Firing patterns of gastrocnemius motor units in the decerebrate cat

Abstract

SUMMARY1. The patterns of medial gastrocnemius (MG) motor unit firing in response to MG muscle stretch have been studied in decerebrate cats, using intracellular recording techniques. In most of the units, the motoneurone axonal conduction velocity and cell input resistance were measured, and the maximum amplitude and wave form of the group Ia composite EPSP evoked by MG nerve stimulation were determined. The mechanical properties of the muscle unit portion of each motor unit were also studied.2. Motor unit firing patterns were classified into two groups, 'tonic' and 'phasic'. With few exceptions, tonic motor units showed sustained firing throughout MG stretches of varying duration, while most phasic units did not fire at all to the same stimulus. Tetanization of the afferents did not convert any of the previously phasic units to tonic firing.3. MG motor units in this study were divided into three groups on the basis of the muscle unit twitch properties. Units with short twitch time to peak values (< 35 msec) were subdivided into two groups according to twitch tension output: (a) type F, with twitch tension > 1*5 g, and (b) type F*, with twitch tension < 1U5 g. Units with slow twitch time to peak (> 35 msec) were classified as type S.4. The presence or absence of tonic firing during sustained MG stretch was found to be significantly related to the following factors: (a) the motor unit twitch type, in that tonic firing was observed in 100 % of type S, 70 % of type F*, and only 10 % of type F units; (b) the apparent motoneurone size, in that tonic units tended to have higher cell input resistance values and slower axonal conduction velocities than phasic units; and (c) the density and spatial organization of the group Ia synaptic input, in that the MG monosynaptic EPSPs found in tonic motor units tended to be both larger in amplitude and longer in duration than those found in phasic units.5. The intrinsic properties of MG motor units, the characteristics of the group I a synaptic input to the motoneurones and the unit firing patterns elicited by MG muscle stretch appear to be mutually interrelated, forming a pattern of motor units within the MG pool which is analogous to the pattern thought to characterize the motor units belonging to 'slow' and 'fast' muscles. In most respects this pattern of MG motor units appears to be a continuum without clearly separable subgroups.

Recruitment of motor units in the medial gastrocnemius muscle during human quiet standing: is recruitment intermittent? What triggers recruitment?

“…The intermittent behavior of these phasic units is governed inherently by their low oxidative metabolism (Bradacs et al 1997). Postural motor units, however, are small, highly excitable units and are thus capable of discharging tonically (Burke 1968). The intermittency of the postural MG motor units, then, is not a consequence of their physiological inability to sustain firing.…”

Group Ia synaptic input to fast and slow twitch motor units of cat triceps surae

“…In the present paper an attempt has been made to examine what may be regarded as an extrinsic characteristic of motor units, that is, the relative amount and organization of synaptic input to triceps surae units. This study has been limited to the monosynaptic group I a input, both for technical simplicity and because this excitatory input to extensor cells is of considerable interest as it relates to motoneurone properties (see Eccles, Eccles & Lundberg, 1957a;Mendell & Henneman, 1967) and to motor unit firing patterns (see Denny-Brown, 1929;Granit, Henatsch & Steg, 1956; Granit, Phillips, Skoglund & Steg 1957;Henneman, Somjen & Carpenter, 1965a, b; Burke, 1968).…”

Contraction time and voluntary discharge properties of individual short toe extensor motor units in man.

“…It has been suggested that the firing rate of a motoneurone, which is controlled by the duration of afterhyperpolarization, is matched to the twitch duration of its muscle fibres (Eccles, Eccles & Lundberg, 1958). The maximum rates of motoneurone discharge obtained by stretching the muscle or by tetanization of its afferents are 0022-3751/79/3020-0586 $01.50 © 1979 The Physiological Society higher for fast twitch motor units than for slow ones, but not high enough to give a completely fused contraction of any type of motor unit (Burke, 1968).…”