Reflex Action and Reflex Arc:
What is Reflex Action?
It was discovered by Marshal Hall in 1883.
A reflex action may be defined as a spontaneous, automatic, involuntary, nerve-mediated activity produced at the unconscious level by stimulating specific receptors. As a matter of fact, reflex or a reflex action is a mechanism by which sensory impulse is automatically converted into a motor effect through the involvement of the central nervous system.
Reflex actions occur below the level of consciousness. These are sudden and automatic. These occur very fast, even before one can feel the stimulus through the brain.
Examples of Reflex Action:
Following examples may be cited in favour of reflex actions-
(1) Gall bladder secretes bile into the duodenum when the food reaches therein.
(2) The eyelids blink when a moving object approaches the face.
(3) Sneezing is a reflex response to a stimulus in the nose.
(4) The lower leg gives a jerk in the forward direction when the doctor taps the tendon of a knee.
(5) Yawing in response to increased CO2 in the blood and skeletal muscles of the face and thorax.
(6) Withdrawl of hand when pricked with a needle.
(7) Withdrawl of leg in a decapitated frog when touched with acid or a live electric wire.
Mechanism of Reflex Action:
A reflex action is brought about in the following way-
(1) Receptor organ, for example- the skin of the toe receives the stimulus when acid is applied to a toe of decapitated frog. Here skin acts as a receptor organ and initiates a nerve impulse.
(2) Sensory or afferent nerve fibres conducts sensory impulse from the receptor organ (skin) to the spinal cord.
(3) Spinal cord acts as a modulator and changes sensory impulse into a motor impulse.
(4) Motor or efferent nerve fibre conducts motor nerve impulse from the spinal cord to the effectors (leg muscles).
(5) Effector organ in this case leg muscles give the response. These contract and the leg is withdrawn to avoid the stimulus.
Significance of Reflex Action:
Reflex Action have the following advantages-
(1) It controls a number of daily body activities.
(2) Response to harmful stimulus is very quick so as to reduce harm to the body.
(3) The response to a stimulus is always accurate, useful and purposeful.
(4) These prevent overloading of the brain, so prevent its fatigue.
What is Reflex Arc?
The path travelled by an impulse in a reflex action is called the reflex arc. It is the structural and functional unit of a reflex action that connects a receptor and an effector. It consists of five components, and all these components are essential for the completion of a reflex arc. Even if one of the components is missing, reflex action will not take place.
Components of Reflex Arc:
Components of reflex arc are as follows-
(1) Receptor to receive the elicit of reflex on applying stimulus. Example- skin, eye, ear etc.
(2) An afferent neuron that transmits the sensory impulses from receptor to the spinal cord through dorsal root where their cell bodies make up the swelling called as dorsal root ganglion.
(3) Interneuron in the grey matter of spinal cord that changes the sensory impulses into the motor impulses.
(4) Efferent Neuron that leaves the spinal cord through ventral root and carries the impulses with the muscles or glands.
(5) The motor or an effector organ that shows a response to impulse received from the efferent neuron.
A reflex pathway results in rapid response to stimuli because of the presence of a small number of synapses. Furthermore, an impulse need not make a lengthy trip to the brain and back to give an appropriate response.
A nerve impulse can be transmitted only in one direction in a reflex arc because the impulse can cross a synapse in one direction only. Further, repeated stimulation of a receptor may temporarily suspend the reflex arc as the synapses get fatigued.
Types of Reflexes:
Reflexes are of two types- unconditioned reflexes and conditioned reflexes.
These are inborn or inherited reflexes shown by all individuals. These reflexes need no training and are initiated immediately after birth. Example- a baby crying for milk, the opening of mouth on hearing a loud sound, peristalsis in the small intestine, breathing movements, sweating during high temperature as well as exercise etc. The reflex arcs of unconditioned reflexes are constant. These are elicited in response to definite stimuli.
These are acquired during the lifetime of an animal through learning or experience to stimuli that originally failed to elicit a reaction. The conditioned reflexes involve the establishment of new reflex arcs. These are of temporary nature and may disappear or reappear again.
Russian physiologist, Ivan Pavlov, demonstrated the occurrence of conditioned reflexes through the “Bell Experiment on dog“. Normally, salivation occurs only when food stimulates the taste buds on the tongue (unconditioned reflex). Salivation can also occur by the sight and smell of food. These involve conditioned reflexes. Pavlov supplied an additional stimulus to a dog by ringing the bell, whenever food was provided to it. In due course of time, the ringing of the bell at the time of lunch produced salivation even in the absence of food.
Pavlov called the sound of the bell conditioned stimulus, salivation in response to the bell, a conditioned response, food itself an unconditioned stimulus, and salivation in response to food an unconditioned response. A conditioned reflex is established when a new sensory clue (the bell) becomes associated with an inborn reflex, i.e., salivation. So a conditioned reflex is only acquired and non-inheritable property.
Significance of Conditioned Reflexes:
These enable the animal to adapt to their environment by experience. Most of the habits like cycling, playing a musical instrument, writing as well as reading, salivation at sight or smell of delicious food, knitting without looking etc. are developed due to conditioned reflexes.