Nervous coordination
Monacia Williams, Contributor

Hello students, welcome to another in our series of lessons. Many of you are now counting down to exams and are probably feeling a bit anxious. Try not to be, because anxiety and studying do not go together well. Ensure that you do at least two hours of private study every day or else you will become overwhelmed by all the work you need to do before the exams.

Now, let us begin our look at the nervous system.

The nervous system consists of:

• The Central Nervous System - CNS, made up of the brain and the spinal cord
• The Peripheral Nervous System - PNS, made up of all the nerves outside of the brain and the spinal cord.

These two parts form a communication network linking the reception of stimuli to responses. Two new words have just been introduced; these are stimuli, which is the plural for the word stimulus, and the other is response. What do these words mean? Please note that it is important that you know their meanings.

• Stimulus - a change in an organism's environment which is detected by a receptor.
• Response - the action which results from the stimulus, for example, contraction of muscles to cause movement which is carried out by the effector.

How is the stimulus received? The stimulus is detected by the receptor.

• Receptor - a sense organ, for example, eye, ear, skin which receives stimuli and converts them to nervous impulses.

The response is carried out by the effector.

• Effector - a part of an organism, a muscle or a gland which carries out a response.

The nervous system is made up of nerve cells or neurones. These are cells which transmit electrical impulses to and from the brain or the spinal cord.

There are three types of nerve cells:

• Motor neurone
• Sensory neurone
• Intermediate or relay neurone

You will need to become familiar with the diagrams of both the motor and the sensory neurone. Be able to draw them and be able to label them. These are favourite diagrams that may appear on both Paper 01 and Paper 02. Let us take a look at them now.

Motor neurone

The motor neurone:

• Conducts impulses from the CNS to the effector.
• Has the cell body at one end with the axon extending from the cell body. The axon can be over a metre long.
• Has at the other end fibres which carry the impulses to the cell body. These are called dendrites.
• Has its axon insulated by the myelin sheath which is broken at regular intervals by gaps known as the Nodes of Ranvier. The presence of the myelin sheath enables the myelinated fibre to carry impulses at a faster rate than one which has no myelin.

Sensory neurone

The sensory neurone:

• Is also myelinated
• Conducts impulses from receptors to the CNS.
• Has its cell body in the middle region of the neurone.
• Has, at one end, nerve endings which end in a sense organ and, at the other, endings which connect with the intermediate neurone at synapses.

The intermediate or relay neurone

This consists of a cell body with dendrites - nerve endings, which connect with the sensory neurone at one end and dendrites which connect with the motor neurone at the other. It is found between the sensory and the motor neurone, hence the name, intermediate. These neurones are not joined together but are connected by gaps which are called synapses. Synapse act like one-way valves. Vesicles with transmitter substances are only found on one side of the synapse so the message can only travel from that side.

The synapse and its mode of action

See you next week as we continue our study of the nervous system.

Monacia Williams teaches at Glenmuir High School. Send questions and comments to kerry-ann.hepburn@gleanerjm.com