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 stimulus.
(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 muscle cells, 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
Reflex actions 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
You don't have to think about
reflex actions, given a stimulus, they just happen!
Reflex action occur in simpler
organisms than humans and in evolutionary terms, they can be
considered an aid to survival,
e.g. 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.
A baby grips a finger placed near
its hand - a grasping reflex.
In these reflex action 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:
receptor ===> coordinator ===> effector
and in a little more detail ...
stimulus ==> receptor cells
==> sensory neurone ==> synapse ==> relay neurone and
synapse in CNS (spinal cord or unconscious brain) ==> motor neurones ==> effector
cells/organ => response
Note the three neurones in
the reflex arc do NOT link physically, there is a gap, the synapse, between each pair enabling lots of neurones to be
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 - descriptive details to go with the
diagram above, are set out below. CNS = central nervous system.
The same reflex action applies to when your hand
accidently touches a hot surface.
1. Your hand touches a hot surface.
2. The pain receptor cells are stimulated by a
relatively high temperature well over the body temperature of 37oC.
3. The pain signal is sent along a sensory neurone.
4. These impulses are passed along via synapses to a
relay neurone in the spinal CNS.
5. The CNS responds by sending a signal along a
motor neurone via synapses to the muscles.
6. When the signal reaches the effector cells, the
muscles respond and you quickly withdraw your hand from the source
of heat to complete the reflex arc autonomic response.
Note that in a reflex arc the synapses ensure the
nerve signal impulses only travel in one direction.
Other examples of the CNS coordinating the response to a
Many animals react instinctively to movement of an
object nearby e.g.
The receptors in the animals eye are stimulated.
Sensory neurons transmit the information from the
receptors to the CNS-brain.
The CNS-brain decides how to respond.
The CNS sends signals through the motor neurons to
the effector cells in the animals muscles (e.g. of arms, legs or
wings - depending on the animal).
The muscles contract and the animal responds by
moving towards a prey or fleeing from danger to safety.