MUSCULAR SYSTEM

Skeletal Muscles

There are over 600 skeletal muscles that serve for body movement through contraction and
relaxation. These muscles are voluntary (under conscious control), made up of striated muscle
fibres (of two types: fast-twitch and slow-twitch), and attached to bones by tendons.  The kinds of
body movement include locomotion, facial expressions, posture, and other movements. Skeletal
muscles also have muscle tone (partial contraction), maintained by ongoing signals from the nervous
system, which helps maintain body posture and readiness for physical activity. Muscles account for
approximately 40 per cent of body weight and the metabolism that occurs in this large mass produces
the heat that is essential to the maintenance of body temperature.



Skeletal muscles form largely a system of pairs of muscles that contract and relax to move or bend
a joint. For instance, when the anterior thigh muscle (the quadriceps) contracts, the knee
straightens whilst the posterior thigh muscle (the biceps) relaxes. In this instance, the
quadriceps of the thigh is referred to as the agonist (or prime mover), i.e. the muscle in state of
contraction producing the prime, first motion. To bend the knee back again and return it to its
original bent position, the biceps of the thigh contracts (pulls) whilst the quadriceps relaxes.
Then, the contracting biceps is termed the antagonist as it produces the opposing motion. Similar
opposition is found in the upper arm between the biceps and the triceps (they bend the elbow
joint). There are also synergists, muscles that help other skeletal muscles in their actions, and
fixators, muscles that stabilise, or fix, ends of other muscles. Some skeletal muscles are also
classified for their ability to straighten (extend) or bend (flex) joints, and thus whole limbs, as
extensors and flexors, respectively.


The Cardiac Muscle

The cardiac muscle (or myocardium, the middle muscular layer of the heart) is found only in the
heart and it forms the atria (sing. atrium) and ventricles, i.e. the walls of the organ. Like the
skeletal muscles, the cardiac muscle contains striated fibres, but unlike the skeletal muscles it
is involuntary (not under conscious control). The muscle cells of the myocardium maintain a
consistent heartbeat (or heart rate). The heart rate as observed in the arteries of the body is
termed pulse rate, and it is recorded as the number of beats per minute.

Smooth Muscles

Smooth muscles are found throughout the body:  in internal organs, blood vessels, and various
glands. Similarly to the cardiac muscle, smooth muscles are involuntary, but in contrast to
skeletal muscles and the cardiac muscle, they are not striated (or nonstriated). Smooth muscles
within the walls of the digestive tract organs cause peristalsis (or vermiculation) which is the
wave-like, worm-like movement of the intestine consisting of alternate circular contractions that
aids in intestinal digestion and transport of food. Other examples of involuntary activities of
smooth muscles are constriction (closing) of the bronchioles, pupils of the eye or skin contraction
in cold conditions (called colloquially gooseflesh or goose pimples; also goosebumps in American
English).