Nervous System and Sense Organs ICSE Class 10: Neuron, Brain, Reflexes, Eye and Ear

What Nervous System and Sense Organs is about

Weightage: CISCE publishes no chapter or unit weightage for ICSE Biology. Nervous System and Sense Organs is in the 3. Human Anatomy and Physiology (iii) part of the 2027 syllabus and can be asked in both Section A and Section B of the 80-mark paper.Source: CISCE ICSE Biology (Science Paper 3) syllabus, examination year 2027

Key concepts

The neuron and the synapse

A neuron has:

• Cell body (cyton): contains the nucleus and granular cytoplasm.
• Dendrites: short, branched processes that carry impulses towards the cell body.
• Axon: a single long fibre that carries impulses away from the cell body; often covered by a fatty myelin sheath (insulates and speeds up conduction), with gaps called nodes of Ranvier.
• Axon endings with synaptic knobs.

A synapse is the junction between the axon ending of one neuron and the dendrite of the next. The impulse crosses the gap (synaptic cleft) by a chemical, a neurotransmitter (for example, acetylcholine).

Types: sensory (receptor to CNS), motor (CNS to effector) and association/relay neurons (within the CNS). A nerve is a bundle of axons; nerves may be sensory, motor or mixed (both kinds of fibres, like all spinal nerves).

Worked example

Arrange in the order an impulse travels: axon, dendrite, synaptic cleft, cell body, axon ending.

Try it first, then show the solutionSolution

1. The impulse enters through the dendrite.
2. Passes through the cell body, along the axon to its endings.
3. Crosses the synaptic cleft to the next neuron.

Answer: Dendrite → cell body → axon → axon ending → synaptic cleft.

Central nervous system: brain

The brain lies in the cranium, covered by three meninges (dura mater, arachnoid, pia mater), with cerebrospinal fluid that cushions it and supplies nutrients.

Part	Position	Main functions
Cerebrum	largest; two hemispheres, folded surface	intelligence, memory, thinking, will; voluntary actions; centres of sight, hearing, speech, touch
Cerebellum	below the back of the cerebrum	balance and posture; coordination of muscular movements
Medulla oblongata	lowest part, continues into the spinal cord	involuntary actions: heartbeat, breathing, swallowing, coughing, sneezing, vomiting
Thalamus	deep inside, below the cerebrum	relay centre for sensory impulses (pain, touch, temperature) to the cerebrum
Hypothalamus	below the thalamus	body temperature, hunger, thirst, sleep, emotions; controls the pituitary
Pons	above the medulla	relays impulses between brain parts; helps regulate breathing

Grey matter (cell bodies) is outside (cortex) and white matter (myelinated axons) inside in the brain; the reverse in the spinal cord, where grey matter is the inner butterfly (H)-shaped region.

Worked example

A basketball player's accurate, balanced throw needs coordination of muscles. Which part of the brain is chiefly responsible?

Try it first, then show the solutionSolution

1. Balance, posture and smooth coordination of voluntary movements are controlled by the cerebellum.
2. The decision to throw comes from the cerebrum.

Answer: Cerebellum (for coordination and balance).

Spinal cord, PNS and ANS

Spinal cord: a cylindrical cord running inside the vertebral column from the medulla; grey matter inside, white matter outside. It conducts impulses to and from the brain and is the centre for spinal reflexes.

Peripheral nervous system (PNS) (in brief): cranial nerves (12 pairs, from the brain) and spinal nerves (31 pairs, from the spinal cord; all mixed).

Autonomic nervous system (ANS) (in brief): controls involuntary organs (heart, glands, gut, blood vessels). Two parts with opposite effects:

• Sympathetic: prepares the body for emergencies ('fight or flight'): faster heartbeat and breathing, dilated pupils, more blood to muscles, slower digestion.
• Parasympathetic: restores normal, restful conditions: slower heartbeat, constricted pupils, active digestion.

Worked example

Name the division of the nervous system that makes the heart beat faster when you are frightened.

Try it first, then show the solutionSolution

1. Fear is an emergency situation.
2. The sympathetic system prepares the body for action.

Answer: Sympathetic nervous system.

