Muscles of the Human Body

Muscles provide the mass and shape of the human body and the joints afford flexibility.

There are three types of muscles in the human body- the skeletal muscles which are used to produce movements of the body, the smooth muscles which are found in the small and large intestines, urinary bladder, stomach, and uterus, and the cardiac muscles which are found in the heart.

The skeletal muscles are also called voluntary muscles and are made up of striped muscle fibers. The muscles have two or more attachments. The attachment, that moves the least, is the origin, and that which moves the most is the insertion. The ends of the muscles are attached to bones, cartilage, or ligaments by cords of fibrous tissue called tendons. Muscles that run obliquely to the line of pull are referred to as pennate muscles resembling a feather. Every muscle is in a partial state of contraction while resting. This is called muscle tone. 

Since muscle fibers are either fully contracted or relaxed, there being no intermediate stage, it follows that a few muscle fibers within the muscle are fully contracted all the time. To bring about this state and to avoid fatigue, different groups of motor units, and thus different groups of muscle fibers, are brought into action at different times. This is accomplished by the asynchronous discharge of nervous impulses in the motor neurons in the anterior gray horn of the spinal cord.

Muscle tone basically is dependent on the integrity of a simple monosynaptic reflex arc composed of two neurons in the nervous system. The nervous impulses travel in the afferent neurons that enter the spinal cord. There they synapse with the motor neurons situated in the anterior gray horn, which in turn, send impulses down their axons to the muscle fibers. Should the afferent or efferent pathways are cut the muscle would immediately lose its tone and become flaccid. A flaccid muscle on palpation feels like a mass of dough and has completely lost its resilience. It quickly atrophies and loses its volume.

Muscle movement is accomplished by bringing into action increasing numbers of motor units and at the same time reducing the activity of the motor units of muscles that will oppose or antagonize the movement. It is important to understand that all movements are the result of the coordinated action of many muscles. A muscle may work in any of four ways- as a prime mover, as an antagonist, as a fixator, and as a synergist.

The Smooth Muscles

In the tubes of the body, smooth muscles provide the motive power for propelling the contents of the lumen. It milks the contents onward and also helps in mixing the contents for digestion.

The Cardiac Muscles

The cardiac muscles are made up of striated muscle fibers that branch and unite with each other. It is found in the myocardium of the heart. Its fibers tend to be arranged in whorls and spirals and have the property of spontaneous and rhythmical contraction. Specialized cardiac muscle fibers form the conducting system of the heart. The cardiac muscle is supplied by autonomic nerve fibers that terminate in the nodes of the conducting system and in the myocardium.