Skeletal muscles contain sensory fibers (muscle spindles; also called intrafusal fibers) that run parallel to the skeletal muscle fibers. The muscle spindles assess the degree of stretch of the muscle, as well as the speed of contraction. In the stretch reflex, rapid stretching of the muscle (e.g., tapping the tendon) lengthens the spindles in the muscle and results in an increased frequency of action potentials in the afferent sensory neurons of the spindle. These afferent fibers in turn excite the α motor neurons in the spinal cord that innervate the stretched muscle. The result is that the reflex arc is a stretch-induced contraction of the muscle that does not require input from high centers in the brain. As the muscle shortens, efferent output is also sent to the spindle, which thereby takes the slack out of the spindle and ensures its ability to respond to stretch at all muscle lengths. By their action, muscle spindles provide feedback to the muscle in terms of its length and thus help maintain a joint at a given angle.
Golgi tendon organs are located in the tendons of muscles and provide feedback regarding contraction of the muscle. The main component of the tendon organ is an elongated fascicle of collagen bundles that is in series with the muscle fibers and can respond to contractions of individual muscle fibers. A given tendon organ may attach to several fast-twitch or slow-twitch muscle fibers (or both) and sends impulses through type Ib afferent nerve fibers in response to muscle contraction. The type Ib afferent impulses enter the spinal cord, which can promote inhibition of α motor neurons to the contracting (and synergistic) muscles while promoting excitation of α motor neurons to antagonistic muscles. The inhibitory actions are mediated through interneurons in the cord that release an inhibitory transmitter to the α motor neuron and create an inhibitory postsynaptic potential. The type Ib afferent impulses are also sent to higher centers of the brain (including the motor cortex and cerebellum). It is hypothesized that feedback from the tendon organs in response to muscle contraction may smooth the progression of muscle contraction by limiting the recruitment of additional motor units. Of interest is that the response of the tendon organ is not linearly related to force; rather, it drops off at higher levels of force, which may facilitate the recruitment of motor units at higher levels of effort.
The skeletal system supports the body in an erect posture with the expenditure of relatively little energy. Nonetheless, even at rest, muscles normally exhibit some level of contractile activity. Isolated (i.e., denervated) unstimulated muscles are in a relaxed state and are said to be flaccid. However, relaxed muscles in the body are comparatively firm. This firmness, or tone, is caused by low levels of contractile activity in some of the motor units and is driven by reflex arcs from the muscle spindles. Interruption of the reflex arc by sectioning of the sensory afferent fibers abolishes this resting muscle tone. The tone in skeletal muscle is distinct from the “tone” in smooth muscle (see
Muscle cells convert chemical energy to mechanical energy. ATP is the energy source used for this conversion. The ATP pool in skeletal muscle is small and capable of supporting only a few contractions if not replenished. This pool, however, is continually replenished during contraction, as described later, so that even when the muscle fatigues, ATP stores are only modestly decreased.
Muscle cells contain creatine phosphate, which is used to convert ADP to ATP and thus replenish the ATP store during muscle contraction. The creatine phosphate store represents the immediate high-energy source for replenishing the ATP supply in skeletal muscle, especially during intense exercise. The enzyme creatine phosphokinase catalyzes the reaction: