The Nervous System and Sense Organs

ICSE Class 10 Biology || Chapter 09 || Detailed Master Notes

Every organism must become aware of what is going on around it and perform actions for its survival. Apart from actions which provide adjustments to the external environment, there are many internal activities (like breathing, digestion, heartbeat) of which we are unaware. All such actions have to be properly timed and coordinated. This coordination occurs via two agencies: the Nervous System and the Hormonal (Endocrine) System.

9.1 Need for a Nervous System

The nervous system performs several vital functions:

Keeps us informed about the outside world through sense organs.
Enables us to remember, think, and reason out.
Controls and harmonises all voluntary muscular activities (e.g., running, writing).
Regulates involuntary activities such as breathing, beating of the heart, and movements of the gut.

9.2 The Unit of the Nervous System: Neuron

Neuron (Nerve Cell): The structural and functional unit of the nervous system. The brain consists of an estimated 100 billion neurons!

Structure of a Neuron

A typical neuron possesses three main parts:

1. Cyton (Cell Body / Perikaryon): The main body of the nerve cell containing a prominent nucleus and cytoplasm. The cytoplasm contains characteristic darkly staining Nissl's granules (which help in protein synthesis).
2. Dendrites: Several short, branched extensions arising from the cyton. They receive nerve impulses from other neurons and conduct them towards the cyton.
3. Axon: A single, long process extending from the cyton. It conducts impulses away from the cyton to another neuron or an effector organ.
- Myelin Sheath: An insulating layer covering many axons, formed by specialized Schwann cells. It speeds up the conduction of nerve impulses.
- Nodes of Ranvier: The gaps throughout the length of the myelin sheath where the axon is exposed.
- Axon Endings (Terminals): The axon ends in fine branches bearing swollen knob-like structures called Synaptic Knobs, which contain neurotransmitters.

Fig 9.1: Structure of a generalized multipolar neuron

Some Basic Terms in Nervous Activity

Stimulus: An agent or a sudden change of the external or internal environment that results in a change in an organism or any of its body parts.
Response: The change in an organism resulting due to stimulus.
Impulse: A wave of irritability (electrical disturbance that sweeps over the nerve cell).
Receptors: The specialised epithelial cells which, on receiving the stimulus, set up waves of impulses towards the central nervous system.
Effectors: Muscles or glands which, on receiving the impulse from the brain or spinal cord, contract or secrete substances.

Transmission of the Nerve Impulse

In the normal (resting) condition, the outer side of the nerve fibre carries positive (+) charge. This is called the polarised state. This polarisation is due to more Na? ions outside the axon membrane.

On stimulation (mechanical, electrical, chemical, or heat), the axon membrane at that spot becomes more permeable to Na? ions which move inwards and cause loss of polarisation (depolarisation). This is known as the excited region. The point of depolarisation becomes a stimulus for the next neighbouring area of the membrane which in turn becomes depolarised.

Meanwhile, the previous area becomes repolarised due to active transport of Na? ions again to the outside. This transport is achieved by what is called the "sodium pump" using energy through ATP.

Conduction of nerve impulse is a wave of depolarisation followed by repolarisation. Note: Electricity is conducted through a wire at a speed of about 150,000 km per second, but the nerve impulse travels at a maximum speed of about 100 metres per second only.

Fig 9.2: Conduction of nerve impulse through a nerve fibre

Synapse

Synapse: The point of contact between the terminal branches of the axon of one neuron and the dendrites of another neuron.

Nerve impulses travel across a synapse chemically through a neurotransmitter (e.g., Acetylcholine), ensuring the impulse travels in only one direction.

Types of Neurons

Sensory Neurons: Carry impulses from sense organs (receptors) to the Central Nervous System (Brain/Spinal Cord).
Motor Neurons: Carry impulses from the CNS to the effector organs (muscles or glands) causing them to react.
Association (Interneurons): Located entirely within the CNS. They connect sensory and motor neurons.

