Nervous System A and P Part 1

Nervous System

Components of the central and peripheral nervous system
The central nervous system (CNS) consists of the brain and spinal cord, the peripheral nervous system (PNS) consists of all of the other nervous tissue in the body such as the nerves going to and from the spinal cord and the nerves in the limbs.

Components of a neurone
A neurone is a nerve cell and we will consider 2 types: motor and sensory

Sensory and motor nerves
Motor nerves carry information out from the central nervous system to the periphery where they initiate some activity for example contraction of a muscle. Sensory neurones carry information from the periphery of the body into the central nervous system. A motor neurone consists of short dendrites relaying information towards the cell body. The cell body contains cytoplasmic granules and a nucleus. From the cell body the nerve impulse is then carried out along the axon. (A dendrite is any nerve fibre which carries information towards the cell body and an axon is any nerve fibre which carries information away from the cell body.) The axon is surrounded by the Schwann cells of the myelin sheath. It is the motor neurones which carry impulses to the muscles via the motor end plates.

A sensory neurone picks up information from peripheral sensory receptors, carries this along the dendrite to the cell body. The information in the form of a nerve impulse is then carried into the central nervous system via the axon of the sensory neurone. Therefore motor neurones tend to have short dendrites and long axons whereas sensory neurones tend to have long dendrites and short axons.

Afferent and efferent nerve pathways
An afferent neurone is any neurone which carries sensory information into the central nervous system. An efferent neurone is any neurone which carries information out from the central nervous system. So afferent information is carried by sensory neurones and efferent information by motor neurones. (One way to remember this E if for effluent or E for exit).

Neuronal transmission
This means the transmission of a nerve impulse. When a section of nerve fibre is at rest it is polarised. This means it is negatively charged on the inside and positively charged on the outside. When a nerve impulse is passing the polarity is reversed and the fibre is negative on the outside and positive on the inside, in other words the fibre has depolarised. After the nerve impulse has passed the polarity reverts back to being negative inside and positive outside by the process of repolarisation. A nerve impulse passing along a nerve fibre is therefore described as a wave of depolarisation.

Relationship between speed of impulse and diameter of an axon
This relationship is very simple; the wider diameter of the axon, the faster the nerve impulse will travel. However, in humans the main factor influencing the rate of transmission is the presence of the myelin sheath.

Function of myelin
Myelin facilitates `bouncing` or salutatory transmission of a nerve impulse; this greatly increases the speed of transmission. In this form of transmission the nerve impulse jumps from one node of Ranvier to the next. Myelin also protects the nerve fibres from damage, insulates a nerve fibre from surrounding fibres in the same nerve and also nourishes the fibres.

Functions of neurotransmitters
Neurotransmitters are chemicals which are released from pre-synaptic neurones. These chemicals then diffuse across the synaptic gap to the post synaptic neurone where they cause a new nerve impulse to be generated by depolarising the post synaptic membrane.