Nervous System and Sense Organs | ICSE Class 10 Biology Notes

This note covers the ICSE Class 10 Biology topic Nervous System and Sense Organs: the divisions of the nervous system, the structure and types of neurons and nerves, how a nerve impulse travels and crosses a synapse, the parts of the brain and their functions, grey and white matter, the sympathetic and parasympathetic systems, reflex, voluntary and involuntary actions, the structure, working and defects of the eye, and the structure of the ear and its roles in hearing and balance.

What makes up the human nervous system?

Coordination is the process through which two or more organs interact and complement the functions of one another. During exercise, for example, the breathing rate, heartbeat and blood flow all rise together to supply the working muscles.

The nervous system and the endocrine system jointly coordinate the activities of all organs. The nervous system gives quick coordination through a network of point-to-point connections, while the endocrine system gives chemical integration through hormones.

The nervous system gets information from our sense organs and acts through our muscles. Information is detected by the specialised tips of some nerve cells, called receptors, usually located in sense organs such as the inner ear, the nose and the tongue. Gustatory receptors detect taste, and olfactory receptors detect smell.

Divisions of the nervous system

• The central nervous system (CNS) is the brain and the spinal cord. It is the site of information processing and control.
• The peripheral nervous system (PNS) is all the nerves associated with the CNS: the cranial nerves arising from the brain and the spinal nerves arising from the spinal cord.
• The PNS has afferent fibres, which carry impulses from tissues and organs to the CNS, and efferent fibres, which carry regulatory impulses from the CNS to tissues and organs.
• The PNS is divided into the somatic nervous system, which relays impulses from the CNS to skeletal muscles, and the autonomic nervous system, which carries impulses from the CNS to the involuntary organs and smooth muscles.
• The autonomic nervous system is further divided into the sympathetic and parasympathetic nervous systems.

The visceral nervous system is the part of the PNS made up of the nerves, fibres, ganglia and plexuses by which impulses travel between the CNS and the viscera, in both directions.

Draw a flow chart. Branch "Nervous system" into "Central nervous system (brain and spinal cord)" and "Peripheral nervous system (cranial and spinal nerves)". Branch the peripheral system into "Somatic" and "Autonomic", and branch "Autonomic" into "Sympathetic" and "Parasympathetic".

What is the structure of a neuron?

The neuron (nerve cell) is the structural and functional unit of the nervous system. It is a microscopic, highly specialised cell that can detect, receive and transmit stimuli. It has three major parts: the cell body, the dendrites and the axon.

• The cell body contains cytoplasm with the typical cell organelles, a nucleus that controls the cell's activities, and granular bodies called Nissl's granules.
• Dendrites are short fibres that branch repeatedly and project out of the cell body. They also contain Nissl's granules. They receive signals and carry impulses towards the cell body.
• The axon is a single long fibre whose far (distal) end is branched. It carries impulses away from the cell body to a synapse or to a neuromuscular junction.
• Each axon branch ends in a bulb-like synaptic knob, containing synaptic vesicles filled with chemicals called neurotransmitters.

Myelinated and non-myelinated fibres

In a myelinated fibre, Schwann cells form a myelin sheath around the axon. The gaps between two adjacent myelin sheaths are the nodes of Ranvier. Myelinated fibres are found in spinal and cranial nerves.

A non-myelinated fibre is enclosed by a Schwann cell that does not form a myelin sheath. Such fibres are common in the autonomic and somatic nervous systems.

Direction of the message

1. Information picked up at the dendritic tip sets off a chemical reaction that creates an electrical impulse.
2. The impulse travels from the dendrite to the cell body.
3. It then travels along the axon to its end.
4. There the impulse sets off the release of chemicals, which cross the gap, or synapse, and start a similar impulse in a dendrite of the next neuron.

