Reflex action | Definition, Types and Mechanism and Important solved questions

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 meaning

Reflex means Rapid, involuntary Motor Response to Stimulus

Reflex actions means something that you do without thinking, as a reaction to a situation.

What is an example of a reflex action ?

One example 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.

They 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.

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 For example, the hand or foot is withdrawn every time it is suddenly pinched or pricked with a needle, or touched by a hot object.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.

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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

Reflex arc

The path travelled by an impulse in a reflex action is called the reflex arc . A reflex arc starts from the sensory neuron which is affected by the stimulus, and ends with the last neuron that synapses with the affector; including all the neurons in between.

Reflex arc is a type of emergency mechanism in your nervous system.

For Example – Suppose you touch a Cactus plant, by mistake. To keep your body safe, your arm has to be moved away from the point of contact with the cactus plant. That can be done when the muscles get the instructions for moving your hand away.

Now, your brain is already busy with controlling all the other metabolic pathways of your body. So, if the required signals for moving your hand has to come from your brain, it will take a long time and your finger will feel hurt or pain . To avoid that, the signals have to be generated from some other source, which is your spinal cord.

So what happens is, once the receptors in your finger tips get the ‘thorns in cactus plant ’ information, the afferent nerves will carry the signals to your spinal cord in the form of electric impulses. There, the information will be processed and the required electric impulses will be generated. Now, the efferent nerves will carry those impulses from your spinal cord to the effectors in your arm muscles. These impulses will then move your hand away from the cactus plant, by contracting and relaxing your muscles.

This whole process of carrying the impulses to your spinal cord, processing the impulses and carrying generated impulses to your muscles is called the Reflex Arc.

What are the components of reflex arc ?

There are five components of reflex arc which are affecting inner organs and muscles.

(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.

Why reflex action is important ?

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.

Important Solved Questions for Exams

1. Who discovered reflex action ?
Ans. Marshall Hall (born Feb. 18, 1790, Basford, Nottinghamshire, Eng. —died Aug. 11, 1857, Brighton, East Sussex), he was an English physician, physiologist and early neurologist who was the first to advance a scientific explanation of reflex action and attached his name to the theory of reflex arc mediated by the spinal cord.

2. What is Reflex action ?
Ans. Reflex action is a fast, sudden, involuntary, unplanned, sequence of action that occur in response to a particular stimulus.

3. Which organ or part controls reflex action ?
Ans. The reflex actions controlled by spinal cord and brain are respectively called spinal reflex actions and cerebral reflex actions.

4. Which action is a reflex action ?
Ans. Involuntary actions, this actions is something that we cannot control. We do not tell our body to do it. Involuntary actions include breathing, blinking, our heart beating . In other words say this  is the extremely quick, automatic, sudden action in response to something in the environment.

5. What happens during reflex action ?
Ans. Reflex action is a sudden response to changes in a enviroment without any thinking of feeling in control of a reaction . Example – I pulled my hand back from the flame suddenly.

6. What is a Reflex arc ?
Ans. A reflex arc is a pathway of nerve impulse during reflex action.
What is a reflex arc ?

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