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Chapter 35 The Senses
Objectives
Introduction:
Humans have 5 senses: touch, taste, smell, sight & hearing
The senses are based on receptor cells or groups of
receptors that form sense organs.
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Receptors are specialised to respond to various stimuli
such as heat, light, pressure & chemicals.
Touch
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The skin contains receptors for touch and
temperature.
These receptors are found in different
concentrations around the body.
e.g. many temperature receptors at the elbow.
Taste
Receptors for taste are located in taste buds.
They are found at the top and sides of the
tongue and in some parts of the lining of the
throat.
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The flavour of food is a combination of taste, texture & temperature.
Smell
The roof of the nasal cavity has about 20 million neurons to detect
smell (olfactory neurons).
These neurons respond to about 50 different chemicals in the
gaseous state.
Sight
Structure of the eye:
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Functions of the parts of the eye
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Function of each part of the eye
Name
Function
Fovea
Optic Nerve
Blind spot
Lens
Irish
Pupil
Ciliary Muscle
Vitreous Humour
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Eye Sight Defects
1.
Short Sight (myopia)
•
Condition: can view close objects clearly
but distant objects are out of focus.
Light rays are focussed short of the retina.
Cause: eyeball is too long or the focussing
elements of the eye are too strong.
Correction: use a concave (divergent) lens
to widen the angle over which the light rays
have to be refracted.
•
•
•
Long Sight (hypermetrpia)
•Condition: can view distant objects clearly
but close objects are out of focus,
•The focal point is long of the retina i.e. is
behind retina.
•Cause: eyeball is too short or the focussing
elements of the eye are too weak.
•Correction: use a convex (convergent) lens
to reduce the angle over which the light rays
have to be refracted.
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Short Sightedness
Object comes into focus in front of the retina. Need to
use a concave lens to focus the image onto the
retina so the person sees clearly
Long sightedness – Object comes to a focus
behind the retina. Need to use a convex lens
to focus the light onto the retina
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Hearing
Functions of the Ear
The functions of the ear are
hearing and balance.
The ear is composed of 3
sections:
•Outer ear
•Middle ear
•Inner ear
The outer and middle ear are
filled with air the inner ear is
filled with a fluid called lymph.
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Functions of the parts of the ear
Part
Function
Pinna
Helps collect & channel vibrations into the auditory canal
Auditory Canal
Tube that carries vibrations to t he eardrum
Eardrum
Small tightly stretched membrane that vibrates due to air
vibrations reaching it. It separates the outer & middle ear
Semi-circular Canals
Make up the vestibular apparatus which is responsible for
balance.
Ossicles
3 tiny bones in the middle ear tat transmit vibrations from
the outer ear to the middle ear and amplify (increase) the
vibrations.
Vestibular Nerve
Nerve that carries impulses from the ear to the brain
Cochlear (auditory) Nerve Carries impulses from the cochlea to the brain
Eustachian Tube
Tube that connects the middle ear and pharynx (throat) and
equalises pressure on either side of eardrum
Cochlea
Spiral tube which is responsible for hearing
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Functions of the parts of the ear
Part
Function
Pinna
Auditory Canal
Eardrum
Semi-circular Canals
Ossicles
Vestibular Nerve
Cochlear Nerve
Eustachian Tube
Cochlea
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The Ear & Hearing
•
•
•
•
The pinna collects vibrations
The auditory canal carries vibrations to the eardrum
The eardrum carries the vibrations to the middle ear.
The ossicles (the hammer, anvil & stirrup bones) amplify (increase) the
vibrations and pass them onto the oval window.
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The Eustachian tube connects the middle ear with the pharynx and
equalises the pressure between the middle and outer ear.
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Inner Ear
The inner ear contains:
1) The Cochlea
2) The Semi-circular canals
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2. The Ear & Balance
Balance is largely detected
by the vestibular apparatus
In the middle ear.
Balance is maintained due to vision,
Receptors in the muscles, ligament &
Tendons, & pressure receptors
In the soles of the feet.
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A person who damages their vestibular apparatus loses his/her
sense of balance but the person may learn to use other receptors to
redevelop their sense of balance.
The vestibular apparatus
Is filled with liquid.
Receptors located in different
Parts of the vestibular apparatus
Detect whether the head is vertical
or not.
All these receptors send impulses to
the cerebellum of the brain through
the vestibular nerve.
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Corrective Measures for a Hearing
Disorder
Disorder:
Glue Ear
Cause:
Surplus sticky fluid collecting in the middle
ear often due to overproduction of fluid as a
result of infection.
Correction:
Nose drops taken to decongest and unblock
the Eustachian Tube
In more severe cases small tubes called
grommets are inserted into the eardrum.
Grommets allow air into the middle ear &
this forces fluid down the eustachian tube.
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Summary of Sense Organs
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