AN INTRODUCTION TO COORDINATION IN ANIMAL
Meaning of co-ordination
In
the organism there are many organs. Each organ has a special function (e.g.
ear, eye, heart, stomach, brain, hand, etc.) as if they are independent of each
other. All there organs need to be connected with each other by the exchange of
substances. The linking together in time and space of the organs and their
activities is called coordination.
Therefore,
coordination is the reception and assembling of sensory information in a
central location and subsequent response of organism.
Importance of coordination
Without proper coordination the bodily
activities would be thrown in chaos and disorder i.e. the ability to detect
changes in their internal and external environment helps the organism to
respond to the changes appropriately e.g.( temperature, hard work)
Structures
detecting the changes may be located far away from the one that respond, hence
need for path way (cordination ) to link these structures.
Cordination
helps the organism to work in harmony, e.g. increase of breathing rate and
heart beat during exercise or work, alternation between swallowing food and
inhaling air.
How co-ordination is brought about.
-In
animals
Co-ordination is effected by the nervous system and the endocrine
(hormonal) system.
-In
plants
it is under the control of hormone, e.g. in the meristem a
chemical called auxin effects the growth of the plant.
Nervous System.
The
units that make up the nervous system are called nerve cells (neurones).
The system runs through out the body. Through irritability the organism detect
and respond to stimuli from the internal and external environment
External
environment like heat, light, sound, pressure, gravity, chemicals, water, food,
and other living organisms. Internal environment include, water, glucose,
minerals and temperature.
Types
of stimuli (1) physical (2) chemical (3) biological.
Response: is the reaction to a stimulus, i.e. towards or away
from it.
Sense organs:
Are
the organs which contains sensory cells (receptors).
Skin: touch, pain, temp, & pressure.
Ear: sound and body position (balance).
Eye: light (sight).
Nose: smell, mucus to trap dust.
Tongue: taste: (sweet, salt, bitter, sour.)
From
the receptor,
the sensory
neurones transmit the impulses to the central nervous system (CNS),
where the relay neurones connects it to the motor neurone which
transmits the nerve impulse from CNS to the effectors like muscles,
glands, cilia, flagella. Effectors can be cells, tissues, or organs that enable
organism to respond.
Neurones:
These
are basic unit of the nervous system. They transmit nerves impulse from one
part of the body to the other.
They
are referred as nerve cell. They are small masses of cytoplasm with a central
nucleus.
Adaptation
of neurones to their roles:
Connected
with a fatty myelin sheath, which offers protection and insulate
nerve.
Have numerous dendrites which help in
transmitting impulse from one nerve to another.
They
are numerous
and spread to all part if the body.
They
have an elongated axon which is an extension of the cytoplasm, which
helps in quick transmission of impulse.
Structural
parts of neurones.
-Cell
body (centron); nucleated part of the neurone.
-Dendrites:
short numerous fibrous receives and conduct impulses towards the cell body.
-Dendron:
single long fibre, receive nerve impulse from dendrites and conducts them to
the cell body.
-Axon: Single long fibre, conducts nerve
impulse away from the cell body.
-Axon
plasm: specialised type of cytoplasm through which the nerve impulse travel.
-Myelin
sheath: fatty sheath which encloses the axon has protective function, and it
insulates the axon and speed up the transmission of the impulses.
-Nodes
of ranvier: are construction which interrupt the myelin sheath at exactly one
millimetre interval. They help to propagate nerve impulse and speed up the
transmission of the impulse.
-Neurilemma.
Is a layer of cells which encloses the myelin sheath.
The synapse (neural function)
Is
a junction between dendrites of two adjacent neurones through which nerve
impulse is passed from one neurone to another.
Neurone,
muscular junction, ie. contact between muscle fibre & dendrites of motor
neurones.
Nerve
impulse: slight electric charge which
travel along the neurone. It can not be reversed. It can travel at a speed of
30m/s or 180 km/hr.
