Reflexes are the body's automatic reaction to some type of sensory stimuli. They involve nerve impulses passing from a receptor to a nerve center and then outward to, for example, a nerve or a gland.
Reflexes are classified as unconditioned and conditioned.
Unlike most human behaviors, unconditioned reflexes occur without specific learning or experience. They are considered involuntary acts, because a response occurs automatically when a stimulus (for example, a pinprick) takes place.
Unconditioned reflexes that protect us from harm are called nociceptive reflexes. For example, sneezing, coughing, and gagging are automatic responses to foreign bodies in the nose and throat. Eye blinking or winking helps protect the eye from harm. Reacting quickly to touching a hot stove is yet another example of a nociceptive reflex.
Most reflex acts are very complicated. However, in simple reflexes four events are involved: reception, conduction, transmission, and response. The stimulation is received by receptors, or sensitive nerve endings. These may be in the eye, ear, nose, tongue, or skin. Energy from the stimulus is changed into nerve impulses and conducted from the receptor to the central nervous system. From there, the nerve impulses are transmitted to the motor nerves that control muscle action. The motor nerves conduct the impulses to the muscles and glands, causing them to respond or act. For example, touching a hot stove stimulates receptors in the skin of the finger. This creates a nerve impulse that travels along a sensory nerve to the spinal cord. In the spinal cord, the sensory nerve fibers interlace with motor nerve fibers. The nerve impulse passes from the sensory fibers to the motor fibers, which relay it to the muscles, causing them to contract. When the muscles contract, the person's hand jerks back.
People have many reflex reactions to emotional stimuli, such as anger or fear, including changes in blood pressure and respiration. Lie detectors measure specific physical reactions to emotional stimuli.
Conditioned reflexes are acquired as the result of experience. When an action is performed repeatedly, the nervous system learns to react automatically. Walking, running, and typing are examples of learned conditioned activities that require a large number of complex muscular coordinations.
Conditioned reflexes work by association. For example, a dog's mouth begins to water when the animal smells food. The Russian physiologist Ivan P. Pavlov showed that the flow of saliva, originally an automatic reaction to the smell of food, may become a conditioned reflex. Pavlov rang a bell each time he brought food to a dog. Eventually, the dog's mouth began to water when Pavlov merely rang the bell without food being present. The dog associated the ringing of the bell with the food, just as it associated the odor with the food.
In a simple reflex, a sensory receptor initiates a nerve impulse in an afferent sensory nerve fiber that conducts it to the spinal cord. In the gray matter of the spinal cord, the afferent nerve impulse is fired over the synaptic gap to an efferent motor fiber that passes along the impulse to the appropriate muscle, producing the reflex.
Role in human health
Nerve cells are sensitive to disturbances caused by tumors, trauma, circulatory problems, metabolic disorders, and a host of other diseases that can be diagnosed by determining which reflexes show abnormalities. Abnormal reflexes may suggest the presence of significant central nervous system or peripheral nerve problems.
Reflex tests measure the presence and strength of a number of reflexes to help assess the integrity of the nerve circuits involved. Reflex tests are performed as part of a neurological exam to quickly confirm the integrity of the spinal cord or to diagnose the presence and location of spinal cord injury or neuromuscular disease.
Common diseases and disorders
Some of the more common reflex-related diseases and disorders include stroke, traumatic brain or spinal cord tumors or injury, multiple sclerosis, Wernicke-Korsakoff syndrome, cerebral palsy, and diabetic neuropathy.
Stroke is a brain disorder involving loss of brain functions due to interruption of the brain's blood supply.
Brain and spinal cord injury
Brain and spinal cord injuries most commonly result from motor vehicle accidents, falls, sports injuries, industrial accidents, gunshot wounds, and criminal assault. Damage to the spinal cord affects all nerve function at and below the level of the injury, including muscle control and sensation.
Brain and spinal tumors
Brain and spinal cord tumors are abnormal growths of tissue found inside the skull or the spinal column. The word tumor is used to describe both abnormal growths that are new (neoplasms) and those present at birth (congenital).
Multiple sclerosis involves inflammation within the central nervous system, followed by demyelination, which is a loss of the protective myelin sheaths that surround nerve fibers. When the myelin is damaged, nerve impulses are not transmitted quickly and efficiently. As a result of the inflammatory process, lesions develop in the brain and spinal cord, causing a variety of neurologic symptoms, such as vision loss, numbness or tingling, weakness, unsteady gait, double vision, fatigue, heat intolerance, partial or complete paralysis, and electric shock sensations when bending the neck. These symptoms may cease or may persist after an attack. Symptoms may become progressively worse over time. For individuals with progressive forms of multiple sclerosis, these symptoms may gradually worsen over time without rapid or abrupt changes.
Wernicke-Korsakoff syndrome usually affects people between 40 and 80 years old. The onset is gradual. The syndrome is actually two disorders that may occur independently or together. Wernicke's disease involves damage to multiple nerves in both the central nervous system and the peripheral nervous system. It may also include symptoms caused by alcohol withdrawal. The cause is generally attributed to malnutrition—especially lack of vitamin B1 (thiamine ), which commonly accompanies habitual alcohol use or alcoholism.
Korsakoff syndrome, or Korsakoff psychosis, involves impairment of memory and intellectual/cognitive skills such as problem solving or learning, along with multiple symptoms of nerve damage. The most distinguishing symptom is confabulation (fabrication), during which the person makes up detailed, believable stories about experiences or situations to cover the gaps in their memory. Korsakoff psychosis involves damage to areas of the brain.
Cerebral palsy is a persistent qualitative motor disorder caused by nonprogressive damage to the brain. Although manifested primarily by motor dysfunction, the disorder also may involve sensory deficits and impairment of the intellect. The majority of cases are caused during labor and delivery or during the first month of infancy. Cerebral palsy may be caused by premature birth, prolonged labor, or traumatic delivery. Any situation that interferes with fetal oxygen supply can produce brain damage and cerebral palsy.
Diabetic neuropathy is a nerve disorder caused by diabetes. Symptoms of neuropathy include numbness and sometimes pain in the hands, feet, or legs. Nerve damage caused by diabetes can also lead to problems with internal organs such as the digestive tract, heart, and sexual organs, causing indigestion, diarrhea or constipation, dizziness, bladder infections, and impotence.
Afferent— Conveying impulses toward a nerve center, such as the brain or spinal cord.
Efferent— Conveying nervous impulses outward to nerves or neurons.
Nociceptive— A stimulus that causes pain or injury.
Synaptic gap— The space between neurons across which a nerve impulse is transmitted by a neurotransmitter. Also referred to as a synaptic cleft.
Schwartz, J. H., ed. Principles of Neural Science. Stamford, CT: Appleton & Lange, 2000.
Simon, R. Clinical Neurology. 4th ed. Stamford, CT: Appleton & Lange, 1998.
National Institute of Neurological Disorders and Stroke. 〈http://www.ninds.nih.gov.
National Stroke Association. 9707 E. Easter Lane, Englewood, CO 80112. (800) STROKES. (303) 649-9299. 〈http://www.stroke.org/〉.