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.

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There are more charged proteins and potassium ions (K+) inside the axon than outside and more sodium ions (Na+), chloride ions (Cl-) outside the axon than inside.

This imbalance produces an electrical gradient that is required to create a resting potential.

The membrane is selectively permeable to different ions and it allows sodium. Potassium and chloride ions to diffuse freely across it. Larger charged proteins are restricted to the inside of the membrane by their size. Potassium ions inside the axon outnumber the sodium ions 10:1 and sodium ions outside the axon membrane always outnumber potassium ions 10:1.

These ions ...

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