Importance of synapse
1. Enable single neurone to have connection with many
other neurones from different parts of the body. eg. Touching hot thing= i. Remove hand quickly, ii.
Jumping up, iii. Crying or loud
sound.
2. Ensure transmission of
impulse in one direction
3. play important role in learning & memory
Types of nervous system.
-Central
nervous system (CNS)
-Peripheral
nervous system (PNS)
-Autonomic
nervous system (ANS).
Central nervous system (CNS)
Is
a concentrated mass of nerves which make up (1) brain (2) spinal cord.
All
impulses from or to the body pass through the CNS. So it is the centre of cross
connection and linkages.
(I). The brain.
Is
the enlarged front end of the spinal cord in vertebrates. The brain is housed
in a system of membranes called meninges and it is encased in a bony structure
called skull or cranium. The nerves arising from the brain are called cranial
nerves.
Parts of the brain.
The
cerebrum (fore brain)
It
is the main cordination and association centre. It is the largest part of the
brain and highly folded in order to increase the surface area.
It
has two lobes called cerebral hemispheres which have inner white matter
(consists of nerve fibres).
In
animals without cerebral hemispheres the behaviour is a matter of simple and
conditional reflex action and inborn behaviour pattern called instinct.
The
cerebrum controls: learning,
thought, intelligence, reason, imagination, memory, will power, vision, speech,
taste.
The
cerebellum (mid brain)
It
is the unconscious motor co-ordination centre. It controls learned movement
like walking, posture, speech and balance.
The
medulla oblongata (hind brain)
It
is the reflex centre which controls involuntary and automatic processes like
heart beat, breathing, swallowing, sneezing, yawing, coughing, vomiting,
swallowing, hearing, regulating body temperature.
The
hypothalamus;
Is
part of vertebral fore brain which concerned with regulating physiological state of the body like,
body temperatures, chemical composition of the body, osmoregulation, metabolic
functions and some involuntary activities like appetite, sleep, water
excretion. It also influence the release of hormones by the pituitary gland.
-Olfactory lobes deals with smell, and
optic lobes deals with sight
Functions of the brain
-Receives
impulse from all the sensory organs of the body.
-Sends
off motor nerve impulse to the effectors (glands & muscles) causing them to
function accordingly.
-Correlates
the various stimuli from different sense organs.
-Co-ordinate
body activities so that the mechanism and chemical reactions of the body work
efficiently together.
-It
stores information so that behaviour can be modified according to the past
experienced.
(II). The spinal cord.
Is
the posterior extension of the central nervous system (CNS) from the brain to
the tail. It conducts mostly involuntary processes. Is encased in meninges and
protected by vertebral column.
The
parts of the spinal cord.
The
grey
matter: the inner part which consists of relay neurones which connects
the information between afferent and efferent neurones.
White
matter: the outer part
surrounding the grey matter. It consists of axons of sensory & motor
neurone.
Dorsal
root: carries sensory nerve
fibres.
Ventral
root: carries motor nerve fibres.
Functions of spinal cord:
-Conducts
sensory nerve impulse from receptors to the brain.
-Conducts
motor nerve impulse from the brain to the receptors.
-Enables
animals to attain an upright posture through the maintenance of muscle tone.
Peripheral nervous system (PNS)
Include
all nerves out side the CNS. Also known as somatic nervous system. It links the
CNS with the receptors and effectors.
The content of PNS.
i.
Spinal nerves: there are 31 pairs
of them in man. Each spinal nerve has two roots dorsal root & ventral root.
Dorsal root- contains sensory nerve fibres.
Ventral root. Contain motor nerve fibres
The
cell body of the sensory neurone lies in the ganglion of the dorsal root, while
cell body of the motor neurone lies in the grey matter of the spinal cord.
ii.
