STRUCTURE AND FUNCTION OF NEURONE

Neurones are specialized cells that transmit impulses (messages) around the body in form of electrical charges. The nervous system consists of millions of neurones (nerve cell). Three main types exist in the body namely: sensory, association (intermediate) and motor neurones. Each neurone contains a cell body (soma), dendrites (afferent nerve fibres) and axon.

 

Cell body (soma)

This is made up of a muscle with nucleolus, and Cytoplasm. The cytoplasm is stellate in shape and contains Nissl’s granules which are rich in RNA and manufacture protein.

 

 

Dendrite (afferent nerve fibres)

These are short hair-like structures originating from the periphery of the cytoplasm. Dendrites are either in close contact with other neurones or the stimulus receptor cells. They conduct impulses (messages) to the cell body.

 

Axon (efferent nerve fibre)

This is a single long fibre arising from the cell body and ending with many branches (terminal dendrites) which almost touch the dendrites of adjacent neurones. Each branch of the axon ends in synaptic knobs. Axon can be up to a metre long as in the sciatic nerve of the leg. Axon is covered with a fatty myelin sheath and the myelin sheath is covered with neurilemma. In fact, neurilema is a membrane of another cell called Schwann cell and not a part of neurone. The myelin sheath insulates and protects the axon and helps to prevent impulses from spreading from one neurone to another. At interval (about one metre), the myelin sheath is interrupted by constriction called nodes of Ranvier. The nodes of Ranvier assist in the transmission of impulses. Axons with myelin sheath are called myelinated axons while those without are called unmyelinated axons.

Each synaptic knob at the terminal end of the dendrite contains many synaptic vesicles which possess substances that transmit impulses. When the substances are released by the synaptic vesicles into the synaptic cleft, the substance diffuses across the synaptic cleft thus altering the polarisation of the postsynaptic membrane of the dendrite or the cell body of the next neurone.

 

 

Each synaptic knob at the terminal end of the dendrite contains many synaptic vesicles which possess substances that transmit impulses. When the substances are released by the synaptic vesicles into the synaptic cleft, the substance diffuses across the synaptic cleft thus altering the polarisation of the postsynaptic membrane of the dendrite or the cell body of the next neurone.

 

Sensory neurones (afferent neurones)

These types of neurones receive stimuli and convert the stimuli into impulses which are carried from stimulus receptors (e.g. sense organs) to the central nervous system. Any nerve composed only of sensory fibres is called sensory nerve. A sensory neurone has two long fibres namely: dendron which conducts impulses from a sensory organ to the cell body (soma) and axon which conducts impulses from the cell body to the central nervous system.

 

Association neurones (intermediate neurones)

These neurones are located only in the central nervous system. They connect sensory and motor neurones with each other and with other nerve cells in the central nervous system. They have no myelin sheath.

They transmit impulses from sensory neurones to motor neurones which enter and leave the brain.

 

Motor neurones (efferent neurones)

They transmit impulses from the central nervous system to the effectors (e.g. muscles and glands). Only one long axon is present. Any nerve composed of only fibres of motor neurones is called motor nerves.

 

Mixed nerves

Any nerve composed of sensory and motor fibres is called mixed nerve. All the nerves connected to the spinal cord are mixed nerves. They are 31 in man.

 

Process by which neurones transmit impulses

Impulses are transmitted along a neurone by electro-chemical means (that is by electrical and chemical transmissions).

 

Electrical transmission of impulses

An unmyelinated neurone at rest, that is, a neurone not transmitting impulses, has a positive charge outside and a negative charge inside the cell membrane. At this resting condition, the neurone is said to be polarized and is referred to as resting potential. The positive and negative charges are equal. As an impulse passes along a particular point of a neurone, the polarity now changes (depolarized), the outer surface of the cell membrane becomes negatively charged while the inner side surface of cell membrane becomes positively charged. This sudden change in polarity is called action potential. The resting potential is obtained when the original polarity is restored.

 

Ionic transmission

When a neuron is at rest (polarized neuron) both inside and outside cell membrane have different concentrations of potassium and sodium ions, that is, excess sodium ions are outside the cell membrane. This condition makes the cell membrane inside negatively charged and outside positively charged.

When an impulse is transmitted along a neuron, the cell membrane becomes permeable to sodium ions, (depolarized), thus sodium ions are pumped inside the cell membrane and potassium ions outside. The cell membrane is now depolarized with inside the cell membrane positively charged and outside negatively charged. A sodium potassium pump located in the cell membrane moves excess sodium ions outside and potassium ions inside the cell membrane, thereby restoring the original resting stage of the neurone. When an impulse reaches the motor end plate or synapse the impulse causes the release of a small amount of a chemical substance called acetylocholine which then transmits the impulse to the muscle fibres to contract. After contraction, another chemical substance called cholinesterase is released to neutralise acetycholine thus causing the muscle fibres to relax.