3. Reception, response and co-ordination.pdf

3. RECEPTION, RESPONSE
AND CO-ORDINATION IN PLANTS AND
ANIMALS.
Definition of terms.
1. Reception- Art of receiving signals of
changes through various organs.
2. Stimulus (plural-stimuli)- this is a
condition in the environment which
produces a change in the activity of part or
the whole organism.
3. External stimuli-condition in the
external environment (surrounding of the
whole organism) which produces a change
in the activity of part or the whole
organism e.g. temperature, light, pH,
humidity, mineral salt concentration,
population density, absence/ presence of
predators.
4. Internal stimuli-condition in the
internal environment (surrounding
of the cells) which produces a
change in the activity of part or the
whole organism e.g. accumulation
of wastes, concentration of gases,
presence of internal parasites,
changes in temperature and
glucose concentration.
5. Response- change in an activity
of an organism.
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6. Irritability/Responsiveness
/Sensitivity- Ability of an
organism to detect and respond
to changes in the environment/
stimulus.
7. Receptors –parts of the body
which receive the stimulus.
8. Effectors- parts of the body
which bring about response.
9. Co-ordination- This is the
working together of different
parts of the body in an orderly
manner to bring about better
performance of physiological,
physical or metabolic actions of
the body.
Types of responses.
A. Tropisms/tropic
responses/movements.
B. Taxis/tactic
C. Nastisms/nastic
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A. Tropic responses/ tropisms.
 Tropism refers to a growth
curvature in response to
unidirectional external stimulus.
Tropic responses are controlled by
auxins hence they are usually slow.
 Growth response towards a stimulus
is called positive tropism while
growth response away from the
stimulus is called negative
tropism.
Types of tropisms/tropic
responses
1. Phototropism- this is growth
unidirectional light.
• Growth towards light is called
positive phototropism e.g.
shoots are positively phototropic.
Growth away from light is called
negative phototropism e.g.
roots are negatively phototropic.
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2. Chemotropism- this is a
growth curvature in response
to chemical concentration e.g.
growth of pollen tube towards
chemical secreted by the
embryo sac hence it is said to
be positively chemotropic.
3. Geotropism/gravitotropi
sm- this is growth curvature
in response to gravity e.g. root
tips/radicles are positively
geotropic while shoot
tips/plumules are negatively
geotropic.
4. Hydrotropism- this is
growth curvature in response
to water/moisture e.g. roots
are positively hydrotropic.
5. Haptotropism/thigmotro
pism- this is growth
contact e.g. tendrils are
positively thigmo/haptotropic
and roots are negatively
thigmo/haptotropic.
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Survival values/ importance
of tropisms.
1. Phototropism exposes the leaves
in position for maximum light
absorption for photosynthesis.
2. Hydrotropism enables the plant
roots to seek for water.
3. Thigmo/haptotropism enables
plants with weak stems
(herbaceous plants) to get support
so as to expose leaves to
photosynthesis, flowers to
pollination and fruits/ seeds for
dispersal.
4. Gravito/geotropism enables
plant roots to grow deep into
the soil thus providing firm
support/anchorage in the soil
and enables the shoot to grow
upwards so as to expose leaves
to sunlight for photosynthesis.
5. Chemotropism enables the
growth of pollen tube towards
the embryo sac facilitating
fertilization.
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B. Taxis/Tactic
 It refers to the movement of the whole
organism/motile/mobile gamete in
response to a unidirectional external
stimulus. It is common in animals.
 Movement towards the stimulus is called
positive taxis while movement away
from the stimulus is called negative
taxis.
Types of taxes.
1. Phototaxis-this is response to
variation in light intensity and
direction e.g. moth, euglena,
chlamydomonas, spirogyra and
fruit flies move toward light
hence are positively phototactic.
Earthworms, maggots, termites
move away from light hence are
negatively phototactic.
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2. Chemotaxis- this is response to
variation in chemicals e.g. male gamete
swim towards the chemicals secreted by
the ovum hence positively chemotactic.
Mosquitoes fly away from insect
repellants hence negatively
chemotactic.
3. Aerotaxis- this is response to variation
in oxygen concentration e.g. amoeba
moves from an area of low oxygen
concentration to an area of high oxygen
concentration hence positive aerotactic.
4. Thermotaxis- this is response
to changes in temperature e.g.
paramecium moves from a region
of lower temperature to a region
of moderate temperature hence
show positive thermotaxis.
5. Osmotaxis- this is response to
changes/ variation in osmotic
pressure e.g. marine crabs
burrow in sand to avoid dilution
of body fluids hence show
negative osmotaxis.
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6. Rheotaxis- this is response to
variation in direction of water and
air currents e.g. fish moves against
water currents hence show
negative rheotaxis. Butterflies and
moths flow into air/wind to
detect the scent of flowers hence
show positive rheotaxis).
7. Hydrotaxis- movement of
whole organism in response to
moisture.
Importance/survival values of
taxes
1. Enable organisms to escape from
harmful stimuli e.g. excessive
heat/ dessication and predators.
2. Enable organisms to seek
favorable habitats and acquire
resources e.g. nutrients and
mates.
3. Chemotaxis enables fertilization
to take place.
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Differences between taxes and tropisms.
1. They are growth responses hence
more permanent
2. They are slow responses.
3. They are brought about by
growth hormones (auxins).
1. They are locomotory responses
therefore are temporary.
2. They are faster.
3. Are not influenced by growth
hormones.
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Tropisms/ tropic responses Taxes/ tactic responses

Similarities between taxis and
tropisms.
i. Both are adaptive responses that
enable organisms to survive
better in their environment.
ii. Both are due to similar external
stimuli e.g. light, water and
temperature.
iii. Both are directional and are due
to unidirectional stimuli.
c. Nastisms/nastic responses.
 These are non-directional
movements of parts of plants in
response to a diffuse stimuli.
 These movements are brought about
by turgor pressure changes in the
leaf and petal changes of certain
plants.
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 The responses include:
i. Folding of leaves in hot weather.
ii. Opening and closing of flowers in response to light
intensity.
iii. Closing of leaves of Mimosa pudica when touched.
Types of nastisms
1. Haptonasty- Movement to touch as in Mimosa
pudica whose leaves close when they or the stem is
touched.
 Also in venus fly trap (Dionaea) which is an
insectivorous/ carnivorous plant which grows in a
nitrogen deficient soil and gets nutrients by trapping
insect.
 The insect is attracted by colour/scent/ sugary baitsa
and lands on the leaves causing the mid rib cells lose
water causing the leaves to close with spines thus
trapping the insect which digested by proteases
secreted by the insect.
2. Nyctinasty- Movements in response to
light intensity and temperature of day and
night as seen in sunflower.
 Examples of Nyctinasty include:
i. Photonasty- Response to light intensity
(flowers open in the presence of light and
close in absence of light).
ii. Thermonasty- Response to temperature
as in flowers of some plants that open with
increase in temperature and close with
decrease in temperature.
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3. Chemonasty- Response to
presence of certain specific
chemical substances e.g. urea and
ammonium compounds found in
insectivorous plants.
 When the insect is trapped it
provides mechanical stimulus for
the release of those chemicals/
proteases by the insect that digest
the insect.
4. Hydronasty- Response to
changes in humidity as seen in
Dandelion plant whose flowers
close when air is moist.
Importance/survival
value of nastisms.
i. It helps to protect the
delicate parts of the flower.
ii. It helps to reduce
transpiration.
iii. Helps to regulate
temperature.
iv. Helps in obtaining some
limited mineral nutrients.
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Study question
 The diagram below represents a
set up during an experiment.
a) Name the type of response investigated.
 Hydrotaxis.
b) What was the experiment investigating?