The cranial nerves: arise from
the brain and are associated with the receptors and effectors in the head. In
man there are 12 pairs of cranial nerves which transmits nerve impulse to the
brain from special sensory organs in the
head. e.g. optical nerves, auditory nerves, olfactory nerves, facial nerves.
Autonomic nervous system (ANS)
They
control involuntary activities of the body, e.g. gut movements, heart beat,
secretion of glands.
The
nerves which comprise the autonomic nervous system arise from the brain or from
the spinal cord and innervate which the body has a little or no voluntary
control.
They
are two
types (i)sympathetic and (ii)parasympathetic system.
The two systems normally oppose each
other in their functions on the same organ.
Parasympathetic
|
sympathetic
|
Slow
heart beat
|
Accelerate
heart beat.
|
Dilates
arterioles
|
Contracts
arterioles.
|
Contracts
bronchioles
|
Dilates
bronchioles
|
Contracts
iris
|
Dilates
iris.
|
Speed
up gut movement
|
Slow
gut movement (peristalsis)
|
Relax
bladder & anal sphincter
|
Contracts
bladder & anal sphincter
|
Relax
erector pili muscles
|
Contract
erector pili muscles
|
Decreases
sweat secretion.
|
Increases
sweat secretion
|
Reflex action
Is
rapid automatic involuntary response to a stimuli by an organ or a group of
organs, e.g. withdrawal of hand from a hot object, sneezing in case of a
foreign body in the nose, blinking of the eye in case an object passes close to
it, secretion of tears when the onion is cut, swallowing, enlargement of pupil
in different light intensity.
Reflex arc
Is
the rout /path followed by the nerve impulse during a reflex action. The arc
composed of
1. Receptor cells
2. Sensory neurone
3. Relay neurone
4. Motor neurone
5. Effectors, i.e. muscles in glands
Types of reflex action
i. Simple reflex action (instinctive Reflex action).
Is a rapid automatic in voluntary response
to a stimulus, by an organ or system of organs.
Which
does not involve the brain directly for its initiation, e.g dilation / contraction of iris in changing
light without being aware, sneezing, withdrawal of hand from hot object, tears
when onion is cut.
-Nothing
can be done to prevent it or modify it.
ii. Conditioned reflex action:
Is an automatic response which can be evoked
from an animal by a related stimulus substituted for the one which usually
initiates the response eg. Dog salivate normally when it sees food (related stimuli); but repeated bell(unrelated stimuli) before food can
condition dog to salivate.
Condition reflex action is formed from
the past experience and invokes modification of
behaviour
throw learning .
Pavlov experiment( Ivan Pavlov – Russian physiologist) did on 1902.
Usually
dog salivates in the sight, smell & test of food. In experiment Pavlov rang
bell immediately before giving food to the dog. The dog leant to associate the
sound of the bell with food, (associative learning). He continued for several
weeks. Later on he rang the bell but did
not present the food, and found that the sound of the bell initiated salivation
in the dog.
In
human beings conditional reflexes are used in riding a bicycle, driving,
swimming, walking, writing, etc.
Differences;
Sense organs
A
sense organ is a mass of specialised sensory receptor cells compacted together,
eg. Eye,
ear, skin, tongue, nose, and hypothalamus
that detect blood sugar level.
The
sensory receptors are composed of specialised cells that detect stimuli acting
on the animal’s environment. Usually a sense organ or cell can respond to only
one kind of stimulus, eg. Cell sensitive to touch will not be affected by
stimulus of heat.
Strong
stimulus produces more sensation than weak stimulus because,
i. It stimulate greater number of sense organs or cells.
ii. Some sensory cells respond only when stimuli is strong
or intense.
Types of receptors.
Interoreceptors:
Are located within the body and respond to stimuli from within the body, eg,
osmoreceptors, ie. Hypothalamus.
Exterorecptors:
Are located near the body surface and respond to stimuli from the external
environment, eg. Photo receptors.
Kinds
of receptor cells.
-Photoreceptors,
are sensitive to light.