9.3 Nerves and Ganglia

A Nerve is a bundle of nerve fibres (axons) enclosed in a tubular medullary sheath, acting like a communication cable. Nerves are of three types:

Sensory Nerve: Contains only sensory fibres (e.g., Optic nerve from the eye to the brain).
Motor Nerve: Contains only motor fibres (e.g., nerve to the muscles of the eyeball).
Mixed Nerve: Contains both sensory and motor fibres (e.g., Spinal nerves).

Ganglia: The aggregates of the cell bodies (cytons) from which the nerve fibres may arise or enter into.

9.4 Divisions of the Nervous System

The nervous system has three main divisions:

Central Nervous System (CNS): Consists of the Brain and Spinal Cord.
Peripheral Nervous System (PNS): Consists of nerves emerging from the CNS (Cranial and Spinal nerves).
Autonomic Nervous System (ANS): Consists of a pair of chain of ganglia running along the spinal cord (Sympathetic and Parasympathetic systems).

9.5 The Central Nervous System: Brain

The brain is the most complex organ, weighing about 1.35 kg (about 2% of body weight) but consuming 25% of the total body oxygen. It is protected by the skull (cranium) and three membranous coverings called Meninges:

Dura mater: Outermost, tough fibrous membrane.
Arachnoid: Middle, delicate, web-like membrane.
Pia mater: Innermost, highly vascular membrane closely investing the brain.

Cerebrospinal Fluid (CSF): The fluid filling the spaces between the meninges and the ventricles of the brain. It acts as a shock absorber and provides nourishment.

Major Parts of the Brain

Cerebrum: The largest part of the brain, divided into right and left Cerebral Hemispheres.
- The two hemispheres are connected by a sheet of fibres called the Corpus Callosum.
- The outer portion (Cerebral Cortex) is Gray matter (cytons) and is highly folded into convolutions (gyri) and grooves (sulci) to increase surface area to accommodate more neurons.
- The inner portion is White matter (myelinated axons).
- Function: Seat of intelligence, consciousness, memory, willpower, and controls all voluntary activities.
Cerebellum ("Little Brain"): Located at the base under the cerebrum. It also has outer gray and inner white matter.
- Function: Coordinates voluntary muscular movements and maintains body balance/posture (e.g., if a drunk person staggers, the cerebellum is affected).
Medulla Oblongata: The lowest part, continuing into the spinal cord.
- Function: Controls vital involuntary activities like heartbeat, breathing, swallowing, coughing, and peristalsis. Injury here is usually fatal.
Diencephalon: Contains the Thalamus (relay centre for pain and pressure) and Hypothalamus (controls body temperature, water balance, hunger, thirst, and regulates the pituitary gland).

Fig 9.4/9.5: Median section of the human brain

9.6 The Central Nervous System: Spinal Cord

The spinal cord extends from the medulla oblongata down to the second lumbar vertebra. It is protected by the vertebral column and the meninges.

Arrangement of Matter (Opposite to the Brain):

Spinal Cord: White matter is on the OUTSIDE, Gray matter is on the INSIDE (forming a butterfly or H-shape).
Brain: Gray matter is on the outside (cortex), White matter is inside.

Functions of Spinal Cord: It is the center for reflexes below the neck, and it conducts sensory impulses to the brain and motor impulses from the brain to effectors.

9.7 The Peripheral Nervous System (PNS)

The PNS connects the CNS to the rest of the body.

Cranial Nerves: Emerge from the brain. Humans have 12 pairs (e.g., Optic nerve, Auditory nerve, Vagus nerve).
Spinal Nerves: Emerge from the spinal cord. Humans have 31 pairs (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal). All spinal nerves are mixed nerves.

9.8 The Autonomic Nervous System (ANS)

The ANS controls the involuntary activities of visceral organs (heart, blood vessels, glands). It consists of two antagonistic systems:

Sympathetic Nervous System: Prepares the body for violent action against abnormal conditions (Fight or Flight). It is stimulated by adrenaline.
Parasympathetic Nervous System: Restores normal conditions after the stress is over (Rest and Digest).

