Journal of Neurophysiology, Vol 39, Issue 3 459-473, Copyright © 1976 by APS
Electrical properties of frog motoneurons in the in situ spinal cord
P. C. Magherini and W. Precht
Electrical properties of the spinal motoneurons of Rana temporaria and R.
esculenta were investigated in the in situ spinal cord at 20-22 degrees C
by means of intracellular recording and current injection. Input resistance
values depended on the method of measurement in a given cell but were
generally inversely related to axon conduction velocity. The
membrane-potential response to a subthreshold current pulse was composed of
at least two exponentials with mean time constants of 2.5 and 20 ms. The
membrance potential reached by the peak of a spike depended on the mode of
spike initiation and membrane potential. Preceding a suprathreshold
depolarization by a hyperpolarizing pulse could delay and eliminate spike
initiation, similar to effects reported in certain invertebrate neurons.
Antidromic invasion frequently failed in motoneurons of normal resting
potential. Antidromic spike components (m,IS, SD) were similar to those of
cat motoneurons. The delayed depolarization and the long
afterhyperpolarization following an antidromic spike had many properties in
common with the analogous afterpotentials of cat motoneurons. The reversal
potential of the short afterhyperpolarization occurring immediately after
the spike varied with resting potential and could not be used to determine
potassium equilibrium potential. Sustained rhythmic firing could be evoked
by continuous synaptic drive or long pulses of injected current. The plot
of firing rate versus current strength had a substantial linear region.
Both steady firing and adaptation properties varied markedly with
motoneuron input resistance.