-Mechanoreceptors,
are sensitive to pressure and vibration.
-Thermoreceptors,
are sensitive to temperature.
-Chemoreceptors,
are sensitive to chemical substances.
-Osmoreceptors,
are sensitive to osmotic pressure and changes of body fluids.
-Painreceptors,
are sensitive to pain on the surface and
in the body.
Sensation
Sensation
arise in the brain. Sensory organ or cell receive the stimulus and sends the
impulse to the brain. The brain record the message and we recognise it as
sensation. The impulse is responded to by a reflex or it is transferred to the
sensory region of the brain.
The skin.
The
mammalian skin consists of two main layers, the epidermis and the dermis (Refer
notes on Excretion). The
dermis carries several types of sensory receptor cells and nerve endings.
Certain parts of the body have more sense cells than others., eg. The finger
tips have more touch receptors than other skin parts, the skin of the lower arm
has many heat and cold receptor cells.
A
particular sense cell responds only to one kind of stimulus, this gives the
name to the sense cell, ie. Pain receptor.
Skin
receptors are more complex consisting of nerve endings surrounded by a
connective tissue called encapsulated nerve endings. These protect the nerve
endings form mechanical damage and also help in generation of nerve impulse.
The
receptor cells and their encapsulated nerve endings.
1. Pressure receptors (Pacinian corpuscles), that detect
pressure and respond to pressure. It also enables us to use tools skilfully.
2. Pain receptors (Ruffini’s end organ), that detect and
respond to pain. It is very important in the survival of animals, ie. Avoid
destruction from whatever is causing pain.
3. Touch receptors (Meissner’s Corpuscles), that detect
and respond to touch. Is found in non-hairy skin, eg. Finger tip and tongue;
also is connected to the hair erector muscles.
4. Thermoreceptors, that detect temperature change, there
two kinds
i.
Heat receptors
(End bulls corpuscles), that respond to heat.
ii. Cold receptors (Krausser’s end organs), that respond
to cold.
The thermoreceptors detect change of up to 0. 5 0C.
The tongue
The
sensation of taste is detected by group of cells called Taste Buds. In mammals
taste buds are found in Lingual papillae, that is upper surface of the tongue.
In other vertebrates taste buds are distributed on the walls of buccal cavity.
Taste buds are also called Gustatory cells.
The combined activity of taste buds and
smell receptors gives the sensation of flavour. The sensation
of taste helps the animal to distinguish between suitable and unsuitable
substances for ingestion. Also stimulate salivary glands and stomach walls to
secrete enzymes.
Types of Taste Sensation
1. Sweet: at the tip of the tongue
2. Sour: at the sides of the tongue
3. Bitter: at the back of the tongue
4. Salt: all over the tongue.
Mechanism
of tasting
A chemical
dissolves on the moisture of the tongue and stimulate sense cell which transmit
impulse to the brain. The brain identifies the particular sensation (Taste).
The Nose
Smell
detectors or receptors are located in the nasal cavity (nose). Smell cells are
grouped together to form Olfactory epithelium. The cells are
stimulated only by chemical substances which have dissolved in moisture (mucus)
on the olfactory region. Impulse from sensory cells is transmitted via Olfactory
nerve to the Olfactory lobe of the brain. Here
the impulse is interpreted as smell.
Diagram.
The Ear
The
Ear has two functions
-Hearing:
detection of sound waves and vibrations in the air.
-Balance:
provides information on the animal’s position in space, ie. Is the animal
upright or at an angle and hence make necessary adjustment.
Parts of the Ear.
The
ear has three parts, Outer ear, Middle ear and Inner ear.
1. The Outer Ear
The outer ear is the air
filled part of the ear which consists of:-
i.
Pinna: is a funnel shaped made of skin and cartilage. It
trap sound waves and directs to the ear tube. Some animals like cattle are able
to rotate their pinna to locate direction of sound waves.
ii.