Fig 9.8: Autonomic nervous system showing opposing effects

Table 9.1: Effects of the two parts of autonomic nervous system

Organs	Sympathetic System	Parasympathetic System
Heart	Accelerates heart beat	Retards heart beat
Blood vessels	Constricts all blood vessels except coronary vessels which are dilated	Dilates all blood vessels except coronary vessels which are constricted
Lungs	Dilates bronchi and bronchioles	Constricts bronchi and bronchioles
Intestines	Peristalsis decreased	Peristalsis increased
Urinary bladder	Sphincter contraction, muscle relaxed	Sphincter relaxation, muscle contraction (feeling to urinate)
Pupil of eye	Dilation	Constriction
Salivary glands	Inhibits secretion of saliva (dryness of mouth)	Stimulates secretion of saliva
Lacrimal (tear) glands	Stimulates secretion	Inhibits secretion
Arrector (or erector) pili muscles of skin	Stimulates contraction (hairs raised)	Relaxes (hairs flattened)
Body (as a whole)	Prepares body for action	Prepares body for relaxation

9.9 Reflex Action and Reflex Arc

Reflex Action: An automatic, quick, and involuntary response in the body brought about by a stimulus. It does not involve the conscious areas of the brain.

The Reflex Arc

The shortest route that an impulse takes from a receptor to an effector is a Reflex Arc. The typical pathway is:

Receptor (Sense Organ) → Sensory Neuron → Dorsal Root Ganglion → CNS (Spinal Cord) → Motor Neuron → Ventral Root → Effector (Muscle/Gland)

Fig 9.7: Cross section of spinal cord showing Reflex Arc

Types of Reflexes

Natural / Inborn Reflex	Conditioned / Acquired Reflex
Inborn, unlearned, inherited from the parents.	Acquired by experience and learning during a lifetime. Brought about due to a previously learned experience.
Similar in all individuals of a species.	Differs from individual to individual.
Examples: Blinking, coughing, sneezing (protective); Salivation, swallowing, peristalsis (provide functional efficiency).	Examples: Salivating at the sight or smell of familiar tasty food, tying shoelaces, playing a keyboard, typing without looking.

Pavlov's Experiment (Fig. 9.9): Russian scientist Ivan Pavlov demonstrated conditioned reflexes by ringing a bell simultaneously with the presentation of food. Eventually, the dog salivated just at the sound of the bell, without seeing any food. A stimulus which previously had no relation with the organism's salivary response, now elicits a response.

Some Common Reflexes in Humans

A. Natural (inborn) Reflexes

Knee-jerk: The leg is involuntarily extended as a result of a sharp tap below the knee-cap in a relaxed leg rested on the other leg while sitting.
Quick closing of the eyelids: When an object suddenly approaches the eye or when a strong beam of light is flashed across.
Withdrawal of the hand: Or any part of the body when suddenly pricked.
Peristaltic reflexes: Chyme (food) is propelled through the small intestine by peristaltic waves. Stretching of the intestinal wall becomes a stimulus and its contraction pushing the food along is the response.
Coughing reflex: When food swallowed enters the wind pipe.
Sneezing reflex: When any irritant enters the nose.

Table 9.2: Differences between reflexes and voluntary actions

Reflexes (Involuntary actions)	Voluntary actions
1. Initiated by some stimulus (touch, pain, pressure, heat, light, etc.)	1. Initiated by a willing thought.
2. Mainly self protective due to environment.	2. Fulfilment of a desired goal.
3. Commands originate mostly in the spinal cord and autonomic nervous system and a few in the brain as well.	3. Commands originate in brain.
4. Involve muscles and glands.	4. Involve only muscles.

The Sense Organs

The sense organs enable us to be aware of the conditions of the environment. The major sense organs are the eyes, ears, tongue, nose, and skin. Sensation is perceived by sensory cells located in these organs categorised as receptors.