Voluntary, involuntary and reflex actions; the reflex arc

• Voluntary actions: under our will, controlled by the cerebrum (writing, walking).
• Involuntary actions: not under our will (heartbeat, peristalsis), controlled by the medulla and ANS.
• Reflex action: a quick, automatic, involuntary response to a stimulus, without conscious thought. Spinal reflexes (knee jerk, pulling the hand away from a hot object) are controlled by the spinal cord; cranial reflexes (blinking, sneezing, change in pupil size) by the brain.

Reflex arc (the pathway of a spinal reflex):
receptor (in skin) → sensory neuron (enters the spinal cord by the dorsal root) → relay (association) neuron in the grey matter → motor neuron (leaves by the ventral root) → effector (muscle or gland).

Natural (unconditioned) reflexes are inborn (blinking, sneezing, knee jerk). Acquired (conditioned) reflexes are learnt by practice or repetition (applying brakes when cycling, mouth watering at the smell of a well-liked dish, typing without looking).

Worked example

Trace the reflex arc when your hand touches a hot pan.

Try it first, then show the solutionSolution

1. Heat receptors in the skin are stimulated.
2. Sensory neuron carries the impulse to the spinal cord.
3. Relay neuron in the grey matter passes it to a motor neuron.
4. Motor neuron carries it to arm muscles (effector), which contract and pull the hand away.

Answer: Receptor → sensory neuron → relay neuron → motor neuron → effector.

The eye: structure and functions

Three layers of the eyeball:

• Sclera: tough, white outer coat; in front it becomes the transparent cornea, which lets light in and helps refract it. A thin conjunctiva covers the front.
• Choroid: middle, dark pigmented layer with blood vessels; prevents internal reflection and nourishes the eye. In front it forms the ciliary body (with ciliary muscles) and the iris (coloured diaphragm with the central opening, the pupil; it controls the amount of light entering).
• Retina: inner, light-sensitive layer with rods (dim light; contain visual purple, rhodopsin) and cones (bright light and colour). The yellow spot (fovea) has the most cones and gives the sharpest vision; the blind spot, where the optic nerve leaves, has no rods or cones.

Lens: transparent, biconvex, held by suspensory ligaments attached to the ciliary body; it focuses light on the retina. Aqueous humour (watery, in front of the lens) and vitreous humour (jelly-like, behind the lens) keep the shape of the eyeball.

Nocturnal animals such as owls have many rods, so they see well in dim light.

Worked example

Name the part of the eye that (a) controls the size of the pupil, (b) has no light-sensitive cells, (c) changes the shape of the lens.

Try it first, then show the solutionSolution

1. (a) Iris.
2. (b) Blind spot.
3. (c) Ciliary muscles (of the ciliary body).

Answer: (a) Iris (b) Blind spot (c) Ciliary muscles

Accommodation, adaptation and stereoscopic vision

Accommodation: the ability of the eye to focus objects at different distances by changing the focal length of the lens.

• Near object: ciliary muscles contract, suspensory ligaments slacken, the lens becomes more convex (thicker).
• Distant object: ciliary muscles relax, ligaments become tight, the lens becomes flatter.

Adaptation: adjusting to changes in light intensity.

• Bright light: pupil constricts; cones work.
• Dim light: pupil dilates; rhodopsin builds up in the rods, so vision slowly improves (dark adaptation).

Stereoscopic vision: humans have both eyes facing forward, so their fields of view overlap; the brain combines the two slightly different images into one three-dimensional image, allowing judgement of depth and distance.

Worked example

What happens to the ciliary muscles and the lens when you look up from a book at a distant tree?

Try it first, then show the solutionSolution

1. For a distant object the lens must be flatter.
2. Ciliary muscles relax, the ligaments are pulled tight, and the lens flattens.

Answer: Ciliary muscles relax; lens becomes thinner (flatter).

