What is a muscle spasm?
A muscle spasm or 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 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 that pumps blood (cardiac muscle),
- skeletal muscle that moves the external body parts,
- and smooth muscle that move portions of hollow structures inside the body, such as the stomach and intestine.
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 require 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 (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.