to the nervous system
the reflex arc and response time experiments
Doc Brown's Biology Revision Notes
Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent
This page will answer many questions e.g.
What do we mean by a sensory organ?
What are your five sense organs?
How are signals from sensory organs sent to
What is a synapse? What is a sensory neuron?
The brain - what the different parts do and the dangers
if damaged gcse biology revision notes
The nervous system and hormones enable us to respond to external changes
- external stimuli.
The nervous system and hormones also help us to control
conditions inside our bodies.
The nervous system enables humans to react to
their surroundings and coordinate their behaviour.
Organisms need to be able to
respond to stimuli from changes in their environment, primarily to
Any change in your surroundings
eg temperature, visual, sound etc. is potentially a detectable stimulus to
one of you sensory organs eg skin, eyes, ears etc. The stimulus might be
chemical, light, pain, position, pressure, sound, temperature, touch etc.
You have five different sense
organs ears, eyes, nose, skin and tongue which contain receptors (groups of
cells) that are sensitive to particular stimuli.
In the receptor cells the
stimulus input is converted into an electrical nerve signal - an electrical
impulse which is sent to the central nervous system (CNS)
The CNS for vertebrates
(animals with backbones) consists of the brain and spinal cord.
The nerve cells (neurones) and
connections are distributed to all parts of the body.
The reflex actions that can
happen by virtue of our central nervous system help prevent injury from
various sources in potentially dangerous situations.
A single celled organism can only
respond to stimuli from its immediate environment.
However, the cells of a
multicellular organism must be able to communicate with each other.
To do this, multicellular
organisms have evolved nerve and hormone communication systems.
With our varied receptor cells,
we as humans can react to our surroundings and coordinate our
behaviour to our best advantage.
Cells called receptors
can detect stimuli (changes in
the environment outside the organism).
Receptors and the stimuli they detect include:
Light receptor cells in the
eyes that are sensitive to light,
the light energy creates electrical signals that are sent to the brain for
Light receptor cells, like
most animal cells, have a nucleus, cytoplasm and cell membrane.
Sound receptors in the ears that are sensitive to sound
vibrations in the air
There are also balance receptors in the ears that are sensitive to changes
in position and enable us to keep our balance.
The receptors on the tongue are
sensitive to chemicals and enable us to taste, and therefore detect, a wide
variety of different foods (bitter, salty, sour, sweet chemical stimuli etc.) or anything
else in contact with the tongue - good or bad!
The receptors in the nose are
also sensitive to chemicals and enable us to smell all sorts of different
things which may be a pleasant or unpleasant experience.
The receptors in the
skin that are sensitive to touch, pressure, pain and to
The central nervous system (CNS)
in the form of an electrical signal, from receptors, passes along cells in nerves
(neurones) to the brain through the central nervous system (spinal
cord ==> brain) and ...
... the brain then coordinates the response,
... reflex actions are
automatic and rapid,
... and often involve sensory,
relay and motor neurones.
The 'components' of the nervous
As mentioned already, the CNS
of vertebrates consists of the brain and spinal cord only.
The CNS is then connected to the
body by sensory neurones and motor neurones.
The structure and function of
different parts of the nervous system are described below.
Nerve cells are called
neurons/neurones, elongated cells that carry electrical signals or impulses all
around the body.
The diagram on the right
shows the basic structure of a nerve cell or neurone.
The axon is covered in a
protective electrically insulating myelin sheath (not shown
see other cell diagrams below).
points about nerve cell structure:
(i) All neurons,
like most other cells, have a cell body with a nucleus in a
membrane surrounded cytoplasm and other subcellular
structures. The cell body has extensions called
dendrites/dendrons that connect to other neurones and carry
the electrical impulses of the nerve signals.
(dendrons) are branched protoplasmic extensions of a
nerve cell that propagate the electrochemical stimulation
received from other neural cells to the cell body, or soma,
of the neuron from which the dendrites project.
