Nerve impulses: action potentials

Nerve impulses: action potentials

1. Neurones at rest- actively pumping 3 sodium out and 2 potassiums into cell. The gated sodium channels are kept open whilst some potassium channels stay open. Potassium ions diffuse out of the cell. The cell cytoplasm contains large negatively charged anions. Interior of cell is maintained at a negative potential compared with the outside- polarised. Potential difference is -60mV, called resting potential

2. Generating an action potential- if some sodium ion channels open, the sodium ions will diffuse down their conc. gradient into the cell- causes depolarisation. Small depolarisations are caused in generator areas whereby a few channels open. Voltage gated channels open by changes in the potential difference across the membrane. When there are sufficient generator potentials to reach the threshold potential they cause the voltage gated channels to open. This allows a large influx of sodium ions and the depolarisation reaches +40mV on the inside of the cell. The AP is self-perpectautin. All or nothing response

3. Stages of an AP: 1. membrane starts at resting state: -60mV, higher conc. of sodium ions outside than inside and a higher concentration of potassium ions inside than outside. 2. sodium ion channels open and some sodium ions diffuse into the cell. 3. the membrane depolarises- it becomes less negative compared to outside and reached the threshold value of -50mV. 4. positive feedback causes nearby voltage gated sodium channels to open and sodium ions flood in- cell becomes positively charged inside compared with outside. 5. the PD is +40mV, inside is positive compared wiith outside. 6. sodium ion channels close and potassium channels open. 7. potassium ions diffuse out of cell bringing the PD back to negative compared with outside- polarisation. 8. The potential difference overshoots slightly, making the cell hyper polarised. 9. the original PD is restored so that the cell returns to its resting state

4. Refractory period- after an AP the sodium and potassium ions are in the wrong places. The concentrations of these ions inside and outside the cell must be restored by the action of the ion pumps. For a short time after each AP it is impossible to stimulate the cell membrane to reach another AP. This is known as the refractory period and allows the cell to recover after an action potential. It also ensures that action potentials are transmitted in only 1 direction