Reactions and Reflexes

Ronald E. Hammond, PhD
Product Manager, Anatomy and Physiology

Figure 1  Schematic representation of the sensorimotor pathway for a volunatry response to a stimulus.

Reflexes and reactions are often confused, but important differences exist. Reactions are voluntary responses whereas reflexes are involuntary or unintentional (and not subject to conscious control in most cases). Each type of response is initiated by a sensory stimulus that may be visual, audible, tactile, olfactory, or gustatory in nature. The stimulus excites specialized sensory receptors that respond specifically to a certain type, quality, and/or intensity of stimulation. Once activated, the receptors propagate nerve impulses that travel toward the brain along sensory (afferent) nerve tracts. The speed of a reflex is greater than that of a voluntary reaction, due largely to the relative complexity of the neural pathway for a reaction (Fig. 1) compared to that for a reflex (Fig. 2).

Reactions or reflexes?

An effective way to help your students differentiate reactions and reflexes is by leading them through a scenario involving a familiar sport. For example, you can describe the events that occur when a baseball player is at bat. He faces the pitcher, who stands 60½ ft away while he prepares to deliver a fastball at 90 mph. The ball will travel from his hand to the plate in approximately 0.45 s. That is all the time the batter will have to respond by either swinging the bat or allowing the ball to pass by. Many hours of practice have provided him with an uncanny ability to judge how the pitch should be played. When he launches a home run over the left field fence, the announcer praises his "unbelievable reflexes." This assessment of the player's ability is not accurate, however, because his response was not truly reflexive as it required him to make a decision and an intentional response.

Now return your students to the batter’s box for a different scenario in which the pitcher loses control and hurls a fastball directly at the batter's face. The batter's eyes close involuntarily as he reacts by ducking sideways to avoid being hit. Again the announcer applauds the player's "terrific reflexes," and this time he is at least partially correct.

Although reflexes are automatic and predictable, they have value in the avoidance of injury. In the above situation, the batter’s eyes closed reflexively in response to the baseball thrown at his face, thereby lessening the likelihood of injury to his eyes. At the same time, he responded to the danger by utilizing voluntary movement to get out of the way.

How much time?

Figure 2  Generalized reflex arc.

Reaction time is the amount of time required for an individual to perceive and respond to a sensory stimulus (such as the act of swinging a bat at a baseball). The average reaction time for a visual stimulus is 200 to 250 ms; for hearing, 150 to 200 ms; and for touch, 130 to 170 ms. Reaction time improves somewhat through repetition, which is a beneficial result of the many hours of practice that athletes endure. Ultimately, however, the speed at which a nerve impulse travels along a neural pathway (called nerve conduction velocity) limits reaction time. The diameter of the nerve and the amount of myelination can affect nerve conduction velocity. Some large myelinated nerves are capable of conducting impulses at speeds up to 120 m/s.

The simplest, least expensive way for students to measure reaction time is with a reaction time ruler. This device is similar to a yardstick, except its scale is in milliseconds (ms) to correspond with the progressive acceleration of a falling object. The subject sits with his thumb and forefinger poised opposite a designated point near the bottom of the ruler while the experimenter holds the ruler vertically. The experimenter allows the ruler to drop and the subject stops it as quickly as possible without moving his hand upward or downward. The time mark at the point where the subject’s fingers catch the ruler is recorded as his visual reaction time. Students can design their own experiments, testing the effects of distraction, fatigue, caffeine, and other factors on their performance. These experiments can be an excellent introduction to the fundamental procedures used in scientific data collection and analysis.

Reaction time rulers, along with materials and instructions for eliciting and studying a variety of reflex mechanisms, are included in the Carolina™ Reflexes and Reactions Kit.