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Explain how nerve impulses are transmitted along and between neurones

Transmitting Along Neurones

When a nerve cell is not stimulated, it is in a resting state. During this state, the inside of the axon is relatively more negative than outside of the axon, this is called the resting potential. The reason behind this negative charge is because of the distribution of the principal ions (Na+ and K+). In reality, there is a higher concentration of sodium ions outside the neurone while there is a higher concentration of potassium inside the neurone. This allows an electrochemical gradient to be generated, which is crucial for an action potential (AP) to be generated. During the resting state, there is a larger amount of positive ions (cations) on the outside of the neurone compared to the inside, thus giving it a relatively negative charge inside the axon. When there is a stimulus, this causes the Sodium (Na) channels located on the surface of the axon to open, causing an influx of sodium ions into the axon. This causes the charge within the axon to reverse and become positive, a process called depolarisation. Once the axon has depolarised over a certain threshold, this causes an AP. 

The AP, a charge in the electrical potential associated with the propagation of an impulse along, in this case, the nerve cell, travels in one direction from the soma to the axon terminals. Thus, once the AP passes a certain area of an axon, that part must return back to its resting state. This is called repolarisation. During this period, the sodium ion channels that were previously open during depolarisation start to close slowly and simultaneously the potassium ion channels open, causing an efflux of potassium ions from within the axon, causing the charge within the axon to become less positive (and therefore more negative - returning back to the resting potential). But here we come to a problem. If all the ions keep being pushed out or into the axon, how can a second AP be generated? Will there be enough ions? In order to prevent a shortage of cations, there is a mechanism called the sodium potassium pump that is constantly working. It works by pumping 3 sodium ions out for every 2 potassium ions being pumped in, therefore maintaining and restoring the axon back to the resting potential.

Transmitting Between Neurones

Once the AP reaches the axon terminals, it must be converted into something else as an AP cannot cross interneuron space between the postsynaptic and presynaptic neurone, which is called the synaptic cleft. Thus, when the AP reaches the axon terminal, it releases a chemical messenger called a neurotransmitter (NT). This NT is released into the synaptic cleft and moves via diffusion from the presynaptic to the postsynaptic neurone where the NT binds to receptors on the postsynaptic neurone. Once they bind to receptors, this allows ion channels to open, causing an influx of cations (sodium ions), which may initiate an action potential depending on the amount of receptors that have been activated. 

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