath during the action potential. They are inconsistent with a number of calculations of electrical characteristics of peripheral axons derived from voltage clamp experiments in isolated fibers. It is argued that the electrical resistance of the myelin lamellae is relatively low, though within the range calculated for other glial membranes. This suggestion is found more compatible with the available morphological data than the alternative proposal that a leakage pathway under the myelin sheath might be responsible for the afterpotential [Barrett and Barrett (1982) J. Physiol., Lond. 323, 117-144]. The significance of this organization for the function of myelinated axons and the electrical basis of the afterpotential are examined further in the accompanying paper [Blight (1985) Neuroscience 15, 13-31].