Nerve Cells (Neurones)

The nerve cell or neurone is the basic functional unit of a nervous system.

It is a ce/ll that is modified to transmit impulses.

Nerve impulse is an electrical signal that is transmitted along a nerve fibre.

A typical neurone is made up of a cell body (centron) and dendrites (extensions).

In some neurones, one of the dendrites is elongated to form an axon.

Cell body

It contains a nucleus and cell organelles.

From it extends several nerve fibres called dendrites and axon.

Dendrites

Are extensions or nerve fibres that transmit impulses towards the cell body.

Axon

It is a long extension of nerve fibres that conducts impulses away from the cell body.

The cytoplasm found within the axon is called axoplasm.

The end of axon branches into several small extensions called dendrites each with a swollen end called synaptic knob.

The axon is surrounded and insulated by a fatty sheath called myelin sheath.

The myelin sheath is surrounded by the neurilemma, which is the membrane of the Schwann-cell but lies in intimate contact with axon.

The myelin sheath is broken at 1mm intervals by constrictions called nodes of Ranvier.

The nodes of Ranvier to propagate the nerve impulse and speed up the transmission of an impulse.

The myelin sheath insulates the axon.

Structure and Functions of a Nerve Cell

Types

Neurones are described as unipolar, bipolar or multipolar according to how many dendrites project from the cell body.

The dendrites of the cell body make contact with neighbouring neurones in the central nervous system (CNS), while the terminal dendrites at the end of the organ make contact with the effector organ.

Several neurones link up to transmit impulses from a sensory organ to the central nervous system, and also from the CNS to effector organs.

Types of Neurones

There are three types of neurones grouped according the direction of impulse conduction;

(a) Sensory neurone (afferent)

(b) Motor neurone (efferent)

(c) Relay neurone (connector, intermediate or associative)

Sensory neurone (afferent)

Conduct impulses from receptors to the central nervous system hence referred to as afferent neurones. 

They link sense organs such as the ear, eye, skin, nose and tongue with the central nervous system.

Have only one nerve fibre extending from the cell body hence they are unipolar.

Its cell body is situated off the axon, and outside the central nervous system (CNS).

Its receptor dendrites are located in the sense organ, while the terminal dendrites are located in the central nervous system.

Function: Transmit nerve impulses from sense organs to the central nervous system.

Motor neurones (Efferent)

 Conduct impulses from the CNS to the effectors such as muscles and glands.

Its cell body is located at the end of the axon and is located in the CNS.

Are multipolar with several dendrons projecting from the cell body.

Function: Transmit nerve impulses from the CNS to the effectors.

Relay neurones(intermediate)

Connect the sensory neurones with the motor neurones through small gaps or neural junctions called synapses.

Are bipolar or multipolar nerve.

Are located in the CNS.

Are non-myelinated and possess a short axon.

Function: Relay nerve impulses between sensory and motor neurones.

Differences between sensory, relay and motor neurones