 How ants respond to moisture/water.
c) State the likely identity for substance K.
 Drying/ dehydrating agent/ silica gel/
anhydrous calcium chloride.
d) Explain your answer in c) above.
 The colour of cobalt (II) chloride remained
blue/ there was no moisture/ water in the
flask to change the colour of cobalt (II)
chloride.
e) Account for the observation made in flask 2.
 More ants moved/ were attracted into the
flask because of the presence of moisture/
water vapour.
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Co-ordination in plants.
 Plant growth responses are co-
ordinated by plat hormones e.g.
auxins, gibberelins, cytokinins and
florigens.Tropisms are influenced
by auxins.
Role of auxins (e.g. Indole acetic
acid) in tropisms.
 Auxins are produced at the tips of
the roots and shoots (apical
meristems).
 They diffuse away from the tips to
the region of cell elongation where
they bring about growth in the
region of cell elongation.
 The concentration/amount of the
auxins present influence how the
shoots and roots grow.
 The concentration that stimulates
growth in the shoots is higher than
the concentration that influences
growth in the roots.
 In shoots, the high auxin
concentration stimulates faster
growth up to a given optimum
while low auxin concentration has
little effect on the growth.
 In roots, low auxin concentration
stimulates rapid/ faster growth
while high concentration inhibits
growth.
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A. AUXINS AND PHOTOTROPISMS
1. Under uniform light distribution,
auxins are produced at the shoot
tips then translocated evenly
down the shoot
 This leads to equal growth hence
normal increase in height of a shoot
upwards.
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2. When a shoot tip/ plumule is
exposed to unidirectional
light, light stimulus causes
auxins to migrate to the
darker side causing higher
concentration of auxins on
the darker side.
 The higher concentration of
auxins on the darker side
stimulates faster growth (than
lit side) hence a shoot
tip/plumule curves/bends
towards the source of light.
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3. When the shoot tip is cut,
auxins are removed hence
there light cannot cause
unequal distribution of
auxins.
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4. If the shoot apex is covered
with an opaque object, the
tip does not sense light
stimulus because the opaque
object prevents the light
from causing uneven
distribution of auxins.
 The auxins are therefore
distributed evenly at the tip
hence the tip continues to
grow upwards as shown
below.
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5. If the shoot apex is cut and the
synthetic auxins (e.g. agar
block), auxins will diffuse into
shoot tip.
 Light stimulus will cause the
auxins to migrate to the darker
side.
 Higher concentration of auxins
on the darker side will stimulate
faster growth (on the darker
side) hence the shoot tip will
bend/ curve towards the source
of light.
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6. If the seedling is grown in total
darkness, more auxins are formed in
the dark causing faster growth
(elongation).
 The seedling will grow tall, thin, with
weak stem and small pale/ yellow leaves.
This is because seedlings/ plants lack
light hence it fails to form chlorophyll.
 This phenomenon is called etiolation.
Significance of etiolation
 It enables the plants/ seedling to grow
faster to expose leaves to sunlight for
photosynthesis and stems grow taller so
that leaves can search for light.
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Study question
1. In an experiment, students
treated seedlings as illustrated
below.
a) Account for the observations
made in seedling I. (3mks)
 The tip bends towards light/
positive phototropism.Light
causes migration of auxins
produced at the tip to the
darker side of the shoot.This
causes faster division cells/
elongation/ growth on the
darker/ opposite side.
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b) Explain the similarity in the end
results made in seedlings II and
III. (2mks)
 Both seedlings remain upright/
erect.Seedling II does not have a
tip that produces auxins while
in seedling III the tip has been
covered by an opaque material
preventing light from causing
unequal distribution of auxins/
causing equal/ even
distribution of auxins.
c) State the likely treatment that
would make seedlings II and III
respond like seedling I. (2mks)
 Fitting an ager block treated with
auxins at the decapitated/ cut
end of seedling II.
 Removing the opaque material
covering the tip of seedling III/
replacing the opaque cap with a
transparent one.
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B. AUXINS AND GEO/
GRAVITOTROPISM
 If a seedling is placed
horizontally on the ground, the
following observations are
made after few days:
i. The shoot tip (plumule) will
curve/ bend upwards hence
negative geotropism).
ii. The root tip (radicle) will
bend/ curve downwards
(hence positive geotropism).
 This is because the force of
gravity will cause auxins to
migrate to lower side of the
seedling.
 Higher concentration of
auxins on the lower side of
the shoot stimulates faster
growth (than the upper side)
hence the shoot tip bends/
curves upwards.
 Higher concentration of
auxins on the lower side of
the root inhibits growth on
the lower side (than on the
upper side) hence root tip
bends downwards.
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Study question 1
 The photographs below show three
bean seedlings that are of the same
age but were grown under different
environmental conditions.
a) Based on external appearance of the
seedlings, suggest the conditions under which
each of them was grown.
Q-with unidirectional/ one sided light.
R- In full light/ adequate/ enough light.
S- In the dark.
b) State the term used to describe the
phenomenon exhibited by specimen S and
state its significance.
 Etiolation. It enables the plants/ seedling to
grow faster to expose leaves to sunlight for
photosynthesis.
c) Explain how the response in specimen S took place.
 More auxins are synthesized in the dark which
stimulate faster growth.
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Study question 2
 The photographs below show an
experiment that was set up to investigate
a certain response in bean seedlings.
Examine them and answer the questions
that follow.
a) Which response was being
investigated?
 Geotropism/ gravitotropism
b) Account for the observation in
seedlingA after 24 hours.
 Force of gravity caused auxins to
migrate to the lower side of the root
tip.Lower concentration on the upper
side stimulates faster growth hence
the root tip bent/ curved downwards.
c) Explain why the root of seedling B
continued growing straight down.
 Force of gravity was acting uniformly
on the root tip hence there was no
unequal distribution of auxins.
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C. AUXINS ANDTHIGMOTROPISM/
HAPTOTROPISM.
 When a tendril/ a climbing stem
comes into contact with a solid hard
object, contact causes auxins to
migrate from the side of contact to
the opposite side.
 Higher auxin concentration on the
outer/ opposite side stimulates
faster growth than on the point of
contact hence the shoot tip coils/
grows around the support structure.
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STUDY QUESTION
Form 4 students set up an experiment as
shown below using enclosed seedling.
a) What type of response is
exhibited by the seedlings in this
experiment:?
 Geotropism
b) State the observations made in
experiment I and II
 In experiment I the shoot and the root
grow horizontally straight.
 In experiment II, the shoot tip/plumule
bends/curves upwards and the root
tip/radicle bends/curves downwards.
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c) Account for the observations
made in experiment I and II
 In experiment I, the clinostat rotates
causing the auxins to be uniformly
distributed in the shoot and root
tips.
 This stimulates uniform growth in
the root and the shoot.
 In experiment II, gravity causes
auxins migrated to the lower side of
the root and shoot tips.
 A higher concentration of auxins on
the lower side of the shoot tip
stimulates faster growth on the
lower side hence the shoot tip/
plumule bends/ curves upwards.
 Higher concentration of auxins on
the lower side of root tips/ plumule
hinders growth on the lower side
hence the radicle/ root tip curves/
bends downwards.
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d) Give a reason why
seedlings were enclosed.
 Helps to prevent exposure to:
i. Wind which may increase the
rate of water loss.
ii. Wind or other external factors
which may shift the alignment
of plant seedlings.
iii. Pests and other insects which
may damage the seedlings thus
interfere with the responses.
3. Study the diagram below and
answer that questions that
follow.
a) Name the type of germination
shown above.
Epigeal germination.
b) Give a reason for your answer.
The cotyledon is brought
above the soil.
c) Name the type of response
exhibited in diagram a.