(iii) An axon
(nerve fibre), is a long, slender projection of a nerve cell
(neuron), in vertebrates, that typically conducts electrical
impulses known as action potentials away from the nerve cell
(iv) Neurones are
relatively long cells which helps the fast electrical
impulse transfer between one neurone and another - one long
nerve cell transfer is faster than through lots of
(v) The myelin sheath
is a fatty electrically insulating tissue layer around the
axon connections between neurones. The myelin sheath also
helps speed up the electrical impulse transfer and the axon in the neurone cells
carries the electrical signal - if there was no myelin insulation, the signal will be lost.
Axon endings (axon
terminals) are button-like endings of axons through which axons make
synaptic contacts with other nerve cells or with effector cells.
Receptors - groups of
cells that respond to a particular stimulus - e.g. they detects stimuli such
as heat, light, pain, sound, taste, smell, pressure (see previous
section for more details).
Receptors often form part of
a larger complex organs e.g. the taste buds on your tongue or
the retina cells of the eye which respond to light.
Receptors start what is known
as the 'reflex arc' described in the next section.
Sensory neurones - the
nerve cells that transmit the electrical impulse signal from the receptors in
the sense organs to the spinal chord and brain of the central nervous system.
A long dendron carries nerve
signals from receptor cells to the cell body which is at the
centre of the neurone. A shorter axon then transfers the
electrical impulse from the cell body to the axon terminals that
connect to the CNS.
Relay neurones - the
nerve cells that transmit the electrical signals through the CNS (brain +
spinal cord) from sensory neurones to the
Lots of dendrites
spreading out from the relay neuron carry
the nerve signals from the sensory neurones to the motor neurones.
Synapse - a connection
between two neurones eg the thin gap of the junction between a sensory neurone and a relay
neurone, it enables the electrical impulse signals from receptors to reach the spinal cord
and brain (ie the central nervous system) and on to the effectors.
Between the end of one neurone,
and the start of another, chemicals are released in the gap that rapidly
diffuse across the gap in the synapse, triggering the transfer
the electrical signal.
You can think of the released
chemical as 'messenger molecule', and technically it is called a
neurotransmitter because it triggers the electrical
signal from one nerve cell (neurone) to another.
The transfer of the nerve
impulses is quite fast, but the diffusion of the
neurotransmitter molecules across the synapse gap does take a
short time, so things are slowed down a bit.
chemicals produced that transmit the electrical signal across a synapse gap
between one neurone cell and another (see text and diagram under synapse).
Motor neurones - the
nerve cells that transmit the electrical signals through the central nervous
system from the brain via the spinal cord to the effector cells of the
muscles or glands from one neurone to another (see diagram above).
Many short dendrites carry
nerve impulses from the CNS to the cell body, then one long axon
carries the signal from the cell body to the effector cells.
have already been discussed.
Effectors - the muscles
or glands that respond in a variety of ways to the electrical signal from
the brain - they respond to the nervous impulses.
Nervous impulses cause
muscles to respond and contract e.g. from receptors
detecting heat or pain.
Nerve impulses cause
glands to secrete hormones - chemical messengers to effect a
Effectors complete what is
known as the 'reflex arc' described in the next section.
The central nervous system (CNS) and
reflex actions - the reflex arc
You should know and understand the role of receptors,
sensory neurones, motor neurones, relay neurones,
synapses and effectors in simple reflex actions.
CNS coordinates the response
when it receives information from the receptors and causes the effectors
to respond to the stimulus detected e.g.
(i) suppose you start to cross
the road, but your eye detects a car coming along - the visual
(ii) Your receptor cells
(retina of eye) send nerve impulses to the brain which builds up an
image of the environment - including the approaching car.
(ii) The sensory neurones
convey the information from the receptor cells of the eye to the
(iv) The CNS then decides what
to do e.g. how you will your brain respond to the
(v) The CNS then sends impulses
via the motor neurones which transmit the 'instructions' from
your brain, through the spinal cord, to your muscles.
(vi) Your effectors, that
is your muscles, contract and you step back from being hit by the
car, job done!
In this example both your brain
and spinal cord of your CNS are involved, and you have made a
conscious decision to avoid being hit by the car.
BUT, sometimes your body reacts
without any apparent conscious thought, but the CNS is still
involved either through the spinal cord or an unconscious part of
the brain (see next section on the reflex arc).
