Question

For action potentials and membrane potentials: Describe how the membrane depolarizes and then repolarizes, how an action potential opens voltage gated channels in front of it and thus passes the action potential further and further down the neuron.

Answer 1

• When a neuron is not conducting an electrical impulse, it is said to be at rest.
• In this stage, the axoplasm has more potassium ions and negatively charged proteins compared to the number of sodium ions. Outside the neuron, there are more number of sodium ions and less number of potassium ions. Therefore, the axoplasm is more negative compared to its external environment.
• In this stage, the axonal membrane is more permeable to potassium ions and less permeable to sodium ions and negatively charged proteins.
• This creates a potential difference which is maintained by the active transport conducted by sodium-potassium pumps found in the axonal membrane.
• For every 3 sodium ions transported outside the axon, 2 potassium ions are transported inside the axon by the sodium-potassium pumps.
• The potential difference hence created is called resting potential and it is -70mV.
• When a stimulus reaches a site on the axon, this site becomes permeable to sodium ions. The sodium ion channels open up and the sodium ions enter the axon. The axoplasm becomes less negatively charged. This is called depolarization.
• When the potential difference reaches threshold value, i.e., -55mV, an action potential is generated and fired along the length of the axon.
• The sodium ion influx continues till membrane potential reaches +30mV.
• This leads to opening up of potassium ion channels. The sodium ion channels close down. The potassium ions move out of the axon and the axoplasm becomes less positively charged and the membrane potential reduces to -70mV. This is called repolarization.
• The potassium ions take longer time to close down. This will result in the axoplasm becoming more negatively charged. The potential difference reduces below -70mV. This is known as hyperpolarization.
• When a neuron is hyperpolarized, it cannot generate action potential. This duration of time is called refractory period and it lasts for about 2 milliseconds, after which the resting potential is restored by active transport conducted by the sodium-potassium pump.