ICSE Class 10 Nervous System — Selina Chapter 9

The ICSE Class 10 Nervous System chapter in Selina Biology covers the neuron, brain, spinal cord, reflexes and the sense organs. It explains how the body senses the outside world, remembers, reasons, moves voluntarily, and regulates breathing and heartbeat. This page walks through every section in book order, with key formulas and worked examples.

Download The Nervous System Class 10 ICSE Biology Chapter 9 PDF

Download the The Nervous System Class 10 ICSE Biology chapter PDF from the Selina Concise textbook to revise offline or print for practice. It is the full chapter as printed in the book — 27 pages, 18 sections, 75 questions.

What is inside Chapter 9

27 pages · 18 sections · 75 exercise questions

• PROGRESS CHECK: 2 questions
• PROGRESS CHECK: 3 questions
• REVIEW QUESTIONS: 22 questions
• PROGRESS CHECK: 3 questions
• PROGRESS CHECK: 6 questions
• REVIEW QUESTIONS: 39 questions

9.1 NEED OF NERVOUS SYSTEM

The nervous system performs four major functions. It keeps you informed about the outside world through the sense organs. It enables you to remember, think and reason. It controls and harmonises voluntary muscular activities, like running or holding a book. It also regulates involuntary activities such as breathing and heartbeat.

Each function plays a clear role in the Class 10 ICSE syllabus. When you read, your eyes send signals to your brain. When you run, your nervous system coordinates the muscles involved. When you breathe, you do not think about it, yet the nervous system keeps it going without conscious effort.

For the complete set of chapters in this subject, see the Class 10 Biology book list.

9.2 NEURON (or NERVE CELL) : THE UNIT OF THE NERVOUS SYSTEM

The nervous system consists of the brain, spinal cord, sense receptors and a network of nerves. The brain and spinal cord are built from neurons, also called nerve cells. A neuron is the structural and functional unit of the nervous system. Understanding its structure is the first step to understanding how the whole system works.

9.2.1 Structure of the neuron (Fig. 9.1 A)

A neuron has three main parts. The cell body, called the perikaryon or cyton, holds a well-defined nucleus surrounded by granular cytoplasm. It contains all the usual cell organelles, except the centrosome. Nerve cells have lost the ability to divide, so the centrosome is absent.

Dendrites are branched cytoplasmic projections of the cell body. They conduct nerve impulses toward the cyton. The axon is a long process that can vary from a few millimetres to over one metre in length.

In most neurons, the axon is surrounded by a white insulating myelin (medullary) sheath, covered by an outer thin sheath called the neurolemma. The myelin sheath has gaps called Nodes of Ranvier. Some axons have side branches called collaterals, and the swollen ends store chemicals called neurotransmitters.

Some basic terms appear in this section. A stimulus is a sudden change in the external or internal environment that causes a change in the organism. A response is the change that results from the stimulus. An impulse is a wave of electrical disturbance that sweeps over a nerve cell. Receptors are specialised epithelial cells that set up waves of impulses toward the central nervous system. Effectors are muscles or glands that contract or secrete when they receive an impulse.

Transmission of the nerve impulse follows a clear sequence. At rest, the outer side of the nerve fibre carries a positive charge, called the polarised state. This polarisation exists because more \mathrm{Na^{+}} ions sit outside the axon membrane than inside.

On stimulation, the membrane at that spot becomes more permeable to \mathrm{Na^{+}} ions, which move inward and cause depolarisation. This is the excited region. The sodium pump then actively transports \mathrm{Na^{+}} ions back to the outside using energy from ATP, repolarising the fibre.

Here is a speed contrast worth remembering. A nerve impulse travels at a maximum speed of about 100 metres per second. Electricity in a wire travels at about 150,000 kilometres per second. A nerve fibre is not an electric wire, because no substance or electron actually moves along the fibre.

9.2.2 Synapse

A synapse is the point of contact between the terminal branches of one axon and the dendrites of another neuron, separated by a fine gap. The nerve impulse jumps across this gap by a chemical process. When the impulse reaches the terminal end of an axon, a chemical called acetylcholine is released.

Acetylcholine sets a new impulse in the dendrites of the adjacent neuron. An enzyme soon breaks down the chemical, making the synapse ready for the next transmission.

9.2.3 Types of neurons (Fig. 9.1 B)

Three types of neurons exist, classified by the direction of impulse flow. Sensory neurons carry impulses from receptors (sense organs) to the central nervous system. Motor neurons carry impulses from the CNS to effectors, which are muscles or glands. Association (connecting) neurons sit in the brain and spinal cord, linking sensory and motor neurons.