Receptors

Receptor is any specialised tissue or cell sensitive to a specific stimulus.

Mechanoreceptors: Receptors for touch, pressure of skin due to mechanical change.
Chemoreceptors: Receptors of taste of the tongue and smell of the nose due to chemical influences.
Photoreceptors: Rods and cones of the retina of eye due to light.
Thermoreceptors: Heat and cold receptors in the skin, due to change in temperature.
Phonoreceptors: Receptors for sound/hearing.

9.10 Sense Organs: The Eye

Orbits: The two eyes are located in deep sockets or orbits on the front side of the head. Each eye is in the form of a ball and can be rotated with the help of six muscles.

Eyelids & Eyebrows: Eyelids protect the outer surface of the eyes and shut out light. Eyelashes prevent falling of larger particles. Eyebrows prevent rain drops or trickling perspiration from getting into the eyes.

Tear Glands (Lacrimal Glands)

Located at the upper sideward portion of the orbit. Six to twelve ducts pour the secretion over the front surface.

Functions of tears:

Lubricate the surface of the eye.
Wash away dust particles.
Help in killing germs due to the enzyme lysozyme which has antiseptic property.
Communicate emotions (grief or extreme joy).

The tears drain off into a sac lying at the inner angle of the eye. A nasolacrimal duct conducts the secretion into the nasal cavity.

Fig 9.10: Lacrimal (tear) apparatus of the human eye

Conjunctiva: A thin membrane covering the entire front part of the eye. It is continuous with the inner lining of the eyelids. Over the cornea, it is reduced to a single layer of transparent epithelium.

Internal Structure of the Eyeball (Fig. 9.11)

The wall of the eyeball is made up of three concentric layers:

Sclerotic Layer (Sclera): The outermost tough, white fibrous layer that gives shape to the eyeball. In the front, it bulges out to form the transparent Cornea. The conjunctiva is a thin membrane covering the front of the eye.
Choroid Layer: The middle, highly vascular layer rich in blood vessels (provides nourishment) and heavily pigmented with melanin (prevents internal reflection of light). In the front, it forms the coloured Iris with a central opening called the Pupil. The iris regulates the amount of light entering by dilating or constricting the pupil.
Retina: The innermost, light-sensitive layer containing photoreceptor cells.
- Rods: Sensitive to dim light. Contain the pigment Rhodopsin. Provide black-and-white vision.
- Cones: Sensitive to bright light and colours. Contain the pigment Iodopsin.

Comparison of rods and cones

Rods	Cones
1. More numerous.	1. Less numerous.
2. Mostly at the periphery of retina.	2. Mostly located in the centre of retina.
3. Very sensitive to low levels of illumination.	3. Only stimulated by bright light.
4. One type of rods only, stimulated by most wavelengths of visible light except red.	4. Three types of cone, each selectively responsive to different wavelengths, therefore, allowing colour perception.
5. Rapid regeneration of light-sensitive pigment, therefore, can perceive flicker well.	5. Slower regeneration of light-sensitive pigment, therefore, less responsive to flicker.

Important Regions on the Retina

Yellow Spot (Macula Lutea) / Fovea Centralis: Located at the centre of the horizontal axis of the eye. It contains a very high concentration of cones and NO rods. It is the region of brightest vision and sharpest image formation.
Blind Spot: The point where the optic nerve leaves the eye. It contains NO rods and NO cones, hence no image is formed here.

The Lens and Chambers

Lens: A transparent, biconvex, flexible crystalline body located just behind the pupil. It is held in place by Suspensory Ligaments, which are attached to the Ciliary Body. The ciliary muscles alter the shape of the lens for focusing.
Aqueous Chamber: The front space between the cornea and lens, filled with watery Aqueous Humour (keeps the lens moist and protects it from physical shock, refracts light).
Vitreous Chamber: The larger space behind the lens, filled with a jelly-like thicker fluid called Vitreous Humour (helps in keeping the shape of the eyeball, protects the retina and its nerve endings).

