Because of the chemical nature of impulse and the axon-dendrite structure.A Nerve electrical impulse only travels in one direction.The most important is synaptic transport. In order for a "nerve impulse" to pass from cell to cell, it must cross synaptic junctions. The nerve cells are lined up head to tail all the way down a nerve track, and are not connected, but have tiny gaps between them and the next cell. These tiny gaps are called synapses.Neurotransmitters are molecules that fit like a lock and key into a specific receptor. The receptor is located on the next cell in the line. When the neurotransmitter hits the receptor on the next cell in line, it signals that cell to begin a firing as well.Therefore, nerve impulses cannot travel in the opposite direction, because nerve cells only have neurotransmitter storage vesicles going one way, and receptors in one place.so answer is option A.

The unidirectional transmission of a nerve impulse through a nerve fibre is primarily due to the release of neurotransmitters by axon endings. This process is essential for proper functioning of the nervous system and allows for rapid communication between neurons.

Neurotransmitters are chemical substances that are synthesized and stored in vesicles within the axon terminals. When a nerve impulse reaches the axon terminal, it triggers the release of these neurotransmitters into the synapse, which is the small gap between the transmitting neuron (presynaptic neuron) and the receiving neuron (postsynaptic neuron).

The release of neurotransmitters occurs through a process known as exocytosis. When the nerve impulse reaches the axon terminal, it causes a depolarization of the membrane, leading to the opening of voltage-gated calcium channels. The influx of calcium ions into the axon terminal triggers the fusion of neurotransmitter-containing vesicles with the presynaptic membrane, resulting in the release of neurotransmitters into the synapse.

- Role of Neurotransmitters in Unidirectional Transmission:
The neurotransmitters released into the synapse then bind to specific receptors on the postsynaptic neuron. These receptors are typically located on the dendrites or cell body of the postsynaptic neuron. The binding of neurotransmitters to their receptors initiates a series of events within the postsynaptic neuron, which ultimately leads to either the generation of a new nerve impulse or the inhibition of further impulses.

- Importance of Unidirectional Transmission:
The unidirectional transmission of nerve impulses is crucial for the proper functioning of the nervous system. It ensures that the signal travels in one direction, from the presynaptic neuron to the postsynaptic neuron. This directionality is essential for the coordination of complex processes such as movement, perception, and memory.

- Other Options and their Incompatibility:
The other options listed - neurotransmitters released by dendrites, nerve fibres insulated by a medullary sheath, and sodium pump operating into the nerve fibre - do play important roles in the functioning of nerve fibres and impulse transmission, but they are not directly responsible for the unidirectional transmission of nerve impulses. Therefore, options B, C, and D are incorrect in the context of this question.