In the body of an animal two types of activities take place: voluntary and involuntary. Voluntary action happens according to the will of the individual while there is not any control of will over involuntary action. Involuntary action may again be divided into two categories. The action of first category is related to internal body organs. It is controlled by medulla oblongata. The second type of action is performed by outer body organs. It is under the control of spinal cord. In broader sense, all the involuntary actions are reflex action. But the term reflex action is applied only to those actions which are performed by external body organs without any control of ones will.
Reflex action is an immediate involuntary response to a stimulus. In an involuntary response, a sensory impulse on reaching central nervous system is itself returned to a specific effector as a motor impulse without neuronal analysis and integration. Reflex responses occur below the level of consciousness. Reflex actions are sudden and autonomic. For instance, if ones foot is trickled the leg is drawn at once and this will happen even when one is busy in other activities upon which all the attention is concentrated and perhaps without even being conscious of the irritation. Therefore, we can say that reflex actions are those actions which result from the conversion of sensory impulses into motor impulse by a nerve center, like spinal cord, without the intervention of brain. Some common examples of reflex actions are:
- The knee jerk reflex, when tendon of the knee cap is tapped, it stimulates the stretch receptors in the tendon and leg is involuntarily and momentarily straightened.
- The quick closing of the eyelids or blinking reflex, when an object approaches the eye.
- Sneezing reflex in response to irritation of lining of nose.
Types of reflexes
Reflex actions may be divided into the following two types:
Inborn or unconditioned reflexes : Inborn reflexes are also celled as simple reflex actions. Some of the reflexes are present even at the time of birth. These reflexes are called inborn or unconditioned reflexes. For example, taste of milk causes salivation even in a new born baby, even though the infant has not tasted the food earlier, the pupil of the eye constricts even if the eye is illuminated by light at the time of birth and so on. Inborn reflexes are transmitted through heredity. Inborn reflexes are elicited in response to definite stimuli. The reflex arcs of unconditioned reflexes are constant.
Conditioned reflexes : Conditioned reflexes are acquired through learning or experience to stimuli which originally failed to elicit a reaction. The conditioned reflexes involve the establishment to new reflex arcs and that close into the cerebral cortex. Conditioned reflexes are temporary in nature and may disappear or reappear again. For example, if an animal smells food which he has not tasted earlier it does not salivate, but if the animal sees and smells food many times before eating, saliva is produced. Seeing and smelling of food has conditioned the nervous system of the animal to produce a reflex. The stimulus which produces such a reflex is termed as conditioned stimulus.
The structural and functional unit in the simple reflex is termed as reflex arc. In its basic form reflex arc is regarded as simple nervous pathway connecting a receptor and an effector. Reflex arc has the following parts:
Receptor : Receptor represented by single sensory cell or a group of cells which receives stimuli.
Sensory or afferent neuron : Sensory connects the receptor to the spinal cord. The cell body of sensory is stimulated in the dorsal root ganglion of the spinal nerve. Sensory conveys impulses from the receptor to the spinal cord.
Interneuron or association neuron : Interneuron is present in the spinal cord. Interneuron connects afferent and efferent neurons and passes impulses from afferent to efferent neuron. Generally, there is only one association neuron in the reflex but sometimes two or more are involved in one reflex arc.
Motor neuron or efferent neuron : Motor neuron is located in the ventral root of spinal cord. Motor neuron transmits impulses to the effecteo organ.
Effector organ : It responds to the impulses received. E.g., a muscle or a gland.
Mechanism of Reflex action
Reflex action happens in so less time that it can hardly be estimated that when was the stimulus generated and when the action took place. In man the stimulus of reflex action travels with a speed of 320 km per hour. This can be understood with an example. If a needle is pricked in the hand or leg of a person, it is instantly pulled out by him. In this short span of time, a number of physiological activities take place. Pricking of needle functions as stimulus. The sensory organ receives this stimulus which is transmitted to the center of nervous system through nerve fibers. The impulse of stimulus is converted into efferent impulse by the brain. This is transmitted from brain to muscles of the receptor sense organ through motor nerve fibers. AS a result the organ (hand or leg) is instantly pulled out from the place.