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Reflex Action


Reflex actions are the movements that occur without necessarily, the intervention of the brain. They are also called spinal reflexes. There are many examples of such reflex actions. For example, if a person places his hands in a boiling water bath, without knowing that it is hot, the hands are immediately withdrawn. Only later, it is realised how hot the water was. Similarly, if the knee is hit lightly below the knee cap, the knee jerks. In all these reactions, the sensation of pain is perceived by several pain receptors in the skin which send impulses by the dorsal sensory roots, to the spinal cord. The sensory neuron synapses with an interneuron in the spinal cord and finally the impulse is transmitted to the muscle by the ventral motor neuron. As a result, the muscle contracts, resulting in the withdrawal of finger or hand as the case may be. The entire reaction occurs within a fraction of a second. When the brain is intact a nerve fibre from the spinal cord carries the stimulus to the pain centre in the brain. It is only then that one feels pain. Before this sensation of pain, the hand or leg has already been withdrawn. Here the nervous response is involuntary and occurs before one is aware of the stimulus. Such actions are reflex actions.

Diagram Showing the Regions of Spinal Cord (in T.S) and the Reflex Arc

Reflex Arc


The path traveled by an impulse from the receptor to the effector organ in a reflex action is called a reflex arc. It consists of the following parts:
  1. Receptor organ,
  2. Afferent neuron,
  3. Interneuron in a part of central nervous system,
  4. Efferent neuron, and
  5. Effector organ.
The reflex actions are very useful to the animals. These are protective behaviours which show quick responses and thus avoid delay in responding to harmful stimuli.
 
Table - Cranial Nerves of Mammals

No

Cranial Nerve

Organ Innervated

Type of fibre

Functions

1.

Olfactory

Olfactory mucous membrane

Sensory

Sense of smell

2.

Optic

Retina

Sensory

Sense of vision

3.

Oculomotor

Four of the six eye muscles

Motor

Eye movement

4.

Trochlear

Superior oblique eye muscle

Motor

Eye movement

5.

Trigeminal

Skin and mucous membrane of the head

Sensory

Sensation

6.

Abducens

Rectus eye muscle

Motor

Eye movement

7.

Facial

Muscle of the face, neck, salivary glands, taste buds

Motor and sensory

Movement, saliva secretion and the sense of taste.

8.

Auditory

Internal ear

Sensory

Equilibrium, hearing

9.

Glossopharyngeal

Pharynx, tongue, salivary gland, pharyngeal muscles

Sensory and motor

Taste, salivary secretion, swallowing

10.

Vagus

Pharynx, larynx, trachea, oesophagus thoracic and abdominal viscera

Sensory and motor

Visceral reflexes

11.

 

 

 

Accessory

 

 

 

Thoracic and abdominal viscera

Muscle of the neck and shoulder

Sensory or motor

 


Motor

Visceral reflexes

 


Movement

12.

Hypoglossal

Muscle of the tongue

Motor

Movement





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