All living organisms are sensitive to changes taking place within their surroundings. They detect the changes (stimuli) and respond to them appropriately. The ability of an animal’s body to detect and respond appropriately to stimuli depends on the nervous system and endocrine system.
The stimuli may be within an animal’s body or in its surrounding. Different parts of the body of an animal do not work independently of each other.
They depend upon one another performing various functions as a single unit.
The nervous system controls all the organs and makes them to work together.

This is a system of nerve cells and sensory organs that carry out co-ordination by transfer of impulses.

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The nervous system consists of;
i) Receptors:
These detect the stimuli e.g. sensory endings in the skin, eye and ear.
ii) The central nervous system (CNS)
This interprets and determines the nature of the response. The CNS consists of the brain and spinal cord.
iii) Peripheral nervous system
This consists of voluntary and involuntary nerves.
iv) Effectors
These are organs that carry out the response.

Nervous system
These are organs that carry out the response.

Functions of the nervous system

  • It receives impulses from all sensory organs of the body.
  • It stores information
  • It correlates various stimuli from different sensory organs
  • It sends messages to all parts of the body making them function accordingly.
  • It’s involved in temperature regulation.
    The nervous system is made up of cells called neurons. A neuron is a functional unit cell of the nervous system that transmits an impulse or an electrical message.
    A neuron is made up of a small mass of cytoplasm, a nucleus in a structure called the cell body, branching cytoplasmic filaments called dendrites and a single long fiber called axon.
    There are three types of neurons.
  • Sensory neuron
  • Motor neuron
  • Interneuron (relay neuron)
    These are neurons that transport impulses from the receptors to the central nervous system. A sensory neuron has a single elongated dendrite called a dendron consisting of a fluid filled cytoplasmic tube. It has a cell body in the middle of a short axon and dendron. It is sometimes surrounded with myelin sheath. The myelin sheath increases the speed of the impulse in the neuron.
    Structure of the sensory neuron
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Characteristics of a sensory neuron

  • It has a cell body as a branch on the axil.
  • It has one dendron
  • The axon and dendron may be covered with myelin sheath.
  • The myelin sheath is broken at points called nodes of Ranvier.
  • It has a short axon
  • It has a long dendron
  • The terminal branches are embedded in the receptor.

This is a neuron that carries impulses from the central nervous system to the effectors. Motor neurons consist of short dendrites with a cell body at one end of a long axon. It is also sometimes surrounded by the myelin sheath.

Structure of the motor neuron

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Characteristics of motor neuron

  • At one end there is a thick part called the cell body which contains the nucleus and cytoplasm.
  • The cell body has dendrones which branch into dendrites.
  • From the cell body is a long fibre called axon.
  • The axon may be covered with myelin sheath.
  • The myelin sheath is broken at points called nodes of Ranvier.
  • The axon ends in branching terminals or end branch.
  • It has many dendrones
  • The terminal branches are connected to an effector.
  • It has a short dendron


This is a neuron found in the central nervous system and carries impulses from the sensory neuron to the motor neuron.

Structure of the relay neuron

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Characteristics of relay neuron

  • Its fibres are not insulated i.e. have no myelin sheath.
  • Its cell body is in the middle of the fiber.

General functions of the parts of a neuron

  • Cell body; this consists of a nucleus surrounded by a mass of cytoplasm. The nucleus controls all activities of the neuron.
  • Axon; this is one or more long cytoplasmic extensions running from the cell body. Axons carry impulses over long distances in the body. Each axon is filled with cytoplasm called axoplasm.
  • Myelin sheath; this is a fatty material that covers the axon. The myelin sheath is secreted by cells called Schwann cells. The myelin sheath insulates the axon and speeds up transmission of impulses.
  • Dendrites; these are fine structures on the neuron that link up nerve cells to form a complex network of communication.
  • Schwann cell; this is a cell which secretes the myelin sheath.
  • Node of Ranvier; this is the space on the axon between two adjacent myelin sheaths. It speeds up nervous transmission.
  • Cytoplasm; this is a site for chemical reactions in the neuron.
  • Dendrone; it is a branch through which impulses are transmitted to the body.
    Comparison between motor and sensory neurons
  • They both transmit impulses.
  • They both have a nucleus.
  • They both have an axon, dendrites and cytoplasm.
  • In both impulses move in one direction.
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