9.3 NERVES

A nerve is a bundle of nerve fibres (axons) from separate neurons, enclosed in a tubular sheath. Nerves appear as thread-like white structures that emerge from the brain and spinal cord. They branch to nearly every part of the body. A nerve works like an underground electric cable with many insulated conducting wires. The myelin sheath acts as insulation, preventing the mixing of impulses in adjacent axons.

Three kinds of nerves exist. Sensory nerves contain only sensory fibres, bringing impulses from receptors to the brain or spinal cord. The optic nerve is an example. Motor nerves contain only motor fibres, carrying impulses from the CNS to effector organs. The nerve supplying the eyeball muscles is an example. Mixed nerves contain both sensory and motor fibres, such as a spinal nerve.

Ganglia (singular: ganglion) are aggregates of nerve cell bodies. Nerve fibres may arise from or enter into them.

9.4 TWO MAJOR DIVISIONS OF THE NERVOUS SYSTEM

The nervous system divides into two major parts. The central nervous system (CNS) includes the brain lying in the skull and the spinal cord contained within the vertebral column. The peripheral nervous system (PNS) includes the nerves that emerge from and enter the brain and spinal cord. The PNS splits into the somatic nervous system, which conveys information to voluntary muscles, and the autonomic nervous system, which controls involuntary actions of internal organs.

One common mistake appears here. In the brain, gray matter lies on the outside and white matter on the inside. In the spinal cord, the arrangement is reversed. Gray matter lies inside and white matter outside. Keep this difference clear when answering questions about the nervous system.

9.4.1 The Brain

The human brain is the largest among all animals in proportion to body size. An adult brain weighs about 1.35 kg, forming about 2% of total body weight. It is 80% water and consumes more than 25% of the oxygen taken into the body. The brain is delicate and sits protected inside the cranium of the skull.

Three membranes called the meninges cover the brain. The dura mater is the outermost tough fibrous membrane. The arachnoid is the thin delicate middle layer, web-like in structure. The pia mater is the innermost highly vascular membrane, richly supplied with blood. Cerebrospinal fluid fills the space between the membranes and acts as a cushion against shocks.

The cerebrum is the largest portion of the brain, divided into right and left cerebral hemispheres. The outer cortex contains neuron cell bodies and is called gray matter. Its folds are gyri and its grooves are sulci. The inner portion consists of white matter, mainly axons. The corpus callosum is a sheet of fibres connecting the two hemispheres. The cerebrum is the seat of intelligence, consciousness and will-power, controlling all voluntary actions.

The cerebellum is smaller, located at the base under the cerebrum. It maintains body balance and coordinates muscular activity. The medulla oblongata is the lowest portion, controlling the activities of internal organs such as breathing, heartbeat and peristalsis. Injury to the medulla generally results in death.

Three primary regions make up the brain. The forebrain contains the cerebrum and the diencephalon, which includes the thalamus and hypothalamus. The midbrain is a small tubular part handling reflexes involving the eyes and ears. The hindbrain contains the cerebellum, pons and medulla oblongata.

9.4.2 The Spinal Cord

The spinal cord extends from the medulla of the brain down almost the whole length of the backbone. It ends at the second lumbar vertebra and lies within the neural canal of the vertebrae. The arrangement of gray and white matter here is reversed from the brain. Gray matter, containing cell bodies of motor and association neurons, lies on the inner side. White matter lies on the outer side, with axons running longitudinally to and from the brain.

A small central canal runs through the centre, filled with cerebrospinal fluid. This fluid acts as a shock-proof cushion and forms a medium for exchanging food materials, waste products and respiratory gases with neurons. The same three meninges cover the spinal cord in continuation with those of the brain.

The spinal cord has three functions. It handles reflexes below the neck. It conducts sensory impulses from the skin and muscles to the brain. It conducts motor responses from the brain to the trunk and limbs.

9.5 PERIPHERAL NERVOUS SYSTEM

The peripheral nervous system includes the nerves that carry impulses to and from the central nervous system. It divides into the somatic nervous system and the autonomic nervous system.

9.5.1 Somatic Nervous System

The somatic nervous system consists of two sets of nerves. Cranial nerves emerge from the brain, and there are twelve pairs. Some are sensory, like the olfactory (nose), optic (eye) and auditory (ear). Some are motor, like the nerves to the eye muscles. Some are mixed, like those to the face and tongue.