A cell body with a round nucleus and speckled Nissl's granules has branched dendrites around it. A long axon runs down from it, wrapped by Schwann cells forming the myelin sheath, with nodes of Ranvier between the sheaths. It ends in branched axon terminals tipped by synaptic knobs.

What are the types of neurons and nerves?

Types of neurons

By the number of axons and dendrites, neurons are of three types.

By function, neurons are also of three types.

• A sensory neuron (afferent neuron) carries impulses from a receptor, such as the heat and pain receptors in the skin, to the CNS.
• A motor neuron (efferent neuron) carries impulses from the CNS to an effector, such as a muscle or a gland.
• A relay neuron (interneuron) lies inside the CNS and links a sensory neuron with a motor neuron.

Nerves

A nerve is a bundle of nerve fibres (axons) enclosed in a sheath of connective tissue. A sensory nerve has only sensory fibres, like the optic nerve, which carries signals from the retina to the brain. A motor nerve has only motor fibres. A mixed nerve has both; all spinal nerves are mixed.

There are 12 pairs of cranial nerves and 31 pairs of spinal nerves. The junction between two neurons is a synapse, and a similar junction between a neuron and a muscle fibre is a neuromuscular junction. Through such junctions, impulses are delivered to muscle cells or gland cells.

How is a nerve impulse produced and passed on?

The resting neuron

Neurons are excitable because their membranes are polarised. At rest, the axon membrane is more permeable to potassium ions (K⁺), nearly impermeable to sodium ions (Na⁺), and impermeable to the negatively charged proteins of the axoplasm.

So the axoplasm has a high concentration of K⁺ and negative proteins and a low concentration of Na⁺, while the fluid outside has low K⁺ and high Na⁺. The sodium-potassium pump maintains these gradients, actively moving 3 Na⁺ out for every 2 K⁺ in.

The outer surface of the membrane is therefore positive and the inner surface negative. This potential difference across the resting membrane is the resting potential.

Generation and conduction of the impulse

1. A stimulus at a site, say point A, makes the membrane there freely permeable to Na⁺, and Na⁺ rushes in.
2. The polarity at A is reversed: the outer surface becomes negative and the inner surface positive. The membrane is depolarised, and this potential is the action potential, or nerve impulse.
3. At site B just ahead, the membrane is still positive outside, so current flows on the inner surface from A to B and on the outer surface from B to A.
4. The polarity at B is reversed and an action potential arises there. Repeated along the axon, this conducts the impulse.
5. The rise in Na⁺ permeability is very short-lived and is followed by a rise in K⁺ permeability. K⁺ diffuses out and, within a fraction of a second, restores the resting potential.

Two drawings of a length of axon with plus and minus signs on its membrane. In the upper one, Na⁺ enters at point A, shaded red, where the charges are reversed. In the lower one, curved arrows show current flowing between A and the next point, B, which is lightly shaded.

Transmission across a synapse

A synapse is formed by the membranes of a pre-synaptic neuron and a post-synaptic neuron, which may or may not be separated by a gap, the synaptic cleft.

At an electrical synapse the membranes are very close and current flows directly across; transmission here is always faster than across a chemical synapse, but electrical synapses are rare in our body. At a chemical synapse a fluid-filled cleft separates the membranes.

1. An impulse arrives at the axon terminal.
2. The synaptic vesicles move to the membrane, fuse with it and release neurotransmitters into the synaptic cleft.
3. The neurotransmitters bind to specific receptors on the post-synaptic membrane.
4. This opens ion channels, ions enter, and a new potential, excitatory or inhibitory, is set up in the post-synaptic neuron.

Because neurotransmitters are released from the axon terminal and their receptors lie on the post-synaptic membrane, a chemical synapse passes the impulse in one direction: from the axon of one neuron to the dendrite of the next.

An axon widens into an axon terminal holding round synaptic vesicles. Some vesicles fuse with the pre-synaptic membrane and release neurotransmitters into the synaptic cleft. The post-synaptic membrane carries receptors, and a bracket marks the two membranes and the cleft as the synapse.