Ear tube
(External auditory canal): is a
tube through which sound waves travel. The canal have lots of hairs and secretes wax which
traps dusts and other dirty particles and micro organisms from entering the
ear. It directs sound waves into the ear drum.
iii. Ear drum (Tympanum): is a thin membrane that forms boundary between the
outer and the middle ear. It vibrates when hit by sound waves and transmit it
to the ear ossicles.
2. The middle ear.
Is an air
filled cavity in the skull. It is composed of three tiny bones called ear ossicles, which link the ear drum
with the oval window of the inner ear.
i.
The three ossicles are: a.
Malleus or Hammer
b. Incus or Anvil
c. Stapes or Stirrup
The
three ossicles transmit sound vibrations from the ear drum to the oval window.
When the ear drum vibrates it causes the ossicles to move forward and back
wards. The Malleus transfer vibration from ear drum to Incus which pass it to
Stapes which then transmits the vibration to oval window.
ii. The middle ear is connected to the
pharynx (The mouth and nose cavity) by a tube called Eustachian tube. The tube
is normally closed but opens in case of swallowing or yawning. It opens to let
air enter or leave the middle ear, this equilizes the pressure between inside
and outside of the ear drum and by that it prevent the ear drum from bursting.
iii.
Oval window: is a membrane which passes on vibrations in the fluids of Cochlea.
3. The Inner Ear.
The
inner part consist of series of fluid filled chambers, like:-
a. Cochlea:
is a coiled tube filled with a liquid called Endolymph. Cochlea contain sensory
cells connected to the brain by Auditory nerve. The part that
respond to sound is called the Organ of Corti. Cochlea detect sound
vibration..
b. Semi circular canals: are three canals which stand at right angle to each
other. At one end of each canal a welling called ampulla contain sensory
cells to detect motion. The movement of the fluid in the canals (by
rotating, nodding or moving our head) stimulate sensory cells of the ampullae.
The impulse sent to the brain is interpreted and the response by which we
automatically keep our balance.
c. Utriculus and Sacculus (Utricle and Saccule): are special structures at the
base of semi circular canals which have the sensory hair cells to detect
gravity.
Diagram of the Ear.
Mechanism of hearing
The
pinna collect and direct the sound waves to the ear tube which direct it to the
ear drum. The ear drum vibrates hence converting sound waves into vibration
which is transmitted to ear ossicles. In the middle ear the vibration is
amplified 20 times. Vibration of the ossicle in the oval window set up
vibration in the liquid of the inner ear and cochlea. The vibration of the
endolymph stimulate the organ of corti, which send the impulse to the brain via
the auditory nerve. The brain sensation of different sound volume, pitch,
source, loudness and melodies.
Organ
of corti discriminates sounds
according to their frequencies; ie. High frequency sounds are detected by the part
at the base of the cochlea, while low frequency sounds are detected by the
organ of corti at the apex of the cochlea.
Middle
ear is able to amplify (magnify) vibrations 20 times because:-
a. high density of the bone
b. The lever like action of the ear ossicle which result
from their arrangement and shape.
c. The oval window is small than the ear drum.
Qualities of sounds
1. Loudness:
Is the magnitude or intensity of the sound. It is determined by amplitude or
strength of the sound waves striking ear drum. Loud sounds will cause a large
displacement while soft sounds will cause a small displacement of basilar
membrane found in the cochlea.
2. Pitch:
is the tone of the sound. Is determined by the frequency of the sound wave.
Sound of low frequencies have long wavelengths, which cause low tones. Sounds
of high frequencies have short wave lengths, which cause high tones.
Sound
localization.
The
direction of sound waves is detected accurately as a result of both ears
functioning simultaneously.
-Vibration
(sound) from front: Both ears pick the sound wave at the same time, and the
intensity of the nerve impulse will be of equal strength.