Positive geotropism.
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d) Explain the response named
above.
 Gravity pulls the auxins
downwards. Higher
concentration of auxins on
the lower side
hinders/inhibits growth on
the lower side than on the
upper side.This causes the
hypocotyl to curve/bend
downwards.
RECEPTION, RESPONSE AND CO-
ORDINATION IN ANIMALS.
 Animals detect changes in the
external and internal environment/
receive the stimulus through
receptor organs that have special
cells called receptor/ receptor
cells.
 Receptor organs include: Eyes,
Ears, Nose,Tongue, Skin and
Hypothalamus
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 The eyes have photoreceptors that
detect light.
 The skin has thermoreceptors that
detect temperature,
mechanoreceptors that detect
pressure.
 Ears have receptors that detect sound,
gravity and orientation of the body.
 Hypothalamus has osmoreceptors
that detect osmotic pressure.
 Response in animals is brought
about by organs called effectors.
 Effectors/effector organs include:
i. Glands e.g. salivary
glands.
ii. Muscles e.g. skeletal
muscles.
 The receptors and effectors are
linked by a co-ordinating centre.
 The co-ordinating centre consists
of:
A. Nervous system.
B. Endocrine system.
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A. NERVOUS SYSTEM
 This is a system consisting of
specialized cells called nerve
cells/neurons for
transmission of electrical
impulses.
 The neurones are linked to one
another and to the receptors and
effectors.
 An electrical impulse is an
electrical signal transmitted
within the nerve cell
fibre/axon.
Parts of a neurone
A. Cell body/centron- has a
cytoplasm enclosing a nucleus.
B. Dendrons- they are extensions
that arise from the cell body and
they transmit impulses
towards the cell body.
C. Axon/nerve fibre- this is the
longest dendron which arises
from the cell body and it
transmits impulses from the
cell body.
D. Dendrites- they are extensions
that arise from the dendrons.
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 The axon contains a cytoplasm
called axoplasm which extends
from the cytoplasm of the cell
body.
 It is also covered with myelin /
medullated sheath- which
insulates it and enhances
the speed of impulse
transmission.
 The myelin sheath is
constricted to form junctions
called nodes of ranvier-
which propagate and
promote faster transmission
of impulse.
 On the outer part of the myelin
sheath are cells called
schwann cells- which give
rise to the myelin sheath.
The schwann cells have a
membrane called neurilemma
N/B. Several axons and dendrons
joined together form a nerve.
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Structural adaptations of the
neurone to enable it carry out its
functions.
1. It has long axon to
transmit/conduct impulses.
2. The axon is enclosed by a myelin
sheath which insulates the axon
and for faster transmission of
impulses.
3. The myelin sheath has junctions
called nodes of ranvier to
enhance the speed of conduction
of the impulse.
4. It has schwann cells for
secretion of myelin sheath.
5. It has numerous dendrites
which receive/ deliver
impulses from neighboring
neurones through synaptic
connections.
6. The cell body has a nucleus
for transmission of impulses.
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TYPES OF NEURONES
1. Sensory/afferent/receptor neurone.
2. Motor/efferent/effector neurone.
3. Relay/intermediate neurone.
1. SENSORY NEURONE/ RECEPTOR
NEURONE/ AFFERENT NEURONE.
 It is joined to the sense organs e.g. eyes, skin,
nose, tongues and carries impulses from the
receptors/receptor cells/ sense organs to the
central nervous system (CNS).
 It has one dendron which arises from the cell
body to form axon hence said to be
unipolar.
 The cell body is situated along but off the
axon and outside the central nervous system
(CNS).
 The dendron transmits impulse from the
receptor to the cell body and the axon
transmits the impulse from the cell body to
the CNS.
 The receptor dendrites are located in the
sense organ and the terminal dendrites are
located in the CNS.The cell body is closer to
the CNS.
 The sensory neurones which transmit
impulses from the internal/visceral organs
e.g. lungs and heart are called visceral
sensory neurones.
 The sensory neurones which transmit
impulses from the skin, skeletal muscles,
joints and bones are called somatic sensory
neurones.
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2. Motor neurone/efferent
neurone.
 It transmits the impulse from
the CNS to the effectors.
 It has two or more dendrons
arising from the cell body hence
said to be multipolar.
 The cell body is located at the
end of the axon (the longest
dendron) but within the CNS.
 The terminal dendrites of the
axon are located within the
effectors e.g. muscles.
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3. Relay neurone
/intermediate
neurone/ interneuron.
 It transmits the impulse from
the sensory neurone to the
motor neurone.
 It lacks the myelin sheath.
 It is found within the CNS.
 It has two dendrons that arise
from the cell body hence said
to be bipolar.
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Sensory
Neuron
Interneuron
Motor
Neuron
Sensory
Neuron
Interneuron
Motor
Neuron
Muscle
Contracts
Synapse
Synapse
Synapse
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Differences between a motor and sensory neurone.
i. Transmits impulses from the
CNS to the effectors.
ii. The cell body is located at the
end of the axon and within the
CNS.
iii. It is multipolar.
i. Transmits impulses from the
receptors to the CNS.
ii. The cell body is located along but
off the axon and outside the
CNS.
iii. It is unipolar.
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Motor neurone Sensory neurone

PARTS/COMPONENTS OF
THE NERVOUS SYSTEM.
 They include:
1. The central nervous system
(CNS).
2. The peripheral nervous
system (PNS).
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1. THE CENTRAL
NERVOUS SYSTEM
(CNS)
 It receives and interprets the
impulses hence called a
coordinating centre.
 It consists of:
A. The brain
B. Spinal cord and
associated nerves
A. THE BRAIN
 This is the anterior part (towards the
head) of the central nervous system.
 It is enclosed by protective membranes called
meninges.
 The meninges include:
i. Dura matter- the outermost layer
covering the brain and spinal cord. It is
composed of connective tissue and rich
network of capillaries.
ii. Pia matter- the innermost layer covering
the brain and spinal cord.
 The infection of the meninges causes
meningitis.
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 The space between the dura
and pia matter is called
Arachnoid layer which is
filled with a fluid called
cerebrospinal fluid which
is secreted by blood.
 It contains oxygen and
nutrients e.g. glucose/blood
sugar, amino acids.
Functions/ roles of the
cerebrospinal fluid.
i. It supplies oxygen and
nutrients to the brain cells.
ii. It removes waste products
from the brain cells.
iii. It maintains pressure of the
brain.
iv. It acts as a shock
absorber/protects the brain
from shock/mechanical
damage.
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PARTS OFTHE BRAIN.
 The brain is made up of two halves
called hemispheres i.e. right and
left hemispheres.
 The two hemispheres are
interconnected by a group of nerves
called corpus callosum.
 The left hemisphere controls the
activities of the right side of the
body while the right hemisphere
controls the activities of the left side
of the body.
 The innermost part of the brain and
spinal cord is called grey matter
and the outer part is called white
matter.
1. Cerebrum-This is the largest part of the brain
it is divided into the left and right hemisphere.
 It has many folds on its outer layer to increase
the surface area for packing of neurones.
 If the plasmodium parasite enters the brain and
affects the cerebrum, it causes cerebral malaria.
Functions/roles.
i. It controls learning, imagination, thinking,
personality, hearing, vision and taste.
ii. It controls voluntary body movements/
controls contraction of skeletal muscles e.g.
walking, dancing, movement of lips/ chewing,
movement of the neck/ nodding and
movement sideways.
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2. Cerebellum- It is found
below and behind the
cerebrum.
Functions.
i. Maintenance of body balance
and posture.
ii. Ensures dexterity in fine
movements e.g. playing a
piano, sewing, typing e.t.c.
