Explain how a series of impulses are initiated and transmitted along a neurone and across a synapse.

creating a highly positive area within the axon with respect to the outside. This depolarisation is known as an action potential. At the end of depolarisation the sodium channels are closed and excess Na+ ions are pumped out of the axon by a sodium pump. The time required for this to happen is known as the refractory period. The nerve cannot carry another impulse until this has occurred, ensuring the impulse only travels in one direction. The movement of the wave of depolarisation down the axon of the nerve results in local currents being set up. Positive Na+ ions are attracted to the negative region of the action potential leaving the membrane in this region slightly negative. The membrane is depolarised sufficiently to open the sodium channels in front of the action potential and set up a new action potential. When an action potential reaches the synaptic knob, the permeability of its membrane to Ca2+ ions is changed. Calcium channels in the presynaptic membrane open allowing the calcium ions to diffuse into the synaptic knob. These Ca2+ ions cause the vesicles in the synaptic knob to fuse with the presynaptic membrane and release transmitter substance into the synaptic cleft. The transmitter substance then diffuses across the synaptic cleft and binds with receptor sites on the postsynaptic membrane. Sodium channels on the postsynaptic membrane open and allow and influx of Na+ ions into the cell. This creates an action potential which travels down the axon of the nerve.