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Nerve Impulses - Action Potentials.

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Nerve Impulses - Action Potentials An impulse or action potential is a temporary and local reversal of the resting potential, arising when an axon is stimulated. During an action potential, the membrane potential falls until the inside of the membrane becomes positively charged with respect to the exterior (-70mv to +40mv). The membrane is then said to be depolarised. The change in potential across a membrane comes about because of ion channels in the membrane that, when open, allow particular ions to pass. One type of protein is permeable to sodium ions and another type is permeable to potassium ions. During the resting potential these channels are closed. When an impulse is transmitted they start to open. Sodium channels open first, allowing a large number of sodium ions to flow in. The axoplasm becomes progressively more positive with respect to the outside of the membrane. ...read more.


Subsequently, in the relative refractory period, the resting potential is progressively restored. The first stage of restoration is due to the outward diffusion of potassium ions. In the next stage, the continuing actions of the potassium/sodium ion pumps and the differing rates of diffusion of sodium and potassium ions re-establish the potential difference across the membrane. During this period it becomes increasingly possible for an action potential to be generated. The Threshold Of Stimulation A stimulus must be at or above a certain minimum strength, known as the threshold of stimulation, in order to initiate the transmission of an action potential. Thus a stimulus evokes either a full response or no response at all. When a stimulus is too weak, the influx of sodium ions into the neurone is slight, and normal polarity is very quickly re-established. Such a stimulus is said to be sub-threshold. ...read more.


In myelinated axons, the action potential jumps from node to node (as depolarisation is restricted to nodes) at high speed. This is referred to as saltatory conduction (as the impulse leaps along). By contrast, depolarisation of an un myelinated axon is referred to as continuous conduction and is much slower. Transmission of an action potential across a myelinated axon Saltatory conduction has two advantages: * In humans, un myelinated fibre nerve impulses travel at 1 to 3 metres per second, while myelinated fibres conduct at speeds of up to 120 metres per second. * Metabolically, saltatory conduction is economical, because fewer ions move across the membrane, so the ion pumps need less energy to restore the ionic balance. The speed of transmission of a nerve impulse is also determined by the number of synapses involved. Communication between neurones across the minute gaps at the synapses involves chemical release and a brief time delay. Therefore, the greater the number of synapses (or number of neurones) in a series of neurones, the slower the conduction velocity. ...read more.

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