The Nervous System
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?
You should appreciate that the nervous system and hormones enable us to respond to external changes.
The nervous system and hormones also help us to control
conditions inside our bodies.
a) You should know that the nervous system enables humans to react to
their surroundings and coordinate their behaviour.
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 signal - an electrical
impulse which is sent to the brain.
The reflex actions that can
happen by virtue of our central nervous system help prevent injury from
various sources in potentially dangerous situations.
b) Know that cells called receptors
can detect stimuli (changes in
the environment outside the organism).
c) Know that light receptor cells, like most animal cells, have a
nucleus, cytoplasm and cell membrane.
d) Know and understand that information
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.
e) You should know and understand the role of receptors,
sensory neurones, motor neurones, relay neurones,
synapses and effectors in simple reflex actions.
Receptors - groups of
cells that respond to a particular stimulus
Nerve cells are called
neurons, elongated cells that carry electrical signals or impulses all
around the body.
Sensory neurones - the
nerve cells that transmit the electrical impulse signal from the receptors in
the sense organs to the spinal chord and brain (central nervous system).
Relay neurones - the
nerve cells that transmit the electrical signals from sensory neurones to the
Synapse - a connection
between two neurones eg the junction between a sensory neurone and a relay
neurone, it enables the electrical impulse signal to reach the spinal cord
and brain (ie the central nervous system). 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, transferring
the electrical signal.
chemicals produced that transmit the electrical signal across a synapse gap
between one neurone cell and another.
Myelin sheath - is a
insulating tissue layer around the axon connections between neurones - the axon in the neurone cells
carries the electrical signal - if there was no myelin insulation, the signal will be lost.
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.
Effectors - the muscles
or glands that respond in a variety of ways to the electrical signal from
Reflex actions are
automatic responses to stimuli detected by the receptors in the organs of
They are an important defence
mechanism of our body eg
If in danger your body releases
the hormone adrenaline to heighten your mental and physical response.
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.
Know that in a simple reflex
from a receptor to an effector (via spinal cord and an unconscious
part of the brain):
A stimulus detected by
receptors (receptor cells) causes impulses from a receptor to pass along a sensory
neurone (nerve cell) to the central nervous system.
At a nerve junction (synapse) between a sensory
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 eg the central nervous systems
decides what is to be done depending on what stimulus is received, so
Muscles in your arm may
contract to withdraw your hand from a heat source or sharp point.
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
cell ==> sensory neurone ==synapse==> relay neurone in central
nervous system =synapse==>
motor neurone ==> effector ==> response
The reflex arc action is
fast, no thinking involved, just a rapid automatic response on the
part of your body!
Note on comparing nerve
and hormone functions
Hormones effectively act as
'chemical messages' to trigger particular biochemical reactions and their
effect is more general and relatively long-lasting compared to eg the
nervous impulses and responses of 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,
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