The document describes the structure and function of the human nervous system. It explains that the nervous system detects stimuli through receptors and transmits signals through neurons to the central nervous system (brain and spinal cord). The CNS coordinates responses by muscles or glands. There are three main types of neurons - sensory neurons carry signals from receptors to the CNS, relay neurons connect sensory and motor neurons in the CNS, and motor neurons carry signals from the CNS to effectors. Neurons transmit signals across synapses using neurotransmitters. Reflex actions provide rapid automatic responses through a reflex arc pathway without involving consciousness.

1. CSEC Biology
Irritability–
Homeostasis and
response

2. The human nervous system Part 1 - Structure and function
The nervous system enables humans to react to their surroundings and
to coordinate their behaviour.
A stimulus is any change in the internal or
external environment of an organism that
initiates a response.. These are detected
by receptors (cells that detect a change)
and information passes along cells
(neurones) as electrical impulses to the
central nervous system (CNS).
The CNS is the brain and spinal cord.
The CNS coordinates the response of
effectors which may be muscles
contracting or glands secreting hormones.
stimulus  receptor  coordinator  effector  response
CNS
Peripheral
Nervous
System

3. Definitions Continued
• Response: a change in an organism or part of an
organism which is brought about by a stimulus.
• Effector: the part of an organism that responds to the
stimulus.
• In animals, the sense organs contain the receptors,
e.g. the eyes, ears, nose, tongue and skin contain
specialized receptor cells that detect stimuli. In plants,
the very tips of roots and shoots act as receptors.
• In animals, muscles and glands are effectors. In
plants, the regions just behind the tips of roots and
shoots and the petioles of leaves are effectors.

4. Response of Plants to Stimuli – Part Movements
• Changes in the turgidity of cells bring about many part
movements:
• The leaves of some plants respond to touch or strong
winds by folding, which protects them from damage, e.g.
Mimosa (the sensitive plant).

5. Response of Plants to Stimuli – Part Movements
• The leaves of some plants respond to changing light
intensities by folding at night and opening in the morning to
access light for photosynthesis, e.g. tamarind.

6. Response of Plants to Stimuli – Part Movements
• The flowers of some plants respond to changing light
intensities by opening in the morning to expose the
stamens and carpels for pollination, and closing at night,
e.g. hibiscus. Others open at night and close in the
morning, e.g. night flowering cactus.

7. Response of Plants to Stimuli – Part Movements
• Parts of insectivorous plants move to trap prey, e.g. Venus
fly trap snaps closed to trap insects.

8. Growth movements
• Shoots grow and bend towards unilateral light. This
maximizes the amount of light they have available for
photosynthesis. In the absence of light, or when illuminated
evenly, shoots grow upwards against gravity to ‘search’ for
light or to maximize the amount of light they receive.
Growing upwards also ensures that flowers are held in the
best position for pollination, and fruits and seeds for seed
dispersal.

9. Growth movements
• Roots grow and bend downwards with the pull of gravity.
This enables them to anchor the plant in
• the ground and to obtain water and minerals. Roots also
grow and bend towards water to maximise
• the amount of water they can obtain for photosynthesis.

10. Responses of Invertebrates
Stimulus Response How the response aids
survival
Light Most move away from the light
into darkness.
Makes the organisms harder to be
seen by predators
Moisture Most move away from dry
areas into areas with moisture.
Prevents desiccation (drying out),
especially if the organisms do not
have waterproof body
coverings.
Temperature Move away from very low or
very high temperatures.
Prevents extremes of temperature
affecting enzyme activity.
Chemicals Move towards chemicals given
off by food and away from
harmful chemicals.
Enables organisms to find food
which is essential for survival and
avoid being harmed by chemicals
such as pesticides.
Touch Move away or curl up if
touched.
Helps the organisms escape from
predators or gives protection against
predators.

11. The human nervous system Part 1 - Structure and function
Neurones make up both the CNS and PNS, and they transmit messages
called nerve impulses. All neurones have a cell body with thin fibres of
cytoplasm extending from it called nerve fibres. Nerve fibres that carry
impulses towards the cell body are called dendrites. Nerve fibres that
carry impulses from the cell body are called axons; each neurone has
only one axon.
B. Relay neurone
A . Sensory neurone
C. Motor neurone
receptor endings
dendrites
cell body
axon
axon terminals
axon
dendrites
axon

12. The human nervous system Part 1 - Structure and function
There are three main types of neurones:
A. Sensory neurones – these carry impulses from the receptors to the central
nervous system (CNS).
B. Relay neurones – these connect the sensory neurones to the motor neurones in
the CNS.
C. Motor neurones – these carry impulses from the CNS to an effector.
B. Relay neurone
A . Sensory neurone
C. Motor neurone
receptor endings
dendrites
cell body
axon
axon terminals
axon
dendrites
axon