How do we see?

Entry of light rays: Light rays from the object enter the eyes through the transparent structures (conjunctiva, cornea, aqueous humour, lens, vitreous humour).
Focusing of image: First, the curvature of the cornea converges the light rays to some extent and the lens converges them further to form an image on the retina. The image on the retina is inverted and real.
Nerve impulse produced in retina transmitted to brain: The light energy of the image produces chemical changes in the sensitive cells (rods and cones). These changes generate nerve impulses which travel through the optic nerve and reach the visual area of the cerebrum.
Brain interprets: Our brain interprets the image in many ways, e.g. it "sees" the objects upright even if the image formed in the eye is inverted.

Fig 9.11: Vertical section of the human eye

Accommodation of the Eye

Accommodation: The process of focusing the eye at different distances by changing the curvature (and thus the focal length) of the lens.

Viewing Near Objects: Ciliary muscles contract → Suspensory ligaments relax → Lens becomes thicker and more convex.
Viewing Distant Objects: Ciliary muscles relax → Suspensory ligaments become taut/tight → Lens is stretched and becomes thinner/flatter.

Light and Dark Adaptation

When you pass from a brightly lighted area to a dark room (such as a cinema hall), you experience difficulty in seeing objects for a short while. Slowly, your vision is improved. This improvement is called Dark Adaptation. This change is due to (a) regeneration of the visual purple (rhodopsin), the pigment of the rods, which was earlier broken down due to bright light, and (b) dilation of the pupil permitting more light to enter the eyes.

Conversely, moving from dark to bright light causes a dazzling effect for a short period. After a few seconds, he comes back to normal viewing through Light Adaptation. The adaptation is due to reverse of the previous changes, i.e., (a) the visual purple of the rods is bleached, reducing their sensitivity, and (b) the pupil constricts (gets narrower), to reduce the amount of light entering the eyes.

Fig 9.14: Size of pupil in dim light and bright light

Colour Vision

Colour vision is possible only through cones of the retina which are stimulated only in bright light. You cannot make out the red, violet or purple flowers in a garden on a moonlit night, because then only the rods function and not the cones.

Stereoscopic (binocular) Vision

All monkeys/apes and particularly humans can perceive depth or the relative distance of the objects. This is due to simultaneous focusing of an object in both eyes, and their images by a kind of "overlapping" in the brain giving the three dimensional effect.

After-images

If one looks at a bright object for a moment and then closes the eyes, the sensation of light persists for a short period. In the same way, if one looks at a brightly coloured object and then looks at a dark surface, an image of the object in the same colour will persist. This is known as persistence image or the after-image. It lasts for about one-tenth of a second. This is the principle on which the technique of motion pictures is based.

9.11 Defects of the Eye

Myopia (Short-sightedness): Can see near objects clearly, but distant objects are blurred.
- Causes: Eyeball is lengthened from front to back, or the lens is too curved/convex.
- Result: The image is formed in front of the retina.
- Correction: Using a Concave (diverging) lens.
Hypermetropia (Long-sightedness): Can see distant objects clearly, but near objects are blurred.
- Causes: Eyeball is shortened from front to back, or the lens is too flat.
- Result: The image is formed behind the retina.
- Correction: Using a Convex (converging) lens.
Astigmatism: Uneven curvature of the cornea or lens, causing some parts of the object to be in focus while others are out of focus. Corrected by Cylindrical lenses.
Presbyopia: Affects older people. The lens loses its flexibility/elasticity, making it hard to focus on near objects. Corrected by convex lenses.
Cataract: The lens turns opaque. Vision is cut off. Corrected by surgical removal of the lens and replacing it with an artificial lens or using spectacles.
Night Blindness: Inability to see in dim light due to non-formation of rhodopsin. Caused by Vitamin A deficiency.
Colour Blindness: A genetic defect (X-linked) where the person cannot distinguish between certain colours (mostly red and green).
Squint: In this defect, the two eyes somewhat converge leading to what is called "cross eye". An opposite condition appears when they diverge and is called the "wide eye". Corrected by surgery and suitable exercise.

