Effectors

The structures that give responses, when they are stimulated by nerve impulse are called effectors. The principal effectors are muscle respond by contracting and glands which respond by secreting.

Reflex Action And Reflex Arc

Three types of actions are performed by our bodies These me involuntary action, voluntary action and reflex action.

Reflex Action

It is also called reflexes. These are the responses to external and internal environmental changes and are immediate or automatic and without the intervention of will. There are two types of reflex action, simple reflex and conditional reflex.

Spinal reflex action: Certain responses are without the help of the brain. In these cases the spinal cord acts as the control center. Such a response Is a reflex act. A reflex act is a simple response which involves no conscious control. Example: If one unexpectedly touches a hot object, the hand Is rapidly removed from the source of heat.

Reflex Arc Showing The Path Of A Spinal Reflex

Reflex Arc

Receptors in the skin of the hand are activated by the heat of the object. These receptors stimulate a sensory neuron leading to the spinal cord via a spinal nerve. The cell body of the sensory neuron is outside the cord. The sensory neuron enters a dorsal nerve root of the spinal cord.

The  impulse then crosses a synapse to an interneuron which lies completely within the cord. The impulse travels along the interneuron and then passes across a synapse to the dendrites and the cell body of a motor neuron and lies ventrally within the cord of a motor neuron. The motor neuron eventually branches to form synapses with several muscle cells i.e. an effector. The nerve impulses then move along the motor neuron to the effector. Acetylcholine is released across the synapses, polarity of the muscle cells is reversed, and the muscle contracts. The path of the Impulse is a reflex act and Is called a reflex arc. In short, a reflex arc includes five fundamental parts. 

The five parts of reflex arc are:

1. Receptors

2. Sensory Neurons

3. Interneurons

4. Motor Neurons

5. Effectors

All the activities of a reflex action take place within a fraction of a second. They are not controlled by brain. However, some of the Interneurons of the spinal cord synapse with other Interneurons with fibers In tracts that run up and down the spinal cord within white matter leading to the brain. The brain therefore, registers an awareness of what happened. This leads to a variety of secondary response in man. A loud 'oh' would be a common secondary response to the stimulus of the hot object.

Examples

(a) Watering of the mouth on the sight or smell of food.

(b) Blinking of eyes.

(c) Knee jerk.

(d) Closing the eyes when a flash of light falls across and so on.