Journal of Neuroscience, Vol 9, 415-425, Copyright © 1989 by Society for Neuroscience
Signaling properties of Ascaris motorneurons: graded active responses, graded synaptic transmission, and tonic transmitter release
RE Davis and AO Stretton
Neurosciences Training Program, University of Wisconsin-Madison 53706.
The commissural motorneurons of the nematode Ascaris are capable of
transmitting signals passively over long distances with little decrement.
This ability is due to the high resistivities of their membranes (Davis and
Stretton, 1989). Although these cells rely on their passive properties for
long-distance signaling, voltage-sensitive channels are present in
commissural membranes. These channels underlie the graded active responses
that can be elicited at the offset of abrupt hyperpolarizing and
depolarizing intracellular current pulses. The inhibitory motorneurons
generate membrane potential oscillations when they are strongly
depolarized. All-or-none action potentials have never been observed to
occur spontaneously, nor has it been possible to evoke them even when the
cells have been strongly hyperpolarized to remove any possible channel
inactivation. Our findings indicate that the typical all-or-none action
potentials so commonly used in nerve cells throughout the animal kingdom do
not occur in these cells. Synaptic transmission is therefore mediated
without spikes and is graded. The resting potentials of Ascaris
motorneurons lie where the synaptic input-output curves are steepest, above
the threshold for release of neurotransmitter. Tonic transmitter release
from commissural motorneurons may be the neural mechanism underlying the
hydrostatic skeleton of Ascaris.
Previous Article | Next Article
