Types and functions of the nerves

Types of nerves

A nerve is a collection of nerve fibres; they emerge from the central nervous system. There are two types of nerves: sensory (receptor) nerves are linked to sensory receptors, whilst motor (effector) nerves end in a gland or muscle.

1. Sensory nerves receive information and then relay it to the brain. These respond to touch, temperature and pain. These are found near to the skin’s surface.
2. Motor nerves act on information received from the brain, which results in a muscle movement response. These are situated in the muscle tissue. When massage is applied to the body the effect on the nerves is usually either relaxing or stimulating.

There are 31 pairs of spinal nerves that emerge from in between the spinal column vertebrae; all of these contain mixed nerves – both motor fibres that supply the muscles and sensory fibres that relay sensations.

Nerve impulses transmissions

Nerve fibres are individual neurones. Messages are passed along the nerve fibres as electrical impulses; these are caused by changes to the charges present on the inside and outside of the membrane of the nerve fibre. The membrane of a nerve fibre that is resting is polarised when there are more positive ions present on the outer surface and more negative ions present inside the membrane. Potassium and sodium ions present in all the body fluids are positively charged, whereas chloride ions are negatively charged. When the neurone is stimulated, a wave of depolarisation passes along the fibre as the charges become temporarily reversed.

Depolarisation can be prevented when a local anaesthetic is administered, as this will block the passage of electrical impulses.

Medical terms which relate to the nervous system are:

• hyperaesthesia: oversensitivity to touch
• anaesthesia : loss of sensation
• analgesia: loss of sensation to pain.

Passage from a neurone to a muscle fibre

Very similar to a synapse, this is a gap between adjacent neurones and takes place with the use of chemical transmitter substances. These are released from the end of the motor neurone into the muscle. All muscle fibres stimulated by a neurone will be contracted. The motor point is the point where the nerve enters a muscle.

Every individual neurone branches, making contact with the individual muscle cells. A neurone together with the muscle fibre is called a motor unit. The area of contact between the neurone and the muscle fibre is called the motor end plate or neuromuscular junction.

Passage from one neurone to another

Neurones pass along information to and from the central nervous system. Inside the central nervous system, impulses pass from one neurone to another without ever touching. When an impulse reaches the end of the nerve fibre a chemical is released known as neurotransmitter substance. An example of this is acetylcholine; the chemical passes along a small gap (synapse) and is then taken up by an adjacent neurone, which causes an electrical impulse within the neurone.

Reflex and voluntary actions

These are the actions we can control and are initiated by the brain, such as speaking. A reflex response is a quick involuntary response to stimuli. If we pick up a hot object we very quickly drop it. This response is because our pain receptors in the hand have been stimulated. Impulses are passed through the sensory nerve fibres to the spinal cord and are then transmitted through relay neurones to motor neurones; impulses pass to the arm muscles which are then stimulated to contract and the hot object is dropped. A reflex involving the spinal cord is called a spinal reflex. Impulses also run up the spinal cord to the brain so you become aware you have touched something hot or cold and therefore a secondary response is initiated which is usually one of shouting out.