How is the brain protected, and what are its main parts?

The brain is the body's central information processing organ, its "command and control system". It controls voluntary movements, balance, vital involuntary organs such as the lungs, heart and kidneys, body temperature, hunger and thirst, 24-hour (circadian) rhythms, several endocrine glands and behaviour. It also processes vision, hearing, speech, memory, intelligence, emotions and thoughts.

Protection

• The brain sits inside a bony box, the skull (cranium).
• It is covered by three cranial meninges: an outer dura mater, a very thin middle arachnoid and an inner pia mater in contact with the brain tissue.
• Inside the skull it is held in a fluid-filled "balloon" that absorbs shocks; the fluid is the cerebrospinal fluid.
• The spinal cord is protected by the vertebral column (backbone).

Regions of the brain

• The forebrain: cerebrum, thalamus and hypothalamus.
• The midbrain: between the thalamus and hypothalamus of the forebrain and the pons of the hindbrain.
• The hindbrain: pons, cerebellum and medulla oblongata.

The midbrain, pons and medulla make up the brain stem, which connects the brain with the spinal cord.

External features

The cerebrum forms the major part of the brain. A deep cleft divides it lengthwise into left and right cerebral hemispheres, joined by a tract of nerve fibres called the corpus callosum. Its surface layer, the cerebral cortex, is thrown into prominent folds.

The cerebellum lies below the back of the cerebrum. Its surface is very convoluted, giving extra space for many more neurons. The medulla continues downward into the spinal cord.

A brain cut down the middle, seen from the side. The folded cerebral hemisphere forms the upper part, with the curved corpus callosum beneath. A forebrain bracket covers the cerebrum, thalamus and hypothalamus. The midbrain, with the cerebral aqueduct, lies below. A hindbrain bracket covers the pons, the cerebellum at the back and the medulla, which runs down into the spinal cord.

What do the main parts of the brain do?

The forebrain is the main thinking part of the brain. The cortex of its cerebrum has sensory areas that receive impulses from receptors, with separate areas for hearing, smell, sight and so on. Association areas interpret this by combining it with information from other receptors and with stored information; they handle memory and communication.

A decision then passes to the motor areas, which control voluntary muscles such as the leg muscles.

The thalamus, wrapped by the cerebrum, is a major coordinating centre for sensory and motor signals.

The hypothalamus, at its base, has centres that control body temperature and the urge to eat and drink. Its neurosecretory cells secrete hypothalamic hormones; for example, when growth hormone is low, it releases growth hormone releasing factor, which stimulates the pituitary gland to release growth hormone.

The midbrain has a canal, the cerebral aqueduct, and four round swellings on its upper side, the corpora quadrigemina. It receives and integrates visual, tactile and auditory inputs.

The cerebellum gives precision to voluntary actions and maintains posture and balance; walking in a straight line, riding a bicycle and picking up a pencil depend on it. It integrates information from the semicircular canals of the ear and the auditory system.

How are grey and white matter arranged in the brain and spinal cord?

Grey matter is where neuron cell bodies are concentrated, giving a greyish colour. White matter is made of nerve fibres covered by myelin sheaths, which give an opaque white look. In terms of the parts of a neuron, the cell bodies and dendrites lie in the grey matter, while the myelinated axons make up the white matter.

In the cerebrum the grey matter lies outside, as the cerebral cortex. The white matter, made of the myelinated fibres of the tracts, forms the inner part of each hemisphere.

The spinal cord runs down from the medulla inside the vertebral column, covered by the meninges. Here the arrangement is reversed: the grey matter lies inside, as a butterfly-shaped region around a narrow central canal, and the white matter lies outside it.

Each spinal nerve joins the cord by two roots. The dorsal root brings in sensory fibres and bears the dorsal root ganglion, holding the cell bodies of sensory neurons. The ventral root carries motor fibres out.

