Chapter in a nutshell: The nervous system coordinates the body using neurons that carry electrical impulses. It has the CNS (brain + spinal cord), PNS (nerves) and ANS (involuntary control). A quick automatic response is a reflex action via a reflex arc. The sense organs β chiefly the eye and ear β detect stimuli; common eye defects (myopia, hypermetropia, presbyopia, astigmatism, cataract) have specific corrections.
1. Need for the Nervous System
It (1) keeps us informed of the outside world via sense organs, (2) enables memory, thought and reasoning, (3) controls voluntary muscular activity, and (4) regulates involuntary activities (heartbeat, breathing). Body coordination is by two systems β nervous (fast, electrical) and hormonal/endocrine (slower, chemical).2. The Neuron (nerve cell) β structural & functional unit
| Part | Description |
|---|---|
| Cyton (cell body) | contains the nucleus + cytoplasm; no centrosome (neurons cannot divide) |
| Dendrites | branched projections that bring impulses to the cyton |
| Axon | long fibre that carries impulses away; may be > 1 m long |
| Myelin sheath | white insulating cover on the axon; gaps = Nodes of Ranvier |
| Axon terminals | swollen ends storing neurotransmitters |
3. The Nerve Impulse
- In the resting (polarised) state the axon's outer surface is positive (more NaβΊ outside).
- On stimulation the membrane becomes permeable, NaβΊ rushes in β depolarisation (excited region). This excites the next region, while the previous region is repolarised by the sodium pump (active transport using ATP).
- An impulse is thus a travelling wave of depolarisation followed by repolarisation (no matter actually moves along the fibre). Max speed β 100 m/s.
4. Types of Neurons & Nerves
- Neurons: Sensory (receptor β CNS), Motor (CNS β effector), Association/relay (connect the two, in CNS).
- Nerves (bundles of axons): Sensory (e.g. optic nerve), Motor (e.g. to eye muscles), Mixed (e.g. spinal nerves β both fibres).
5. Divisions of the Nervous System
- Central NS (CNS): brain + spinal cord β the control centre.
- Peripheral NS (PNS): cranial nerves (12 pairs) + spinal nerves (31 pairs) connecting the CNS to the body.
- Autonomic NS (ANS): controls involuntary organs; sympathetic ("fight or flight" β speeds heart, etc.) and parasympathetic ("rest" β calms, opposite effects).
6. The Brain
Protected by the skull and three membranes (meninges), and cushioned by cerebrospinal fluid (CSF). Outer grey matter, inner white matter (reverse of the spinal cord).| Part | Function |
|---|---|
| Cerebrum | thinking, memory, intelligence, will, voluntary actions, sensations |
| Cerebellum | balance, posture and coordination of muscular movement |
| Medulla oblongata | controls involuntary actions β heartbeat, breathing, blood pressure |
| Thalamus | relay centre for sensory impulses |
| Hypothalamus | controls body temperature, hunger, thirst, emotions; links to endocrine system |
7. The Spinal Cord
A tube of nervous tissue running down the backbone from the medulla, protected by the vertebral column. Outer white matter, inner grey matter (butterfly-shaped). It conducts impulses to and from the brain and is the centre for reflex actions below the neck.8. Reflex Action & Reflex Arc
A reflex action is a rapid, automatic, involuntary response to a stimulus (e.g. withdrawing a hand from a pin, knee jerk, blinking).- Reflex arc (pathway): Receptor β Sensory neuron β Spinal cord (relay neuron) β Motor neuron β Effector (muscle/gland). The brain is informed afterwards.
| Natural (inborn) reflex | Acquired (conditioned) reflex |
|---|---|
| present from birth | learnt through experience |
| e.g. blinking, sneezing, knee-jerk | e.g. cycling, salivating at the sight of food |
9. The Eye (sense of sight)
| Part | Function |
|---|---|
| Cornea | transparent front; refracts (bends) light in |
| Sclera | tough white outer protective coat |
| Choroid | middle pigmented layer; prevents internal reflection; nourishes |
| Iris | coloured ring; controls the size of the pupil |
| Pupil | central opening; regulates light entering |
| Lens | focuses light onto the retina (changes shape = accommodation) |
| Retina | light-sensitive layer with rods (dim light) and cones (colour) |
| Yellow spot (fovea) | point of sharpest vision (most cones) |
| Blind spot | where the optic nerve leaves β no rods/cones, no image |
| Aqueous & vitreous humour | fluids that maintain eye shape & refract light |
- Accommodation: the ciliary muscles change the lens thickness to focus near/far objects.
10. Eye Defects & Corrections
| Defect | Problem | Correction |
|---|---|---|
| Myopia (short-sight) | image forms in front of retina; can't see distant clearly | concave lens |
| Hypermetropia (long-sight) | image forms behind retina; can't see near clearly | convex lens |
| Presbyopia | lens loses elasticity with age | bifocal lenses |
| Astigmatism | uneven cornea curvature β blurred | cylindrical lens |
| Cataract | lens becomes opaque/cloudy | surgical removal + lens implant |
11. The Ear (hearing & balance)
- Outer ear: pinna + auditory canal β collects sound to the eardrum (tympanum).
- Middle ear: three ear ossicles (malleus, incus, stapes) amplify and pass vibrations; the Eustachian tube equalises air pressure.
- Inner ear: cochlea (converts vibrations to nerve impulses β auditory nerve β brain) and semicircular canals + vestibule (control balance/equilibrium).
- Hearing pathway: sound β pinna β eardrum β ossicles β cochlea β auditory nerve β brain.