Reflex actions are
automatic responses to stimuli detected by the receptors in the organs of
Reflexes are rapid automatic
responses to particular stimuli, that do NOT involve the conscious
part of the brain - they are an important defence
mechanism of our body to prevent injury eg
The transfer of information in
a reflex action, from a receptor to an effector, is called a reflex
If in danger, especially if you
get a shock - experience a traumatic situation, your body
the hormone adrenaline to heighten your mental and physical response
to the new situation.
If the intensity of light
impacting on your eye is too great, your pupil automatically gets smaller to
allow less light. In a dimly lit room, the opposite response occurs and your
pupil widens to let more light in.
If something hot touches your
skin, on feeling pain you immediately try to recoil from the heat source eg
on burning your hand, the muscles rapidly contract to take your hand away.
In these situations, not
involving the conscious brain functions, the transfer of
information from the receptor to the effector is called a reflex
Know and understand that in a simple reflex
from a receptor to an effector - by way the spinal cord or an unconscious
part of the brain):
A stimulus detected by
receptors (receptor cells) causes impulses from a receptor to pass along a
neurone (nerve cell) to the central nervous system.
At a nerve junction (synapse) between a
neurone and a relay neurone in the central
nervous system, a chemical is released that
causes an impulse to be transmitted by a relay
A chemical is then released at the
between a relay neurone and motor neurone in
the central nervous system, causing impulses to
be sent along by a motor neurone to the organ
(the effector) that brings about the response (of the effector cells).
The effector is either a muscle or a gland, a muscle
responds by contracting or a gland responds
by releasing (secreting) chemical substances called hormones.
The central nervous systems
decides what is to be done depending on what stimulus is received
Examples of reflex arc
Muscles in your arm may
contract to withdraw your hand from a heat source, sharp point
or wasp/bee sting!
Glands may secrete a
particular hormone in response to a particular stimulus eg adrenalin
in a 'flight response' from a dangerous situation.
The pupils in your
eyes respond by decreasing/increasing in size if the light level is
Summary of the reflex arc
sequence via the central nervous system:
stimulus => receptor
cells => sensory neurones + synapses => relay neurones +
synapses in CNS (spinal chord or unconscious brain) => motor neurones + synapses => effector
cells/organ => response
The reflex arc action is
automatic and fast, no thinking involved - doesn't involve the conscious brain, just a rapid automatic response on the
part of your body!
Another good example is when facing
and experience a threat situation! When an insect bites your hand, the
reflex arc goes into action and your body muscles (e.g. in your arm) rapidly
withdraw your hand from the threat.
The pain receptor cells are
stimulated by the insect bite!
(ii) Sensory neurones
The sensory neurones send
electrical nerve signals to the relay neurones in the CNS.
(iii) Transmission - synapses
When the impulses reach a
synapse between a sensory neurone and a relay neurone in the CNS they
trigger the release of a chemical (neurotransmitter) causing the
impulse to be sent along relay neurons.
Note that a relay
neurones connect sensory neurones to a motor neurones.
(iv) Pain experienced -
decision automatically made!
The CNS processes the nerve
signals and starts the response 'procedure'.
When the impulses reach a
synapse between a relay neuron in the CNS and a motor neurone they trigger
the release of a chemical (neurotransmitter) causing the impulse
to be sent along motor neurones.
(v) Motor neurones
The motor neurons convey the
response signal to the effectors, in this case the muscle cells of
(vi) Effector response
The effectors act i.e. your
muscles contract to produce the automatic response - the rapid
recoil of your arm and hand from the vicinity of the insect.
and this is how a reflex
arc works and its faster than normal conscious decision making
processes BECAUSE you don't have to think about it !!!!
NOT part of a reflex arc action!)
See also the iris reflex action
The eye -
structure, function and vision defects GCSE biology revosion
and hormone functions
Hormones effectively act as
'chemical messages' to trigger particular biochemical reactions and their
effects are ..
more general around the body, but
tend to affect particular cells in particular organs,
and relatively long-lasting compared to eg the
fast but short-term nervous impulses and responses of a reflex arc.
Compared to the hormone
system of response and control in the body, nerve signals are
electrical (not chemical), the nerves act very fast - a short burst of a
nervous impulse for a short time, acting from one precise area to
another in the body.