3. Medulla oblangata- It is
located below the cerebellum
and connected to the spinal
cord.
Functions.
i. Controls involuntary
responses e.g. breathing,
heartbeat, coughing,
sneezing, swallowing,
vomiting, salivation.
ii. Controls dilation and
constriction of the blood
vessels hence influencing
blood pressure.
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4. Hypothalamus- It is found below the
thalamus and closely associated with the
pituitary gland.
 It controls the autonomic nervous system
which transmits impulses from the internal
organs.
Functions.
i. Controls appetite, sleep, wakefulness.
ii. It controls homeostasis (thermoregulation,
osmo-regulation)
5. Thalamus- It integrates the sensory impulses
from the eyes, ears and the skin and then sends
the them to the cerebrum.
Function.
 It contains receptors for pain and pleasure.
6. Pituitary gland- It is attached to the
hypothalamus and projects downwards.
Functions/ roles
i) It is the master gland and secretes various
hormones e.g. oxytocin, prolactin, follicle
stimulating hormone, luteinizing hormone.
ii) Controls other endocrine glands.
7. Pons- They work together with the medulla
oblangata to control involuntary activities.
8. Corpus callosum- It is composed of the
axons/ nerve cells that connect the left and
right hemispheres of the brain.
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Cerebrum

brain
Spinal Cord
Cerebellum
Cerebrum
Medulla Oblongata
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Cerebrum
Cerebellum
Medulla
oblangata
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B. SPINAL CORD
 This is the posterior/back
extension of the CNS from the
brain to the tail.
 It is enclosed by the meninges
that protect it against mechanical
damage.
 It is also protected by the
vertebral column/back
bone.
 It is made of the grey matter the
white matter.
 There is a narrow canal called
the central canal which runs
down the spinal cord.
 The central canal is filled with
cerebrospinal fluid.
 The grey matter is H-shaped and
is surrounded by the white
matter.
 It is grey/dark in colour because
it consists of the cell bodies of
the motor and relay neurones,
dendrites and synapses.
Functions of spinal cord.
i. Coordinates some automatic
responses/reflex actions.
ii. It links the nerves of the
peripheral nervous system
and the brain.
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 The white matter
surrounds the grey matter
and consists of the axons of
the sensory and motor
neurones.
 The myelin sheaths/
medulated sheath/
myelinated sheath of
neurons/ axons which are
made up of fats that make it a
shiny appearance.
SYNAPSE
 This is a gap between two
adjacent neurones
 The substance that transmits
the impulse within the
synapse/from one synapse to
the other is called
acetylcholine which is
broken down by enzyme
acetylcholinesterase/
cholinesterase/
acetylhydrolase into acetic
acid and choline.
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Cell body
Synapse
Synapse
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2. PERIPHERAL NERVOUS
SYSTEM (PNS)
 This is the system of nerves that
connect the CNS and other parts
of the body.
 It consists of Cranial nerves and
Spinal nerves.
1. Cranial nerves- They
transmit impulses from the
sense organs and muscles of
the head and neck.They arise
from the brain and innervate
organs such as ears and
eyes. Examples of such
nerves include:
i. Optic nerves- which innervate
the eyes.
ii. Olfatory nerves- which
innervate the nose.
iii. Auditory nerves- which
innervate the ears.
2. Spinal nerves- They are nerves
that arise from the spinal cord and
innervate the surrounding parts
of the body such as hands, legs,
ribs, abdomen e.t.c.
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Parts/ components of the
peripheral nervous system (PNS)
a) Voluntary (somatic) nervous
system- It consists of nerves
that control voluntary activities
of the body e.g. raising of the leg,
nodding the head e.t.c.
b) Involuntary (autonomic)
nervous system- It consists of
nerves that control involuntary
activities of the body e.g. beating
of the heart, secretion of
hormones and enzymes, gut
movements e.t.c.
Parts/ components of the
involuntary/ autonomic
nervous system.
1. Parasympathetic nervous
system- Its stimulation is aimed
at energy conservation activities
e.g. Slowing of the heart beat,
Stimulation of the digestive
tract/gut, Decreased force of
muscle contraction.
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2. Sympathetic nervous system-
Its stimulation produces effects
that prepare the animal for
emergency e.g. Increased
heartbeat rate to supply enough
blood containing oxygen to the
muscles and remove carbon (IV)
oxide from the muscles, Increased
blood pressure for faster
movement of blood, Dilation of
the pupil to locate the enemy,
Closing of the sphincter muscles
of the anus and bladder and
conversion of glycogen into
glucose in the liver.
REFLEX ACTION.
 This is a rapid/ faster and
automatic/ involuntary response to a
stimulus.
 It is processed in the spinal cord not
in the brain.
 It helps protect the body
before the brain knows what is
going on.
Types of reflex action
a) Simple reflex action.
b) Conditioned reflex action.
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A. SIMPLE REFLEX ACTION.
 Simple reflex actions e.g. withdrawal
of finger from a hot or sharp object, is
an automatic response to a specific
stimulus.
 During simple reflex action, an
impulse passes through a reflex arc.
 A reflex arc refers to the path taken
by the impulse during a reflex action.
 A reflex arc consists of sensory,
intermediate and motor neurons.
 When a finger touches a sharp/hot
object, the stimulus/ heat/ pain is
detected by temperature/
thermoreceptors/ pain receptors in
the skin/ dermis.
 An impulse is generated which is
transmitted along the sensory neurone
to the grey matter/ central nervous
system/ spinal cord.
 The chemical transmission of impulse
is done across a synapse to relay
neurone and through another synapse
to motor neurone and across another
synapse to muscles/ effectors/ biceps
muscles.
 The biceps muscles contract and the
hand is withdrawn.
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B. CONDITIONED REFLEX
ACTION
 Conditional reflex actions e.g.
salivation of a dog/humans/students
in response to sound, is an automatic
response evoked from an animal by
unrelated stimulus, substituted for
the one which normally elicits the
response.
 It develops from past experience,
and involves modification of
behaviour/learning.
 It weakens with time, and must be
reinforced by repeating the related
original/primary stimulus.
 Dogs/Students salivate when the
bell for meals rings because they
have learned to associate the ringing
of the bell at meal time with food,
every time it rings, they are offered
food.
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Differences between simple reflex action and conditioned
reflex action.
1. It is not learned/ it is
inborn/ requires no
experience.
2. Primary/ related stimulus is
involved.
3. Impulse follows appropriate
sensory and motor neurone
or sensory and motor
components are the same all
times.
4. A single stimulus brings
about a desired response.
1. It is learned/ requires
experience.
2. Secondary/ unrelated stimulus
is involved.
3. Impulse follows inappropriate
motor neurone or primary
sensory component is replaced
by secondary component but
the motor component remains
unchanged.
4. Repeated stimulus is needed to
bring about response.
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Simple reflex action Conditioned reflex action

Transmission of the nerve impulse.
 A nerve impulse is an electrical wave
arising from changes in ionic
concentration across the membrane of the
nerve fibre/axon.
 The metallic ions involved are sodium ions
(Na+) and potassium ions (K+). It occurs
in two phases:
(i) Resting phase/potential- It is the
non-conducting phase in the nerve
fibre.
 Sodium (Na+) ions which are more are
on the outside the nerve fibre/
axoplasm than potassium [K + ) ions
which are more inside the nerve fibre.
 There are also more negative charges
(anions) within the axoplasm.
 As a result the outer side of nerve fibre
acquires a relative positive charge while the
inner side acquires a relative negative
charge.
 The nerve fibre is said to be polarized.
 During resting potential Sodium ions are
actively pumped out while potasium ions
diffuse freely in and out of the axoplasm.
