Spinal Cord Reflexes That Cause Muscle Spasm
In human beings, local muscle spasm is often observed. In many, if not most, instances, localized pain is the cause of the local spasm.
Muscle Spasm Resulting from a Broken Bone. One type ofclinically important spasm occurs in muscles that sur-round a broken bone. This results from pain impulses initiated from the broken edges of the bone, which cause the muscles that surround the area to contract tonically. Pain relief obtained by injecting a local anes-thetic at the broken edges of the bone relieves the spasm; a deep general anesthetic of the entire body, such as ether anesthesia, also relieves the spasm. One of these two anesthetic procedures is often necessary before the spasm can be overcome sufficiently for the two ends of the bone to be set back into their appro-priate positions.
Abdominal Muscle Spasm in Peritonitis. Another type oflocal spasm caused by cord reflexes is abdominal spasm resulting from irritation of the parietal peritoneum by peritonitis. Here again, relief of the pain caused by the peritonitis allows the spastic muscle to relax. The same type of spasm often occurs during surgical operations; for instance, during abdominal operations, pain im-pulses from the parietal peritoneum often cause the abdominal muscles to contract extensively, sometimes extruding the intestines through the surgical wound. For this reason, deep anesthesia is usually required for intra-abdominal operations.
Muscle Cramps. Still another type of local spasm is thetypical muscle cramp. Electromyographic studies indi-cate that the cause of at least some muscle cramps is as follows: Any local irritating factor or metabolic abnor-mality of a muscle, such as severe cold, lack of blood flow, or overexercise, can elicit pain or other sensory signals transmitted from the muscle to the spinal cord, which in turn cause reflex feedback muscle contraction. The contraction is believed to stimulate the same sensory receptors even more, which causes the spinal cord to increase the intensity of contraction. Thus, pos-itive feedback develops, so that a small amount of initial irritation causes more and more contraction until a full-blown muscle cramp ensues.