Nerves from all over the body meet in a bundle in the spinal cord on their way to the brain, so the cord carries messages to and from the brain. Reflex arcs are also formed in the spinal cord.

Draw an oval section with a butterfly-shaped grey matter region and a small central canal at its middle, surrounded by white matter. Add a dorsal root with a swollen ganglion on the upper side and a ventral root on the lower side, joining outside the cord as a spinal nerve.

How do the sympathetic and parasympathetic systems regulate the body?

The autonomic nervous system carries impulses from the CNS to the involuntary organs and smooth muscles. It works without conscious control. Most internal organs receive nerves from both of its divisions, which usually have opposite effects, so their balance keeps each organ working at the right level.

• The sympathetic nervous system arises from the thoracic and lumbar regions of the spinal cord. It prepares the body for stress or emergency, the "fight or flight" response.
• The parasympathetic nervous system arises from the brain and the sacral region of the spinal cord. It returns the body to a calm, resting state and favours digestion.

In an emergency the hormone adrenaline from the adrenal glands has similar effects to sympathetic stimulation. The heart beats faster, blood is diverted from the digestive system and skin to the skeletal muscles, and breathing speeds up, preparing the body to fight or run away.

The somatic nervous system, by contrast, carries impulses to skeletal muscles, and most of its actions are under voluntary control.

What is a reflex action, and how does it differ from other actions?

A reflex action is a quick, automatic response to a stimulus, made without thinking and without feeling in control of the reaction. Pulling the hand back from a flame and the change in pupil size in bright light are examples.

If we had to think about touching a flame, the brain would have to receive signals, think and send instructions back, and we might get burnt.

Instead, the nerves detecting heat connect to the nerves that move muscles in a simpler way, a reflex arc. Reflex arcs form in the spinal cord, where nerves first meet on their way to the brain, although the information also reaches the brain.

Reflex arcs evolved because the brain's thinking is not fast enough, and they remain more efficient for quick responses.

Pathway of the reflex arc

1. Receptor: heat and pain receptors in the skin detect the hot object.
2. Sensory neuron: carries the impulse to the spinal cord.
3. Relay neuron: in the spinal cord, passes it to a motor neuron.
4. Motor neuron: carries it back to the arm.
5. Effector: the arm muscle contracts and the hand is pulled away. A message also travels up the cord to the brain.

A hand touches a steaming pan, with heat and pain receptors in the skin labelled. A red sensory neuron runs to a cross-section of the spinal cord (CNS), where a relay neuron lies in the pink, butterfly-shaped inner region. A motor neuron returns to the effector, the muscle in the arm. A red arrow labelled "message to brain" points up the cord.

Natural and acquired reflexes

A natural reflex (unconditioned reflex) is inborn. An acquired reflex (conditioned reflex) is learnt through practice and experience until it becomes automatic.

Voluntary and involuntary actions

Voluntary actions, such as writing, talking, moving a chair and clapping at the end of a programme, are based on deciding what to do next. The brain sends messages to the muscles through the motor areas of the forebrain.

Involuntary actions happen without thinking control: the heart beats, we breathe and digest food, and the mouth waters at the sight of food we like. Many are controlled by the midbrain and hindbrain; blood pressure, salivation and vomiting are controlled by the medulla.

What is the structure of the human eye?

The eye is one of the most valuable and sensitive sense organs; with our eyes closed we cannot identify colours. It works like a camera: its lens system forms an image on a light-sensitive screen, the retina. The eyeball is approximately spherical, about 2.3 cm in diameter.

External and protective parts

• Each eyeball sits in a bony socket of the skull, the orbit.
• Eyebrows, eyelids and eyelashes protect the eye from sweat, dust and strong light.
• The conjunctiva is a thin, transparent membrane lining the eyelids and covering the front of the eyeball.
• Lacrimal glands secrete tears, which keep the eye moist, wash away dust and contain lysozyme, an enzyme that kills bacteria.