12. Worked / Structured Examples (ICSE pattern)
Q1. Name the functional unit of the nervous system and its three parts. Solution: Neuron β cyton, dendrites, axon. Q2. Give the reflex-arc pathway for withdrawing a hand from a hot object. Solution: Receptor (skin) β sensory neuron β spinal cord β motor neuron β effector (arm muscle). Q3. Which part of the brain controls heartbeat and breathing? Solution: Medulla oblongata. Q4. A person cannot see distant objects clearly. Name the defect and its correction. Solution: Myopia; corrected with a concave lens. Q5. Name the part of the eye with the sharpest vision and the part with no vision. Solution: Yellow spot (fovea); blind spot.13. Key Terms β Quick Glossary
| Term | One-line definition |
|---|---|
| Neuron | nerve cell; unit of the nervous system. |
| Synapse | junction between two neurons (chemical transfer). |
| Reflex action | rapid involuntary response to a stimulus. |
| Reflex arc | receptorβsensoryβspinal cordβmotorβeffector. |
| CNS / PNS / ANS | brain+cord / nerves / involuntary control. |
| Cerebrum / cerebellum / medulla | thinking / balance / vital involuntary acts. |
| Accommodation | lens changing shape to focus. |
| Myopia / hypermetropia | short-sight / long-sight. |
| Cochlea | hearing organ of the inner ear. |
| Acetylcholine | neurotransmitter that crosses the synapse. |
14. Common Mistakes to Avoid
- Reversing grey/white matter: brain = grey outside; spinal cord = white outside.
- Saying impulse conduction is like electricity in a wire β no particles/electrons actually move along the fibre.
- Confusing myopia (concave lens) with hypermetropia (convex lens).
- Mixing up dendrites (carry to cyton) and axon (carry away).
- Forgetting the relay/association neuron in the reflex arc.
- Thinking the brain triggers a reflex β the spinal cord does; the brain is informed after.
15. Likely Exam Questions (with crisp answers)
- Name the two systems that coordinate the body. β Nervous and endocrine (hormonal) systems.
- Draw/label the three parts of a neuron. β Cyton, dendrites, axon.
- What is a synapse? β The junction between two neurons where the impulse passes chemically.
- Name the neurotransmitter at a synapse. β Acetylcholine.
- List the three types of neurons. β Sensory, motor, association (relay).
- Name the parts of the CNS. β Brain and spinal cord.
- State the function of the cerebellum. β Maintains balance, posture and coordinates muscular movement.
- Define reflex action with an example. β Rapid automatic involuntary response, e.g. knee-jerk.
- Give the reflex-arc sequence. β Receptor β sensory neuron β spinal cord β motor neuron β effector.
- Differentiate natural and acquired reflexes. β Inborn (blinking) vs learnt (cycling).
- Name the light-sensitive layer of the eye and its two receptor cells. β Retina; rods and cones.
- What is accommodation of the eye? β Adjusting the lens's focal length to focus near or distant objects.
- Name the defect corrected by a convex lens. β Hypermetropia (long-sight).
- Name the three ear ossicles. β Malleus, incus, stapes.
- Which part of the ear maintains balance? β The semicircular canals (with the vestibule).
- What is the function of the Eustachian tube? β Equalises air pressure on both sides of the eardrum.
16. Autonomic Nervous System β sympathetic vs parasympathetic
| Organ | Sympathetic ("fight/flight") | Parasympathetic ("rest") |
|---|---|---|
| Heart rate | increases | decreases |
| Pupil | dilates | constricts |
| Digestion | slows | speeds up |
| Breathing | faster | normal |
17. Protection of the CNS
- The brain is protected by the cranium (skull), three meninges and cerebrospinal fluid (CSF) which cushions shocks and supplies nutrients.
- The spinal cord is protected by the vertebral column and meninges.
18. Image Formation & Stereoscopic Vision
Light is refracted by the cornea and lens to form a real, inverted, diminished image on the retina; rods/cones convert it to impulses sent by the optic nerve to the cerebrum, which interprets it as upright. Having two front-facing eyes gives overlapping fields β binocular (stereoscopic) vision β judging distance/depth.19. Adjustment of the Pupil
In bright light the iris contracts the pupil (less light in, protects retina); in dim light the pupil dilates (more light in). This is itself a reflex.20. More Worked / Structured Examples
Q6. Why can't we see clearly for a moment on entering a dark room from sunlight? Solution: The pupil needs time to dilate and the rods to adjust to low light (dark adaptation). Q7. Name the structure that converts sound vibrations into nerve impulses. Solution: The cochlea (inner ear). Q8. Which lens corrects presbyopia and why? Solution: Bifocal lenses β the ageing lens cannot focus both near and far. Q9. State the function of the iris. Solution: Controls the size of the pupil, regulating light entering the eye.21. More Exam Questions (with crisp answers)
- Name the fluid that cushions the brain. β Cerebrospinal fluid (CSF).
- What is binocular vision? β Use of both eyes to give a single 3-D image with depth perception.
- Name the part of the eye that adjusts the pupil. β The iris.
- Differentiate sympathetic and parasympathetic actions on the heart. β Sympathetic speeds it up; parasympathetic slows it down.
- Name the receptor cells for dim light and for colour. β Rods (dim light); cones (colour).
- Why is the image on the retina inverted but we see upright? β The brain (cerebrum) interprets and corrects the inverted image.
- Which canal equalises pressure across the eardrum? β The Eustachian tube.
- Name the part of the brain that is the seat of intelligence and memory. β The cerebrum.