After reading this article you will learn about the Stimuli Act to the Body through Organs:- 1. Meaning of Receptors 2. The Reflex Arc 3. Types of Muscles.
Meaning of Receptors:
The environmental stimuli act upon an individual through the sense organs, and an individual reacts to the environment through the muscles and glands. The sense-organs are called ‘receptors’, because they receive stimuli from the environment. The muscles and glands are at present called ‘effectors’, because an individual reacts to the environment through them.
The eyes, the ears, the nose, the tongue, and the skin are called the exteroceptors, because they receive stimuli which are external to the body. Among them, the eyes, the ears, and the nose are called distance receptors because they are stimulated by objects at a distance from the body; and the skin is called the contact receptor because it is stimulated by objects in contact with the body.
The sense-organs excited by stimuli within the organism are called the interoceptors. The sense of taste is not really a contact receptor because the surface of the tongue is not the real organ of taste. The taste cells in the taste buds below the papillae are the real organ of taste.
So the sense of taste is an interoceptor. Organs of hunger and thirst, organs of pain in the viscera organs of sensations of breathing, circulation, sex, etc., are interoceptors.
The receptors in the joints, tendons and muscles, and semicircular canals of the internal ear, which are connected with peripheral sensory neurons, are called the proprioceptors. They are so called because they inform the organism about itself. Proprio means ‘self’.
The Reflex Arc:
The reflex arc is the physiological mechanism of a reflex action. A sense-organ is stimulated by a stimulus. A sensory neuron carries the nerve impulse from the sense-organ to a sensory centre or nerve-cell in the spinal cord or the brain stem.
There is a synaptic connection between a sensory neuron and a motor neuron. The motor neuron carries the nerve current from the central switching station to a muscle or a gland. The muscle or the gland acts according to the impulse received through the motor neuron.
This combination of a sensory neuron with a motor neuron through a synaptic junction in the grey matter in a lower centre is called the ‘reflex arc’. The nature of reflex action will be considered later. We see a flash of lighting. At once our eyes are closed. It is a reflex action.
Types of Muscles:
There are two kinds of muscles, voluntary, and involuntary. Voluntary muscles are under the control of the will. The muscles of the limbs, trunk and face are voluntary. Involuntary muscles are not under the control of the will. The muscles composing the wall of the stomach, intestines, and the heart are involuntary.
The voluntary muscles are long drawn out and marked across or striped. The involuntary muscles are shorter and taper at both ends, and are un-striped. But the muscles composing the heart, though involuntary are striped; but they are very much shorter than ordinary striped muscle fibres.
Striped muscles are attached to the framework of bones, and so called skeletal muscles. They are chiefly involved in all overt actions. They are supplied chiefly with nerve-fibres from the cerebrospinal system, and react more quickly than smooth muscles.
Smooth muscles are called visceral muscles because they are attached to the internal organs. They are supplied with nerve-fibres from the autonomic nervous system. They have to do with internal adjustments of the body.
Contraction is the function of all muscles, smooth or striped. The excitation comes from the cerebrospinal system to the muscles, and releases the energy stored in them in the form of contraction. Muscular contraction determines the behaviour of an organism. The muscles are one kind of effectors.