Internal structure

• The sclera is the tough, white outer layer that protects the eyeball. At the front it becomes the transparent cornea, which bulges forward. Light enters through the cornea, and most refraction happens at its outer surface.
• The choroid is the middle layer, rich in blood vessels that nourish the eye, with dark pigment that stops light being reflected inside. At the front it forms the ciliary body and iris.
• The retina is the inner layer, a delicate membrane with an enormous number of light-sensitive cells. When lit, they generate electrical signals, which the optic nerve carries to the brain.

The iris is a dark muscular diaphragm that controls the size of the pupil, which regulates the amount of light entering the eye.

The crystalline lens, of fibrous, jelly-like material, is held by suspensory ligaments. The ciliary muscles change its curvature, giving the fine adjustment of focal length needed to focus objects at different distances. It forms an inverted, real image on the retina.

The watery aqueous humour fills the space between the cornea and the lens, and the jelly-like vitreous humour fills the chamber behind the lens. Both help keep the eyeball in shape.

Cells of the retina

• Rods respond to dim light. Their pigment, rhodopsin (visual purple), needs vitamin A to form.
• Cones work in bright light and detect colour. Their pigment is iodopsin.
• The yellow spot (fovea), opposite the lens, is packed with cones and gives the sharpest vision.
• The blind spot, where the optic nerve leaves, has no rods or cones, so no image is seen there.

A section of the eyeball. The cornea bulges at the front, with the aqueous humour behind it. The iris surrounds the pupil, with the crystalline lens just behind and the ciliary muscles around its edge. The vitreous humour fills the large space behind the lens, the retina lines the back, and the optic nerve leaves from there.

How does the eye focus, adjust to light and judge depth?

How we see an object

1. Light from the object enters through the cornea, where most refraction takes place.
2. It passes through the aqueous humour and the pupil to the lens.
3. The lens focuses it through the vitreous humour into an inverted, real image on the retina.
4. The light-sensitive cells generate electrical signals, which the optic nerve carries to the brain.
5. The brain interprets the signals, so that we perceive objects as they are.

Accommodation

The ability of the eye lens to adjust its focal length is called accommodation.

The focal length cannot be decreased below a certain limit. The near point, the least distance of distinct vision, is about 25 cm for a young adult with normal vision. The far point is infinity for a normal eye, so a normal eye sees clearly from 25 cm to infinity.

Adaptation

Adaptation is the eye's adjustment to changes in brightness. The change in pupil size is itself a reflex.

• In bright light the circular muscles of the iris contract, the pupil narrows and the cones are active.
• In dim light the radial muscles of the iris contract, the pupil widens and the rods take over.
• Entering a dark room from bright light, we see little at first. Rhodopsin bleached by bright light takes time to re-form; as it does, vision improves. This is dark adaptation.

Stereoscopic vision

Both eyes face forward, so their fields of view overlap, and each sees an object from a slightly different angle. The brain combines the two images into one three-dimensional picture. This stereoscopic (binocular) vision helps us judge depth and distance.

What are the defects of the eye, and how are they corrected?

When the eye gradually loses its power of accommodation, vision becomes blurred because of refractive defects. The three common ones, myopia, hypermetropia and presbyopia, are corrected with suitable spherical lenses.

Myopia (near-sightedness)

A person with myopia sees nearby objects clearly but not distant ones; the far point is nearer than infinity. The image of a distant object forms in front of the retina, owing to excessive curvature of the eye lens or elongation of the eyeball. A concave lens of suitable power brings the image back on to the retina.

In (a), rays from an object O at the far point of a myopic eye focus on the retina. In (b), parallel rays from a distant object meet in front of the retina. In (c), a concave lens spreads the parallel rays so they seem to come from O, and they focus on the retina.

