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Nerve Impulses.

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Introduction

Access - Biological Science Fri a.m. Cathy Hogg March 2003 Nerve Impulses 1. a) Polarised membrane The membrane of a resting neurone must always be polarised. This means that the fluid on the inside must be negatively charged with respect to the outside therefore, an electrical potential difference exists across it. 1. b) Resting potential The difference in the charge of the membranes inner and outer fluid is about -70mv (millivolts) i.e. the inside of the cell is negative relative to the outside. When the cell is not transmitting an impulse, the transmembrane potential is called the resting potential. 2. The resting potential of a neurone is maintained by the neurone being able to maintain an internal fluid composition different to that outside the cell. ...read more.

Middle

It harnesses energy from ATP to move Na+ out and K+ in; it is a large protein molecule that forms a channel across the membrane. The membrane is 50 times more permeable to K+ than Na+. 3. An action potential is triggered when a stimulus reaches a neurone and a subtle electrical change takes place at the point of contact on the cell body. Small changes in potential spread to the axon hillock and they combine to exceed the threshold level and the membrane potential becomes positive. The membrane becomes more permeable to sodium ions, some of the extracellular sodium ions enter the cytoplasm by diffusion more easily than before. ...read more.

Conclusion

If the depolarisation isn't great enough to reach threshold then the action potential and subsequent impulse aren't produced. Rest neurone Permeable to Na+ at site of stimulation Na+ ions in membrane potential less negative Threshold -50mv Influx of Na+ Inside = +_ve +40mv Electrical charge reversed = Depolarisation 4. Electrical impulses are conducted by molecules hitting other molecules and are positively charged Sodium potassium exchange pump causes the "Mexican wave"type response where once the threshold is reached in an axon it passes from one axon to the next. 5. All or nothing means that is the stimulus is stronger than that necessary to produce an impulse is applied then the frequency of impulse production usually increases. The nervous system discriminates against weak and strong stimuli. Either the impulse is strong enough to cause the sodium potassium pump to work effectively or the resting potential is maintained. ...read more.

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