-Vibration
(sound) from the sides: one ear will pick the sound waves earlier than the
other, the intensity of the nerve impulse to the brain will be different from
one ear to other ear. This allows to determine the direction of sound waves,
ie. The ear on the opposite side will receive less sound and slightly later
than the ear on the direction of the source.
-Equidistant
sound. Is usually difficult to locate the sound direction. The animal will try
to rotate the pinna or the whole head in attempt to locate the source of sound.
Ear disorders
Proper
hearing depends on the efficient transmission of sound waves from the external
ear through middle ear to inner ear and the response of the sensory cells. Any
interference with these will result to certain degree of deafness.
Hearing
defects.
Loss
of hearing and deafness can be caused by:-
a. Blockage of the Ear tube by too much wax or foreign
bodies
b. Rapture (perforation) of Ear drum, The burst of ear
drum due to physical blow, loud noise or infections, ie. When children stick
sharp object in the ears.
c. Fusion of the Ear ossicles: caused by abnormal growth
of connective tissues in the middle ear which fuses the ear ossicles together
and prevent them from moving.
d. Nerve destruction: caused by damage of auditory nerve
due to nervous diseases, over dose of antibiotics, accidents, or exposure to
loud sounds.
Disease
of the Ear.
a. Acute Labyrinthitis: Is an inflammation of the middle
ear and cochlea. It may lead to deafness if not treated. Can be treated by
using drugs.
b. Tinnitus: caused by accumulation of wax in the ear or
use of certain drugs, eg. Quinine. Treatment is by removal of wax and
abstaining from causative drugs.
c. Vertigo: caused by disorientation of body in space.
This is due to dilation of endolymph.
The Eye.
The eye is a spherical structure composed
of different tissues all essential to focus the light on the light sensitive
retina layer where the pictures of the environment are made. The large part of
the eye is enclosed in a protective bony socket in the skull called orbit
or eye socket. A thick layer of fat is deposited around the
eyeball which provide further protection as a shock absorber against mechanical
damage or injuries. There are set of muscles which attach the eyeball to the
walls of the socket. These muscles are:
(i)
Superior and inferior oblique
muscles; move the eyeball left and right.
(ii)
Superior and inferior rectus
muscles; move the eyeball up and down.
These muscles can move the eyeball in
many directions to increase the field of view.
Functions of eye’s components.
(i)
Eye lids.
Two thin folds of the skin in-front of the eye ball. They protect
external surface of the eye. By blinking (movement of the eye lids) they keep
the surface of the eyes moist. Saline liquid (solution of Sodium Chloride and
Hydrogen Carbonate) from the tear gland( below the upper eyelid) contains an
enzyme which kills micro organisms. Blinking washes this liquid across the
eyeball.
(ii)
Eye lashes:
Many long hair s on the edge of the eye lids. They protect the eye
from entries of small particles.
(iii)
Eye brows: the raised potion of the skin above the eyeball,
thickly covered with hair. They prevent the entry of dust particles and sweat
in the eye.
(iv)
Sclera: (sclerotic layer). The outer most layer of the eye. Is
white in colour and is formed of tough non- elastic connective tissue to
protect and maintain the shape of the eye.
(v)
Tear glands: (lachrymal glands): they are under the top eye lid.
They secrete tears which keep the surface of the eye ball moist and wipe away
the dust and other foreign materials when blinking. They also contain Lysozyme.
An enzyme that kills bacteria. Excess
tears drain through the lachrymal duct and released through
the nasal
cavity.
(vi)
Cornea: is the transparent front part of the eye (is the
continuation of the sclera). It refracts or bends light rays.
(vii) Conjunctiva: Is a transparent membrane (epithelium) which cover
and protect the cornea.
(viii) Choroid:
Is a heavily pigmented layer beneath the sclera. It contains blood vessels to
supply the eye with oxygen and nutrients. It has black pigment to absorb
scattered light. Thus reducing reflection of light within the inner eye.
Choroid extends to the front of the eye to form ciliary body and iris.