Any practical work and
investigations you did should also be revised
(which should also
be revised, helps in understanding 'how science works' and context
reaction times – measuring
reaction times using metre rules, stop clocks or ICT,
using forehead thermometers
before and after exercise,
demonstrating the speed of
transmission along nerves by candidates standing in a semi-circle and
holding hands and squeezing with eyes closed,
designing an investigation to
measure the sensitivity of the skin,
demonstrating the knee jerk
investigation to measure the
amount of sweat produced during exercise,
Simple physical response tests
Copied and re-edited from
Reaction times and vehicle stopping distances
gcse physics revision notes
Your reaction time to a situation may be typically 0.2 to
0.8 seconds when fully alert. However your reaction time can be affected by
tiredness, feeling unwell, drugs, alcohol, in other words anything that
affects the speed of your brain function.
You can conduct quite simple experiments to test your
reaction time to a particular situation. However, since the reaction time is
(a) Computer screen reaction test - responding as quickly as possible to
something appearing on the screen.
In this situation, the computer
software generates something up on the screen and your click the mouse
or tap the keypad in response to the visual (or sound?) stimulus.
The computer automatically times
your response by monitoring your contact with the keyboard or by
clicking the mouse - its more accurate, especially as it can measure
reaction times in milliseconds.
Computer generated stimuli give more
accurate response reaction times than e.g. the dropped ruler experiment
described in section (b) which potentially involves human error -
computer experiments avoid the possibility of the person anticipating
when the event is to happen e.g. reading the body language of someone
dropping the ruler in experiment (b) described below.
I've quickly written an extremely
simple computer programme to test your response to a X appearing on
test: It probably only works on Microsoft platforms, and
maybe not all of them?
Your anti-virus protection might
query it, because it is a .exe file, but its written with
compiled BBC BASIC and should not pose any threat. Unfortunately I
never learned to write in a multi-platform professional computer
programming language, but I'm not exactly short of website projects!
(b) Catching a falling object test
Fraught with human error, but a bit
of classroom fun!
You get someone to hold a ruler vertically, with
thumb and first finger, above someone else's hand, who is ready to catch
it with their thumb and first finger.
First image on the right. The
ruler should be held at the top of the scale and steady hands from
The catching person should have
the middle of their thumb and finger adjacent to zero on the cm
scale - squat down to make sure you are reading the scale
Then, without warning, the person holding the ruler,
lets go of it. The second person has to react as fast as possible and
catch the dropped ruler between their thumb and first finger.
Second image on the right. The
longer the distance, the slower your reaction time!
When caught, you then read how far
the ruler as fallen by taking the reading, to the nearest centimetre,
from where the middle of their thumb and finger are.
You repeat the experiment a number
of times to get an average, but its not a particularly accurate
You need to have steady hands and not
let the ruler wobble about or fall at an angle other than vertical.
Controlling variables - fair test
You should drop the ruler from
the same height each time the experiment is performed.
should also use the same ruler and the same hand to catch the ruler.
Use the same person/people
dropping the ruler and catching it though, obviously, you can compare
one person's results with another.
The slower your response time, the
further the ruler falls before being caught.
You might repeat the
experiment by having e.g.
having some background distractions - a group of people
talking nearby, or somebody trying to engage you in conversation or
or taking a caffeinated drink
like coffee or cola to act as a stimulant. - a drug that speeds up
neural activity in the central nervous system.
The brain - what the different parts do and the dangers
if damaged gcse biology revision notes
TOP OF PAGE
notes nervous system KS4 biology Science notes on nervous system GCSE biology guide
notes on nervous system for schools colleges academies science course tutors images
pictures diagrams for nervous system science revision notes on
nervous system for revising biology modules biology topics notes to help on understanding of
nervous system university courses in biological science
careers in science biology jobs in the pharmaceutical industry
biological laboratory assistant
apprenticeships technical internships in biology USA US grade 8 grade 9 grade10 AQA
GCSE 9-1 biology science GCSE
notes on nervous system Edexcel GCSE 9-1
biology science notes on nervous system for OCR GCSE 9-1 21st century biology science OCR GCSE
notes WJEC gcse science CCEA/CEA gcse science