ABSTRACT
Two basic types of mechanical response can be elicited from the lancelet myotome by stimulation of the nerve cord by single shocks: a fast twitch with a rise time of 70-80 msec., and a slow twitch a rise time of 200-250 msec.
The two twitch components can easily be elicited together to produce a biphasic contraction. A second type of biphasic contraction occurs more rarely in which the slow phase has a faster rise time and a higher peak tension.
The mean of all resting potentials was 48.94 mV. ± 15.5 mV. The commonest values were near 50 mV. and this must refer to the deep lamellae which form the bulk of the myotome.
Spontaneous potentials are a striking feature of the lancelet muscles.
The fast twitch is accompanied by an all-or-nothing, spike-like potential of between 30 and 70 mV. The slow twitch is accompanied by one or more potentials of between 1 and 10 mV. The type-2 biphasic mechanical response mentioned above is accompanied by two large potentials which attain similar amplitude, but differ in that the second is a graded response.
Some of the features of lancelet myotomal muscle have also been described in cyclostomes, nematodes and echinoderms.
