The sensory system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory receptors, neural pathways, and parts of the brain involved in sensory perception. Commonly recognized sensory system organs are those for vision, hearing, touch, taste, smell, and balance. In short, senses are transducers from the physical world to the realm of the mind where we interpret the information, developing our perception of the world round us.
The sensory system consists of simple to complex structures called sensory receptors. The sensory receptors enable us to detect changes in our own body and in objects and events in the world around us. The information about the changes within the body is used to maintain homeostasis. The sensory receptors which report sight, sound and chemicals in the outer world are used to find food, shelter, mate, safety from enemies, and other adaptive responses to environment.
BASIC STRUCTURE OF SENSORY RECEPTORS
All sensory receptors are similar in basic structure. The simplest and most primitive type of sensory receptor is a single unspecialised sensory neuron whose terminal end is capable of detecting stimuli. It is called primary sense cell. Olfactory cells belong to this category. The more complex sensory receptors are modified epithelial cells able to detect stimuli. These receptors are termed secondary sense cells. They form synaptic connections with the sensory neurons which transmit impulses to the CNS. Mammalian taste buds are receptors of this type. The most complex sensory receptors consist of numerous sense cells, sensory neurons and associated accessory structures. They are known the sense organs. Eye and ear have a level of complexity of sense organs. The accessory structures may have a double role : eliminating the effect of unwanted stimuli and amplifying the effects of desired stimuli.
Working of Sensory Receptors
All sensory receptors resemble in mode of working too. The receptors receive particular stimuli and set up appropriate electrical impulses in the nerves. A stimulus is some form of energy: light, sound, pressure, heat, osmotic potential, electric current, and chemical changes. Each type of receptor is sensitive to a particular kind of stimulus. An animal responds to a stimulus in a four-step process-
(1) Sensory Transduction. Sensory receptors transduce (transform) the energy of a stimulus into a localized nonpropagated electrical response which initiates nerve impulses in the neuron leaving the receptor.
(ii) Transmission. The sensory neuron relays the impulse to the brain directly or through the spinal cord.
Nerve impulses (action potentials, often called receptor potentials) that reach the brain via sensory neurons are termed sensations. In the brain, the sensations are analysed and interpreted as perceptions, and we come to know of the stimuli acting on the receptors. The brain transmits motor impulses to appropriate effectors- muscles or glands.
(iv) Response. Effectors produce suitable response . Muscles contract,or glands secrete chemicals, in response to the information sent to the brain by the receptors.
The sensory receptors, thus, merely receive the stimuli and set up similar nerve impulses. They cannot interpret the impulses generated in them by the stimuli. This power lies with the brain. Actually, therefore, we feel, taste, smell, see and hear with the help of specific functional areas of the brain. Of course, the brain without sensory receptors cannot detect environmental changes. Similarly, the sensory receptors alone are of no use. Hence, what actually matters is where the nerve impulses reach, and not what generates them.
Special and General Senses
The receptors for sight, hearing, smell and taste are localized in specific organs and are, therefore, called special sensory receptors, and these senses are termed special senses. The sense of touch has receptors scattered all over the skin. Hence, it is termed a general sense.
Sensory Adaptation. If a stimulus once noted by the receptors remains constant, it becomes less noticeable. This mechanism is called sensory adaptation. It checks the nervous system from becoming too sensitive. We quickly note a strange odour, but we also cease to perceive it quickly. For example, you soon cease to smell a perfume applied to the body or clothes, but others approaching you can still note its fragrance. Without sensory adaptation, you would be distinctly aware of the touch of your clothes and of every sound reaching your ears. Concentrating on a single stimulus, such as a person talking to you, would be difficult.
Types of Receptors.
The receptor cells are of many types. They may be classified according to their position in the body or regarding the form of stimulus energy they can detect.
A. Regarding their location, the receptors are of 3 types : exteroceptors, proprioceptors and visceroceptors or interoceptors.
1. Exteroceptors or External Receptors. These are located at or near the body surface, and are stimulated by changes in the environmental factors. They include the receptors for touch, taste and smell, and sense organs for sight and hearing.
2. Proprioceptors. The proprioceptors are located in the skeletal muscles, joints, tendons, etc. It is from these receptors that we know the position of our arm or leg without having to look at it.
3. Visceroceptors or interoceptors or Internal Receptors.
These are located in the viscera. They are affected by stimuli originating within the body itself and cause sensations, such as pain, hunger, thirst, fatigue, nausea, sex, etc. They also monitor blood pressure, carbon dioxide level, body temperature, osmotic relationships, pH, etc.
B. According to the form of the stimulus energy they detect and transform (transduce), the receptors are of 5 main types. These are given in table –