Detection of Environmental Conditions in Mammals

Irritability

• The ability to respond to stimuli

• Stimuli (external or internal) are detected by receptors or sense organs

• Receptors generate nerve impulses after being stimulated

• Nerve impulses are sent to the brain for interpretation

• The brain then generates nerve impulses carrying suitable responses to the effectors

• The effectors (muscles or glands) then produces suitable responses

Eye

• Sense organ for receiving light

• Protected by the skull

Structures around the eye-ball

• Tear glands: secrete tears which– wash away dust– contain lyzozyme which kills bacteria– moisten the eye surface

• Eyelids– protect eye from damage

• Eyelashes– protect the eye from large particles to enter it

Internal structure of the eye-ball

• Wall of eye-ball consists of 3 layers

1. Sclera• maintain the shape

of the eye-ball• protects the inner

structures• provides anchorage

to eye muscles

Internal structure of the eye-ball

2. Choroid• With many blood

vessels and pigment

• Blood vessels supplies oxygen and nutrients to the eyes, and to remove metabolic wastes from them

• Pigment absorbs extra light to prevent reflection of light inside the eyeball which may blur the image

Internal structure of the eye-ball

3. Retina• Contains light

sensitive cells (photo-receptors) and nerves• Rods for black-

and-white vision• Cones for colour

vision

Internal structure of the eye-ball

Yellow spot• densely packed with

cones• no rod is present• gives the most

distinct image and the greatest colour discrimination

Internal structure of the eye-ball

Blind spot• the point where the

nerve fibres leave the eye-ball

• no photo-receptorscannot detect any

image

Internal structure of the eye-ball

Cornea• Continuous with sclera• Protected by

conjunctiva• To allow light to enter• To refract light onto

the retina

Internal structure of the eye-ball

Pupil• The opening which

allows light to enter the eye-ball

Iris• To control the size of

the pupil

Change in the pupil size

• Iris is made of circular muscles and radial muscles –antagonistic pair

Increase in the pupil size

• At dim light : circular muscles relax; radial muscles contract• increase in size of pupil

Decrease in the pupil size

• At bright light : circular muscles contract; radial muscles relax• decrease in size of pupil

Internal structure of the eye-ball

Lens

• Transparent, elastic, biconvex structure

• To focus light rays on the retina by changing its convexity

Internal structure of the eye-ball

Suspensory ligaments

• Hold the lens in position

Ciliary body

• Regulates the curvature of the lens by contraction and relaxation of the ciliary muscles

Accommodation

• The ability of the eye to focus objects at varying distances onto the retina

• Light entering the eye is refracted successively at the cornea, the aqueous humour, the lens and the vitreous humour.

• The image is formed on the retina and the retina sends signal along the optical nerve to the brain, causing the sensation of sight.

• The image formed on the retina is inverted but is interpreted as erect.

Focusing near object

Light from near object

Focusing on near objects

Ciliary muscles contract Decrease in circumferenceTension of suspensory ligaments is decreasedLens become more convex

Focusing far away object

Light from far away object

Focusing on distant objects

Ciliary muscles relax Increase in circumferenceTension of suspensory ligaments is increasedLens become less convex

Accommodation

Object CiliaryMuscle

Shape Focal length

Near Contract Thicker Shortened

Distant Relax Thinner Lengthened

Near Point and Far Point

• The average normal eye can focus objects easily from about 25 cm, i.e., the near point, to infinity, i.e. the far point.

• This range of distance of clear vision varies from one person to another and decreases with age.

Short sight

Light from distant object

• The eyeball is a bit too long.

• The lens lacks the ability to accommodate for a distant object.

Correction of short sight

Light from distant object

Diverging lens

( Concave lens )

Long sight

Light from near object

• The eyeball is a bit too short.

• The lens lacks the ability to accommodate for a near object.

Correction of long sight

Light from near object

Converging lens

( Convex lens )

Causes

Defect Eye lens Eye ball Correction

Short sight Too thick Too long Concave lens

Long sight Too thin Too short Convex lens

Eye defects

• Short-sighted– Image of a distant object formed in front of the

retina– Lens too thick– Eye-ball too long– Correction: wear concave lens

Eye defects

• Long-sighted– Image of a near object formed behind the retina– Lens too thin– Eye-ball too short– Correction: wear conves lens

Eye defects

• Colour blindness– Defect of one or more of the three types of cone

cells– Unable to distinguish between colours– Inherited

Class Practice

1. Which of the following statements about the lens is/are correct ?

(1) The image formed on the retina is real.(2) The pupil is smaller in bright light than in dim light.(3) When the object distance changes, the eye focuses an object by chaning the focal length of the eye lens.

Internal structure of the eye-ball

Anterior chamber• Filled with aqueous

humour– to refract light onto the

retina

– to maintain the shape of the eye-ball

– to supply nutrients to the conjunctiva, conera and lens

Internal structure of the eye-ball

Posterior chamber• Filled with vitreous

humour– to refract light onto the

retina

– to maintain the shape of the eye-ball

Internal structure of the eye-ball

Optic nerve• To transmit nerve

impulses to the optic centre in the cerebral cortex of the brain for interpretation

Basic Fact of EAR

• Ears are used to detect SOUND in environment.