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(ii) Action phase/potential-The
membrane becomes permeable to the sodium
(Na+) ions that diffuse inwards and the
potassium {K+) ions diffuse outwards.
 Also negative ions diffuse out of the
axoplasm and the inside becomes
positively charged while the outside
becomes negatively charged hence the
nerve fibre is depolarized.
 An action potential is in form of a wave
along the nerve fibre. It takes place in
milli seconds.
Transmission of the impulse across a
synapse/ neuro-Junction.
 A synapse- is a the junction between
the dendrites of two adjacent neurones.
Function- allows transmission of nerve
impulses from neurone to neurone.
 When an impulse reaches the synaptic
knob, it causes the synaptic vesicles to
move to the pre-synaptic membrane.
 The vesicles attach to the membrane
and discharge the transmitter substance
called acetylcholine into the synaptic
cleft.
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Sodium ions
Potassium ions

 The acetylcholine diffuses across the
membrane and attaches to the post-
synaptic membrane. Here it causes
depolarization of ions (sodium and
potassium ions).
 Sodium ions move from the cleft to
the post synaptic knob through the
post- synaptic membrane causing
an action potential and the nerve
impulse is generated in the adjacent
neuron.
 An enzyme Cholinesterase/
acetylcholinesterase/ acetyl hydrolase
present at the synapse breaks down
acetylcholine into acetic acid/ ethanoic
acid/ vinegar and choline.
 These substances are absorbed actively into the
knob and used to re-synthesize
acetylcholine using energy in form of ATP.
Significance/ importance of breakdown of
acetylcholine.
 To re-polarise the pre-synaptic membrane for
the next impulse propagation/ transmission to
avoid merging/ joining successive nerve
impulse from neurone to neurone.
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Synaptic vesicles
Synaptic cleft
Pre-synaptic
membrane
Post-synaptic
membrane
Synaptic knob
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B. ENDOCRINE SYSTEM
 It consists of glands which are
ductless that produce hormones.
 Hormones are organic compounds
which are either protein or steroids
in nature secreted by ductless glands
and shed into the blood stream.
 They are produced in small
quantities and transported by blood
to other parts of the body/ target
organs where they cause a response.
 In animals , hormones regulate
growth and development, control
the behavior during breeding and
control proper performance of cells.
 Secretion of hormones is controlled
by the nervous system.
 The main glands include:
A. Pituitary gland.
B. Adrenal gland.
C. Thyroid gland.
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A. PITUITARY GLAND
 It secretes the following hormones:
1. Thyroid stimulating hormone
(thyrotropin)- which stimulates the
thyroid gland to secrete thyroxine
hormone.
2. Gonadotropins e.g. FSH and LH.
3. Adrenocorticotropic hormone
(ACTH)-which stimulates the
adrenal cortex to produce
adrenaline.
4. Growth hormone
(somatotrophin).
5. Antidiuretic hormone-
which enhances the re-
absorption of water in the
kidney tubules.
6. Oxytocin- which stimulates the
contraction of muscles of uterus
during birth and muscles of milk
ducts to allow milk let-down.
7. Prolactin-stimulates milk
production and is responsible for
maternal instinct.
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B. ADRENAL GLANDS.
 It is located above kidneys. It
consists of adrenal cortex (on the
outside and adrenal medulla (on
the inside).
 Adrenal cortex secretes
aldosterone- which stimulates the
reabsorption of sodium ions.
 Adrenal medulla receives nerve
impulse from the brain and secretes
adrenaline.
 Adrenaline is secreted during
anxiety, excitement/emergency
hence called fight/flight
hormone.
Effects of adrenaline.
1. Increases heart beat, increasing the rate of
blood circulation.
2. Stimulates the conversion of glycogen to
glucose in the liver hence increasing blood
sugar level.
3. It increases metabolic rate.
4. It causes constriction of arterioles to the skin
and digestive system.
5. Breathing rate becomes faster and deeper.
6. Fats are converted into fatty acids which are
made available for muscle contraction.
7. It stimulates the skeletal muscles to contract
and relax causing movement.
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Effects of over secretion of
adrenaline.
1. Hypertension/high blood
pressure.
2. Severe headache.
3. Racing heart.
4. Sweating.
5. Faintness.
6. Aging of major body organs e.g.
kidney, heart and liver.
Effects of undersecretion/
underproduction of adrenaline.
1. Reduced metabolism.
2. Decreased heart beat rate.
3. Reduced rate of breathing.
4. Reduced activity of the animal.
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C. THYROID GLAND
 It is stimulated by thyroid
stimulating hormone to secrete
thyroxine.
 Thyroxine is a complex organic
compound that contains iodine.
Functions of thyroxine.
1. Controls basal metabolic rate by
increasing glucose oxidation.
2. Enhances the effect of growth
hormone (somatotrophin) ensuring
normal growth and mental
development.
3. Works together with adrenaline to
control involuntary activities e.g.
increased blood circulation.
Effects of under-secretion of thyroxine
/hypothyroidism.
 Under-secretion of thyroxine may be due to
insufficient iodine in the diet or defective
enzymatic reactions concerned with iodine
production. Iodine is a component of
thyroxine.
 Under-secretion leads to Cretinism in
children and Myxoedema in adults.
1. Cretin children have the following
characteristics:
i. Poor mental development/ low intelligence.
ii. Deformed legs.
iii. Dry leathery skin.
iv. General body sluggishness.
v. Large tongue.
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2. Myxoedema causes goitre / swelling of
thyroid gland.This is due to overworking
of thyroid gland in an attempt to
synthesize enough thyroxine.
3. Reduced metabolic rate shown by
reduced heart beat, reduced breathing
rate and low body temperature.
4. Individuals are mentally and physically
sluggish which can lead to obesity and
oedema (swelling of feet).
 Under-secretion is controlled by use of
balanced diet supplemented by iodinised
salt and administration of iodine tablets.
Effects of over-secretion of
thyroxine/ hyperthyroidism
• Overproduction of thyroxine
may result from defective
enzymatic activity.
i. Increased body temperature.
ii. Increased breathing rate.
iii. Increased body metabolism
leading to body weight loss.
iv. Excessive energy production.
v. Extreme can lead to heart failure
(thyrotoxicosis).
• It is controlled by surgical
removal of a part of thyroid gland
and use of antithyroid drugs.
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Functional differences between nervous and endocrine system.
1. Uses hormones to bring about
response.
2. Hormones are transported in
blood.
3. Responses are slow and affect
several parts of the body.
4. Effects are long-lasting.
5. Responses take place
involuntarily.
1. Uses electrical impulses to bring about
response.
2. Impulses are transported through
neurons.
3. Responses are quick, specific and
localised.
4. Effects short-lived.
5. Responses take place voluntarily and
involuntarily.
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Endocrine system. Nervous system.

Similarities between endocrine
and nervous system.
1. Both provide a means of
communication and
coordination within the body of
an organism.
2. Both involve response to a
stimulus.
3. Both bring about survival
response.
4. In both chemical transmission is
involved.
5. The target organs of hormones
are like the effector organs.
DRUG ABUSE
 A drug- this is a chemical substance
which when taken has psychological
and physiological effects in the body.
Drugs taken to cure diseases are
called medicinal drugs.
 Drug addiction-This is the state
of being mentally and physically
dependent on drugs.Addiction
requires increase in dosage to
experience the desired feelings.
 Drug abuse- This refers to use of
the drug for the purpose not meant
for it to bring about
physical/physiological effects which
are not curative.
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Classification of drugs.
A. Hallucinations.
B. Stimulants.
C. Depressants.
D. Narcotics.
E. Inhalants.
A. Hallucinations
 They cause one to have hallucinations i.e.
to see something imagined/not present.
 Examples include:
a) Bhang (Cannabis sativa).
