Nervous systems - Neurones

Neurone cell membranes are polarised at rest.

• In a neurones resting state the outside of the membrane is positively charged compared to the inside. This is due to the fact that there are more positive ions outside the cell than inside.
• This polarises the membrane.
• The voltage across the membrane when its at rest is called the resting potential. (-70mV)
• The resting potential is created and maintained by sodium potassium pumps and potassium ion channels in a neurones membrane.

Stimulus - This excites the neurone cell membrane, causing sodium ion channels to open. The membrane becomes more permeable to sodium, therefore sodium ions diffuse into the neurone down the sodium into electrochemical gradient. the makes the inside of the neurone less negative.

Depolarisation - If the potential difference reaches the threshold (-55mV) More sodium ion channels open. more sodium ions diffuse into the neurone.

Repolarisation - At a PD at (+30mV) the sodium ion channels close and potassium ion channels open. The membrane is more permeable to potassium so potassium ions diffuse out of the neurone down the potassium ion concentration gradient. This starts to bring the membrane back to its resting potential.

Hyperpolarisation - Potassium ion channels are slow to close so there is a slight overshoot where too many potassium ions diffuse out of the neurone. The potential difference becomes more negative than the resting potential.

Resting Potential - The ion channels are reset. the sodium potassium pump returns the membrane to its resting potential and maintains it until the membrane is excited by another stimulus.