Autonomic Reflexes in the Spinal Cord
Many types of segmental autonomic reflexes are integrated in the spinal cord. Briefly, these include (1) changes in vas-cular tone resulting from changes in local skin heat; (2) sweating, which results from localized heat on the surface of the body; (3) intestinointestinal reflexes that control some motor functions of the gut; (4) peritoneoin-testinal reflexes that inhibit gastrointestinal motility in response to peritoneal irritation; and (5) evacuation reflexes for emptying the full bladder or the colon. In addi-tion, all the segmental reflexes can at times be elicited simultaneously in the form of the so-called mass reflex, described next.Mass Reflex. In a spinal animal or human being, some-times the spinal cord suddenly becomes excessively active, causing massive discharge in large portions of the cord. The usual stimulus that causes this is a strong pain stimulus to the skin or excessive filling of a viscus, such as overdistention of the bladder or the gut. Regardless of the type of stimulus, the resulting reflex, called the mass reflex, involves large portions or even all of thecord. The effects are (1) a major portion of the body’s skeletal muscles goes into strong flexor spasm; (2) the colon and bladder are likely to evacuate; (3) the arte-rial pressure often rises to maximal values, sometimes to a systolic pressure well over 200 mm Hg; and (4) large areas of the body break out into profuse sweating.
Because the mass reflex can last for minutes, it pre-sumably results from activation of great numbers of reverberating circuits that excite large areas of the cord at once. This is similar to the mechanism of epileptic seizures, which involve reverberating circuits that occur in the brain instead of in the cord.