Hypermetropia (far-sightedness)

A person with hypermetropia sees distant objects clearly but not nearby ones; the near point is farther than 25 cm. Rays from a close object focus behind the retina, because the focal length of the lens is too long or the eyeball is too small. A convex lens of suitable power adds the focusing power needed.

In (a), rays from N, the near point of a hypermetropic eye, focus on the retina; N', the normal near point, is closer. In (b), rays from N' meet behind the retina. In (c), a convex lens makes rays from N' seem to come from N, and they focus on the retina.

Presbyopia, astigmatism and cataract

• Presbyopia: the power of accommodation usually decreases with age as the ciliary muscles weaken and the lens becomes less flexible, so the near point recedes. People with both myopia and hypermetropia often need bifocal lenses: concave above for distant vision, convex below for near vision.
• Astigmatism: the cornea is curved unevenly, so lines in one direction look sharp and lines in another look blurred. It is corrected with cylindrical lenses.
• Cataract: in some older people the lens becomes milky and cloudy, causing partial or complete loss of vision, which cataract surgery can restore.

How is the ear built, and how does it help us hear and balance?

Structure of the ear

• External ear: the pinna, a flap of skin-covered cartilage, collects sound waves. The external auditory canal leads them inward; its hairs and wax trap dust. The eardrum (tympanic membrane), a thin membrane at its inner end, vibrates when sound enters.
• Middle ear: an air-filled cavity with three tiny bones, the ear ossicles: the malleus (hammer) touching the eardrum, the incus (anvil), and the stapes (stirrup) fitting into the oval window. The Eustachian tube links the middle ear to the throat and keeps air pressure equal on both sides of the eardrum.
• Internal ear: fluid-filled canals, the labyrinth, containing endolymph and surrounded by perilymph. The coiled cochlea holds the organ of Corti, with sensory hair cells for hearing. The vestibule's two sacs, the utriculus and sacculus, sense the position of the head. Three semicircular canals at right angles to one another, each with a swollen ampulla, sense movement of the head.

The auditory nerve carries impulses from the internal ear to the brain.

The outer ear leads into a canal ending at the labelled eardrum. Tiny bones lie beyond it, and further in is the coiled, snail-like cochlea, labelled, with looped canals above it and a nerve leading away.

Label the pinna, auditory canal and eardrum; the malleus, incus and stapes in the middle ear, with the Eustachian tube running down; and the semicircular canals, vestibule, cochlea and auditory nerve of the internal ear.

Course of perception of sound

1. The pinna collects sound waves and directs them along the auditory canal.
2. The waves strike the eardrum and make it vibrate.
3. The malleus, incus and stapes pass on the vibrations and amplify them.
4. The stapes vibrates the oval window, setting up waves in the fluids of the cochlea.
5. These stimulate the hair cells of the organ of Corti, and the cochlea converts the vibrations into electrical signals.
6. The auditory nerve carries the signals to the hearing area of the cerebrum, which perceives them as sound.

Two ears let the brain find a sound's direction by noting which ear hears it first; the time gap is often less than a thousandth of a second. The human audible range is 20 Hz to 20,000 Hz; it varies between people and decreases with age. Prolonged exposure to loud sound can cause hearing loss.

Role of the ear in balance

1. When the head turns, the endolymph in the semicircular canals moves and bends the hair cells in the ampullae; lying in three planes, the canals detect turning in any direction.
2. When the head tilts, small chalky particles in the utriculus and sacculus press on their hair cells under gravity.
3. The stimulated hair cells send impulses along the auditory nerve to the brain.
4. The cerebellum integrates this information and adjusts the muscles to keep posture and balance.

