Question

Explain the process of depolarisation of a membrane of a nerve fibre.

Answer 1

Hint: The generation of an action potential due to a stimulus reverses the polarity. The membrane is negatively charged on the outside and positively charged on the inside.

Complete Answer:
- The neural system is a highly organised network that allows for point-to-point connections for quick coordination. Animals have a neural system composed of highly specialised cells which can sense, receive and transfer various types of stimulus. These cells are known as neurons. Each neuron receives an impulse and must transfer it onto the next neuron to ensure that its path proceeds with the right impulse. The dendrites take up an impulse through a series of chemical events that is carried through the axon and transferred to the next neuron.

- When it is not activated by any impulse, the nerve fibre is at a resting stage. The concentration of potassium ($K^+$)ions is more within the axoplasm when a neuron is not conducting any impulse, that is, it is at rest; while the concentration of Sodium ($Na^+$) ions is more outside the axoplasm. As a result, in contrast to sodium ions, potassium ions travel faster from the inside to outside. The membrane outside then becomes positively charged and negatively charged inside. This is known as membrane polarisation or polarised nerve.

- As a stimulus is introduced to the polarised membrane, it becomes free-permeable to $Na^+$. This results in a sudden release of $Na^+$ followed by the polarity reversal at that site. The membrane's outer surface becomes negatively charged and the inner surface becomes positively charged. The membrane polarity is thereby reversed and thus depolarised.

Note: A nerve impulse is transferred from one neuron to the other via junctions called synapses. The membranes of a presynaptic neuron and a postsynaptic neuron make up a synapse.