Neuron — Structure (Cell Body, Dendrites, Axon)

NCERT grounding

Section 18.3 of NCERT Class 11 Biology opens with the line: "A neuron is a microscopic structure composed of three major parts, namely, cell body, dendrites and axon." The chapter then defines Nissl's granules in the cyton, distinguishes dendrites (toward the cell body) from axon (away from the cell body), classifies neurons by polarity (multipolar, bipolar, unipolar) and by myelin coat (myelinated, non-myelinated), and names the Schwann cell, the myelin sheath and the nodes of Ranvier. NIOS Biology Lesson 17 (Section 17.5) supplements this with the functional classification — sensory, motor and intermediate neurons — that NCERT defers to its discussion of reflex arcs.

"Dendrites transmit impulses towards the cell body, whereas axons transmit nerve impulses away from the cell body to a synapse or to a neuro-muscular junction."

— NCERT, Class 11 Biology, §18.3

Anatomy of the neuron

A neuron is an excitable cell specialised for one job: receive a stimulus at one end, conduct it as an all-or-none electrical event along its length, and pass it on at the other end. To do that it has dropped most of its capacity to divide (mature neurons are amitotic) and has instead expanded its surface area in two opposite directions away from a central cell body. The receiver end is a bouquet of short, branched processes called dendrites. The transmitter end is a single, long fibre called the axon. The integrating hub between them is the cell body — also called the cyton, perikaryon or soma.

This three-part design is non-negotiable in NEET. The cell body holds the nucleus and the bulk of the protein-synthesis machinery, the dendrites are the input antennae, and the single axon is the output cable. Every diagram, every PYQ, every confusion cluster works off this scaffold.

Cell body (cyton / soma / perikaryon)

The cell body is roughly polyhedral or stellate, with a large central nucleus that has a prominent nucleolus. Its cytoplasm — sometimes called neuroplasm — contains all the usual eukaryotic organelles: mitochondria, a well-developed Golgi apparatus, lysosomes and a dense cytoskeleton of neurofilaments and microtubules. What sets it apart from other cells is one specific organelle: large, deeply basophilic clumps scattered through the cytoplasm called Nissl's granules (or Nissl bodies). Nissl granules are simply the rough endoplasmic reticulum of a neuron seen by light microscopy — aggregations of RER membranes studded with ribosomes — and chemically they are made up of RNA (ribosomal and messenger) and protein. They are the engine of the neuron's heavy protein-synthesis economy.

Dendrites — the input branches

From the cell body project several short, tapering, branched fibres called dendrites. NCERT describes them as "short fibres which branch repeatedly and project out of the cell body" and notes — importantly — that they also contain Nissl's granules. The two key adjectives are therefore short and branched; the two key facts are that they contain Nissl bodies and that they transmit impulses towards the cell body (afferent direction with respect to the soma). NIOS adds that a single neuron may carry as many as 200 dendrites, allowing it to receive synaptic input from hundreds of other neurons in parallel. The fine, knob-like protrusions on dendritic surfaces — dendritic spines — are the post-synaptic docking points for those inputs (beyond syllabus, but useful when reading reflex arc figures).

Axon — the conducting cable

From the opposite pole of the cell body emerges a single long fibre — the axon. The cone-shaped region where the axon leaves the soma is called the axon hillock; it is where the action potential is actually triggered because its membrane has the highest density of voltage-gated sodium channels and the lowest threshold of any part of the neuron. Axons can be remarkably long — the motor axons running from the spinal cord to the toes are nearly a metre long, even though their cell body fits inside the spinal cord.

The axon's cytoplasm is called the axoplasm and is enclosed by the axolemma (axonal membrane). It is the axolemma that is polarised in the resting state and that depolarises during an action potential. Unlike the dendrites and the cyton, the axon does not contain Nissl granules — protein made in the cell body has to be ferried down the axon by axonal transport. The axon's distal end is divided into many fine branches called telodendria, and each branch terminates in a swelling — the synaptic knob (axon terminal, terminal bouton) — which makes contact with the next neuron, a muscle fibre or a gland cell.

Structural types: multipolar, bipolar, unipolar

NCERT classifies neurons by the number of processes arising from the cell body. This is the structural (morphological) classification and is one of the highest-frequency MCQ targets in the chapter. Memorise the three by the pair "polarity → location".

Functional types: sensory, motor, interneuron

NIOS Lesson 17 — and every Class-12 textbook reflex-arc figure — uses a parallel functional classification based on the direction of conduction. NEET aspirants must hold both classifications in mind because a single neuron always has both a structural and a functional label.

The third functional class is the interneuron (also called association neuron, intermediate neuron or relay neuron). Interneurons lie entirely within the central nervous system, between a sensory input and a motor output, and they integrate the signal. NIOS notes that "mostly there occurs an intermediate neuron between the axon ending of the afferent fibre and the motor neuron inside the spinal cord" — this single line is the canonical NCERT/NIOS justification for the reflex-arc third neuron. Interneurons are numerically the largest class in the human nervous system.

Myelinated vs non-myelinated fibres

NCERT recognises two types of axons, myelinated and non-myelinated. The myelin sheath is a multi-layered, lipid-rich wrapping around the axon laid down — in the peripheral nervous system — by Schwann cells. Each Schwann cell wraps a short segment of the axon, and the small unwrapped intervals between two adjacent Schwann cells expose the bare axolemma; these gaps are the nodes of Ranvier. The myelin sheath insulates the axon electrically and forces the action potential to "leap" from one node to the next, a mode of conduction called saltatory. In the central nervous system the equivalent myelinating cell is the oligodendrocyte — a NEET-favourite distinction that has been asked verbatim (NEET 2017, Q.114).

The synaptic knob

NCERT's last anatomical sentence on the neuron is on the axon terminal: "Each branch terminates as a bulb-like structure called synaptic knob which possess synaptic vesicles containing chemicals called neurotransmitters." The synaptic knob (also called the axon terminal, end-bulb or terminal bouton) is the output device of the neuron. Inside it sit hundreds of small, membrane-bound synaptic vesicles each pre-loaded with a neurotransmitter — most commonly acetylcholine at motor end-plates. The pre-synaptic membrane is densely packed with voltage-gated Ca²⁺ channels. When an action potential arrives at the knob, Ca²⁺ influx triggers vesicle fusion and exocytosis of neurotransmitter into the synaptic cleft. The neurotransmitter then binds receptors on the post-synaptic membrane — a fact NEET 2017 Q.107 tested directly.

Input → output flow inside a single neuron

cyton not the conducting unit

1. Step 1

   Reception

   Dendrites and cyton receive neurotransmitter from upstream neurons.

2. Step 2

   Integration

   Graded potentials summate at the axon hillock.

3. Step 3

   Trigger

   Threshold reached → action potential born at the hillock.

4. Step 4

   Conduction

   Impulse travels down the axon (saltatory if myelinated).

5. Step 5

   Release

   Synaptic knob releases neurotransmitter onto next cell.

Worked examples

Common confusion & NEET traps