Firing patterns of gastrocnemius motor units in the decerebrate cat

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Abstract
The patterns of medial gastrocnemius (MG) motor unit firing in response to MG [medial gastrocnemius] muscle stretch were studied in decerebrate cats, using intracellular recording techniques. In most of the units, the motoneuron 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. Motor unit firing patterns were classified into 2 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. 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 2 groups according to twitch tension output type F, with twitch tension > 1.5 g, and type F*, with twitch tension > 1.5 g. Units with slow twitch time to peak (> 35 msec) were classified as type S. The presence or absence of tonic firing during sustained MG stretch was found to be significantly related to the following factors: 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; the apparent motoneuron size, in that tonic units tended to have higher cell input resistance values and slower axonal conduction velocities than phasic units; and 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. The intrinsic properties of MG motor units, the characteristics of the group Ia synaptic input to the motoneurons 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.