Chapter 26

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Chapter 26

Reception of Stimuli

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3.5.3 Responses in the Human Nervous SystemObjectives – What you will need to know from this section1. The sense organs contain receptors,

with the brain as an interpreting centre for received information.

2. Knowledge of the five senses and related organs.

3. Study the eye and the ear – recognition and function of the main parts. 

4. Corrective measures for long and short sight or for a hearing defect.

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Sense Organs• A sense organ consists of many sense

receptors grouped together.

The five traditional senses are:1. smell2. taste3. touch4. hearing5. sight

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Smell

Taste

Touch

Hearing

Sight

Sense

Smell

Taste

Touch

Hearing

Sight

Sense

Nose

Tongue

Skin

Ear

Eye

Organ

Smell

Taste

Touch

Hearing

Sight

Sense

Nose

Tongue

Skin

Ear

Eye

Organ

chemicals [receptors in the nasal cavity detect vapours]

chemicals [taste buds detect sweet, sour, salt and bitter].

touch, pressure, temperature and pain [receptors spread throughout body]

sound [receptors in cochlea]

light [by rods and cones in the retina]

Stimulus detected

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1. The nose

• The nose is the organ of smell. • Olfactory neurons in the nose detect many

smells.

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How do smell and taste work?

Smell and taste belong to our chemical sensing system (chemosensation). The process of smelling and tasting begins when molecules released by the substances around us stimulate special nerve cells in the nose, mouth, or throat. These cells transmit messages to the brain, where specific smells or tastes are identified.

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• Olfactory (smell nerve) cells are stimulated by the odors around us—the fragrance from a rose, the smell of bread baking.

• These nerve cells are found in a tiny patch of tissue high up in the nose,and they connect directly to the brain.

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2. TasteThe tongue is the organ of taste. Taste buds in the tongue can detect the following tastes:1. sweet2. sour3. bitter4. salty

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• Many flavors are recognized through the sense of smell. If you hold your nose while eating chocolate, for example, you will have trouble identifying the chocolate flavor, even though you can distinguish the food’s sweetness or bitterness.

• This is because the familiar flavor of chocolate is sensed largely by odor.

• This is why a person who wishes to fully savor a delicious flavor (e.g., an expert chef testing his own creation) will exhale through his nose after each swallow.

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The Skin

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3. Touch

The skin is the organ of touch and temperature.The sense of touch is distributed throughout the body. Nerve endings in the skin and other parts of the body transmit sensations to the brain. Some parts of the body have a larger number of nerve endings and, therefore, are more sensitive.

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Four kinds of touch sensations can be identified: 1. cold, 2. heat, 3. contact, and4. pain.

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4. The Ear

The Ear has 2 separate functions:1. Hearing2. Balance

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The Outer EarIs filled with air.It consists of the1. Pinna2. Auditory canal &3. Eardrum

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1. Pinna – funnels vibrations into the ear canal.

2. Ear canal –carries vibrations to the eardrum. It has hairs and wax glands to trap dirt and germs.

3. Eardrum – a membrane of skin that carries the vibrations to the middle ear.

EardrumPinna

Ear Canal

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The Middle earIs filled with air.It consists of the1. Eustachian tube 2. three small bones [ossicles] &3. the oval window

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1. Eustachian tube – connects the middle ear with the pharynx and equalises pressure between the middle and outer ear.

2. Ossicles - hammer, anvil and stirrup, that amplify(increase) the vibrations & pass them on to the oval window.

Middle Ear

Ossicles

Eustachian tube

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3. Oval window –

the stirrup vibrates against it, causing fluid in the cochlea of the inner ear to move

Middle Ear

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The Inner earIt is filled with fluid.It consists of the1. cochlea and 2. semi-circular canals.

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1. Cochlea – a coiled tube that contains nerve cells that convert fluid movements into nerve impulses.

2. Semi-circular canals – 3 fluid filled tubes responsible for keeping our balance.

Inner Ear

Cochlea

Semi-circular canals

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• The pinna (ear lobe) channels the sound (vibrations in the air) towards the eardrum, which then vibrates.

• In turn, this vibrates the hammer, anvil and stirrup bones, which amplify the sound.

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• The stirrup pushes on the oval window of the cochlea, moving the liquid inside.

• Special hairs on 30,000 receptor cells detect the movement and send signals to the brain along the auditory nerve.

• The brain interprets these as sounds, and we ‘hear’.

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• Deafness can be caused by long exposure to a high level of noise, drugs, or ear infections.

• Damage to the eardrum, ossicles [bones], and cochlea, which can be caused by loud sounds, produces incurable deafness.

• Workers exposed to prolonged sounds of over 90 decibels [dB] are obliged by law to wear ear protection.

• Any exposure to 140 dB causes immediate damage to hearing.

