Worm Breeder's Gazette 7(2): 55

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

Membrane Properties of Ascaris Commissures

R.E. Davis, A.O.W. Stretton

Figure 1

Microscopic studies (Stretton et al., PNAS, 75:3493, 1978; White et 
al., Phil.  Trans., 275:327, 1916) have revealed the morphological 
similarities between motorneuronal types found in Ascaris 
des and those found in C.  elegans.  Based on the 
assumption that the function of morphologically analogous elements 
will be similar, we have previously predicted that types DAS, DB and 
DA (Ascaris types DE1, DE2 and DE3) are excitatory motorneurons 
whereas types VD and DD (Ascaris VI and DI) are inhibitory 
At the last C.  elegans meeting (1981), we described techniques 
which allowed us to make the first intracellular recordings from 
Ascaris neurons.  These recordings were made from the commissures of 
identified motorneurons.  (Commissures are single motorneuron 
processes which connect the ventral and dorsal nerve cords.  They 
occur in both Ascaris and C.  elegans.)  At that time, we noted that 
spontaneously occurring excitatory and inhibitory potentials were 
conducted with very little decrement for long distances along the 
commissure (~.5 cm).  Using two intracellular microelectrodes for 
stimulation and recording, we have now determined the electrical (
cable) properties of 
[See Figure 1]
Commissural membranes are similar to excitable membranes found in 
most other organisms in two of their properties: internal (axoplasmic) 
resistivity (Ri) and membrane capacitance (Cm).  Commissures differ 
rather markedly from most other excitable membranes in their space 
constant (lambda) and membrane resistance (Rm).  We suspect that the 
unusually high Rm is an inherent property of the commissural membrane 
itself since the Cm is similar to that of a single biological membrane.
(Myelinated fibers with their wrapping of multiple membranes have a 
much lower Cm.) We are now examining this anatomically.  The high Rm 
produces a long lambda which accounts for the ability of the 
commissural membrane to conduct spontaneous passive signals over long 
distances with only slight decrement.  We have also examined 
electrical signals evoked by stimulation of the ventral nerve cord.  
To date, all of the signals we have observed are graded, i.e., the 
signals lack a clear threshold and their amplitude is a continuous 
function of stimulus strength.  We have never observed spontaneous all-
or-none action potentials nor have we been able to evoke them (at 
normal resting potential or after hyperpolarization).  Thus, lacking 
this classical long-distance signalling mechanism, Ascaris  
motorneurons appear to rely on their unusual membrane properties.  to 
convey information over the long distances separating the two nerve 
cords.  Nonetheless, active voltage-dependent membrane channels do 
appear to be present as indicated by our ability to elicit anode-break 
responses.  These responses, however, are themselves graded (i.e., 
they increase in amplitude with increasing strength of the 
hyperpolarizing pulse).  The signalling, properties of the 
interneurons are as yet unknown, though we are in the process of 
investigating them.  At this point, it is not possible to rule out the 
presence of an action potential mechanism in other nematodes (
including C.  elegans).  Such an active mechanism may, however, not be 
necessary; in fact, one might expect passive signalling to be the 
modus operandi in a system where only short distance signalling (on 
the order of one or a few millimeters) is required.  If the Rm of 
neuronal membranes in C.  elegans is the same as that in Ascaris 
commissures, a lambda of 0.9mm for a 0.5 m fiber (in adult C.e.) or 0.
4mm for a 0.1 m fiber (in the L1) would be expected.  Thus passive 
signalling over the length of the animal is entirely feasible.
We have also examined neuromuscular synaptic transmission.  For both 
excitatory and inhibitory motorneurons, synaptic transmission is 
graded.  In addition, we have evidence that Ascaris motorneurons 
tonically release neurotransmitter.  It is easy to see how graded 
synaptic transmission can be an effective way to generate the 
continuous gradation of muscle contraction which underlies a 
locomotory wave (in contrast to integration of multiple non-graded 
neuronal elements which together could produce the same effect).  It 
is also tempting to speculate that graded synaptic transmission and/or 
tonic neurotransmitter release may be important in the production and 
variation of muscle tonus comprising the hydrostatic skeleton.

Figure 1