Introduction to Muscle

Skeletal muscle is a classic example of a biological structure-function relationship. At both macro- and micro-scopic levels, skeletal muscle is exquisitely tailored for force generation and movement. This page is an attempt to at least mention some of the important aspects of the basic science of the neuromuscular system, a kind of table of contents in prose.

Development

The development of the neuromuscular system is typically divided into at least three phases. Myogenesis (perhaps synchronous with axonal outgrowth) refers to the fusion of the precursor myoblasts into true muscle fibers. Nerves attach to, or innervate, fibers as (or perhaps just before) they develop during the phase of synaptogenesis. This process results in most fibers being multiply innervated. The several axons in contact with each fiber compete for control during synapse elimination until each fiber is synapsed with only one axon. Single innervation is believed to be very important, as the axon is thought to have a strong influence on the properties of the fiber.

Muscle Fiber Structure

Muscle cells are roughly cylindrical, with diameters between 10 and 100 µm but up to a few centimeters long. Each cell is embedded in a basal lamina of collagen and large glycoproteins. Between the fiber and the basal lamina are large numbers of satellite cells, that are important in the growth and repair of the fiber. The fiber itself contains specialized structures for excitation-contraction coupling to ensure that a contractile stimulus (received at the synapse) is rapidly and evenly communicated to the whole fiber. Contractile and performance characteristics vary, but are closely linked to the myosin heavy chain isoform expressed by the fiber. Force production occurs in the myofibrils, which are chains of sarcomeres running from one end of the fiber to the other. Energy for contraction comes from metabolism of fats and sugars.

Muscle Architecture

The properties of a whole muscle depend not only on the properties of the fibers, but also on the organization of those fibers: the muscle architecture. Fibers rarely run the whole length of the muscle, tending to be somewhat oblique to the muscle's line of action. Peak force production is related to the physiological cross sectional area (PCSA), which estimates the sum of the cross sectional area of all the fibers. Contraction velocity and excursion range are related to fiber length.

Control of Contraction

Although each fiber is innervated by a single axon, a motorneuron may have a hundred or more axons. A single motorneuron, with all the fibers it controls, is called a motor unit. As the brain's signal for contraction increases, it both recruits more motor units and increases the "firing frequency" of those units already recruited. Even during a "maximal voluntary contraction", it is unlikely that all the motor units (and hence muscle fibers) are activated.

Biomechanics of Strength

The above discussion focused on the muscle itself. All joints, however, are set up as lever systems: the fulcrum where two bones meet, one force produced by the muscle, and the other by a load. Strength is not just muscle force, but muscle force as modified by the mechanical advantage of the joint. To complicate matters further, this mechanical advantage usually varies with joint rotation (as does the muscle force). The net result is strength that varies with joint angle and may be somewhat decoupled from muscle force. Joint strength can (obviously) be increased with exercise.