Question
Draw and label the structure of a neuron. Describe the function of each part in the transmission of a nerve impulse.
(NCERT Class 11, Chapter 21 — Neural Control and Coordination. High-weightage topic for NEET — appears almost every year.)
Solution — Step by Step
A neuron has three main regions: the cell body (soma), dendrites, and the axon. Think of the neuron as a tree — the roots receive signals (dendrites), the trunk carries them (axon), and the branches at the end pass them on (axon terminals).
The cell body contains the nucleus, Nissl granules (rough ER), and other organelles. Nissl granules are unique to neurons and are responsible for protein synthesis.
Dendrites are short, branched extensions of the cell body. Their job is to receive incoming signals from other neurons or sensory receptors and carry them towards the cell body.
The surface area created by branching is the key — more branches means the neuron can receive signals from more sources simultaneously.
The axon is a single, long fibre that carries the nerve impulse away from the cell body. It begins at a thickened region called the axon hillock, which is where the impulse is actually generated.
In vertebrates, many axons are covered by a myelin sheath — a fatty white covering produced by Schwann cells. Between adjacent Schwann cells are gaps called Nodes of Ranvier. These nodes are critical: they allow the impulse to “jump” from node to node (saltatory conduction), making transmission much faster.
At the far end of the axon, the fibre branches into axon terminals, each ending in a synaptic knob (also called the terminal bouton). These knobs contain vesicles filled with neurotransmitters — chemicals that carry the signal across the synapse to the next neuron.
When an impulse arrives, the vesicles fuse with the membrane and release neurotransmitters into the synaptic cleft.
Why This Works
The structural design of a neuron is perfectly matched to its function of rapid, directional communication. Dendrites have high surface area to maximise signal reception, the axon provides a dedicated one-way highway for the impulse, and the synaptic knob handles the handover to the next cell.
The myelin sheath deserves special attention. Since myelin is a poor conductor, the impulse cannot travel continuously along a myelinated axon. Instead, it “jumps” between Nodes of Ranvier — this is saltatory conduction and it can be up to 70× faster than conduction along an unmyelinated fibre. This is why your reflex responses are so fast.
Unmyelinated fibres (like those carrying dull, aching pain) conduct slowly. Myelinated fibres (like those in your motor neurons) conduct rapidly. NEET frequently tests whether you can connect myelin to speed.
Alternative Method — Functional Classification
Instead of remembering parts anatomically, you can remember the neuron by signal flow:
This flow diagram is enough to reconstruct labels in a diagram-based question. Draw the arrow path first, then fill in the structure names. For NEET diagram questions, this approach is faster than trying to recall names in isolation.
NEET 2023 had a direct question on which part of the neuron generates the action potential. The answer is the axon hillock, not the dendrite or the cell body. Many students write “cell body” — the hillock is where threshold depolarisation happens.
Common Mistake
Confusing the direction of signal flow. Students often write that axons receive signals and dendrites send them — exactly backwards. Remember: Dendrites = Delivery inward (towards the cell body). Axon = Away from the cell body. This single error has cost many NEET students a mark they could have easily secured.
Also, do not write that the myelin sheath is produced by the neuron itself — it is produced by Schwann cells (in the peripheral nervous system) or oligodendrocytes (in the CNS). NEET has tested this distinction.
Quick Reference
| Part | Direction | Key Feature |
|---|---|---|
| Dendrites | Towards cell body | High branching, large surface area |
| Cell body | — | Nucleus, Nissl granules |
| Axon hillock | — | Site of impulse initiation |
| Axon | Away from cell body | Covered by myelin sheath |
| Nodes of Ranvier | — | Gaps in myelin; enable saltatory conduction |
| Synaptic knob | — | Releases neurotransmitters |
Final answer: A neuron consists of dendrites (signal reception), cell body (integration), and axon (signal transmission). The myelin sheath accelerates conduction via saltatory conduction at Nodes of Ranvier. Signal is passed to the next neuron via neurotransmitter release from the synaptic knob.