Ear Defects - Deafness

Glue ear is:• a hearing disorder• caused by too much sticky fluid in the

middle ear• corrected by decongestants or grommets

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LEARNING CHECK1. What is the function of the

[a] pinna, [b] 3 ossicles, [c] cochlea, [d] semi-circular canals, [e] Eustachian tube?

2. Outline how vibrations in the air are eventually “heard” by our brain.

3. Name a common ear defect.4. Give some possible causes & treatments.5. How might you reduce your risks of this

defect?

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• Conjunctiva - transparent membrane that protects the eye.

• Cornea - front transparent part of the sclera. It focuses light rays on the retina.

Conjunctiva

Cornea

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• Sclera – is a tough, white coat that holds the eye in place

• Choroid – nourishes the eye & prevents reflection of light.

• Retina - contains light receptor cells, rods (for black & white vision) and cones.

ScleraChoroid

Retina

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Iris

• Fovea - mainly cones, where most images are focused.

• Iris – Coloured part of the eye, controls the amount of light entering the eye, by altering the size of the pupil.

Fovea

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• The Aqueous humour - watery liquid that supplies the lens and cornea with nutrients and helps keep the shape of the eye.

• Vitreous humour - gel that helps maintain the shape of the eye.

Aqueous humour Vitreous humour

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The Retina• Contains light receptor cells - rods and cones

except for at the Blind spot - where the optic nerve leaves the eye.

• When light rays focus on the retina, these receptor cells are stimulated and impulses are carried by the optic nerve to the brain.

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Rods and Cones The photoreceptors - rods and cones• The cones are activated in bright conditions • while the rods are activated in dark conditions • The highest concentration of cones is in the

Fovea / yellow spot• Defective or damaged cones result in colour

deficiency; whereas, defective or damaged rods result in problems seeing in the dark and at night.

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In bright light,

pupil constricts.

In dim light,

the pupil dilates.

How we see

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• Ciliary muscle - controls the shape of the lens.

• Suspensory ligaments - hold the lens in place.

• Lens - focuses the light rays on the retina.

Ciliary muscle

Suspensory ligamentsLens

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Accommodation is the changing of the shape of the lens to focus light onto the retina.

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LEARNING CHECK1. Name the 5 senses and the organs

involved.2. Name the 3 main layers of the eye and the

function of each.3. What is the function of the

[a] iris, [b] lens, [c] cornea, [d] fovea4. What is accommodation?

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For close vision:1. the Ciliary muscle contracts,

2. the suspensory ligaments relax,

3. the lens becomes thicker.

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For Distant Vision:1. the Ciliary muscle relaxes

2. the suspensory ligaments contract,

3. pulling the lens thinner.

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Reflex Action and Antagonistic Muscle Action

Circular muscles contract

Radial muscles relax

Pupils constricted

Radial muscles contract

Circular muscles relax

Pupils dilated

Why is it important that the amount of light entering the eye is regulated?

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Seeing things at different distancesFor distant objects, the ciliary muscle relaxes and so the suspensory ligaments pull tight, pulling the lens thinner – the light doesn’t bend as much.

For close objects the ciliary muscle contracts, allowing the lens to go fat, thus bending the light more.

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Eye DefectsThere are 2 Eye Defects:1. long-sighted, eyeball is too short. The image is focused behind the retina.2. Short - sighted, eyeball is too long. The image is focused infront of the retina.

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• You are long-sighted if you can clearly see objects a long way off, but you cannot see things close by.

• Convex lenses are used to correct the problem. They bend the light before it enters the eye.

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• You are short-sighted if you can clearly see objects close to you, but you cannot see things in the distance.

• Concave lenses are used to correct the problem. They diverge the light before it enters the eye

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LEARNING CHECK1. Explain how the ciliary body and

suspensory ligaments alter the lens.2. What is the function of the

[a] humours, [b] optic nerve?3. If you are longsighted, what does it

mean?4. What could be a possible cause?5. What type of lens can rectify it?

CHAPTER 35THE SENSES

The eye is the organ of sight. The main parts of the eye and their functions are:

• The conjunctiva is the membrane around the eye. It protects the eye.

• The sclera is a tough, white coat that holds the eye in shape.• The cornea is the front part of the sclera. It allows light into the

eye and bonds it to help focus it on the retina.• The choroid nourishes the eye and prevents internal reflection of

light.• The retina is light sensitive. It contains light receptors, rods (for

black and white vision, work in dim light) and cones (for colour vision, work in bright light).

• The fovea is the part of the retina where most images are focused.• The blind spot is where the optic nerve leaves the retina. It has no

rods or cones.• The optic nerve carries impulses to the brain.

CHAPTER 35THE SENSES

• The lens focuses light on the retina.

• The iris is the coloured part of the eye. It controls the amount of light entering the eye.

• The pupil is the black circle at the front of the eye. It lets light into the eye.

• Ciliary muscles change the shape of the lens (called accommodation) to focus the image on the retina.

• The aqueous and vitreous humours keep the eye in shape.

CHAPTER 35THE SENSES

CHAPTER 35THE SENSES