13. The human nervous system Part 1 - Structure and function
Neurones are not joined together; they have small gap between them.
• When an impulse reaches
the end of a neurone, a
chemical (neurotransmitter)
is released across the gap.
• The chemical then diffuses
across the synapse.
• When the chemical reaches
the next neurone this starts
another impulse.
The gap is called
a synapse

14. The human nervous system Part 1 - Structure and function
Reflex action
Source of heat
Receptor cells in
the finger
Effector – muscles in
the arm
Motor neurone
Sensory neurone
Spinal cord
Reflex actions
are important as
they protect the
body from injury.
Reflex actions are automatic and rapid; they do not involve the
conscious part of the brain. The path that a reflex action takes is
called a REFLEX ARC.

15. QuestionIT!
The human nervous
system
Part 1
• Structure and function

16. 1. What is a stimulus?
2. What is a receptor?
3. Name the two parts of the central nervous system.
4. What is an effector?
5. What does the CNS coordinate?
6. Put these in the correct order: receptor, stimulus, response,
coordinator, effector.
7. What is the role of the sensory neurone?
8. What is the role of the relay neurone?
9. What is the role of the motor neurone?
10. What is a synapse?
11. Describe what happens at the synapse.
12. Why are reflex actions important?
13. Recall the pathway of the reflex arc.
The human nervous system Part 1 -
Question IT

17. AnswerIT!
The human nervous
system
Part 1
• Structure and function

18. The human nervous system Part 1 -
AnswerIT
1. What is a stimulus? Any change in the surroundings
2. What is a receptor? Cells that detect a change
3. Name the two parts of the central nervous system. Brain and spinal
cord
4. What is an effector? A muscle or gland
5. What does the CNS coordinate? The response of effectors
6. Put these in the correct order: receptor, stimulus, response,
coordinator, effector. stimulus  receptor  coordinator 
effector  response
7. What is the role of the sensory neurone? Carry impulses from the
receptors to the central nervous system (CNS)
8. What is the role of the relay neurone? Connect the sensory
neurones to the motor neurones in the CNS.
9. What is the role of the motor neurone? Connect the sensory
neurones to the motor neurones in the CNS.

19. The human nervous system Part 1 -
AnswerIT
10. What is a synapse? A gap between two neurones
11. Describe what happens at the synapse. An impulse reaches the end
of a neurone; neurotransmitter is released across the gap. It then
diffuses across the synapse and when it reaches the next neurone this
starts another impulse.
12. Why are reflex actions important? They protect the body from
injury
13. Recall the pathway of the reflex arc. receptor  sensory neurone
 spinal cord  motor neurone  effector

20. LearnIT!
KnowIT!
The human nervous
system
Part 2
• The brain (biology only)
• The eye (biology only)
• Control of body
temperature (biology only)

21. The human nervous system Part 2 - The brain (biology only)
The brain controls complex behaviour. It is made of billions of
interconnected neurones and has different regions that carry out
different functions.
cerebral cortex
cerebellum
Three of the main structures in the brain are:
• cerebral cortex – is the outer
‘wrinkled’ layer of the brain it is
responsible for intelligence, language,
memory and consciousness
• cerebellum – is located at the lower
part of the back of the brain and is
responsible for voluntary coordination of
the muscles
• medulla – is located in the lower part of
the brain stem it is responsible for
involuntary coordination such as
breathing, swallowing and heart rate.
medulla
front
back
Video - The Brain

22. The human nervous system Part 2 - The brain (HT biology only)
A neuroscientist is a scientist that studies the structure and function
of the brain. The complexity and delicacy of the brain makes
investigating the brain and treating brain disorders very difficult.
Neuroscientists use
different methods to study
how the brain works:
• studying patients with
brain damage
• electrically stimulating
different parts of the
brain
• using magnetic
resonance imaging
(MRI) scanning.
Studying brain damage
Damage to different brain areas
produces different behavioural and
psychological effects.
For example, damage in the front end
of the brain disturbs the ability to
make decisions, whereas damage to
the back of the brain disturbs vision.

23. The human nervous system Part 2 - The brain (HT biology only)
Electrically stimulating different parts of the brain
An electroencephalogram (EEG) is a test used to find problems
related to electrical activity of the brain. An EEG tracks and records
brain wave patterns. By stimulating different parts of the brain and
observing what happens, it is possible to get a greater understanding
of what different areas of the brain do.

