Naples Sports Medicine

Understanding Muscles

Muscles are largely composed of protein, with a hierarchical system of organization from large groups of small fibers. A muscle is a group of motor units that are physically separated by a membrane from other groups of motor units. A muscle is connected to bones trough tendons.

A motor unit consists of a single neuron and all of the muscle fibers innervated by it. The ratio of nerves to fibers determines the fine motor control available to that muscle. For example, the hand has fewer fibers per motor unit than do the muscles of the calf.

The muscle fiber is composed of myofibrils, which are small bundles of myofilaments. These are the elements of the muscle that actually shorten upon contraction. Myofilaments are made up mainly of two types of protein, myosin and actin. Myosin are short and thick while actin are long and thin. Two other important proteins comprising myofibrils are troponin and tropomyosin, involved in the contractile response.

The main function of muscle tissue is contraction. This contractions can be brought by either involuntary or voluntary stimuli. Voluntary muscle tissues receive nerve fibers from the somatic nervous system. Therefore, their contraction can be voluntary controlled. Skeletal muscles are the major voluntary muscle tissue. Involuntary muscle tissue receive nerve fibers from the autonomic nervous system and cannot be voluntary controlled, except in a few rare cases. The heart is the principal example of involuntary muscle tissue.

With few exceptions, single muscles never contract by themselves. Rather, specific sets of muscles contract together or in a sequence. The production of complex movements responsible for even the simplest tasks is dependent upon a correspondingly subtle control mechanism. This is the responsibility of the nervous system, which neutralizes the actions of muscles that are not required and causes the contraction of muscles that are required. The spinal cord and the brain exercise this control through the motor nerve fibers.

Each muscle cell does not have an individual line from the central nervous system (CNS). Impulses travel down the nerve axon from the CNS, branching off to supply a group of muscle cells, which contract together. In order to coordinate muscular movement, the CNS must be supplied with information about the length of the muscle and the tension of the tendons, which attach it to the skeleton. This information is provided by special sense organs called “muscle spindles” which measure the strain in the muscle and can be used to pre-set the tension of muscles.

Skeletal muscles must contract rapidly in response to signals from the CNS, and they must develop adequate tension at the same time to produce and effective mechanical force. Examination of skeletal muscle reveals a junction between the nerve fiber and the muscle surface. The surface acts as an amplifier, increasing the effect of the tiny current coming from the nerve fiber to stimulate the larger muscle fiber. The arrival of the nerve impulse triggers the release of a chemical called acetylcholine from the motor nerve ending. This passes across the gap to stimulate the membrane of muscle fiber. This stimulation, in the form of an electric current, passes along the surface of the muscle and causes it to contract. It takes only 1/1,000th of a second for the current to pass along the surface of the muscle fiber. The fiber releases unless yet another impulse arrives. If tis chemical mechanism is blocked, the result would be paralysis.

Orthopedic Corner | Leon Mead MD Orthopedic Doctor | 730 Goodlette Road North, Suite 201  Naples Florida 34102 | Phone: (239) 262-1119