• Ears help to detect movement & position.

• Ear is divided into Outer Ear, Middle Ear & Inner Ear.

Structure of EAR

• Outer Ear: EAR PINNA, EAR CANAL & EAR DRUM.

• Middle Ear: EAR BONES

• Inner Ear: EUSTACHIAN TUBE & ADENOIDS.

Outer Ear

• It is the part which is visible and is made of folds of skin and cartilage.

• It leads into the ear canal, which is about one inch long in adults and is closed at the inner end by the eardrum.

• The eardrum is a thin, fibrous, circular membrane covered with a thin layer of skin.

• It vibrates in response to changes in the air pressure that constitute sound.

• The eardrum separates the outer ear from the middle ear.

Middle Ear• It is a small cavity which

conducts sound to the inner ear by means of three tiny, linked, movable bones called "ossicles."

• These are the smallest bones in the human body and are named for their shape.

• The hammer (malleus) joins the inside of the eardrum.

• The incus joint with the hammer and to the stapes.

• The base of the stapes fills the oval window which leads to the inner ear.

Inner Ear

• The inner ear is a very delicate series of structures deep within the bones of the skull.

• It consists of a maze of winding passages, called the "labyrinth".

• The front (see cochlea) is a tube resembling a snail's shell and is concerned with hearing.

• The rear part is concerned with balance.

Detection of SOUND

1. Sound waves (air vibrations) are collected by the OUTER EAR.

2. Sound waves vibrate the EAR DRUM.

3. Vibrations are amplified by the EAR BONES.

4. Vibrations change the pressure of the FLUID of the INNER EAR.

5. Vibrations are transmitted to signals to the brain via nerve impulses.

Intensity Cues in Stereo

• When the volume of two speakers are equal, we will hear the sound as come from the centre.

Structure of the ear

• Three regions:• Outer ear• Middle ear• Inner ear

Process of hearing

• Sound waves are collected by the ear pinna

Process of hearing

• Sound waves pass along the external auditory canal to the ear drum

Process of hearing

• Sound waves make the ear drum to vibrate• Ear drum converts sound waves into mechanical vibrations

Process of hearing

• Ear drum transmits vibration to the ear bones

• Ear bones transmit and amplify vibrations

Process of hearing

• Ear bones transmit vibration to the oval windows

Process of hearing

• Oval window causes the perilymph in the upper canal of the cochlea to vibrate

Process of hearing

• Perilymph transmits vibrations to the endolymph in the middle canal

Process of hearing

• The sensory hair cells on the bottom membrane of the middle canal are stimulated

• The sensory hair cells send off nerve impulses

Process of hearing

• The auditory nerve transmits the impulses to the auditory centre of the cerebral cortex

• The auditory centre interprets the nerve impulses and produce the sensation of hearing

Process of hearing

• The vibrations of perilymph are transmitted to the round window

• Round window bulges outwards into the middle ear cavity to release pressure

Equalizing the pressure on both sides of the eardrum

• The middle ear is air-filled– The atmospheric pressure may become higher

or lower than the air pressure in the middle ear– This causes the ear drum to curve inwards or

outwards– The ear drum cannot vibrate properly and

causes pain and temporarily deaf

Equalizing the pressure on both sides of the eardrum

• The pressure on both sides of the ear drum can be equalized by the Eustachian tube• Eustachian tube is connected to the pharynx• It is opened only during swallowing or yawning

Detection of movement by the ear

• Above the cochlea are three semi-circular canals– They are responsible for detecting head movement

• At the base of each canal is a swelling called ampulla

• The semi-circular canals are perpendicular to each other to detect head movement in all planes

Detection of movement by the ear

• Semi-circular canals are filled with endolymph • Gelatinous mass (cupula) is found inside each ampulla

Detection of movement by the ear

• When the head move, the semi-circular canals will move in the same direction

• However, the endolymph in the canals will move in opposite direction due to inertia

• The endolymph displaces the gelatinous mass inside the ampulla

• The sensory hair cells under the gelatinous mass is stimulated

• Nerve impulses are generated and transmitted along the auditory nerve

Detection of movement by the ear

• Nerve impulses are generated and transmitted along the auditory nerve to• the cerebrum: aware of the direction of head

movement

• the cerebellum: leads to appropriate responses of the muscles to maintain body balance

Nose - the olfactory organ

• For detection of smell

• By olfactory cells on the upper part of nasal cavity

• Covered with mucus– to dissolve chemicals in air which stimulate the

olfactory cells to produce nerve impulses to the cerebrum

Tongue - the taste organ

• Detected by taste buds on the upper surface of the tongue which are stimulated by chemicals dissolved in saliva

• Different regions detect different tastes

sweetsalty

sour

bitter• Flavour of food is

given by both the sense of taste and odour of it

The skin

• Contains many receptors for the sensation of touch, cold, hot, pain and pressure

• The distribution of them are uneven throughout the skin