Table 12.1 ). Slow-twitch muscle fibers express type I myosin heavy chain, whereas fast-twitch skeletal muscle fibers could contain type IIa, type IIx, or type IIb myosin heavy chains. Some fast-twitch muscle fibers may contain a mixture of type II myosin isoforms.
Slow-twitch skeletal muscles are also characterized by a high oxidative capacity (see
Table 12.1 ), which in combination with the low myosin ATPase activity contribute to the fatigue resistance of slow-twitch muscle fibers. The oxidative capacity of the fast-twitch muscle fiber ranges from relatively high (in muscle fibers expressing type IIa myosin heavy chain) to low (in muscle fibers expressing type IIb myosin heavy chains). The low oxidative capacity of fast type IIb muscle fibers, coupled with the high myosin ATPase activity, increases the susceptibility of these muscle fibers to fatigue. Humans, however, rarely express the type IIb myosin heavy chain. Type IIb myosins are expressed in small animals such as rabbit and rats.
Motor units are generally composed of only one type of muscle fiber (
Table 12.2
Fig. 12.15), unless the muscle fibers are undergoing a transition. Conditions that may trigger a change in the type of myosin expressed in a muscle fiber within a motor unit include chronic conditions such as microgravity (in space flight), denervation, and chronic unloading, which are associated with severe atrophy and promote the gradual transition from the expression of slow muscle myosin (type I) in the fiber to the expression of fast muscle myosin (types IIa and IIx).
An important function of slow motor units is in the maintenance of posture (see
Fig. 12.15 ). The low ATPase activity of myosin in slow motor units, coupled with their high oxidative capacity, facilitates the ability of these slow motor units to maintain posture at low energy cost and thus resist fatigue. The smaller diameter of slow muscle fibers, and the higher capillary density in slow muscle, also helps slow muscle resist fatigue.
Fast muscle, in contrast, is recruited for activities that require faster movements, more force, or both (see
Fig.
12.15 ). Weightlifting, for example, can require a lot of power for short duration. In order to meet the demands for more force, additional motor units are recruited. In comparison with slow motor units, the fast motor units typically contain more muscle fibers (see
Table 12.2 ). Fast muscle fibers also have a larger diameter than do slow muscle fibers. Thus recruitment of fast motor units can help meet the increased demands of burst activities such as weightlifting. The high myosin ATPase activity in fast muscle fibers and the increase in diffusion distance (resulting from the large diameter of the fast muscle fibers), however, increase the susceptibility of fast muscle fibers to fatigue.
Additional differences between fast and slow muscles include the following: 1.
The motor neuron in slow muscle is more easily excited than that in fast muscle, and so slow muscles are typically recruited first. As stated previously, the high oxidative capacity of slow muscle, coupled with the low myosin ATPase activity, helps reduce the susceptibility to fatigue in slow muscle.
2.
The neuromuscular junction of fast muscle differs from that in slow muscle in terms of acetylcholine vesicle content, the amount of acetylcholine released, the density of nicotinic acetylcholine receptors, the acetylcholine esterase activity, and Na channel density, all of which endow the fast muscle with a higher safety factor for initiation of an action potential. During repetitive stimulation, however, the safety factor in fast muscle drops quickly (faster than that seen in slow muscle).
3.