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Excitation from SAN (sino-atrial node) spreads across atria, causing them to contract first. The excitation is conducted to the AV (atri...

 - An electrical impulse travels down the axon of a pre-synaptic neurone. - The movement of this electrical impulse initiates the release of chemical messengers which are...

The transmission of an action potential- the signal- from one neurone to another occurs over the synapse. The structure of a synapse is as follows; there is a presynaptic bulb, a post synaptic bulb and th...

Natural selection is when certain characteristic are 'selected' for. Each species have simiar characteristics but differences in genes means that their characteristics can vary for example individual bird...

Muscle contraction can be explained using the 'Sliding Filament Hypothesis'. Following nervous stimulation, a nerve impulse will be transmitted to a neuromuscular junction. This will cause a release of th...