Defects of the eye

Defect	Cause	Image	Correction
Myopia (short-sightedness)	eyeball too long, or lens too curved	in front of the retina for distant objects	concave lens
Hypermetropia (long-sightedness)	eyeball too short, or lens too flat	behind the retina for near objects	convex lens
Presbyopia	ageing: lens loses elasticity, ciliary muscles weaken	near objects blurred	convex (often bifocal) lens
Astigmatism	uneven curvature of the cornea (or lens)	parts blurred	cylindrical lens
Cataract	lens becomes opaque (cloudy), usually with age	blurred, dim vision	surgical removal of the lens and an artificial lens implant

Diagrams are required only for myopia and hypermetropia, with and without correction.

Worked example

A student cannot read the blackboard clearly but can read his book. Name the defect, where the image forms, and the corrective lens.

Try it first, then show the solutionSolution

1. Distant objects are blurred, near objects clear: myopia.
2. Light from distant objects focuses in front of the retina.
3. A concave lens diverges the rays so they focus on the retina.

Answer: Myopia; in front of the retina; concave lens.

The ear: hearing and balance

External ear: pinna (collects sound waves), external auditory canal, and the tympanum (eardrum), which vibrates.

Middle ear: three ear ossicles: malleus (hammer, touching the eardrum), incus (anvil), stapes (stirrup, fitting into the oval window of the inner ear). The Eustachian tube joins the middle ear to the throat (pharynx) and equalises air pressure on both sides of the eardrum. Infections of the throat can spread to the middle ear through it.

Inner ear (membranous labyrinth, filled with fluids):

• Cochlea: spirally coiled; contains the organ of Corti with sensory hair cells: hearing.
• Vestibule (utriculus and sacculus): static balance, the position of the head with respect to gravity.
• Three semicircular canals with swellings called ampullae: dynamic balance, detecting rotational movement of the head.

Course of hearing: sound → pinna → auditory canal → eardrum vibrates → malleus → incus → stapes → oval window → fluid in the cochlea → hair cells of the organ of Corti → auditory nerve → cerebrum (hearing centre).

Worked example

Which parts of the ear are responsible for (a) balance when the head rotates, (b) balance when the head is still but tilted?

Try it first, then show the solutionSolution

1. (a) Rotational movement is sensed by the semicircular canals (ampullae): dynamic balance.
2. (b) Position relative to gravity is sensed by the utriculus and sacculus: static balance.

Answer: (a) Semicircular canals (b) Utriculus and sacculus

Nervous System and Sense Organs in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• The Human Eye and Its Defects
• The Human Ear: Hearing and Balance
• The Brain and Autonomic Nervous System

Nervous System and Sense Organs: key facts

Reflex arc

$\text{receptor}\to \text{sensory neuron}\to \text{relay neuron}\to \text{motor neuron}\to \text{effector}$

Through the spinal cord.

Path of sound

$\text{pinna}\to \text{canal}\to \text{tympanum}\to \text{malleus}\to \text{incus}\to \text{stapes}\to \text{cochlea}\to \text{auditory nerve}\to \text{cerebrum}$

Hearing pathway.

Accommodation

$\text{near: ciliary contract, lens thicker};\text{far: ciliary relax, lens thinner}$

Changes focal length.

Lens for defects

$\text{myopia: concave};\text{hypermetropia: convex};\text{astigmatism: cylindrical}$

Presbyopia: convex/bifocal; cataract: surgery.

Nervous System and Sense Organs: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the board asks Nervous System and Sense Organs

The question patterns to expect, and the method that works for each.

Tricks to remember Nervous System and Sense Organs

Short memory aids. Say each one aloud once, then use it on a question the same day.

• MIS for the ossicles

  Malleus, Incus, Stapes: 'My Inner Stirrup' from eardrum to oval window.

  Use it when: Order of ossicles.

• Myopia: Minus lens

  MYopia needs a MINUS (concave) lens; HYPERmetropia needs a PLUS (convex) lens.

  Use it when: Correcting defects.

• Rods for Rough (dim) light, Cones for Colour

  Rods work in dim light; Cones see Colour in bright light.

  Use it when: Retina cells.

