The fibre containing Nissl’s granules is a dendrite. NCERT states that the short fibres which branch repeatedly and project out of the cell body also contain Nissl’s granules and are called dendrites. Dendrites transmit impulses towards the cell body.

The fibre without Nissl’s granules is the axon — a long fibre whose distal end is branched. Axons transmit nerve impulses away from the cell body to a synapse or to a neuro-muscular junction.

The presence of Nissl’s granules is therefore a practical microscopic criterion for distinguishing the two kinds of process, quite apart from their difference in length and branching pattern.

The pattern follows directly from where each type of axon occurs.

Cranial (and spinal) nerves are myelinated. Their axons are enveloped with Schwann cells which form a myelin sheath, with nodes of Ranvier in the gaps between adjacent sheaths. Their conduction depends on that sheath, so destroying the myelin removes exactly the structure that made their conduction fast — and the slowing is marked.

Fibres of the autonomous and somatic neural systems are commonly unmyelinated. Such a fibre is enclosed by a Schwann cell that does not form a myelin sheath around the axon in the first place. A condition that destroys myelin therefore has little myelin to destroy there, and conduction in those fibres is comparatively unchanged.

The general lesson: the functional consequence of a lesion depends on which structures the affected tissue actually possesses.

Multipolar neurons in the cerebral cortex. A multipolar neuron has one axon and two or more dendrites. Since dendrites transmit impulses towards the cell body, having many of them means the cell can receive input from many different sources at once. The cerebral cortex carries out integration — comparing, combining and associating information — so a cell that gathers many inputs and produces one integrated output along its single axon is exactly what that work requires.

Bipolar neurons in the retina. A bipolar neuron has one axon and one dendrite. Its task in the retina is to receive a visual signal and pass it on faithfully along the pathway, not to combine many unrelated inputs. A straight-through, one-in one-out design suits a relay in a sensory chain.

So the number of dendrites tracks how much the neuron is expected to integrate as opposed to simply relay.

The statement is wrong on both counts. A unipolar neuron is a cell body with one axon only — the single process is the axon, and it is the dendrites that are absent. Such neurons are found usually in the embryonic stage.

The general principle the student has missed is that every neuron has exactly one axon, in all three types. Multipolar neurons have one axon and two or more dendrites; bipolar neurons have one axon and one dendrite; unipolar neurons have one axon and no dendrite. So the classification into unipolar, bipolar and multipolar is effectively a count of dendrites, not of all processes indiscriminately.

This also makes functional sense: the axon is the neuron's only output channel, so a cell without one could not transmit an impulse to a synapse or a neuro-muscular junction at all.

Common: in both cases the axon's branched distal end terminates in a structure containing vesicles filled with a chemical messenger — synaptic vesicles containing neurotransmitters. In both cases the arriving impulse causes release of that chemical, which then acts on the membrane of the next cell.

Different: the target. Axons transmit nerve impulses away from the cell body either to a synapse — where the next cell is another neuron — or to a neuro-muscular junction, where the next cell is a muscle fibre and the neurotransmitter is acetyl choline, generating an action potential in the sarcolemma and ultimately a contraction.

Why a chemical step is needed: the two cells are separate cells, with their own membranes and, at a chemical synapse, a fluid-filled gap between them. An electrical impulse travels along a continuous membrane; it cannot simply jump across a cleft between two different cells. Converting the signal into a released chemical bridges that gap. The conversion also brings an advantage: the new potential produced in the receiving cell may be either excitatory or inhibitory, so a chemical step allows the signal to be modulated rather than merely relayed.