(ix)
Ciliary body: Are the elastic muscles (containing blood vessels)
which relax and contract changing the shape of the lens. This allows vertebrate
to focus both near and distance objects i.e. accommodation.
(x)
Iris: A pigmented circular and radial muscles which
regulate the amount of light entering the inner eye by changing the size of the
pupil. This adaptation help adjust the iris to vision in bright and dim light.
The iris determine the colour of the eye.
(xi)
Pupil: Is a hole or opening of the iris which allows light
to enter the eye.
(xii) Lens: Is a
transparent biconvex structure filled with jelly like substance. It is held in
place by suspensory ligament. The lens help the eye to focus for the different
substances(near or far), a process called accommodation.
(xiii) Suspensory ligaments: Are fibers which hold the lens in position by
attaching it to the ciliary muscles.
(xiv) Retina:
Is the inner most part of the eye that contain light sensitive cells
(photoreceptors). It has two types of cells: cones and rods.
(a) cones: sensitive to white colour and function in bright
light. It contains iodopsin, adapted
for bright light and colour vision.
(b) rods: sensitive in black colour and function in dim light.
It contains rhodopsin, adapted for
low illumination and does not perceive colours. Cones are less sensitive than
rods.
(xv) Fovea:
special region of the Retina with the highest concentration of cones, where
most of the light is focused, and is the point of best seeing. Is also
called Yellow Spot.
(xvi) Blind Spot:
A spot in the retina where all sensory neurones enter the optic nerve. Here are
no sensory cells (no cones or rods). So image falling on the blind spot can not
be perceived by the brain (no image or picture can be made.)
(xvii) Optic nerve: The nerve consists of many sensory neurones transmitting nerve impulse
from the retina to the brain.
(xviii)Anterior chamber: Is the space between cornea and lens. It contains
watery fluid called aqueous humour which supply nutrients and oxygen to the lens
and the cornea.
(xix) Posterior chamber: Is the space between the lens and the retina. It
contains jelly like transparent material called vitreous humour, which
maintain the round shape of the eye.
(xx) Eye muscles: Are attached to the sclera and the bony orbit. Their contractions
serve to rotate the eye ball in various directions.
Comparison between mammalian eye and camera
Mammalian eye
|
Camera
|
function
|
Iris
|
Diaphragm
|
Regulates amount of light.
|
Lens
|
Convex lens
|
Focuses light.
|
Retina
|
Sensitive film
|
Formation of image.
|
Lens
changes in Thickness
|
Lens move back and forward
|
accommodation (adjustment
to different for focuses)
|
Choroid
|
Black surface
|
Prevents internal
reflection of light.
|
Mechanism of image formation (Seeing)
When
light rays pass from one medium to another of different densities are bent, ie.
Refracted. Light rays from external objects enter the eye; they pass through
cornea, aqueous humour, pupil, lens, vitreous humour and finally to the fovea
centralis of the retina, where the image is recorded as real, upside down,
smaller than the object. All these media have different densities and refract
light rays focusing them on the fovea centralis of the retina.
The
inverted image (upside down and smaller) on the retina stimulates receptor
cells and nerve impulse is transmitted to the right and left eye, the brain
receives the impression of different distances. The brain gives a sensation of
object’s nature, colour, brightness, distance and direction. In our impression
the objects ‘’Seem to be upright’’
Accommodation of the Eye (Focusing)
Accommodation
is the ability of the eye to focus both near and distant objects. It is
accomplished by a change in the shape of the lens.
Focusing
a distant object; The ciliary
muscles relax while the tension of the suspensory ligaments is increased. This
decreases the curvature of the lens. The lens become thin (flattened shape), thus
allowing light rays from a distant object to be focused into the retina.
Focusing
a near object; The ciliary
muscles contract while the tension of suspensory ligaments is decreased
(relaxed). This increase the curvature of the lens. The lens become thick
(convex). This allows light rays from a near object to focused into the retina.
