NCERT grounding
NCERT Class XI Biology, Chapter 7 — Structural Organisation in Animals — opens with the statement that "all complex animals consist of only four basic types of tissues" and treats neural tissue as the last of the four. The chapter's one-line definition is that neural tissue exerts the greatest control over the body's responsiveness to changing conditions; it contains neurons, the unit of the neural system, which are excitable cells, while the neuroglia — non-excitable — make up more than one-half the volume of neural tissue in the vertebrate nervous system. NIOS Biology, Lesson 5 (Tissues and other Levels of Organization), §5.3.4, supplies the structural detail: the neuron's cyton, dendrites, axon, Nissl bodies, medullary (myelin) sheath, sheath cells and nodes of Ranvier.
Two cell types — neurons and neuroglia
Neural tissue has only two cell families. Neurons are excitable: their plasma membrane can generate and propagate an electrical signal, the nerve impulse. Neuroglia, or glial cells, are non-excitable; they cannot fire impulses. Despite this asymmetry of function, neuroglia are far more numerous: NCERT records that they protect and support neurons and constitute more than one-half the volume of neural tissue in the vertebrate nervous system. The same volumetric dominance is exactly what NEET 2022 tested when it listed neuroglia among possible connective tissues and asked which of four options was not a connective tissue.
Rule: Neural tissue = neurons (excitable, signal-generating) + neuroglia (non-excitable, supportive). Neuroglia > 50% of nervous-tissue volume.
Neuron
Status: Excitable; the structural & functional unit of the nervous system.
Job: Receives stimuli at dendrites, conducts an impulse down the axon, hands it to the next cell.
Output: Nerve impulse → muscle (contract) or gland (secrete) or next neuron.
NEET 2018 — Nissl bodies = RER + free ribosomesNeuroglia
Status: Non-excitable supporting cells of neural tissue.
Job: Protect, insulate (myelinate) and nourish neurons; maintain the extracellular environment.
Bulk: > ½ the volume of vertebrate neural tissue.
NEET 2022 trap: neuroglia is NOT a connective tissueAnatomy of a typical neuron
A neuron is a single cell drawn out into an extraordinary shape. Like any cell it is bounded by a plasma membrane and contains a nucleus and the usual organelles — but those organelles are concentrated in a central swelling called the cyton (also called the cell body or soma), and the rest of the cell is stretched into thin processes that may run a metre or more. NIOS §5.3.4 lays down three structural parts: cyton, dendrites and axon.
Figure 1. Generalised plan of a multipolar neuron — cyton with central nucleus and dark Nissl granules, several short branching dendrites that receive stimuli, and a single long axon insulated by segments of myelin produced by Schwann cells. The gaps between segments are the nodes of Ranvier; the axon ends as fine branches bearing synaptic knobs.
Cyton (soma)
The cyton is the metabolic centre of the neuron. It is bounded by a plasma membrane and possesses a nucleus and the usual organelles — including mitochondria. Its most diagnostic feature is the presence of Nissl bodies: dark granules of the cytoplasm composed of RNA and protein (NIOS) and, when resolved electronically, recognisable as stacks of rough endoplasmic reticulum studded with free ribosomes — the answer choice NEET picked in 2018. Nissl bodies are absent from the axon and from the axon hillock and they are therefore used in histology to distinguish a nerve cell body from its processes.
Dendrites
Dendrites are the smaller branching processes of the cyton. The Greek root dendros means "tree" — and a cyton typically bears many short, tapering, repeatedly forked processes that look exactly like the crown of a tree. Functionally, dendrites are the input zone of the neuron: they receive stimuli, either from sensory cells or from the axon terminals of upstream neurons, and conduct that signal towards the cyton.
Axon
The axon is a single, much longer process that arises from the cyton — usually from a slight conical elevation called the axon hillock. It carries the impulse away from the cyton towards its target. The axon may branch only at its far end, where it gives rise to fine terminal branches that end on a muscle fibre, a gland cell or another neuron. NIOS records the functional sequence in one sentence: "the branching dendrites receive the stimulus and transmit it through the cyton to the axon, which finally transmits it through its variously branched ends into either a muscle (to order it to contract) or to a gland (to order it to secrete)."
Direction of impulse within a single neuron
-
Step 1
Stimulus arrives
Sensory input or transmitter from upstream neuron contacts a dendrite.
Input zone -
Step 2
Dendrites → cyton
Signal propagates along the branching dendrites into the cell body.
Integration -
Step 3
Axon conducts
An action potential travels down the axon, jumping node-to-node if myelinated.
Output line -
Step 4
Terminal handoff
Terminal branches deliver the impulse to muscle, gland or next neuron at the synapse.
