Motor unit types of cat triceps surae muscle

Abstract

SUMMARY1. Motor units, defined as including a motoneurone (cell body, dendrites and axon) plus the muscle unit innervated, have been examined in the triceps surae motor pool of pentobarbital anaesthetized cats.2. The technique of intracellular stimulation and recording which was used permitted measurement of the axonal conduction velocity, post-spike hyperpolarization duration and input resistance of individual motoneurones, and the correlation of these properties with the characteristics of the twitch and tetanus responses of the muscle unit innervated by the cell elicited by direct intracellular stimulation.3. On the basis of muscle unit speed of contraction, motor units were divided into two groups: (a) fast twitch, or F, type with twitch time to peak (TwTp) less than or equal to 30 msec, and (b) slow twitch, or S, type with TwTp of 40 msec or greater. The twitch tensions (TwTen) produced by type F units were significantly larger (median value = 18 g) than the tensions generated by type S units (TwTen median value = 1*6 g). Type F muscle units had much higher tetanus fusion frequencies (median = 85 pulses/see) than the S type (median 25 pulses/sec), and tended to have smaller tetanus to twitch tension ratios (Tet/Tw) (median = 2.6) than type S units (median = 5.4).4. The gastrocnemius heads contained a mixture of F and S types of muscle units, the proportions found being about 3 to 1 respectively. Units encountered in the soleus muscle were uniformly of type S. The characteristics of gastrocnemius and soleus type S motor units were not identical but appeared to represent quantitative differences in units of the same qualitative type.5. Motoneurones innervating type F muscle units had faster axonal conduction velocities, shorter post-spike hyperpolarizations and lower input resistances than those supplying type S units. However, no combination of motoneurone properties alone was sufficient to separate unambiguously types F and S motor units.

Firing patterns of gastrocnemius motor units in the decerebrate cat

“…Recent control experiments have shown considerable diminution of twitch tension, plus variable but relatively small changes in twitch time to peak, in type F units studied both before and after tetanic MG contraction with the muscle unloaded. The distribution of motor unit twitch characteristics obtained in further control experiments in which the muscles were never allowed to shorten without load has been similar to that reported in the first paper of this series (Burke, 1967a), with the addition of a few type F units with twitch tension output of less than 1-5 g. Thus it can be concluded that the twitch tension values recorded for many units in the present investigation were in all likelihood abnormally low, and that in addition the distinction between fast and slow twitch units may have been somewhat blurred, so that the twitch characteristics of the motor units in the present sample may match those of the earlier sample much more closely than I had appreciated at first. With this reservation, the conclusions drawn from the present investigation stand essentially umnodified.…”

“…Given other, more intense stimulation, the larger tension type F units undoubtedly participate but such states could not be tested because of the technical limitations of the intracellular approach. As has been found true of soleus motor units (Denny-Brown, 1929;Granit, 1958), the maximum firing frequencies of both type S and type F* units during muscle stretch, although different from one another (see Table 1), were in each case lower than that required for complete fusion of tetanic tension of the individual muscle units (Burke, 1967a, see also Kernell, 1966b). However, the summated tension profile of the whole muscle can remain relatively smooth because of the small individual unit tension outputs (see Fig.…”

“…1). (Burke, 1967a). In the present experiments, the MG muscle was disconnected from the myograph during MG nerve tetanization in order to avoid dislocation of the intracellular electrode (see Methods).…”

“…Therefore, the individual twitch responses were studied during steady tetanic stimulation of the ipsilateral DP nerve, which silenced most or all of the background activity. The passive resting tension was adjusted during the tetanus to about 100 g as in earlier work (Burke, 1967a) produced by intracellular stimulation (at artifact in upper trace); muscle at about 100 g passive initial tension, with background active tension suppressed by DP tetanus (see Fig. 1, B).…”

“…The results suggest that, under the present experimental conditions, type F* units tend to fire at somewhat more rapid rates during homonymous muscle stretch than do the type S units in the same animal. Unfortunately, the post-spike hyperpolarization durations were not measured in these units so that it is not known whether this factor may have correlated with the firing rates (see Kernell, 1965 Rall, 1959;Henneman et al 1965a, b;Kernell, 1966a;Burke, 1967a). The graph in Fig.…”