generating an action potential

• while the neuron is at rest, it maintains a concentration gradient of sodium ions across the plasma membrane, the concentration outside is higher than the inside. Equally, the concentration of potassium ions is higher outside of the neuron than the inside
• If some of the sodium ion channels are opened, then ions will quickly diffuse down the concentration gradient into the cell from the surrounding tissue fluid; this causes depolarisation.
• In the generator region of a neuron, the gated channels are opened by the action of a synapse. when a few gated channels open, they allow a few sodium ions into the cell- producing a small depolarisation. This is known as a generator potential, it may go no further. But, if more gates are opened then the generator potentials are combined to create a larger depolarisation- this passes the threshold of -50 mV and causes an action potential.
• Most of the sodium ion channels in a neuron are opened by changes in the potential difference across the membrane, hence they are called voltage-gated channels. When there are sufficient generator potentials to reach the -50 mV threshold, they cause the voltage-gated channels to open. this is an example of positive feedback because a small depolarisation causes a change that increases the depolarisation further.
• the opening of the voltage-gated sodium ion channels allows a large influx of sodium ions and the depolarisation reaches +40 mV on the inside of the cell. Once +40 mV is reached, the neuron will transmit the action potential. The action potential is self-perpetuating, once it starts at one point it will continue to the end of the neuron. All action potentials are the same magnitude of +40 mV, thus it is referred to as an all-or-nothing response.