Reflex

Reflexes are set motor responses to specific sensory stimuli. All reflexes share three classical characteristics: they have a sensory inflow pathway, a central relay site, and a motor outflow pathway. Together, these three elements make up the reflex arc. Reflexes can also be characterized according to how much neural processing is involved in eliciting a response. Some reflexes, like the short reflex in the gastrointestinal mucous membranes that secrete digestive enzymes, involve very local neural pathways. Other reflexes relay information through the spinal cord or other higher brain regions. However, reflexes rarely involve lengthy processing. Just as some reflexes result from neutral stimuli, others result from neuroendocrine stimuli.

The human body has numerous essential reflexes. Among them are the reflexes for swallowing, lactation (the secretion of milk), digestion, elimination of body waste, and self-preservation. Chemical sensory neurons in the stomach trigger reflexive secretion of digestive enzymes.

Reflexes can be inborn or conditioned. Although the majority of reflexes are inborn responses, some reflexes are conditioned into a person as the result of life experiences. The classical example of a conditioned reflex would be a dog's salivating in response to a dinner bell. Inborn reflexes in adults include the knee-jerk reflex and various skin reflexes to heat or pressure. Other reflexes include shivering, pupil constriction in bright light, the plantar reflex (curling up of the toes when the sole of the foot is irritated), and vomiting. Blinking can also occur reflexively as a defense mechanism; for example, as a response to air being blown on the eye.

Newborn reflexes are inborn primitive reflexes that are present in the first few months of life. Because they are so highly conserved in humans, these reflexes are thought to have provided some advantage to humans during evolution. The rooting reflex—the turning of the infant's head toward a touch stimulus in response to a stroke on the cheek—allows the infant's mouth to locate the nipple for nursing. The suckling reflex—initiated by touching the mucous membranes on the inside of the mouth with any object—also serves to facilitate nursing. The grasping reflex is seen when an infant tightly grasps an object placed firmly in its hand. The walking reflex is obvious when a young baby is held upright with feet barely touching the surface below; the infant alternately puts weight on each foot. And the Moro (or startle) reflex is evident when the baby throws out and wriggles its arms as if to hold on to something when the baby's head is left momentarily unsupported. Each of these reflexes is routinely checked by a physician during the baby's physical examinations.

Reflexes utilize or affect different types of muscle tissue, including smooth, cardiac, or skeletal muscle tissue. Reflexes operating in conjunction with smooth muscle tissue include those found in the urinary bladder, colon, and rectum. Typically, when an organ surrounded by smooth muscle expands as it is filled, stretch receptors respond to initiate reflexive movement, emptying the organ. For example, in the bladder, as urinary volume increases, stretch receptors in the urinary smooth muscles signal relaxation of the bladder that opens to release urine. Some reflexes, such as the urinary reflex, can be consciously regulated. For example, someone can intentionally resist urinating until a later time; however, eventually the reflex will win out.

The swallowing reflex involves both smooth and skeletal muscle responses. A mass of food in the throat stimulates mechanoreceptors of the pharynx which relay impulses to the medulla in the nervous system. The medulla, in turn, signals skeletal muscles in the upper esophagus and smooth muscles in the lower esophagus to swallow.

Some reflexes effect skeletal muscle responses. The flexor withdrawal reflex involves cutaneous (skin) receptors and skeletal muscles. A good example of this reflex is observed when someone steps on a sharp tack. Pain receptors in the skin send a rapid message to the dorsal (back) side of the spinal cord that sends out immediate signals from the ventral (front) side of the spinal cord to muscles in both legs causing them to cooperate simultaneously to avoid stepping on the tack. The leg that stepped on the tack must flex (close) its knee joint and raise the thigh to lift the foot off the tack. The opposite leg immediately must bear the body's full weight. Most reflexes, such as this one, are mediated by the spinal cord in vertebrates (backbone animals). The dorsal side of the spinal cord receives sensory input, while the ventral side sends out motor commands. As such, most reflexes are under autonomic (involuntary) control.

Some reflexes orchestrate a response to a stimulus across multiple systems. The diving response is a breathing reflex that is triggered by submergence. Although this reflex is most pronounced in infants, it has also been documented in young children. This reflex prompts the subject to hold its breath when the face is submerged in water. The heart rate slows down, and blood flow to peripheral tissue decreases. The resulting accumulation of oxygenated blood in the central (critical) body regions helps preserve life during water submergence. Victims of prolonged submergence, however, can survive only if the water temperature (which decreases the metabolic rate) is exceptionally low. Reflexes are often assessed during a physical examination to determine appropriate reflex function or indicate problems with either the nervous or muscular system.