Question

Explain the following processes:

1. Polarisation of the membrane of a nerve fiber
2. Depolarisation of the membrane of a nerve fiber
3. Transmission of a nerve impulse across a chemical synapse.

Answer 1

a. Polarisation of the membrane of a nerve fiber

When a nerve fiber is at rest, it is considered to be in a polarised state. The nerve fiber’s membrane has a resting potential when it is polarised. The following steps occur during the polarization of a nerve fiber’s membrane:

• When a depolarised area of a nerve fiber first begins to become polarised, there are more K⁺ ions outside the nerve fiber and a considerable amount of Na⁺  ions in the axon membrane.
• As the membrane region gets polarised, it becomes more permeable to K⁺  ions and impenetrable to Na⁺  ions and negatively charged proteins.
• A sodium-potassium pump sends 3 Na⁺ ions outside the axon and 2 K⁺ ions into the axon via active transport.
• Because of the flow of sodium and potassium ions, the inner side of the membrane becomes electronegative (negatively charged), while the outer side becomes electropositive (positively charged). This causes the nerve fiber to become polarised.

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b. Depolarisation of the membrane of a nerve fiber
 When a nerve fiber is activated, it is said to be in a depolarised state. The membrane of the nerve fiber undergoes an action potential when it is depolarized. During the process of depolarisation of a nerve fibers membrane, the following phases occur:

• The axon has a higher concentration of K⁺ ions in a polarised condition, while the concentration of Na⁺ ions is higher outside the axon.
• When a nerve fiber is stimulated, the permeability of the membrane for Na⁺ ions and K⁺ ions reverses.
• The membrane becomes highly permeable to Na⁺ ions.
• There is a fast influx of Na⁺ ions into the axon.
• As a result, the inside of the membrane becomes positively charged, while the exterior of the membrane gets negatively charged.
  Finally, the nerve fibers membrane depolarizes, and it experiences an action potential.

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c. Transmission of a nerve impulse across a chemical synapse

• The membranes of the pre-synaptic and post-synaptic neurons combine to form a synapse.
• A gap known as the synaptic cleft may or may not divide two synapses.
• The synaptic cleft separates the pre-and post-synaptic neurons at a chemical synapse.
• The calcium ions in the synaptic cleft enter the synaptic knobs at the axon terminals of the pre-synaptic neuron when an impulse reaches the axon terminal.
• The synaptic vesicles in the pre-synaptic neuron’s synaptic knobs migrate in its direction and fuse with the plasma membrane.
• Acetylcholine, a neurotransmitter, is released by the vesicles in the synaptic cleft. (Empty synaptic vesicles are filled when they return to the cytoplasm of the pre-synaptic neuron.)
• The protein receptors found on the plasma membrane of post-synaptic neurons are where the molecules of acetylcholine bind.
• As a result of this interaction, potassium ions exit the post-synaptic membrane and sodium ions enter the post-synaptic neuron.
  This causes the membrane of the post-synaptic neuron to create an action potential, which then transmits the impulse to the post-synaptic neuron.