Effector
Structural classes of neurons
Neurons are also classified by the number of processes leaving the cyton. Multipolar neurons have one axon and many dendrites and are typical of the brain and spinal cord. Bipolar neurons have one axon and a single dendrite and occur, for example, in the retina of the eye. Unipolar neurons have only one process arising from the cyton and are usually found in the embryonic stage. The cyton-plus-processes plan is the same in all three classes; only the branching pattern differs.
Myelinated vs non-myelinated fibres
Once an axon leaves the cyton it constitutes a nerve fibre. The fibre may or may not be wrapped in an extra insulating layer. NIOS calls this layer the medullary sheath; it is produced by the sheath cells — known in vertebrates as Schwann cells — and is made chiefly of myelin, a lipid-like substance. According to whether this sheath is present or absent, the fibre is described as medullated (myelinated) or non-medullated (non-myelinated).
The medullary sheath does not run unbroken from cyton to terminal. Each Schwann cell forms only one short segment of myelin; between two consecutive Schwann cells the axon membrane is bare. These regular breaks are the nodes of Ranvier. At a node, the axon membrane is exposed to the extracellular fluid, and the impulse is regenerated each time it reaches one; in effect the action potential leaps from node to node and the fibre conducts much faster than an equivalent non-medullated one of the same diameter.
Figure 2. The myelin sheath is interrupted at regular intervals by nodes of Ranvier. In the myelinated fibre, the impulse is regenerated only at the nodes and effectively jumps along the axon; in the non-myelinated fibre the impulse moves continuously and more slowly.
Impulse flow and the synapse
A single neuron is rarely the whole story. To produce a reflex or a thought the nervous system stitches neurons together end to end, and the place at which the axon terminals of one cell meet the dendrites or cyton of the next is the synapse. At a synapse the two membranes are extremely close but do not actually fuse — the impulse is handed across as a chemical messenger or, in some places, as a direct electrical current. The axon terminals likewise relay the impulse to muscle fibres (ordering them to contract) and to gland cells (ordering them to secrete). NCERT therefore positions neural tissue as the control tissue: epithelia protect and absorb, connective tissues bind and transport, muscle moves — but neural tissue tells the others when.
Inside the central nervous system, the cytons and the dendrites of neurons are concentrated in the grey matter, while the myelinated axons run together as tracts to form the white matter. Outside the brain and spinal cord, the same axons travel as cranial nerves and spinal nerves, and they are again wrapped in Schwann-cell myelin and broken at nodes of Ranvier — the basic plan you draw in Figure 1.
Worked examples
Identify the cell type and tissue: "Excitable, generates and conducts a nerve impulse, possesses Nissl bodies in its cytoplasm." Is this a connective, muscular or neural tissue cell?
Answer. A neuron, which is the unit of neural tissue. The diagnostic clue is the presence of Nissl bodies — dark RNA + protein granules of the cyton seen in nerve cell bodies only. Muscle fibres are excitable but contain sarcomeres and lack Nissl bodies; connective-tissue cells (fibroblasts, mast cells, etc.) are non-excitable.
In a histology slide of nervous tissue, a student counts roughly twice as many small support cells as neurons. Is this ratio surprising? Justify with reference to NCERT.
Answer. No — it is exactly what NCERT predicts. The text states that neuroglia constitute more than half the volume of the neural tissue in the vertebrate nervous system. Since glial cells are also individually smaller than neurons, their numerical share is even higher than their volumetric share. A 2:1 (or higher) glia-to-neuron ratio is therefore consistent with the source.
A myelinated axon of length 10 cm has internodes of about 1 mm separated by nodes of Ranvier. Approximately how many Schwann-cell segments lie along this axon, and why is myelination useful here?
Answer. If each internode (the bit of myelin between two consecutive nodes) is ≈ 1 mm and the axon is 100 mm long, there are ≈ 100 Schwann-cell segments along its length, each laying down its own piece of myelin. The sheath is interrupted at each node, exposing the axon membrane. This is useful because the action potential is regenerated only at the nodes and effectively jumps from one to the next, so a myelinated fibre conducts much faster than a bare axon of the same diameter.
Match the part of a neuron with its role: (A) dendrite, (B) cyton, (C) axon, (D) Nissl body — with (i) protein synthesis, (ii) receives stimulus, (iii) conducts impulse away from cell body, (iv) houses nucleus and main organelles.
Answer. A–(ii); B–(iv); C–(iii); D–(i). The dendrites receive the stimulus; the cyton holds the nucleus and most organelles; the axon carries the impulse outward; Nissl bodies are the protein-synthesis machinery (rough ER + free ribosomes) of the cyton.