• Cerebellum = Balance-ellum

  Cerebellum keeps the body balanced and coordinated; cerebrum thinks.

  Use it when: Not mixing the two.

Nervous System and Sense Organs: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In the board paper this is a 5-mark question split into 1 + 2 + 1 + 1 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Nervous System and Sense Organs

The slips examiners see most often, and what to write instead.

• ✗Mistake: Malleus for stapes

  CISCE examiners' comments, 2024: for the ossicle that passes vibrations to the inner ear, candidates chose malleus.

  Wrong working

  Malleus touches the oval window.

  Right working

  Stapes fits into the oval window; malleus touches the eardrum.

  ✓Do this: MIS: Malleus-Incus-Stapes, eardrum to inner ear.

  Usually costs: 1 mark.

• ✗Mistake: Iris for ciliary muscles

  CISCE examiners' comments, 2024: for adjusting the focal length of the lens, candidates wrote iris.

  Wrong working

  Iris changes the focal length.

  Right working

  Ciliary muscles change the lens's curvature; the iris changes the pupil size.

  ✓Do this: Iris = light amount; ciliary = focus.

  Usually costs: 1 mark.

• ✗Mistake: Myopia and hypermetropia confused

  CISCE examiners' comments, 2024: for a student who could not see the blackboard, candidates wrote hyperopia, said the image forms beyond the retina, and suggested a convex lens.

  Wrong working

  Hypermetropia; image behind retina; convex lens.

  Right working

  Myopia; image in front of the retina; concave lens.

  ✓Do this: Far blurred = myopia = minus lens.

  Usually costs: 1-3 marks.

• ✗Mistake: Cerebrum for cerebellum

  CISCE examiners' comments, 2024: in a basketball case, candidates confused cerebrum and cerebellum.

  Wrong working

  Coordination and balance: cerebrum.

  Right working

  Cerebellum.

  ✓Do this: Cerebellum balances.

  Usually costs: 1 mark.

• ✗Mistake: Mixing up static and dynamic balance

  CISCE examiners' comments, 2024 and 2025: candidates paired semicircular canals with static equilibrium and the vestibule with dynamic equilibrium, or wrote only 'ampulla'.

  Wrong working

  Semicircular canals: static balance.

  Right working

  Semicircular canals (with ampullae): dynamic balance; utriculus and sacculus: static balance.

  ✓Do this: Spinning = canals; tilting at rest = vestibule.

  Usually costs: 1-2 marks.

• ✗Mistake: Vague cataract answers

  CISCE examiners' comments, 2025: candidates wrote cornea, retina or iris instead of lens, or 'eye is weak'.

  Wrong working

  The cornea becomes opaque.

  Right working

  The lens becomes opaque.

  ✓Do this: Cataract = cloudy lens.

  Usually costs: 1 mark.

• ✗Mistake: Mixed nerve misnamed

  CISCE examiners' comments, 2024: for a nerve with both sensory and motor fibres, candidates wrote association neuron or spinal nerve.

  Wrong working

  Association neuron.

  Right working

  Mixed nerve.

  ✓Do this: A nerve with both fibre types is mixed.

  Usually costs: 1 mark.

Practice questions on Nervous System and Sense Organs

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   A cat hunting at dusk relies mainly on which retinal cells, and which pigment do they contain?
   (a) cones; rhodopsin
   (b) rods; rhodopsin
   (c) cones; melanin
   (d) rods; haemoglobin

   Hint

   Dim light cells and visual purple.

   Answer

   (b)

2. Q2 · 1 mark · MCQHarder board level

   Which of these is NOT a part of the choroid layer of the eye?
   (a) iris
   (b) ciliary body
   (c) cornea
   (d) the pigmented middle coat

   Hint

   Which coat becomes transparent in front?

   Answer

   (c): the cornea is the transparent front part of the sclera.

3. Q3 · 1 mark · Assertion-reasonBoard level

   Assertion (A): When a person is frightened, the pupils dilate and the heart beats faster.
   Reason (R): The parasympathetic nervous system prepares the body for emergencies.
   (a) Both (A) and (R) are true, and (R) is the correct explanation for (A).
   (b) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
   (c) (A) is true and (R) is false.
   (d) (A) is false and (R) is true.