Amount
of light; The Iris controls
amount of light entering the eye.
-In
bright
light, the circular muscles of the iris contract. Thus the iris expand hence
the pupil becomes smaller.
-In
dim
light, the radial muscles of the iris expand, thus the iris contact hence
increasing size of pupil which allow more light to inter.
These reflex actions protect the retina from damage by
excessive light and improve visibility in dim light. The size of pupil also
reduce when viewing near objects and enlarge when viewing distant (far)
objects.
Defects of mammalian eye.
Are
structural deviations of the eye which alter (change) the focusing mechanism of
the eye.
1. Myopia: Short sightedness or near sighted. Person can not focus distant object properly. Can
only focus near objects properly. This is because the light rays of a distant
object converge at a point in-front of the retina because the eyeball is too
long. Distant object appear blurred, ie. Not clear.
-Is
corrected by spectacles with concave (diverging) lenses. The
lenses diverge the light rays before reaching the eye enabling them to be
focused on the retina.
2. Hypermetropia: Long sightedness or far sighted. Person can not focus near objects properly. Can only
focus distant or far objects properly. This is because the light rays of a near
object converge at a point behind the retina because the eye ball is too short.
Near object appear blurred. He will put book too far from the eyes when
reading.
-Is
corrected by spectacles with convex (converging) lenses. Convex lenses converge
the light rays before they reach the eye.
3. Presbyopia.
Old
age problem. Is a defect caused by loss of flexibility or elasticity of
the lens and weakening of ciliary muscles. Person can not focus near object
properly. It affects people of about forty (40-45) onwards.
-Can
be corrected by using spectacles with bifocal lenses.
4. Astigmatism. Is caused by unequal curvature of cornea or lens which produce unequal
refraction of light rays entering the eye, hence images are not properly
focused on the retina.
-Can
be corrected by spectacles with special cylindrical lenses.
5. Squintedness. It affects the paired rectus muscles which turn eyeball up and down
and the oblique muscles which moves the eyeball left and right. Focusing and
accommodation are achieved with difficulties, because the eyeballs face
different directions
-It
is difficult to correct.
6. Colour blindness. Is a genetic disorder in which certain colour can not
be distinguished by human being. Common type is Red-green colour blindness
where person can not distinguish between red and green colours. Is due to
defect in genes that control red and green colours.
7. Claucoma.
Is common in old people. Is caused by pressure in the eye. Individual suffers
from blockage in the eye where aqueous humour builds up and distorts the shape,
leading to the blurred images.
Diseases of the Eye.
1. Nigh blindness. The disease is characterized by inability to see clearly in dim light.
Is caused by lack of vitamin A in the diet or poor absorption in the intestine
of vitamin A, which leads to decrease in rhodopsin and retinal.
-Condition
is reversed by good intake of diet rich in vitamin A or administration of
vitamin A.
2. Cataracts.
Is a worm disease, in which worms invade the lens. The lens become opaque
preventing light to pass through easily, hence one can not see properly.
-It
is curable in early stage.
-In
chronic stage: Surgical removal of the lens and replacing it by a plastic lens
inside the eye or a good lens from a donor. Or: Using spectacles with strong
lens that can take the role of natural lens after removing the damaged one from
the eye.
3. Conjunctivitis. Is the inflammation of conjunctiva (Transparent membrane which covers
and protect cornea.) is caused by micro-organism, dusts particles and smoke.
-It
can be treated using antibiotics like Tetracycline.
4. Trachoma.
Is a viral disease, which affects the lining of the eyelids. If not treated it
can cause blindness. It is a highly contagious disease.
-Is
treated by using broad spectrum antibiotics.
5. Xerophthalmia. Is caused by lack of vitamin A. The cornea becomes dry and scaly and
thick.
-Is
treated by taking diet rich in vitamin A.
6. Exophthalmia. Is a situation where by eyeball protrude. Is usually caused by
secondary infections.
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