Neurotransmission

Action potential is the maximum positive charge generated within the axon as a result of a nerve impulse.

Synapses are specialised junctions where impulses pass from one neuron to the next.

• A sensory neurone carries a nerve impulse from a receptor into the central nervous system.
• A relay neurone carries a nerve impulse from the central nervous system to a motor neurone. 
• A motor neurone carries a nerve impulse from the central nervous system to a skeletal muscle. 

There are two main types of cells in the nervous system:

1. Neurones, that are adapted to carry nerve impulses

2. Neuroglia, that provides structural and metabolic support to the neurones.

The basic structure of the neurone is:

1. A cell body containing a nucleus surrounded by a granular cytoplasm (perikaryon). The granules in the cytoplasm are referred to as Nissl substance and consist of dense clusters of rough endoplasmic reticulum.

2. An axon which conducts impulses away from the cell body to other neurones or to effectors.

3. One or more dendrites that are highly branched processes which carry impulses from specialised receptors or from adjacent neurones.

There are three types of neurones:

Type	Structure	Function
Multipolar neurones	Most common type. Have many dendrites.	They act as motor neurones.
Bipolar neurones	Uncommon. Have one dendrite.	They act as receptors for sight, smell and balance
Pseudo-unipolar neurones	Have one dendrite	Act as sensory neurones.

• Axons are surrounded by Schwann cells, which is called a myelin sheath.
• Myelinated neurones are mostly found in sensory and motor neurones.
• Unmyelinated are found in relay neurons and the autonomic nervous system.
• Between the Schwann cells there are small gaps where the axon is not covered by myelin, known as nodes of Ranvier.
• The conduction velocity of the nerve impulses is proportional to the diameter of the axon.
• The conduction velocity of myelinated neurons is faster than that of unmyelinated neurons of the same diameter.
• When a neurone is not conducting an impulse it is in resting state.

In resting state:

1. Large negatively charged organic ions are mainly on the inside the axon. This is because the membrane is impermeable to them. They cause the overall negative charge inside the axon.

2. Potassium ions are in greater concentration inside the axon. This is because there protein channels that allow K+ to pass through. They do not diffuse out down their concentration gradient because they are attracted by the overall negative charge inside.

3. Sodium ions are in greater concentration outside. This is because the permeability of the axon membrane to Na+ is low and also because they are expelled by ion pumps.

4. Chloride ion concentration is greater outside. Their concentration gradient is inward but they are repelled by the overall negative charge.

When an impulse is generated:

1. Sodium channels open and Na+ enter faster than they are expelled.

2. The overall charge inside the axon becomes positive. This is known as the action potential.

3. The permeability of the membrane to Na+ decreases and the permeability to K+ increases.

4. K+ flow out. Overall charge inside becomes negative.

5. Potassium ions continue to leave and the overall charge inside becomes slightly more negative than the when in resting potential. This is called hyperpolarisation.

6. Resting potential is restored as K+ and Na+ return to their resting concentration.

Saltatory conduction is when the action potential jumps from one node of Ranvier to the next and takes place in myelinated axons, greatly increasing the conduction velocity.

At a synapse an impulse travels from one neurone to the next.

There are two main types of synapse:

1. Electrical. This occurs when the two neurons are very close together.

2. Chemical. This is the most common.

In chemical synaptic transmission:

1. An action potential reaches the synaptic knob of the presynaptic neurone and calcium channels open. Calcium ions diffuse into the knob.

2. This increase in calcium ions stimulates the movement of vesicles containing a transmitter substance towards the presynaptic membrane.

3. The vesicles fuse with the membrane and release their substance by exocytosis into the synaptic cleft.

4. The transmitter substance diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane. This causes ion channels to open.

5. The movement of Cl, Na+, K+ in or out of the postsynaptic neurone generates a postsynaptic potential.

6. Depending on which ion moves in or out the postsynaptic membrane is either depolarised or hyperpolarised.

7. Depolarisation results in the development of an excitatory postsynaptic potential and hyperpolarisation results in an inhibitory postsynaptic potential.

8. The transmitter substance is quickly removed by enzyme action and diffusion.

There are four main groups of transmitter substances:

Transmitter substance	Function
Acetylcholine	Found at neuromuscular junctions and many parts of the brain
Amines e.g. noradrenaline.	In the sympathetic nervous system and some parts of the central nervous system
Amino acids e.g. glycine	Inhibitory substance in nerve pathways in the spinal cord.
Neuropeptides e.g. endorphins	Regions of the brain. Block pain.