Spinal nerves emerge from the spinal cord, with thirty-one pairs in total. These break down as 8 pairs in the neck region, 12 in the thorax, 5 in the lumbar region, 5 in the sacral region and 1 in the coccygeal region. Each spinal nerve has a dorsal root with a dorsal ganglion, carrying sensory fibres, and a ventral root carrying motor fibres. Every spinal nerve is a mixed nerve.

9.5.2 Autonomic Nervous System

The autonomic nervous system (ANS) consists of a pair of chains of nerves and ganglia on either side of the backbone. It controls the involuntary actions of internal organs. Two parts exist, opposite in action: the sympathetic and parasympathetic systems. The sympathetic system arises from the spinal cord between the neck and waist. The parasympathetic system sits in the head, neck and sacral regions.

The sympathetic system prepares the body for violent action against abnormal conditions. The parasympathetic system re-establishes normal conditions after the violent act. The hormone adrenaline, secreted by the adrenal glands on the kidneys, stimulates the sympathetic system. Emotions such as grief, anger and fear strongly influence the ANS. Long-continued emotional stress may cause high blood pressure and stomach ulcers.

9.6 REFLEXES (Involuntary actions)

A reflex action is an automatic, quick and involuntary response brought about by a stimulus. It differs from a voluntary action, which starts with a willing thought. A reflex is self-protective, while a voluntary action fulfils a desired goal. Reflexes involve muscles and glands; voluntary actions involve only muscles.

Reflexes divide into two types. Natural (inborn) reflexes need no learning, like blinking, coughing and sneezing. Conditioned (acquired) reflexes develop through experience or learning, like salivating at the sight of a favourite food. Pavlov’s experiment demonstrated this: a dog trained to associate a bell with food salivated at the bell alone, even without food.

The reflex arc follows a fixed path: stimulus → receptor → sensory (afferent) fibre → CNS → motor (efferent) fibre → effector (muscle or gland).

Here is a worked example with original values. You touch a hot plate. The heat is the stimulus. Receptors in your fingertip detect it. A sensory neuron carries the impulse to your spinal cord. The spinal cord processes the signal and generates a motor impulse. A motor neuron carries the response. Your hand muscles contract, pulling your hand away. The whole sequence takes a fraction of a second, without conscious thought.

Remember this rule: all reflexes are involuntary, but not all involuntary actions are reflexes. Some voluntary actions, repeated with specific stimuli, become conditioned reflexes.

You can browse all Class 10 ICSE books for other subjects on this site.

9.7 THE SENSE ORGANS

The sense organs are the eyes, ears, tongue, nose and skin. They respond to light, sound, taste, smell and touch. Other senses include balance, body movements, hunger, thirst and pain. Receptors are specialised tissues or cells sensitive to specific stimuli. Mechanoreceptors handle touch and pressure. Chemoreceptors handle taste and smell. Photoreceptors are the rods and cones of the retina. Thermoreceptors detect heat and cold. Phonoreceptors detect sound.

9.7.1 The Eyes

Protective structures surround each eye. The orbits are deep sockets on the front of the head. Eyelids protect the front surface and shut out light. Eyelashes prevent larger particles from falling in. Eyebrows stop rain and perspiration from entering the eyes. Tear glands sit at the upper side of each orbit, producing tears that lubricate the eye, wash away dust, kill germs through the enzyme lysozyme, and communicate emotions.

The eyeball wall has three concentric layers. The outer sclerotic layer is tough and white, becoming the transparent cornea at the front. The middle choroid layer is rich in blood vessels and contains melanin, which prevents light from scattering. It expands forward to form the ciliary body and the iris. The inner retina is light-sensitive, containing rods and cones. Rods contain rhodopsin and work in dim light. Cones contain iodopsin and handle bright light and colour.

The yellow spot (macula lutea) lies at the back of the eye and contains the maximum number of cones, giving the best vision. The blind spot has no sensory cells and is where the optic nerve leaves the eyeball. The lens is a transparent biconvex body held by the suspensory ligament. The aqueous humour fills the front chamber, keeping the lens moist and refracting light. The vitreous humour fills the back chamber, maintaining the eyeball shape.

Accommodation is the focusing of the eye for objects at different distances. For distant vision, the lens becomes thinner. For near vision, the lens becomes more convex. The ciliary muscles control these changes. Dark adaptation happens when moving from bright to dark light: rhodopsin regenerates and the pupil dilates. Light adaptation is the reverse: rhodopsin bleaches and the pupil constricts.

9.7.2 Common Defects of the Eye

Eight common eye defects appear in this chapter. Each has a specific cause and correction.

