2-4. TYPES OF MUSCLE TISSUE
a. Skeletal Muscle. Each skeletal muscle is an individual organ of the human
body. Each is composed of several types of tissues, mainly striated muscle fibers, and
fibrous connective tissue (FCT). Each is attached to and moves bones. Bones are
parts of the skeleton serving as levers. The large portion of a muscle is known as its
belly or fleshy belly. The muscle is attached to bones by tendons or aponeuroses.
Tendons and aponeuroses are similar to each other. However, tendons are cord-like,
and aponeuroses are broad and flat. The fleshy portion may be directly connected to
the bone. If it is attached to the bone, it is called a "fleshy attachment."
(1) Anatomy. The muscle cells of skeletal muscles are elongated and are
called fibers. The fibers of the skeletal muscles are striated (a striped appearance) to
give strength. Movement of the skeleton, such as lifting a leg, is voluntary, as are all of
the movements characterized by the skeletal system.
(2) Physiology. The neuromuscular junction consists of a nerve fiber and a
skeletal muscle fiber. The nerve fiber is branched at the end to form a structure called
the end plate. This end plate invaginates into the muscle fiber, but it always stays
outside the membrane of the muscle. The sole feet are located at the tips of the
numerous branches of the end plate. The space between the fiber membrane and the
sole foot are referred to as the synaptic cleft. A gelatinous substance fills the synaptic
cleft. Mitochondria that supposedly synthesize the substance acetylcholine are located
in the sole foot. Numerous small vesicles (bags) serve as storage locations for acetyl-
choline. The enzyme cholinesterase, which is used to destroy acetylcholine, is also
found in the area of the synaptic cleft.
(a) Secretion of acetylcholine. The vesicles release acetylcholine
when a nerve impulse reaches the neuromuscular junction. Shortly after the
acetylcholine is released (around two milliseconds), it diffuses and no longer has any
effect upon the muscle. During the short time, the acetylcholine produces its effects
upon the muscle; the muscle becomes very permeable to sodium ions (Nat). Because
of the influx of sodium ions into the muscle, the electrical potential of the membrane
increases. Hence, the muscle fiber is stimulated. Figure 2-1 illustrates the contraction
of skeletal muscle.
(b) Destruction of acetylcholine. Shortly after the acetylcholine is
released, cholinesterase begins to destroy it. Such a rapid destruction of the
acetylcholine prevents it from re-stimulating the muscle until another nerve impulse
reaches the neuromuscular junction. Figure 2-2 illustrates the relaxation of the muscle