Reflex action

Reflex Action Definition

A reflex action may be defined as a spontaneous, automatic and mechanical response to a stimulus acting on a specific receptor without the will of an animal. 

Reflex Action Examples. One example of reflex action has been cited above. Other common examples of reflex actions in man are knee-jerk reflex, movement of diaphragm during respiration, blinking of eyes, coughing, yawning, sneezing etc. In knee-jerk reflex, a gentle strike below the knee cap, while sitting with freely hanging legs, kicks the leg forward. Another very good example is afforded by the withdrawal of the leg of a decapitated frog*, also called a “spinal frog”, when touched with an acid or a live electric wire, Here the action of the frog does not at all involve its will as it is without a brain.

Reflex actions are performed in the presence of the brain also, e.g., closing of the eyes if strong light is suddenly flashed on them or some object suddenly comes too near them, watering of mouth on seeing delicious food. These reflex actions, though performed without our will, are in our knowledge. There are many reflex actions which on without our knowledge, e.g., flow of bile from the gall- bladder into the duodenum when the food reaches there, peristalsis of the alimentary canal, beating of the heart, etc. These and other physiological actions are controlled by simple reflexes of autonomic nervous system.

The reflex actions controlled by spinal cord and brain are respectively called spinal reflex actions and cerebral reflex actions.

Reflex action diagram

Types of Actions

Animals show two types of actions: voluntary and involuntary. A voluntary action is performed by the animal with its will. In this action, the animal exercises its choice, so that the same stimulus may depending upon the situation. For example, seeing a snake in the way, one may run away, or call for help, or try to kill it to save oneself. An involuntary action, on the other hand, is performed by the animal without its will. It is very quick and the animal has no choice in it. Therefore, the same stimulus always gets the same response just as pressing a particular button of a machine brings into action a definite part of it. For example, the hand or foot is withdrawn every time it is suddenly pinched or pricked with a needle, or touched by a hot object. The involuntary actions are known as the reflex actions.

Mechanism of Reflex Action 

A reflex action is brought about in the following way. When acid is applied to a toe of a decapitated frog, the stimulus is received by a receptor in the skin. Receptor is a general term for any type of sense organ. On receiving a stimulus, the receptor sets up a sensory impulse. The latter is carried to the spinal cord through the dorsal sensory root of a spinal nerve, i.e., sciatic nerve, in the above example. The spinal cord transforms the sensory impulse into a motor impulse. The latter is transmitted to the leg muscles. The muscles then contract and the leg is withdrawn to avoid the stimulus. The muscles are referred to as the effectors, where the impulse ends and response is given. 

Reflex Arc

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The path travelled by an impulse in a reflex action is called the reflex arc . 

It consists of five parts : 

(i) A specific receptor, the neurons of which receive a stimulus and set up a sensory impulse

(ii) An afferent nerve, which brings the sensory impulse from the receptor to the central nervous system. 

(iii) A portion of the central nervous system, brain or spinal cord, the neurons of which analyse and interpret the sensory impulse and set up an appropriate motor impulse. Brain and spinal cord are called modulators.

(iv) An efferent nerve, which carries the motor impulse from the central nervous system to the specific effectors (muscle fibres or gland cells.

(v) An effector, where impulse terminates and response is given as per instructions received from the modulator.

There may be connector (intermediate, relaying) neurons between the sensory and motor neurons.

A reflex pathway results in rapid responses to stimuli because it has a small number of synapses. In addition, the message need not make a lengthy trip to brain and back to give an appropriate response.

A nerve impulse can flow only in a single direction in a reflex arc (afferent to efferent neuron), because the nerve impulse can cross a synapse in one way only. Therefore, stimulating an effector or an efferent neuron cannot produce a reflex: response in a receptor.

Repeated stimulation of a receptor may temporarily suspend the reflex response because the synapses in the reflex arc are fatigued.

Importance of Reflex Actions 

Reflex action is very important. It has two advantages 

(i) It enables the animal to respond immediately to the harmful stimuli so that no harm is caused to it.

(ii) It relieves the brain of too much work as the responses of routine nature take the form of reflex actions. If the animal were to exercise its will every time a wave of peristalsis started in the intestine, the brain would soon be exhausted.

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