9.7.3 The Ear — Senses of Hearing and Balance

The ear handles two functions: hearing and body balance. It has three divisions. The outer ear consists of the pinna, the auditory canal and the ear drum. The middle ear contains three tiny bones called the malleus (hammer), incus (anvil) and stapes (stirrup), collectively called the ear ossicles. It also has the oval window, round window and the eustachian tube, which connects the middle ear to the throat and equalises air pressure.

The inner ear has the cochlea, semicircular canals, utriculus and sacculus.

The hearing path is clear. The pinna collects sound waves. The ear drum vibrates. The ossicles magnify the vibration through a lever-like action. The stirrup moves the oval window. Fluid in the cochlea vibrates. Sensory cells in the organ of Corti send impulses through the auditory nerve to the brain. Humans hear frequencies from 20 to 20,000 Hertz, with the keenest hearing between 1000 and 4000 Hz.

Balance comes from two sources. The semicircular canals provide dynamic balance when the body is in motion. The utriculus and sacculus provide static balance when the body is stationary, such as when standing.

9.7.4 The Sense of Taste (Gustation) (Extra – not specified in syllabus)

Taste buds lie mainly on the upper surface of the tongue. Each taste bud is an ovoid group of sensory and supporting cells. Sensory hairs project into taste pores on the surface. The four elementary tastes are sweet, salt, bitter and sour, perceived in different areas of the tongue. This section is marked extra and not specified in the syllabus, so it needs only a light read.

9.7.5 The Sense of Smell (Extra – not specified in syllabus)

The sense of smell lies in the delicate epithelial layers of the nasal chamber. Sensory cells have hair-like projections that respond to particles dissolved in the mucous secretion. The olfactory nerve transmits impulses to the brain. Flavour is a combination of taste and smell. This section is marked extra and not specified in the syllabus, so read it lightly.

What to practise from ICSE Class 10 Nervous System

The chapter has six practice blocks. Work through them in order to test your understanding of each section.

• PROGRESS CHECK: 2 questions on neuron terms and stimulus versus response
• PROGRESS CHECK: 3 questions on CNS parts and reflex types
• REVIEW QUESTIONS: 22 questions covering multiple choice, very short, short and long answer types
• PROGRESS CHECK: 3 questions on eye functions and the path of light
• PROGRESS CHECK: 6 questions on ear parts, tastes and true or false statements
• REVIEW QUESTIONS: 39 questions covering eye defects, ear, taste and smell

The long answer and diagram questions concentrate on the neuron, reflex arc, eye defects and the ear. For syllabus updates and specimen papers, check the official CISCE website.

ICSE Class 10 Nervous System frequently asked questions

What is a reflex arc in the ICSE Class 10 nervous system chapter?

A reflex arc is the shortest route an impulse takes from a receptor to an effector. The path is stimulus → receptor → sensory (afferent) nerve fibre → CNS → motor (efferent) nerve fibre → muscle or gland. It may involve two neurons or three, with an association neuron in between.

Which part of the brain controls body balance, and which controls heartbeat and breathing?

The cerebellum maintains body balance and coordinates muscular activity. The medulla oblongata controls the activities of internal organs, including breathing and the beating of the heart.

Why do we see objects upright when the image on the retina is inverted?

The image formed on the retina is real and inverted. The brain interprets the image and perceives the object as upright. This interpretation happens in the visual area of the cerebrum.

Which eye defect is corrected by a concave lens and which by a convex lens?

Myopia, or near-sightedness, is corrected by a concave lens because the image forms in front of the retina. Hyperopia, or long-sightedness, is corrected by a convex lens because the image forms behind the retina.

How do the sympathetic and parasympathetic systems differ in their effect on the body?

The sympathetic system prepares the body for violent action, accelerating the heart and dilating the pupil. The parasympathetic system restores normal conditions, retarding the heart and constricting the pupil. Their effects are opposite in action.

Reference: ICSE Class 10 Biology textbook material, chapter 9: The Nervous System — CISCE-aligned.

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• Chapter 3: Genetics Some Basic Fundamentals
• Chapter 4: Absorption By Roots the Processes Involved
• Chapter 5: Transpiration
• Chapter 6: Photosynthesis
• Chapter 7: The Circulatory System
• Chapter 8: The Excretory System
• Chapter 10: Endocrine Glands
• Chapter 11: The Reproductive System
• Chapter 12: Population
• Chapter 13: Aids to Health
• Chapter 14: Health Organisations
• Chapter 15: Pollution