Fig 9.15: Short-sightedness (Myopia) and its correction

Fig 9.16: Long-sightedness (Hyperopia) and its correction

9.12 Sense Organs: The Ear

The human ear performs two completely different, yet vital functions:

Hearing
Body Balance (Equilibrium)

Structure of the Ear

The ear is divided into three main divisions:

1. Outer Ear

Pinna (Auricle): The projecting cartilaginous part. It collects and directs sound waves.
Auditory Canal: The passage leading inwards. It contains sebaceous glands that secrete wax (cerumen) to trap dust and insects.

2. Middle Ear

An air-filled cavity containing:

Tympanum (Eardrum): A tightly stretched membrane that vibrates when struck by sound waves.
Ear Ossicles: A chain of three tiny bones: Malleus (Hammer), Incus (Anvil), and Stapes (Stirrup). They amplify and transmit vibrations to the inner ear.
Eustachian Tube: A tube connecting the middle ear to the throat. It equalizes air pressure on both sides of the eardrum.
Oval Window (Fenestra ovalis): A membrane-covered opening leading to the inner ear. The stapes fits into it.
Round Window (Fenestra rotunda): Another opening covered by a membrane, allowing for fluid displacement in the cochlea.

3. Inner Ear (Membranous Labyrinth)

The inner ear is a complex system of interconnected tubes and sacs filled with a fluid called endolymph, surrounded by a bony cavity filled with perilymph. It has two main parts:

Cochlea (For Hearing): A snail-shell-like coiled tube. Its central canal contains the sensory Organ of Corti, which has hair cells that act as hearing receptors. It connects to the Auditory Nerve.
Vestibular Apparatus (For Balance): Consists of:
- Three Semicircular Canals: Arranged at right angles to each other. Their swollen bases (ampullae) contain sensory cells that detect Dynamic Balance (movement, turning, spinning).
- Utriculus and Sacculus: Sac-like structures that contain sensory cells to detect Static Balance (gravity, posture when standing still). Connects to the Vestibular Nerve.

Fig 9.18: Diagrammatic view of the human ear

Fig 9.19: The course of perception of sound in human ear

Mechanism of Hearing

Sound waves → Pinna → Auditory Canal → Tympanum (vibrates) → Ear Ossicles (amplify vibrations) → Oval Window → Cochlear fluid (endolymph/perilymph) → Organ of Corti (hair cells stimulated) → Auditory Nerve → Brain (Cerebrum) where sound is interpreted.

Mechanism of Balancing

As the head is turned in different directions, the fluid inside the semicircular canals is also shaken. The moving fluid pushes against sensory hair cells sending the nerve impulse through the auditory nerve to the brain. The sensory cells in the semicircular canals are concerned with dynamic equilibrium i.e. while the body is in motion. Similar sensory patches are also located in the utriculus and sacculus which register the static (positional) balance with respect to gravity.

If you spin round and round, the fluid in the semicircular canals continues to spin for a short time even after you stop, and you feel dizzy and at the same time, your eyes perform to-and-fro movements caused due to stimulation of semi-circular canals. Sea-sickness, air-sickness, and car-sickness are often due to these unusual sensations of equilibrium.

9.13 The Sense of Taste (Gustation)

(Extra - not specified in syllabus)

The sense of taste is located in the taste buds of the tongue. The sensory cells end in hair-like processes and have nerve fibres extending from their bases. Substances in solution enter the taste pores and stimulate the sensory hairs.

The four fundamental tastes are: Sweet, Salt, Bitter, and Sour.

Fig 9.20: Different taste areas of tongue showing bitter (back), sour (sides), salty (front sides), and sweet (tip).

9.14 The Sense of Smell

(Extra - not specified in syllabus)

The sense of smell is located in the delicate epithelial layers of the nasal chamber. The sensory cells have hair-like projections which respond to particles dissolved in the mucous secretion of the nose. The impulse is transmitted to the brain by the olfactory nerve.