1. Causes euphoria or pleasurable
relaxation
2. Causes depressed hearing, lowered eye
and skin sensation.
3. Causes acute pain and anxiety.
4. Causes depressed mental ability.
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b) Lysergic acid diethylamide.
1. causes hallucinations and dilated
pupil
2. Leads to increased body
temperature, heart beat, blood
pressure and sweating
3. Causes lack of appetite and sleep
4. Causes anxiety and helplessness
5. Causes mental confusion and
impaired memory
6. It causes dry mouth and muscle
shaking.
c) Miraa (khat).
1. Causes increased blood pressure,
body temperature and respiratory
rate.
2. Reduces sexual potency.
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B. Stimulants- They stimulate the central
nervous system i.e. causes the CNS to
increase the rate of chemical
reactions/physiological activities of the
body
 They include:
a) Cocaine
1. Causes increased heart beat.
2. Causes vomiting.
3. Causes convulsions and respiratory
failures.
b) Amphetamines.
1. Causes euphoria (pleasurable
relaxation).
2. Causes hallucinations.
3. Causes panic.
C. Depressants- They suppress the
CNS causing decreased rate of
physiological activities of the body.
 They include:
a) Alcohol
1. Causes impaired physical and
mental functioning of the body.
2. Can cause brain damage.
3. It causes poor judgment, memory
and perception.
4. Can cause liver cirrhosis hence
liver destruction.
5. Increased incidences of cancer and
heart attack.
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D. Narcotics- They are drugs used to
cause insensible condition in the body.
They can be used in surgical operations.
 They include:
a) Opium.
1. It depresses the nervous system.
2. Can cause confusion and convulsions.
b) Morphine.
1. Causes restlessness and sleeplessness.
2. Can cause watery eyes and running nose.
3. Can cause nausea (feeling of sickness in
the stomach).
4. Can cause diarrhoea and vomiting.
5. Causes raised body temperature
(fever) and respiration.
6. Can cause abdominal and muscle
cramps.
7. Can cause dehydration and loss of
body weight.
E. Inhalant- They are inhaled.
 Example is glue which:
1. Damages the nervous system,
kidney and liver.
2. Reduces mental and physical
capabilities.
3. Causes fatigue and weight loss.
4. Can cause excitation.
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GENERAL EFFECTS OF DRUG ABUSE
1. Reduced appetite and poor feeding
leading to emaciation.
2. Poor absorption of vitamin K and E
leading to sterility and blindness.
3. Lowered nervous co-ordination leading
to loss of posture and balance hence
poor performance in sports and manual
activity.
4. Irritation of lungs and respiratory tracts
leading to frequent coughs and
infections.
5. It may lead to cancer of lungs, throat and
urinary bladder.
6. It may lead to stomach ulcers.
7. Damage to the heart and liver leading to
heart attack and liver cirrhosis.
8. Interference with temperature
regulation leading to excessive heat loss.
9. Damage to the brain causing
sleeplessness (insomnia), hallucinations,
madness and loss of memory (amnesia).
10. Poor foetal development and pregnancy
complications in women.
11. Damage to vital body organs leading to
death.
12. Impaired/ poor judgment leading to
accidents and infections e.g. HIV/AIDS
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 Effects of drugs on the society
include:
1. Marital conflict.
2. Violence.
3. Theft to sustain the habit.
4. Reduced productivity at
work.
5. Loss of jobs/income.
6. Misuse of family
resources.
Study questions
1. Explain how drug abuse exposes one to
the dangers of HIV/AIDS infection.
Answers.
 Drugs impair reasoning hence heightening sexual
arousal.
 Drugs impair judgment leading to indiscriminate
sex.
 Impaired judgment leads to engaging in
unprotected sex.
 Intoxicated persons are likely to engage in sexual
perversions e.g. rape and homosexuality.
 Intoxicated persons may lose strength and become
victims of sexual abuse.
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2. Discuss the socio- economic
problems associated with drug
abuse.
Answers
a) At family:
 Wastage of family resources and theft
to acquire drugs.
 Lack of family care including poor
care of children and spouses.
 Family break-up due to tensions,
poverty and lack of love.
b) In schools:
 Lateness, indecent behavior, poor
study habits and failure in
examinations.
 Violence against students, teachers
and community.
 Riots and strikes.
c) Increased crime.
d) Premature termination of careers
and other professions
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SENSE ORGANS.
A. THE EYE
 This is a sense organ for sight.
 It is spherical in shape and
located within the socket in the
orbit found within the skull.
 It has photoreceptors stimulated
by light from the object.
 The image of the object is
formed inside the eye,
transmitted to the brain as an
electrical impulse through the
optic nerve where it is
interpreted.
External structure.
 It consists of the following parts:
1. Eyelids.
Adaptation to function.
i. They are made of folds of skin
covering the eye ball protecting
it against mechanical /physical
damage.
ii. Have sweat glands and
sebaceous gland for oily
secretions that keep the eye
moist.
iii. Have eye lashes that trap dust
particles preventing them from
entering the eye.
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2. Tear/Lachrymal glands.
Adaptation to function-
They secrete tears that clean,
moisten, lubricate the eye,
wash foreign materials out of
the eye.Tears also have
lysozyme hormone that
protect the eye against
infections / bacteria/diseases.
3. Eyebrows- are thickly
covered with hair to prevent
sweat and dust from
entering the eye.
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Internal parts of the eye.
1. Conjunctiva
2. Sclera/sclerotic layer
3. Cornea
4. Choroid layer
5. Retina
6. Yellow spot
7. Blind spot
8. Optic nerve
9. Lens
10. Ciliary body
11. Suspensory ligaments
12. Iris
13. Pupil
14. Aqueous humour
15. Vitreous humour
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1. Conjunctiva.
 It is a thin epithelium to protect the cornea.
 It is transparent to allow light pass through.
 It is glandular/ has goblet cells which secrete
mucus for lubrication.
2. Sclerotic layer/sclera layer- It
contains tough fibres/ inelastic collagen
fibres/ it is fibrous to maintain the shape
of the eyeball and protect inner parts of
the eye/ eye ball.
3. Cornea.
 It is transparent to allow light pass through.
 It is curved to refract/ bend light rays onto
the retina.
4. Choroid layer.
 It is rich in blood vessels/ highly
vascularized/ has numerous capillaries
to supply the retina/ eye with nutrients
and oxygen (nourish the eye) and
remove metabolic wastes.
 It has pigmented/ black cells to prevent
reflection of light in the eye
5. Retina- It is the innermost layer of
the eye ball.
 It contains photoreceptors/ cones and
rods to perceive light/ where the image
is formed/ focused/ image formation.
 It has cones for colour vision, bright
light vision, colour perception and rods
for dim light vision.
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6. Fovea centralis-Has numerous
cones only for accurate vision/
visual acuity.
7. Blind spot- It lacks
photoreceptors / cones and rods
to allow passage of blood vessels
and for exit of optic nerve.
8. Optic nerve-it has sensory
neurons/ nerve cells that transmit
impulse to the brain
9. Lens.
 It is biconvex to refract/ focus light
rays onto the retina.
 It is transparent to allow light pass
through.
 It is elastic to allow adjustment of
the shape of lens.
10. Ciliary body.
 It is made up of ciliary muscles that
contract and relax/ are contractile
to change the shape of the lens
during accommodation.
 It is glandular/has secretory cells to
produce/ secrete aqueous humour.
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11. Suspensory ligaments- They
are inelastic and hold the lens in
position to the ciliary body.
12. Iris- This is the coloured part of
the eye.
 It has radial and circular muscles to
control the size of the pupil.
 It has melanin to absorb strong light
and prevent blurred vision.