Glossary

• Neuron — The structural and functional unit of the nervous system, made of a cell body, dendrites and an axon.
• Axon — The single long fibre of a neuron that carries impulses away from the cell body.
• Dendrite — A short, branched fibre of a neuron that receives signals and carries impulses towards the cell body.
• Synapse — The junction between two neurons, formed by the pre-synaptic and post-synaptic membranes, which may be separated by a synaptic cleft.
• Neurotransmitter — A chemical released from synaptic vesicles into the synaptic cleft that passes the impulse to the next neuron.
• Resting potential — The potential difference across the membrane of a resting neuron, positive outside and negative inside.
• Action potential — The reversed potential at a stimulated site of the axon membrane; it is the nerve impulse.
• Reflex arc — The pathway of a reflex: receptor, sensory neuron, relay neuron in the spinal cord, motor neuron and effector.
• Grey matter — Nervous tissue where neuron cell bodies are concentrated; it forms the cerebral cortex.
• White matter — Nervous tissue of myelinated nerve fibres, which give it an opaque white appearance.
• Accommodation — The ability of the eye lens to adjust its focal length to focus near and distant objects.
• Blind spot — The point on the retina where the optic nerve leaves the eye; it has no rods or cones.
• Cochlea — The coiled part of the internal ear that converts sound vibrations into electrical signals.

Common errors and misconceptions

• Misconception: Dendrites carry impulses away from the cell body. Correct: Dendrites carry impulses towards the cell body; the axon carries them away.
• Misconception: Reflex actions are worked out by the brain. Correct: Reflex arcs are formed in the spinal cord, although the information also reaches the brain.
• Misconception: Grey matter lies outside in both the brain and the spinal cord. Correct: It lies outside in the cerebrum but inside, around the central canal, in the spinal cord.
• Misconception: The cerebrum maintains the balance of the body. Correct: The cerebellum maintains posture and balance and makes voluntary actions precise.
• Misconception: In myopia the image forms behind the retina. Correct: In myopia the image of a distant object forms in front of the retina; in hypermetropia a near object's image forms behind it.
• Misconception: The lens does most of the focusing. Correct: Most refraction happens at the outer surface of the cornea; the lens gives the fine adjustment.
• Misconception: The ear is only for hearing. Correct: The semicircular canals, utriculus and sacculus of the internal ear also maintain balance.

Exam-style questions with model answers

Q1. Name the part of the brain that maintains the posture and balance of the body, and give one other function of it. [2 marks]

1. The cerebellum maintains the posture and balance of the body.
2. It also gives precision to voluntary actions, such as walking in a straight line or picking up a pencil.

Q2. State the function of (a) the Eustachian tube, (b) the iris. [2 marks]

1. (a) The Eustachian tube connects the middle ear with the throat and keeps the air pressure equal on both sides of the eardrum.
2. (b) The iris controls the size of the pupil and so regulates the amount of light entering the eye.

Q3. Trace the pathway of the reflex arc when your hand touches a hot object. [3 marks]

1. Heat and pain receptors in the skin detect the stimulus, and a sensory neuron carries the impulse to the spinal cord.
2. A relay neuron in the spinal cord passes it to a motor neuron, which carries it to the effector, the arm muscle.
3. The muscle contracts and the hand is pulled away, while a message also travels up the cord to the brain.

Q4. Give three differences between the effects of the sympathetic and parasympathetic nervous systems. [3 marks]

1. The sympathetic system dilates the pupil of the eye, while the parasympathetic system constricts it.
2. The sympathetic system speeds up the heartbeat, while the parasympathetic system slows it down.
3. The sympathetic system slows down peristalsis and the secretion of digestive juices, while the parasympathetic system stimulates them.
4. Overall, the sympathetic system prepares the body for stress or emergency, and the parasympathetic system returns it to a calm, resting state.

Q5. Distinguish between myopia and hypermetropia with respect to (a) what is blurred, (b) where the image forms, (c) cause and (d) correction. [4 marks]

1. (a) In myopia distant objects are blurred; in hypermetropia nearby objects are blurred.
2. (b) In myopia a distant object's image forms in front of the retina; in hypermetropia a near object's image forms behind it.
3. (c) Myopia is caused by excessive curvature of the lens or an elongated eyeball; hypermetropia by too long a focal length or too small an eyeball.
4. (d) Myopia is corrected with a concave lens, hypermetropia with a convex lens.