Flavour is a combination of taste and smell while eating or sipping. If you hold your nose, you will find that grated apple and grated onion taste alike - slightly sweet. A cold with a blocked nose has the same effect and makes food tasteless/flavourless.

Exam Practice Questions (ICSE PYQ Trends)

Frequently Asked (1 Mark)

NAME THE FOLLOWING The fluid present between the meninges of the brain.
Ans: Cerebrospinal Fluid (CSF).
NAME THE FOLLOWING The part of the brain that maintains posture and equilibrium of the body.
Ans: Cerebellum.
NAME THE FOLLOWING The part of the eye responsible for its shape and providing a transparent window in the front.
Ans: Sclerotic layer / Cornea.
NAME THE FOLLOWING The photosensitive pigment present in the rod cells of the retina.
Ans: Rhodopsin.
NAME THE FOLLOWING The tube connecting the middle ear to the throat.
Ans: Eustachian Tube.
NAME THE FOLLOWING The structural and functional unit of the nervous system.
Ans: Neuron.

Give Biological Reasons

REASONING Answer the following:

Why is the arrangement of white and gray matter different in the brain and spinal cord?
Ans: In the brain, the cytons (gray matter) are on the outside (cortex) to maximize surface area via convolutions for higher intelligence. In the spinal cord, axons (white matter) are on the outside to facilitate rapid conduction of impulses up and down the spinal tracts.
A person feels dizzy for a moment after spinning around. Why?
Ans: Spinning causes the endolymph fluid inside the semicircular canals to move. When the spinning stops, the fluid continues to move for a few seconds due to inertia, stimulating the sensory hair cells and sending false signals of movement to the brain, causing dizziness.
Why do older people generally require convex lenses for reading?
Ans: Older people suffer from Presbyopia. As age advances, the crystalline lens loses its elasticity and the ciliary muscles weaken, preventing the lens from becoming convex enough to focus on near objects. A convex lens corrects this.

Application & Skill Based Questions

APPLICATION Think and answer:

Two hungry boys (A and B) enter a restaurant and find a table decorated as follows: Boy A has a raw whole chicken on his plate, Boy B has a roasted chicken. Boy B starts salivating but not A. Explain the reason for this difference.
Ans: Boy B salivates because of a Conditioned reflex. He has previous experience and memory of the taste of roasted/cooked food. The sight and smell of the roasted chicken acts as a stimulus for salivation. Boy A does not salivate because raw chicken is not associated with a pleasant food memory or experience.

Identify the part of the autonomic nervous system involved in the following situations:

Situation	Organ	Change/Action	Autonomic System
You have entered a dark room	Eye	Dilation of pupil	Sympathetic
Consuming lot of glucose while running a race	Liver	Conversion of glycogen to glucose	Sympathetic
You are chewing a tasty food	Salivary gland	Increased secretion of saliva	Parasympathetic
You are running a race	Adrenal gland	Secretes adrenaline	Sympathetic
You are retiring to bed for sleep	Heart	Heartbeat slows down	Parasympathetic
You are shivering in intense cold	Body hairs	Erection of hair	Sympathetic

Differentiate

DIFFERENCES Differentiate between the following pairs:

Myopia and Hypermetropia: In Myopia, the eyeball is too long, the image forms in front of the retina, and it is corrected by a concave lens. In Hypermetropia, the eyeball is too short, the image forms behind the retina, and it is corrected by a convex lens.
Rods and Cones: Rods contain rhodopsin, function in dim light, and provide black-and-white vision. Cones contain iodopsin, function in bright light, and are responsible for colour vision.
Sympathetic and Parasympathetic Nervous System: Sympathetic prepares the body for stress (fight or flight) by increasing heartbeat and dilating pupils. Parasympathetic restores normal conditions (rest and digest) by slowing heartbeat and constricting pupils.