24. The human nervous system Part 2 - The brain (HT biology only)
Using magnetic resonance imaging (MRI) scanning.
Uses strong magnetic fields and radio waves to produce detailed
images of the brain and spinal cord. It consists of a large tube
containing powerful magnets. MRI can distinguish between
different types of tissues, including detecting cancerous cells. It is a
painless and safe procedure, but uses very expensive equipment
and needs highly trained staff. Video - MRI

25. The human nervous system Part 2 - The eye (biology only)
The eye is a sense organ containing receptors
sensitive to light intensity and colour.
sclera
cornea
iris
ciliary muscles
suspensory ligaments
retina
optic nerve
pupil
lens
Video - The eye

26. The human nervous system Part 2 - The eye (biology only)
Part of the eye Description and function
cornea Transparent layer at the front of the eye, it refracts light
into the eye.
iris The coloured part of the eye, contains muscles that
control the amount of light entering the eye.
pupil The hole in the middle of the iris that lets light in. Its
diameter is controlled by the iris.
lens A transparent, biconvex structure in the eye that refracts
light onto the retina.
retina Thin layer of tissue at the back of the eye that contains
receptor cells for light and colour.
sclera Tough white supporting wall of the eye.
ciliary muscles Muscles that are connected to the lens by the suspensory
ligaments; they change the shape of the lens.
suspensory ligaments These connect the lens to the ciliary muscles.
optic nerve Carries impulses from the retina to the brain.

27. The human nervous system Part 2 - The eye (biology only)
Accommodation is the process of changing the shape of the lens to
focus on near or distant objects.
cornea
retina
lens
To focus on a near object:
• the ciliary muscles contract
• the suspensory ligaments loosen
• the lens is then thicker and
refracts light rays strongly.
To focus on a distant object:
• the ciliary muscles relax
• the suspensory ligaments are
pulled tight
• the lens is then pulled thin and
only slightly refracts light rays. Video - accommodation

28. The human nervous system Part 2 - The eye (biology only)
Two common defects of the eyes are myopia (short sightedness) and
hyperopia (long sightedness) in which rays of light do not focus on the retina.
Myopia (short sightedness)
• People who are short
sighted cannot focus on
objects that are far away.
• It usually occurs when the
eyeball is too long.
• The light rays from distant
objects focus in the eyeball
in front of the retina.
• Myopia can be corrected
with concave lenses so that
the light rays focus on the
retina.
Concave lens

29. The human nervous system Part 2 - The eye (biology only)
Convex lens
Hyperopia (long sightedness)
• People who are long sighted
cannot focus on objects
that are near.
• It usually occurs when the
eyeball is too short.
• The light rays from distant
objects focus in the eyeball
behind the retina.
• Hyperopia can be corrected
with convex lenses so that
the light rays focus on the
retina.

30. The human nervous system Part 2 - The eye (biology only)
There are new technologies available to correct vision these include:
Contact lenses – these can be hard or soft and are
inserted directly into the eye and sit on the cornea.
This then refracts the light rays. Advantages -
lightweight, almost invisible, good for people playing
sports. Disadvantages – not a permanent solution
and they can cause irritation and infections.
Hard
contact lens
Soft
contact lens
Inserting a soft
contact lens
Laser surgery – the other layer of the cornea is peeled back
and lasers are used to change the shape of the cornea so
that the light rays will refract and form an image on the
retina. Advantages – permanent solution to correct vision.
Disadvantages – a surgical procedure, can cause infections.
Lens replacement – surgery refers to a medical procedure
where an artificial acrylic or silicone lens is implanted into
the eye to correct problems with vision. Advantages –
permanent solution to correct vision. Disadvantages – a
surgical procedure, can cause infections.

31. The human nervous system Part 2 - Control of body
temperature (biology only)
Human body temperature is 37oC, this is
the optimum temperature for enzymes
to work effectively.
Body temperature is monitored and
controlled by the thermoregulatory
centre (hypothalamus) in the brain.
Thermoregulatory centre
The thermoregulatory
centre contains receptors
sensitive to the temperature
of the blood. The skin
contains temperature
receptors and sends nervous
impulses to the
thermoregulatory centre.
Free nerve endings
for pain and
temperature
Surface of the skin
Hair

32. The human nervous system Part 2 - Control of body
temperature (biology only)
If the body temperature is too low,
blood vessels constrict
(vasoconstriction), sweating stops
and skeletal muscles contract
(shiver).
If the body temperature is too high,
blood vessels dilate (vasodilation)
and sweat is produced from the
sweat glands. Both these mechanisms
cause a transfer of energy from the
skin to the environment.
Vasoconstriction Vasodilation
Video - Temperature regulation

33. QuestionIT!
The human nervous
system
Part 2
• The brain (biology only)
• The eye (biology only)
• Control of body temperature
(biology only)

34. 1. Name the three main parts of the brain and state the role of each.
2. How do neuroscientists study the brain?
3. What is an EEG?
4. What is an MRI?
5. What does an MRI do?
6. Label parts A – I on the diagram below:
The human nervous system Part 2 -
Question IT
F.
G.
A.
I.
H.
D.
B.
E.
C.