13. Pupil-This is an opening on the
iris through which light passes
into the eye.
14. Aqueous humour- It the fluid
found at the front chamber of
the eye
 It is jelly like which maintains the
shape of the eye ball.
 It contains oxygen and nutrients
for the cornea and the lens.
 It is transparent to allow light to
pass through and refract it onto
the retina.
15. Vitreous humour- This is a
clear fluid at the back of the eye.
 It is jelly-like to maintain the shape
of the eye ball.
 It is transparent to allow light to
pass through and refract it onto the
retina.
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PHOTORECEPTORS.
 They are sensitive to light stimulus.
They include Cones and Rods.
a) Cones-They contain a
photochemical pigment called
iodopsin which perceives light of
high intensity, perception of fine
details and for colour vision.
 Cones are highly concentrated on the
fovea centralis.
 When strong light rays from the
object strike the iodopsin it is
broken down to iodine and opsin
thus initiating a nerve impulse.
 The nerve impulse is then
transmitted to the brain for
interpretation through the optic
nerve.
 Iodopsin is resynthesised by the
energy produced by the
mitochondria in the
photoreceptors.
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 Each cone has its own bipolar
neurone which in turn links it
with the optic nerve fibre i.e.
lack retinal convergence.
 This enables them to have high
visual acuity (ability of the eye
to distinguish objects that lie close
to each other).
 We shift our eyes when observing
objects so that the light rays from
the object can fall on the fovea for
them to be clear.
b) Rods.
 They are used for vision under light of
low intensity.They have a pigment
called rhodopsin which is sensitive to
light of low intensity.
 When light of low intensity strikes the
rod, rhodopsin splits into opsin and
retinine thus initiating an impulse.
 The impulse is transmitted to the brain
through the optic nerve for
interpretation. Retinine is a derivative
of vitamin A whose lack leads to
night blindness.
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 The broken down rhodopsin is
re-synthesised slowly in the dark
light for continued photochemical
reactions in the rods.
 This is experienced when one
enters a dimly lit room from bright
light.
 At first the room looks dark and
nothing is visible but gradually
things start becoming visible.
 Rods have retinal convergence
i.e. several rods converge/
joined to one neurone
therefore they have low visual
acuity hence cannot distinguish
fine detail.
 Retinal convergence also enables any
little stimulation of several rods to
initiate/generate an impulse.
 Rods are in greater concentration
round the periphery of the retina and
are absent in the fovea centralis.
 At night is possible to see an object
clearly when looking from the corner
of the eye.
 This is because when one looks at the
object directly the light rays falls on
the fovea centralis which has cones
only which are operational at light of
strong intensity.
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 Nocturnal animals e.g. cat, owl have
are capable of seeing at night
because of;
i) High concentration of rods than
cones in the retina that are
sensitive to low light intensity.
ii) Retinal convergence of rods to
enable any little stimulation of
rods to generate an impulse.
 At night the low light intensity is
not able to activate the cones.
 By looking from the corner of
the eye the light rays from the
object fall on the periphery of
the retina outside the fovea.
 The periphery contains only rods
which are sensitive to light of low
intensity hence enabling one to
see the object clearly.
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Advantages of having two eyes in
human beings/ binocular vision.
1. Clarity of vision/better focus/
Improved visual acuity.
2. Three dimensional vision/
more accurate assessment of
distance, depth, height and
width of an object.
3. Larger visual field.
4. If one eye is damaged the
human being is not blinded.
RESPONSE TO LIGHT INTENSITY.
 It is brought about the iris that
controls the size of the pupil.
 The iris has circular and radial
muscles that are antagonistic i.e. do
not contract and relax at the same
time.
a) In bright light.
 The circular muscles in the iris
contract and the radial muscles
relax.
 As a result the pupil becomes
smaller in size/ constricts
preventing excess light from
entering which can destroy the
retina.
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b) In dim light.
 The circular muscles in the iris
relax while the radial muscles
contract.
 As a result the pupil becomes
larger to allow more light into
the eye.
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ACCOMMODATION OFTHE
EYE.
 Eye accommodation is the ability
of the eye to focus both near and
distant objects on the retina or
 It is the adjustment of the eye
structures to bring the image
from a near or far objects into
sharp focus on the retina.
 This is achieved by adjusting the
curvature or the shape of the lens
and size of the pupil.
a) Eye accommodation to far/distant
objects.
 When light rays from a distant object strike the
eye, the following events take place:
i. Ciliary muscles relax.
ii. Suspensory ligaments become taut/pulled.
iii. Lens decrease in curvature/ becomes thinner
and long increasing focal length.
iv. Radial muscles of iris contract and circular
muscles relax.
v. The pupil size increases/ dilates.
vi. Light rays from distant objects are refracted
onto the retina.
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b) Eye accommodation to near
objects.
 When light rays from near object strike the
eye, the following events take place:
i. Ciliary muscles contract.
ii. Tension in suspensory ligaments
reduces.
iii. Lens curvature increases/lens becomes
thicker decreasing focal length.
iv. Radial muscles of iris relax and circular
muscles contract.
v. Pupil reduces in size/ constricts.
vi. Light rays from a near object are
refracted onto the retina.
Study question
 Imagine you are sitting outside in
the shade of a tree reading a
book, and you look up to a
distant sun-lit aeroplane in the air
and then you look down to the
book again.Tabulate the sequence
of events in your eyes as the two
actions take place.
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IMAGE FORMATION.
 The image is formed on the retina when
the photoreceptors are stimulated.
 The light rays from the object pass through
the conjunctiva, cornea, aqueous humour,
pupil, lens, vitreous humour and finally
onto the retina.
 The image formed on the fovea centralis is
small, upside down and virtual (not real).
 The photoreceptors are stimulated by light
rays, initiating a nerve impulse which is
sent to the cerebrum part of the brain
through optic nerve for interpretation.
 The brain interprets the image as
being upright, real and allocates
its actual size.
 Impulses from the right eye are
transmitted to left side of the
brain and those from the left eye
to the right side of the brain.
 The colour of the object is
interpreted by the brain and it
depends on the wavelengths of
light stimulating the cones.
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EYE DEFECTS ANDTHEIR CORRECTION
1. Short-sightedness
(myopia)
 This is a condition in which light
rays from a distant object are
brought to focus in front of the
retina/ are blurred/ not clear
while those from near objects are
focused on the retina/ become
clear.
 It is caused by long eyeball or
high refractive power of the eye
lens.
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Rays from distant object

Correction
 It is done by use of/ wearing
spectacles with biconcave
lens to diverge the light rays
from the distant objects
hence they are refracted onto
the retina.
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2. Long-sightedness
(hypermetropia)
 This is a condition in which light
rays from a near object are not
clear/ are not brought to focus
while those from a distant object
are clear/ focused on the retina.
 It is caused by short aye ball or
weak lens system.
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Correction
 It is done by use of/ wearing
spectacles with biconvex lens to
converge light rays from nearby
objects onto the retina hence
near objects are seen clearly.
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3. Astigmatism-This is a congenital defect
whereby the curvature of the cornea is
uneven/not smooth.
 Light rays are refracted differently leading
to blurred vision. It is corrected by use of
spectacles with even lens/astigmatic lens.
4. Squintedness- this is a condition where
external muscles of the eye do not co-
ordinate hence the eye balls face different
directions affecting accommodation.The
defect is difficult to correct.
5. Old sight/presbyopia- This is a
defect where the ability of the lens to
change its thickness is affected which
affects aye accommodation.
 This is caused by hardening of the lens. It is
corrected by use of bifocal lens (containing
both convex and concave lens.
6. Cataracts-This is a defect whereby
the lens becomes stiff and opaque as
a result of damage by the ultraviolet
rays from the sun.
