Reflex

Reflex, in physiology, involuntary response to a stimulus by the animal organism. In its simplest form, it consists of the stimulation of a sensory (afferent) nerve through a sense organ, or receptor, followed by transmission of the stimulus, usually through a nerve centre, to a motor (efferent) nerve, resulting in action of a muscle or gland, called the effector. In most reflex actions, however, the stimulus passes through one or more intermediate nerve cells, which modify and direct its action—sometimes to the extent of involving the muscular activity of the entire organism. For example, a painful stimulus applied to the hand causes a reflex withdrawal of the hand, which involves contraction of the group of muscles which close the joint's angle (flexor muscles) and relaxation of the opposing group of muscles which normally keep the angle of the joint open (extensor muscles). If the stimulus is strong, the coordinating nerve cells pass it to the arm muscles and also to the muscles of the trunk and legs, the result being a jump that removes not only the arm, but the entire person from the vicinity of the painful stimulus.

The system of coordinating nerve cells is such that several different kinds of stimuli may produce the same result. For example, the stimulus produced by the sight of food and that caused by the smell of food travel along different afferent pathways, but both have a common final path that stimulates the salivary glands to secretion. The final common path may also be activated through associated nerve tracts by a stimulus that is not usually directly connected with the response. This type of reflex was named conditioned reflex by the Russian physiologist Ivan Pavlov. In 1904 Pavlov discovered that sounding a bell every time a dog was about to be given food eventually caused a reflex flow of saliva, which later persisted even when no food was produced. Elaborations of this habituative type of reflex are regarded by some physiologists and psychologists as an important basis for many types of behaviour, both voluntary and involuntary.

The normal pathways of many reflexes are generally known, and the presence, absence, or exaggeration of the normal physical responses to certain stimuli are symptoms used by neurologists to determine the condition of the neural pathways involved. A familiar reflex commonly tested by doctors is the patellar reflex, in which an involuntary jerk of the knee is evoked by lightly striking the tendon of the patella, or kneecap, indicating the efficiency of certain nerve tracts in the spinal cord.

See Nervous System; Neurophysiology.