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What is a muscle spasm?
A muscle spasm, or muscle cramp, is an involuntary contraction of a muscle. Muscle
spasms occur suddenly, usually resolve quickly, and are often painful.
A muscle spasm is different than a muscle twitch. A muscle twitch, or
fasciculation, is an uncontrolled fine movement of a small segment of a larger
muscle that can be seen under the skin.
Muscles are complex structures that cause movement in the body. There are
three types of muscle in the body:
Heart muscle pumps blood (cardiac muscle).
Skeletal muscle moves the external body parts, like the arms and legs, and the neck, back and trunk.
Smooth muscle moves portions of hollow structures inside the body. Examples include the muscles that line the esophagus, stomach and intestine, muscles that line large arteries and the muscles of the uterus.
Skeletal muscles are anchored to bone, either directly or by a tendon. When
the muscle contracts, the associated structure moves. This allows arms to lift,
legs to run, and the face to smile. Most of these muscles are under willful or
conscious control of the brain. This type of muscle is striated or striped with
dark-colored muscles fibers containing large amounts of myoglobin, the protein
that helps carry oxygen and light-colored fibers that have lesser amounts of the
protein. The contraction of a skeletal muscle requires numerous steps within
cells and fibers that need oxygen, electrolytes, and glucose, which are
supplied by the bloodstream.
Smooth muscle is located in the walls of hollow internal structures in the body,
like the arteries, intestines, bladder, and iris of the eye. They tend to circle
the structure and when they contract, the hollow structure is squeezed. These
muscles are involuntary and are controlled by the unconscious part of our brain
function using the autonomic nervous system. The autonomic nervous system can
run in the background, regulating body processes automatically for us. There is
a balance between the sympathetic system (adrenergic nerves) that speed things
up and the parasympathetic system (cholinergic nerves) that slow things down.
These names are based on the type of chemical that is used to transmit signals
at the nerve endings. Adrenaline (epinephrine from the sympathetic nervous system) allows
the body to respond to stress. Imagine seeing a bear in the woods; your heart
beats faster, your palms get sweaty, your eyes dilate, your hair stands on end,
and your bowels move. Acetylcholine is the chemical that is the
anti-adrenaline and is involved in the parasympathetic nervous system. Smooth
muscle has the same basic contraction mechanism as skeletal muscle, though
different proteins are involved.