Explain the following processes :
(a) Polarization of the membrane of a nerve fibre
(b) Depolarization of the membrane of a nerve fibre
(c) Conduction of a nerve impulse along a nerve fibre
(d) Transmission of a nerve impulse across a chemical synapse

Options:

None

Solution:
(a) Polarization of the membrane of a nerve fibre:
- When a neuron isnot conducting anyimpulse, i.e., at rest the axon membrane is more permeable topotassium ions $\left(\mathrm{K}^{+}\right)$.
- It is impermeable to sodium ions $\left(\mathrm{Na}^{+}\right)$, negatively charged proteins and $\mathrm{Cl}^{-}$ions.
- Consequently, the axoplasm inside the axon contains high concentration of $\mathrm{K}^{+}$and negatively charged proteins and low concentration of $\mathrm{Na}^{+}$.
- In contrast, the fluid outside the axon contains a low concentration of $\mathrm{K}^{+}$, a high concentration of $\mathrm{Na}^{+}$and thus form a concentration gradient. These ionic gradients across the resting membrane are maintained by the active transport of ions by the sodium - potassium pump which transports $3 \mathrm{Na}^{+}$outwards for $2 \mathrm{~K}^{+}$into the cell.
- As a result, the outer surface of the axonal membrane possesses a positive charge while its inner surface becomes negatively charged and therefore is polarised.
The electrical potential difference across the resting plasma membrane is called resting potential.
- The electrical potential that occurs across the membrane of an axon, when stimulated by threshold stimulus is called depolarisation.
(b) Depolarisation of the membrane of a nerve fibre:
- When a stimulus is applied at a site on the polarised membrane, the membrane becomes more permeable to sodium ions $\left(\mathrm{Na}^{+}\right)$than to potassium ions $\left(\mathrm{K}^{+}\right)$.
- The potential difference in the stimulated/depolarised membrane is called action potential.
- The action potential spreads like a wave along the membrane in the form of impulse or spike.
(c) Conduction of nerve impulse along a nerve fibre:
- When a stimulus is applied at a site on the polarised membrane the membrane at the site $\mathrm{A}$ becomes freely permeable to $\mathrm{Na}^{+}$.
- As a result polarity gets reversed by a rapid inflow of $\mathrm{Na}^{+}$.
- After the reversal of polarity of the membrane, the membrane becomes depolarised.
- The electrical potential difference across the plasma membrane at the site $\mathrm{A}$ is called the action potential; which is termed as nerve impulse.
- The axon membrane (site B) has a positive charge on the outer surface and a negative charge on its inner surface. As a result, a current flows on the inner surface from site A to site B.$-\quad$ On the outer surface current flows from site B to site A, to complete the circuit of current flow.
- Hence, the polarity at the site is reversed, and an action potential is generated at site B.
- Thus, the impulse generated at site A arrives at site B. The sequence is repeated along the length of the axon and consequently impulse is conducted.
- The rise in the stimulus-induced permeability to $\mathrm{Na}^{+}$ is extremely short-lived.
- It is quickly followed by a rise in permeability to $\mathrm{K}^{+}$. Within a fraction of a second, $\mathrm{K}^{+}$diffuses outside the membrane and restores the resting potential of the membrane at the site of excitation and the fibre becomes once more responsive to further stimulation.
(d) Transmission of a nerve impulse across chemical synapse:
- A nerve impulse is transmitted from one neuron to another through junction called synapses.
- Electrical current can flow directly from one neuron to the other across these synapses.
- The membrane of the pre-and post-synaptic neurons are separated by fluid-filled space called synaptic deft.
- Chemicals called neurotransmitters are involved in the transmission of impulses at these synapses.
- The axon terminals contain vesicles filled these neurotransmitters.
- When an impulse (action potential) arrives at the axon terminal, it stimulates the movement of the synaptic vesicles towards the membrane where they fuse with the plasma membrane and release their neurotransmitters in the synaptic cleft. The released neurotransmitters bind to their specific receptors, present on the post-synaptic membrane. This binding opens ion channels allowing the entry of ions which can generate a new potential in the post-synaptic neuron. The new potential developed may be either excitatory or inhibitory.

Topic: Biology, Neural Control and Coordination