 The defective lens is called a cataract
which causes blurred
vision/blindness. It is caused by old
age, eye injury or diabetes mellitus.
 It is corrected by surgical
replacement of the defective lens.
7. Color blindness- this is a genetic
defect in which individuals are not
able to distinguish colors e.g. red
and green colors.
 This is because the retina lacks cones
with pigments that respond to red and
green colors.
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B. MAMMALIAN EAR.
 It performs the following functions:
i. Hearing.
ii. Maintenance of balance and posture.
 It is composed of three main parts:
A. Outer ear.
B. Middle ear.
C. Inner ear.
A. Outer ear
 It consists of:
1. Pinna- It is large and funnel shaped to
collect and concentrate sound waves to
the external auditory meatus /canal/
tube.
2. Auditory canal/meatus/tube-
It is tubular to direct sound waves
into the ear drum/tympanic
membrane.
 It also has hairs to trap solid particles
preventing then from entering the
ear.
 It has wax secreting cells which
secrete wax that traps dust and
prevents entry of solid particles.Wax
also maintains the flexibility of the
ear drum.
 It has sebaceous gland that secrete
sebum which softens the canal/keeps
it supple.
 Sound velocity is highest/fastest in
the auditory canal.
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B. Middle ear
 It consists of:
1. Ear drum/tympanic membrane- It is
a thin membrane consisting of connective
tissue which vibrates and transforms
sound waves into vibrations.
 It then transmits the vibrations to the ear
ossicles.
2. Ear ossicles (malleus, incus and
stapes)- They are bony structures
attached to each other and suspended in
the cavity by the muscles.
 The muscles prevent excessive vibrations
which could damage the inner delicate
parts.
 They form a system of levers
which amplifies and transmits the
vibrations from the tympanic
membrane to the oval window.
3. Oval and round window-
They are membranes that cover
the small holes that lead to the
inner ear.
 They vibrate and transmit sound
vibrations to endolymph and
perilymph of the cochlea.
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4. Eustachian tube- This is the
tube that connects the
pharynx/nasal cavity and the
middle ear. It is normally closed
but opens during swallowing,
yawning and chewing.
Function
i. The Eustachian tube can provide
passage of pathogens from the
pharynx to the middle ear causing
ear infections.
ii. It equalizes the air pressure
between the inner ear and the
outer ear/ atmosphere to prevent
distortion of the ear drum.
 For example, if you go higher up in
an aeroplane, the atmospheric air
pressure outside falls below that of
the middle ear.This causes the ear
drum to bulge outwards. The
condition is rectified by yawing or
swallowing.This opens the
Eustachian tube to equalize the
pressure on both sides of ear drum.
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C. Inner ear.
 It consists of 3 semi-circular canals,
Cochlea andAuditory nerve.
1. Semi-circular canals- They
are 3 tubular cavities containing
the fluid and located at right
angles to each other.
 They open into the
utricule/utriculus which in
turn opens to the
saccule/sacculus then the
succulus opens into the cochlea.
 The utricule and saccule are
collectively called vestibular
apparatus/ vestibule.
 The vestibule and 3 semi-
circular canals are used to
maintain balance and posture.These
cavities in the inner ear are filled
with fluids called perilymph and
endolymph.
 The fluids conduct sound vibrations
from the middle ear to the cochlea
for hearing.The displacement of
fluids in the vestibule and 3 semi-
circular canals leads to restoration of
body balance.
 The fluids also absorb mechanical
shock hence protect the delicate
sensory structures.
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2. Cochlea- This is a coiled tube
consisting of canals, membranes
and sensory cells.The canals are
filled with endolymph and
perilymph which transmit sound
vibrations.
 It is highly coiled to increase the
surface area for attachment sensory
cells for hearing.The sensory
receptors/cells are sensitive to
sound waves, generate impulse
which is transmitted to the brain
through auditory nerve.
3. Auditory nerve- It is made up
of several nerve cells to transmit
impulses to the brain for
interpretation.
THE PROCESS OF HEARING.
 The pinna concentrates sound waves
into the ear canal/ meatus.
 The sound waves strike the ear drum
causing it to vibrate and transform
sound waves into vibrations.
 Vibrations from the eardrum are
transmitted to the malleus which
amplifies and transmits to the incus
up to the stapes.
 The stapes vibrates, amplifies and
passes the vibrations to the oval
window.
 From the oval window the vibrations
are transmitted to the perilymph of
the cochlea.
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 Vibrations in the fluids of the
cochlea disturbs the sensory hair
cells which generate nerve impulses.
 These impulses are transmitted to
the brain through the auditory
nerve.
 The brain interprets the impulses as
sound of different pitches.
Example
 When one spins and then stops
suddenly the person feels dizzy.
 This is because the endolymph
is still in motion and
continuous stimulating the
sensory cells.
MAINTENANCE OF BALANCE
AND POSTURE.
 Balance and posture are brought
about by the 3 semi-circular
canals and vestibule. The 3
semi-circular canals and vestibule
are collectively called vestibular
apparatus.
 The 3 semi-circular canals are
cavities filled with endolymph and
lie at right angles to each other.
 Each semi-circular canal has a
swelling called ampulla at one end
containing sensory hair cells
projecting out to form cupula.
 The hairs are disturbed by the fluid/
endolymph.
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 The 3 semi-circular canals maintain
body balance and posture in relation
to change in position of the head.
 Change of head position causes the
movement of endolymph which
stimulates the sensory hairs initiating
a nerve impulse which is transmitted
to the brain for interpretation.
 The brain sends impulses the
muscles of the body to restore the
body balance.
 The vestibule consists of utriculus
and sacculus which contain chalky
granules called otoliths attached to
the fine sensory hairs of sensory
cells.
 The vestibule (sacculus and
utriculus) maintain body balance and
posture in relation to gravity.
 The change of body posture disturbs
the chalky granules causing
stimulation of the sensory hairs.
 This initiates a nerve impulse which
is transmitted to the brain for
interpretation.
 The brain interprets the impulse
according to the position of the
body in relation to gravity.
 The brain then sends impulse to the
muscles through motor neurone to
restore the correct posture.
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DEFECTS OFTHE EAR.
1. Deafness.- Deafness is a hearing
defect in which an individual is
unable to perceive sound.This
may be temporary/partial or
permanent.
 Partial deafness is caused by:
i. Blockage of external auditory
canal by a layer of wax or
foreign objects which can be
corrected by removal of the wax
or foreign object.
ii. Damage to eardrum by
injection, perforation using
sharp objects or its thickening
which causes rigidity.
iii. Abnormal growth of
connective tissues (fibrosis) in
the ear ossicles which interferes
with their movement.
 Partial deafness can be corrected by
surgery or by using a hearing aid.
 Permanent deafness results due to
damage of the cochlea.
 This may be due to overdose of the
antibiotics or exposure to loud
sounds.
 This damages the sensory cells in the
cochlea.The individual can be
assisted by use of sign language.
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2. Vertigo- This is a condition
where an individual has a
problem with maintenance of
balance and posture. It may be
as a result of infection to the
vestibular apparatus or
receiving many impulses on
balance and posture. It is
corrected through proper
medical care.
3. Absence of pinna- This is a
hereditary defect where the
child is born without the
pinna. It is corrected by fitting
artificial pinna.
4. Tinnitus. This is a defect
characterized by hissing/ringing
sound of the ear which leads to
distortion of sound perception.
 It is caused by infections of the
outer and middle ear.
 It is corrected by use of hearing
aids and treatment of ear
infections.
5. Presbycusis- This is loss of
hearing due to old age which
results from wearing out of the
cochlea hairs and ossicles.
 It is corrected by use of hearing
aids.
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