Changes

===Nerve Impulse Propagation===

===Nerve Impulse Propagation===

Nerves are able to create, amplify and propogate electrical impulses that run along their axon. These nerve impulses are a form of action potential that is carried by ions. The nerve impulse is in effect an electrical difference between the inside and outside of the axon and is caused by ion movements across the membrane. Nerve impulses can occur from a number of sources including sensiry cells, action potentials from other connected nerves or spontaneous depolarisation of the nerve cell membrane.

Nerves are able to create, amplify and propogate electrical impulses that run along their axon. These nerve impulses are a form of action potential that is carried by ions. The nerve impulse is in effect an electrical difference between the inside and outside of the axon and is caused by ion movements across the membrane. Nerve impulses can occur from a number of sources including sensory cells, action potentials from other connected nerves or spontaneous depolarisation of the nerve cell membrane.

====Unmyelinated Axons====

====Unmyelinated Axons====

Conduction of an action potential along an unmyelinated axon, i.e. an axon not covered by some form of glial cell, is a much slower form of nerve impulse propagation than that of a myelinated axon.  

Conduction of an action potential along an unmyelinated axon, i.e. an axon not covered by some form of glial cell, is a much slower form of nerve impulse propagation than that of a myelinated axon.  

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At a given moment voltage-gated sodium channels within the membrane of the axon will open resulting in an influx of ions following their electro-chemical gradient. This Na⁺ influx causes the axon to accumulate a positive charge and results in cellular depolarisation. The extracellular area of the axon therefore looses its positive charge, becoming more negative resulting in a current of positive charge that flows  through the tissue towards the axon. The membrane of the axon is not a perfect insulator and at some regions on the axon, particularly within the area of the axon infront of the action potential, the voltage-dependant ion channels have not activated yet. This means that some of the positive charge is able to flow out of the axon membrane in these regions and this positive charge outflow is mainly via potassium. This outward leakage of potassium results in the current within the axon only being able to travel a short distance before the nerve impulse decays. However the effect of the current locally on the voltage-gated channels means that the nerve impulse is able to open voltage-gated channels within it's immediate vicinity and this is enough for the signal to propagate along the nerve.

At a given moment when the action-potential threshold is reached voltage-gated sodium channels within the membrane of the axon will open resulting in an influx of ions following their electro-chemical gradient. This Na⁺ influx causes the axon to accumulate a positive charge and results in cellular depolarisation. The extracellular area of the axon therefore looses its positive charge, becoming more negative resulting in a current of positive charge that flows  through the tissue towards the axon. The membrane of the axon is not a perfect insulator and at some regions on the axon, particularly within the area of the axon infront of the action potential, the voltage-dependant ion channels have not activated yet. This means that some of the positive charge is able to flow out of the axon membrane in these regions and this positive charge outflow is mainly via potassium. This outward leakage of potassium results in the current within the axon only being able to travel a short distance before the nerve impulse decays. However the effect of the current locally on the voltage-gated channels means that the nerve impulse is able to open voltage-gated channels within it's immediate vicinity and this is enough for the signal to propagate along the nerve.

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The influx of sodium ions via voltage-gated channels is only possible at these nodes  and therefore the nerve impulse is able to effectively 'jump' from node to node. This type of impulse propogation is called '''saltatory conduction'''. Myelination of axons in mammals means that the nervous system can sustain a large number of high velocity axons within a relatively small space.

The influx of sodium ions via voltage-gated channels is only possible at these nodes  and therefore the nerve impulse is able to effectively 'jump' from node to node. This type of impulse propogation is called '''saltatory conduction'''. Myelination of axons in mammals means that the nervous system can sustain a large number of high velocity axons within a relatively small space.

===Synapses===

===Synapses===

The synapses found at the end of axons are fundamental to the functioning of the nervous system as they facilitate communication between nerves and provide an interconnected network for many of the complex processes required by organisms. Synapses are required as the lipid bi-layer of the cell membrane has a relatively large electrical resistance making electrical impulse propagation directly between cells difficult.  

The synapses found at the end of axons are fundamental to the functioning of the nervous system as they facilitate communication between nerves and provide an interconnected network for many of the complex processes required by organisms. Synapses are required as the lipid bi-layer of the cell membrane has a relatively large electrical resistance making electrical impulse propagation directly between cells difficult.