Reflex action and reflex arc

Reflexes are fast predictable, automatic responses to change the environment. First of all Marshal Hall (1833) studied the reflex action. Best and Taylor defined reflex action "simplest form of irritability associated with the nervous system is reflex actions or a reflex reaction is an immediate involuntary response to a stimulus." The reflex actions are involuntary actions because these are not under the conscious control of the brain. Central nervous system is responsible for the control of reflex action.

 Reflex arc is formed by the neurons forming the pathway taken by the nerve impulses in reflex action. The simplest reflexes are found in animals involving a single neuron and the following pathway –

Component of reflex action :
The whole of the reflex are includes five parts —
(i) Receptor organs : Receptors are windows of the body or guards of the body. These are situated on all, important organs, for example eyes, nose, ear, tongue, integument etc. These perceive the stimuli from out side the body.
(ii) Sensory neurons:  These are also termed afferent neurons. These carry the stimuli from receptors to spinal cord. These neurons are situated in the ganglion on the dorsal side of spinal cord gray matter or brain stem.
    (iii)Association neurons or Integrating center : These are also called intermediate neurons or interstitial neurons. These are found in spinal cord. They transfer the impulses from sensory neurons to motor neurons. Association neurons forms monosynaptic and polysynaptic reflex arc.
(iv) Motor neurons : These are situated in the ventral horn of spinal cord. These carry the impulses to effector organs.
(v)Effector organs:  These are the organs, which react and behave in response to various stimuli, for example — muscles and glands.
Mechanism of reflex action :
The time taken by a reflex action is too short, for example — in frog it is 0.3 meter per second and in man 5-120 meter per second.
 Whenever, a part of the body is stimulated by any stimulus, for example — pin pricking, then the stimu-lus is converted into impulse.
 This impulse is perceived by the dendrites of sensory neurons. From here, the stimulus reaches the spinal cord through axonic fibres.
 In the spinal cord, this stimulus passes through synaptic junctions and reaches the intermediate neurons, from where this stimulus reaches the effector organs through motor nerve fibres.
As soon as the stimulus reaches the effector organs, it is stimulated and that part of the body is immediately withdrawn.
 The whole reflex action takes place so rapidly and quickly that we know it when it is completed.
   Type of reflexes :
The reflexes are of following types —
(i) Monosynaptic reflex :
This is the simplest reflex found in vertebrates.
 The simplest reflex found in vertebrates.
 The sensory neuron synapses directly on to the motor neuron cell body.
 In this case the reflex action takes place without the involvement of brain.
(ii) Polysynaptic spinal reflex :
This has at least two synapses situated within the spinal cord. It involves a third type of neuron also — the internuncial or inter-mediate relay neuron.
 The synapses take place between the sensory neuron and intermediate neuron, and between interme-diate neuron and the motor neuron.
These two reflex arcs allow the body to make automatic, involuntary, homeostatic adjustments, to changes in the external environment, such as the iris pupil reflex and balance during locomotion, and also in the internal environment such as breathing rate and blood pressure.
(iii) Polysynaptic spinal/brain reflexes :
 In this case the sensory neuron synapses in the spinal cord with a second sensory neuron, which passes to the brain.
The latter sensory neurons are part of the ascending nerve fibre tract and have their origin in preintermediate neuron synapse. The brain is capable of identifying this sensory information and stores it for further use. The motor activity may be initiated by the brain anytime and the impulses are transmitted down the motor neurons in descending nerve fibre tract, to synapse directly with spinal motor neurons in the postintermediate synaptic region.
(iv) Simple reflex :
Simple reflex is also known as unconditioned reflex.
 It is inborn, unlearned, reflex to a stimulus. Simple reflex is mostly protective in function. Example of simple reflex are
(a) Knee jerk — Tendon of patella tapped, also called patellar reflex.
(b) Corneal reflex (blinking reflex) — closing of eyelids.
(c) Rapid withdrawal of hand while burned or pricked.
(d) Quick recovery of balance while falling.
(e) Scratch reflex of frog — in pitched frog with acetic acid.
(f) Coughing, sneezing and yawning.
(v) Acquired reflex :
Acquired reflex is also known as conditioned reflex. It is not inborn, but acquired and dependent on past experience, training and learning.
Demonstration of conditioned reflex was first made by Russian physiologist Ivan Petrovitch Pavlov (1846-1936) in hungry dog. Pavlov rang the bell while feeding dog, thus associated the unconditioned response with additional stimulus.
 Examples of conditioned reflex are learning of dancing, cycling, swimming, singing,, driving, etc. These actions are under cerebral control during learning.