35. The human nervous system Part 2 -
Question IT
7. Write the name of each part of the eye in the table below:
Part of the eye Description and function
Transparent layer at the front of the eye, it refracts light into the eye.
The coloured part of the eye, contains muscles that control the amount of
light entering the eye.
The hole in the middle of the iris that lets light in. Its diameter is
controlled by the iris.
A transparent, biconvex structure in the eye that refracts light onto the
retina.
Thin layer of tissue at the back of the eye that contains receptor cells for
light and colour.
Tough white supporting wall of the eye.
Muscles that are connected to the lens by the suspensory ligaments; they
change the shape of the lens.
These connect the lens to the ciliary muscles.
Carries impulses from the retina to the brain.

36. The human nervous system Part 2 -
Question IT
8. What is accommodation?
9. How does the eye focus on distant objects?
10. How does the eye focus on near objects?
11. What is myopia? How can it be corrected?
12. What is hyperopia? How can it be corrected?
13. Name three ways that vision can be corrected.
14. State the optimum human body temperature.
15.What part of the brain monitors and controls temperature?
BIOLOGY ONLY:
16. Describe what happens in vasoconstriction and vasodilation.

37. AnswerIT!
The human nervous
system
Part 2
• The brain (biology only)
• The eye (biology only)
• Control of body
temperature (biology only)

38. The human nervous system Part 2 -
AnswerIT
1. Name the three main parts of the brain and state the role of each. cerebral
cortex – is the outer ‘wrinkled’ layer of the brain it is responsible for
intelligence, language, memory and consciousness; cerebellum – is
located at the lower part of the back of the brain and is responsible for
voluntary coordination of the muscles; medulla – is located in the lower
part of the brain stem. It is responsible for involuntary coordination such
as breathing, swallowing and heart rate
2. How do neuroscientists study the brain? Studying patients with brain
damage, electrically stimulating the brain and using magnetic resonance
imaging (MRI) scanning.
3. What is an EEG? Electroencephalogram (EEG) is a test used to find
problems related to electrical activity of the brain.
4. What is an MRI? A device that uses strong magnetic fields and radio
waves to produce detailed images of the brain and spinal cord.
5. What does an MRI do? MRI can distinguish between different types of
tissues, including detecting cancerous cells.

39. The human nervous system Part 2 -
AnswerIT
6. Label parts A – I on the diagram below:
G. ciliary muscles
A. suspensoryligaments
I. retina
H. optic nerve
B. lens
C. cornea
D. pupil
E. iris
F. sclera

40. The human nervous system Part 2 -
AnswerIT
7. Write the name of each part of the eye in the table below:
Part of the eye Description and function
cornea Transparent layer at the front of the eye, it refracts light into the
eye.
iris The coloured part of the eye, contains muscles that control the
amount of light entering the eye.
pupil The hole in the middle of the iris that lets light in. Its diameter is
controlled by the iris.
lens A transparent, biconvex structure in the eye that refracts light
onto the retina.
retina Thin layer of tissue at the back of the eye that contains receptor
cells for light and colour.
sclera Tough white supporting wall of the eye.
ciliary muscles Muscles that are connected to the lens by the suspensory
ligaments; they change the shape of the lens.
suspensory ligaments These connect the lens to the ciliary muscles.
optic nerve Carries impulses fro the retina to the brain.

41. The human nervous system Part 2 -
AnswerIT
8. What is accommodation? The process of changing the shape of the lens focus on
near or distant objects
9. How does the eye focus on distant objects? The ciliary muscles relax, the
suspensory ligaments are pulled tight, the lens is then pulled thin and only slightly
refracts light rays
10. How does the eye focus on near objects? The ciliary muscles contract, the
suspensory ligaments loosen, the lens is then thicker and refracts light rays strongly
11. What is myopia? How can it be corrected? Short-sightedness, it can be corrected
with concave lenses
12. What is hyperopia? How can it be corrected? Long-sightedness, it can be
corrected with convex lenses
13. Name three ways that vision can be corrected? Contact lenses, laser surgery and
lens replacement
14. State the optimum human body temperature. 37oC
15. What part of the brain monitors and controls temperature? Thermoregulatory
centre

42. The human nervous system Part 2 -
AnswerIT
BIOLOGY ONLY:
16. Describe what happens in vasoconstriction and vasodilation.
• If the body temperature is too low, blood vessels constrict (vasoconstriction),
sweating stops and skeletal muscles contract (shiver).
• If the body temperature is too high, blood vessels dilate (vasodilation) and
sweat is produced from the sweat glands. Both these mechanisms cause a
transfer of energy from the skin to the environment.