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Introduction
Animals are different from plants because of their ability of locomotion. This ability probably developed as they have to search for food, unlike the plants that are autotrophic. Since they move from place to place, the animals have to continuously encounter changes in their environment. All animals, in order to maintain a steady state within the body (homeostasis), should be able to perceive these changes and adapt to them.
Human Nervous System - Functions
It controls and coordinates all the activities of the muscles in response to the changes outside.
Units Of Nervous System
The units of nervous system are specialized cells called the neurons.
Transmission Of Messages
The messages are transmitted in form of electrical impulses along the fibres of the neurons. There are three types of transmission of impulses - along the nerve fibre, from one nerve fibre to the other and from one nerve fibre to the muscle fibre or gland.
Types of Neurons
The neurons that conduct impulses from the receptors or sense organs to the central nervous system are called the sensory neurons. They evoke sensations of touch, pain, heat, cold, vision, hearing and taste.
Nerve Fibres and Nerves
The long axons of neurons along with the associated structures are called the nerve fibres. The fibres may be enclosed within sheaths called as myelin sheath.
Central Nervous system
The nervous system in cockroach, though not as advanced as the human nervous system, has the three main divisions - the central nervous system, peripheral nervous system and the sympathetic nervous system.
Peripheral Nervous System
The peripheral nervous system consists of the nerves originating from the ganglia of the nerve ring and the ventral nerve cord.
Autonomic Nervous System
The autonomic nervous system (ANS) consists of two sets of motor neurons and a collection of ganglia.
Reflex Action - Simple Or Unconditioned Or Natural Reflex
When the stimulation of a receptor results in a spontaneous, involuntary reaction, it is called reflex action or simply reflex. These are described as involuntary actions as no conscious awareness, throught or decision is required for their operation.
Complex Or Conditioned Reflex
This type of reflex involves the brain but it is also as fast as the simple reflex. Salivation on smelling one's favourite food is an example of conditional reflex. The individual recognisesrecognizes the smell and based on a previous experience, the response (salivation) occurs. The recognition of the previous experience involves the association centres of the brain.
Sense Organs - Eyes
There are five sense organs in humans - eyes, ears, nose, tongue and skin. Sensation is the conscious or unconscious awareness of external or internal stimuli. Sensing external environment is extroception.
Mechanism of Vision
The light rays from the object pass through the conjuctiva, cornea, aqueous humour, lens and vitreous humour in that order. All these structures refract the light such that it falls on the retina. This is called focussing. Maximum focussing is done by the cornea and the lens. The light then falls on the retina.
Ear - Structure
Man has a pair of ears that function as the organs of hearing and the organs for equilibrium. It has two different structures for each of these functions. Both these structures have receptors that convert the vibrations into nervous impulses that are transmitted via the auditory nerve to the brain.
Mechanism of Hearing
The sound waves are directed towards the ear canal by the pinna. The waves that enter the canal are concentrated and made to strike against the tympanum.
Nose - Structure
Nose is the sense organ for the sense of olfaction or smell. The receptor cells for olfaction are chemoreceptors, stimulated by chemicals. The receptor cells along with other cells form the olfactory epithelium.
Mechanism of Olfaction
The nasal passage is lined by the nasal mucosa that have hairs and mucou-secreting cells. The hairs act like filters to remove the dust particles from the inhaled air. The mucous secreted by Bowman's gland keeps the inhaled air moist. The chemicals released by the various odoriferous substances are dissolved in this moisture. Thus, moisture is essential for the perception of smell.
Tongue - Structure
Tongue is a muscular structure that has the organs of taste reception. The organs for sense of taste are the taste buds.
Mechanism of Taste Reception
The taste buds are stimulated by certain ions and compounds present in the food. Cations like Na+ present in the inorganic salts evoke the salty taste and those like H+ present in the acids evoke a sour taste. Organic compounds like sugars, dextrins, glycerol, saccharine, etc. result in sweet taste and those like nicotine, morphine, caffeine, quinine, etc. result in bitter taste.
Central Nervous System
The nervous system in cockroach, though not as advanced as the human nervous system, has the three main divisions - the central nervous system, peripheral nervous system and the sympathetic nervous system.
Peripheral Nervous System
The peripheral nervous system consists of the nerves originating from the ganglia of the nerve ring and the ventral nerve cord.
Sympathetic Nervous System
It is also called the autonomic or stomatogastric or visceral nervous system. It is made up of a collection of ganglia and nerves. The ganglia that form a part of this system are the frontal, occipital, visceral and proventricular.
Summary
With increasing complexity, the organisms have had to develop means of control and coordination between the different parts of the body.