The nervous system includes all the organs and tissues which enable animals to respond to changes in their environment. A change in the environment is called a response.
The nervous system of mammals is composed of two parts: the central nervous system and the peripheral nervous system.
Central Nervous System
This is made up of the brain and the spinal cord. Both are made up of thousands of nerve cells or neurones. The functions of the central nervous system is correlation of impulses from various sense organs and storage of impulses as information or impressions in the brain for reaction to future similar stimuli.
a) The Brain: The brain is enclosed in a bony case called the cranium. It is divided into fore, mid and hind brain.
- The fore brain consists of olfactory lobes which receive sensory impulses for smell and the cerebrum which is the seat of consciousness, intelligence, memory and all voluntary actions.
- The mid brain consists mainly of optic lobes which control sight.
- The hind brain consists of the cerebellum which is concerned with balance and body posture, receives impulses and coordinates action, respiration, heartbeat, digestive movements and blood supply.
(b) The Spinal Cord: The spinal cord stems from the medulla oblongata and runs through the neural canal of the vertebral column or backbone. It communicates between the brain and other parts of the body. It controls all the reflex (involuntary) actions of the body.
The Peripheral Nervous System: The peripheral nervous system consists of the cranial and spinal nerves and the autonomic nervous system. The cranial and spinal nerves of the peripheral nervous system together with the central nervous system mediate relations between the animals and its external environment. On the other hand, the autonomic nervous system, consisting of the sympathetic and parasympathetic nervous systems regulates events within the animal.
A neurone (nerve cell) is the structural and functional unit of the nervous system. It helps in receiving and relaying or transmission of impulses. A neurone has three main parts: The dendrite, cell body and the axon (nerve fibre or axis cylinder).
The cell body has dendrites extending from it, and a central nucleus. The axon is surrounded by myelin sheath which is interrupted at intervals, forming nodes of ranvier. The axon terminates in dendrites.
Types of Neurones
- Sensory or Afferent Neurone: These neurones carry impulses from receptors, such as the eyes, skin, ears, etc to the central nervous system.
- Motor or Efferent neurone: These neurones carry impulses from the central nervous system to effectors, such as muscles and glands in the body.
- Intermediate or Relay neurones: These neurones receive, transmit and interpret messages in the spinal cord and brain.
Transmission of Nerve Impulse
The dendrites usually receive messages and pass them to the cell body which then passes them (messages) out through the long axon to the dendrites of another nerve cell. However, there is no direct connection between the fibres of adjacent neurones. The junction between one end of a dendrite of one neurone and that of an axon of the next neurone is known as a synapse. The message passes from cell to cell across these synapses.
Reflex actions: These are actions carried out by animals in response to certain stimuli without first thinking or planning for them. They are not under the control of the brain. They are quick, automatic responses and entirely stereotype in nature. Examples of simple reflex or involuntary actions include:
- the blinking of the eyes.
- the beating of the heart.
- sudden removal of hands, legs or skin from hot object.
- the jerking of the legs on tapping the knee cap.
Reflex Arc: A reflex arc is the path traced out by the impulses concerned in a simple reflex action. It consists of three neurones – an afferent, intermediate and efferent neurones.
The message from the external stimuli is received by the sensory or receptor organs such as the skin in the case of sudden removal of leg from hot object. The stimulus is sent to the brain or spinal cord through the sensory or efferent neurone. The intermediate or relay neurone in the brain and spinal cord receives, transmits and interpretes the message. The interpreted message is then sent through the motor or efferent neurone to the organ or effector such as the muscle, which then causes the appropriate response, leading to the sudden withdrawal of the leg from the hot object.
The Sense Organs
The sense organs are the organs which receive stimuli and with the help of the brain (voluntary action), the message is received and interpreted. Hence, such organs enable animals to respond to stimuli within the environment.
The sense organs include:
- Nose – sense of smell.
- Eyes – sense of vision (sight).
- Ears – sense of sound (hearing).
- Tongue – sense of taste.
- Skin – sense of touch and heat.
Functions of the Nervous System
- It coordinates body functions.
- It is responsible for bringing about locomotion or movement.
- It enables the body to respond to external stimuli.