   Hint

   Which division is 'fight or flight'?

   Answer

   (c): the sympathetic system, not the parasympathetic, prepares the body for emergencies.

4. Q4 · 3 marks · Very short answerBoard level

   Name: (a) the fluid filling the space behind the lens; (b) the brain part relaying sensory impulses to the cerebrum; (c) the nerve with both sensory and motor fibres.

   Hint

   One term each.

   Answer

   (a) Vitreous humour (b) Thalamus (c) Mixed nerve

5. Q5 · 2 marks · Short answerHarder board level

   Explain how the eye adjusts when you move from bright sunlight into a dark room.

   Hint

   Pupil and rods.

   Answer

   The pupil dilates to let in more light, and rhodopsin builds up again in the rods, so vision in dim light improves slowly (dark adaptation).

6. Q6 · 2 marks · Short answerHarder board level

   Give two differences between a voluntary action and a reflex action.

   Hint

   Control and speed.

   Answer

   Voluntary: under conscious control of the cerebrum, slower. Reflex: automatic, controlled by spinal cord (or brain stem), very quick, no conscious thought.

7. Q7 · 3 marks · Short answerHarder board level

   Describe the course of perception of sound in the human ear.

   Hint

   Pinna to cerebrum.

   Answer

   Pinna collects sound → auditory canal → eardrum vibrates → malleus, incus, stapes amplify → oval window → fluid in cochlea → hair cells of organ of Corti stimulated → auditory nerve → hearing centre of cerebrum.

8. Q8 · 3 marks · Case-basedHarder board level

   An elderly woman can see distant hills clearly but cannot thread a needle. Name the defect, its cause and correction.

   Hint

   Age-related.

   Answer

   Presbyopia: with age the lens loses elasticity and ciliary muscles weaken, so the eye cannot accommodate for near objects. Corrected by convex (reading or bifocal) lenses.

9. Q9 · 3 marks · Short answerHOTS / competency

   A person's medulla oblongata is damaged in an accident. Predict two functions likely to be affected and explain why damage here is life-threatening.

   Hint

   Involuntary vital functions.

   Answer

   Breathing and heartbeat (also swallowing, coughing). The medulla controls these vital involuntary activities, so damage can stop breathing or circulation.

10. Q10 · 2 marks · Short answerHOTS / competency

    Explain why food 'tastes' bland when you have a blocked nose and why your ears 'pop' in an aeroplane. Which structure equalises pressure in the second case?

    Hint

    Eustachian tube for the ears.

    Answer

    Much of flavour depends on smell, which is blocked. Ears pop when air pressure outside changes and the Eustachian tube opens to equalise pressure on both sides of the eardrum.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Part of a neuron carrying impulses towards the cell body?Show answer

   Dendrite.

2. 2.Junction between two neurons?Show answer

   Synapse.

3. 3.Brain part controlling heartbeat and breathing?Show answer

   Medulla oblongata.

4. 4.Brain part controlling body temperature?Show answer

   Hypothalamus.

5. 5.Cells of the retina for colour vision?Show answer

   Cones.

6. 6.Lens to correct myopia?Show answer

   Concave lens.

7. 7.Defect corrected by a cylindrical lens?Show answer

   Astigmatism.

8. 8.Tube connecting middle ear and throat?Show answer

   Eustachian tube.

9. 9.Part of the cochlea with sensory hair cells?Show answer

   Organ of Corti.

10. 10.Three meninges?Show answer

    Dura mater, arachnoid, pia mater.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you draw a neuron and the reflex arc?
• ☐Can you name each brain part from a function?
• ☐Can you give three effects each of sympathetic and parasympathetic action?
• ☐Can you draw and label the eye with 12 labels?
• ☐Can you explain accommodation for near and far objects?
• ☐Can you give cause and correction of five eye defects?
• ☐Can you trace sound from pinna to brain and name the balance organs?

Nervous System and Sense Organs: frequently asked questions