Explain how a nerve impulse passes along a neuron (8 marks)

Neurons transmit electrical impulses. Electrical impulses are action potentials propagated along the axons of neurons from one end to another. At a resting potential, sodium-potassium pumps pump out 3Na+ for every 2K+ ions in which results in a potential difference (p.d) of around -70mV across the axon membrane (relative to outside, the inside is more negative). An action potential occurs when the p.d. inside the axon reaches a threshold potential of around -55mV. This results in the rapid depolarisation and subsequent repolarisation of a section in the axon. During depolarisation, sodium voltage gated channels open allowing Na+ ions to diffuse down the concentration gradient and into the axon. At a p.d. of +40mV, the sodium voltage gated channels close and the potassium voltage gated channels open allowing K+ ions to diffuse down a concentration gradient out of the axon, allowing the membrane potential to return to around -70mV.
For propagation of action potentials to occur, ion movements depolarising one part of the neuron trigger depolarisation of neighbouring sections. Nerve impulses only flow in ONE DIRECTION. This is maintained by a refractive period which is a slight pause (1-2ms) inbetween action potentials. Local currents involve the diffusion of ions within the axon which help propagate nerve impulses by causing non-polarised regions of the axon to reach a threshold potential. Myelinated neurons have much quicker nerve impusles than un-myelinated neurons as they participate in saltatory conduction. Saltatory conduction is where local currents jump underneath the myelin sheath causing quicker depolarisation in a region further along the axon.