Q6. Describe how a nerve impulse crosses a chemical synapse. [4 marks]

1. When an impulse reaches the axon terminal of the pre-synaptic neuron, the synaptic vesicles move towards the membrane.
2. They fuse with the membrane and release neurotransmitters into the fluid-filled synaptic cleft.
3. The neurotransmitters bind to specific receptors on the post-synaptic membrane.
4. This opens ion channels, ions enter, and a new potential, excitatory or inhibitory, arises in the post-synaptic neuron.

Q7. Explain how the eye focuses on distant and near objects, and define the near point and far point of a normal eye. [5 marks]

1. Accommodation is the ability of the eye lens to adjust its focal length; the ciliary muscles change the curvature of the lens.
2. For a distant object the ciliary muscles relax, the suspensory ligaments pull on the lens and it becomes thin, so its focal length increases.
3. For a near object the ciliary muscles contract, the ligaments slacken and the lens becomes thicker and more curved, so its focal length decreases.
4. The near point is the minimum distance at which objects are seen most distinctly without strain, about 25 cm for a young adult.
5. The far point is the farthest point up to which the eye sees clearly; it is infinity for a normal eye.

Q8. Trace the course of perception of sound from the pinna to the brain. [5 marks]

1. The pinna collects sound waves and directs them along the external auditory canal to the eardrum, which vibrates.
2. The three ear ossicles, the malleus, incus and stapes, pass on the vibrations and amplify them.
3. The stapes vibrates the oval window and sets up waves in the fluids of the cochlea.
4. These waves stimulate the sensory hair cells of the organ of Corti, and the cochlea converts the vibrations into electrical signals.
5. The auditory nerve carries the signals to the hearing area of the cerebrum, where they are perceived as sound.

Key takeaways

• The central nervous system is the brain and spinal cord; the peripheral nervous system is the cranial and spinal nerves.
• A neuron has a cell body with Nissl's granules, dendrites carrying impulses towards it, and an axon carrying impulses away.
• At rest the axon is positive outside and negative inside; Na⁺ rushing in reverses this, creating the action potential.
• At a chemical synapse, neurotransmitters cross the synaptic cleft and bind to receptors on the post-synaptic membrane.
• The forebrain has the cerebrum, thalamus and hypothalamus; the hindbrain has the pons, cerebellum and medulla oblongata.
• Grey matter is outside in the cerebrum but inside in the spinal cord; white matter is made of myelinated fibres.
• A reflex arc runs from receptor to sensory neuron, relay neuron in the spinal cord, motor neuron and effector.
• The ciliary muscles change the lens curvature, so a normal eye sees clearly from 25 cm to infinity.
• Myopia needs a concave lens, hypermetropia a convex lens, and cataract surgery.
• The cochlea turns sound vibrations into electrical signals, while the semicircular canals, utriculus and sacculus keep us balanced.

Test yourself

What are the three meninges covering the brain?

An outer dura mater, a very thin middle arachnoid, and an inner pia mater in contact with the brain.

Where are bipolar neurons found?

Bipolar neurons, with one axon and one dendrite, are found in the retina of the eye.

What are the nodes of Ranvier?

They are the gaps between two adjacent myelin sheaths along a myelinated axon.

Which part of the brain controls salivation, vomiting and blood pressure?

The medulla oblongata in the hindbrain controls these involuntary actions.

Give one example each of a natural and an acquired reflex.

Sneezing when an irritant enters the nose is natural; braking when something suddenly appears ahead is acquired.

Why is no image seen on the blind spot?

The blind spot is where the optic nerve leaves the eye, and it has no rods or cones to detect light.

Which signals are disrupted if the spinal cord is injured?

Messages between the brain and the body below the injury are cut, and reflex arcs through that part of the cord are lost.