It is of interest to know, that all sensory information does not have to go to the - brain before we can react to a stimulus. Our responses to certain stimuli are simple unvaried and quick. For instance, if a finger is too close to a burning match, receptor cells on the skin send the infarmation on a nerve to the spinal cord, and a 'reflex action' is taken to move the finger away fm the fire. Such 'reflex drcuits' are well distributed and one is shown in Fig.
Now let us see what happens when a pin pricks the finger.
1 The pin p-rick is the stimulus which activates a receptor cell at that point.
2 A nerve impulse is transmitted away from the skin along a sensory neuron to the spinal cord.
3 The impulse is now passed through the inter neuron to the motor neuron and finally passes to the-muscle which pulls the hand back from the painful event.
The brain is not normally involved in such simple reflex actions. Only after the event, is the brain informed and we become conscious of what has happened. The time gap between the stimulus and the response is about 1115th of a second and this is often crucial for survival. For instance, a person who is driving a car has to apply his brakes to avoid a sudden collision with another car. His reflex action timing is important if he ( has to avoid the accident. Similarly, the reflexes of an airplane pilot have to be fast to avoid disaster. Human capacity to handle equipment and machines is limited by the time taken by one's reflexes.