1. it around the body. This is called a double circulation.
The functions of interaction are those that permit a living being to receive information
from the exterior or from its interior and respond to the information with the most
adequate response in order to assure its survival.
All of the functions of relation are determined by the stimulus, the receptor that
captures the stimulus and transmits the information to the efector, that executes the
most adequate response.
In large multi-cellular organisms like humans, a complex system is required to co-
ordinate the necessary responses to the hundreds of changes in the surroundings that
we detect. The nervous system enables humans to react to their surroundings and to
co-ordinate their behaviour. The nervous system consists of receptors, nerve cells, and
the central nervous system or CNS for short. Receptors detect changes in the
environment, called stimuli. Nerve cells, called neurones, transmit messages to and
from the CNS.
Cells called receptors are located in our sense organs, which are the eyes, ears, nose,
tongue, and skin. Receptors detect changes in our surroundings or our interior. These
changes are called stimuli. These changes can be temperature, light, sound, the
concentration of a certain substance, etc.
The nervous system allows us
to respond to the stimuli. The
principles of a response to a
stimulus can be summarised in
a key sequence. In this
sequence, the stimulus is the
change we feel, for example, a
change in temperature, a
sound, or a sight. The receptor
is the specific receptor cell
sensitive to one type of
stimulus, for example receptors
in the eye are sensitive to the
stimulus light. The co-ordinator
is the brain or spinal cord,
which processes the information and makes a decision. The effector is a muscle or a
gland. Sometimes there is more than one effector involved in a response. The
response is the contraction of muscle tissue causing movement or the production of a
secretion by the gland. If more than one effector is involved there will be more than one
response.
When someone with a fear
of spiders sees a spider,
the stimulus is the sight of
the spider. The receptor is
TheTheTheThe Nervous systemNervous systemNervous systemNervous system
The interaction function
2. in the eye and the co-ordinator is the brain. The effectors are the leg muscles, the jaw
muscles, and the adrenal glands. The responses are running away, opening the mouth
to scream, and the production of adrenaline. Sensory neurones carry messages as
electrical impulses from the receptor to the co-ordinator. Motor neurones carry
messages as electrical impulses from the co-ordinator to the effectors.
1. See if you can match the sense organs to the stimuli they respond to.
2. The following table shows the steps involved in the response pathway. If the
stimulus is the hot surface of an oven, can you complete the table using the list
below? Drag the steps to the correct boxes in the table.
1. Define stimulus, efector and receptor.
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
_________________________________________________________________
Lesson Review
3. 2. Explain the differences between glands and muscles in relation to the response.
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
3. Look at the conceptual map carefully and fill in the blanks with the words in the box
below:
Stimuli
responses (2)
nervous impulses
effectors (glands)
nervous system
endocrine system
receptors
hormones
effectors (muscles)
messages to
4. Each nerve is a bundle of many neurones lying alongside each other. Each neurone
sends electrical impulses from one place to another in the body. Some neurones are
very long cells, like the ones that reach from the bottom of your back to the tip of your
toes. Others are short, like the neurones in your brain.
Neurons are cells which have a
strange and variable shape. They
have three different parts: the body of
the cell, the axon and the dendrites.
- The body of the cell is the
thickest part of the neuron.
The nucleus and the majority
of the organelles are in the
body of the cell.
- The axon is a large prolongation which comes out of the body of the cell and it
branches out at the end. The axon is normally protected by Schwann cells.
These cells protect and isolate the axon.
- The dendrites are small branching prolongations of the body of the cell.
There are three types of neurone: sensory, relay, and motor neurones.
The sensory neurone transmits
impulses from a receptor into
the CNS. They can be over a
metre long. The long axon is a
special adaptation of the basic
animal cell, which ensures very
fast transmission of an impulse.
The myelin sheath acts as an
insulator and it also speeds up
the transmission of impulses
along the axon.
Relay neurones are found in the
CNS and, as the name implies,
pass messages from one neurone to another, just as runners pass a baton in a relay
race. A relay neurone is much shorter than a sensory or motor neurone.
A motor neurone transmits impulses away from the CNS to a muscle or a gland, which
then brings about a response to the original stimulus.
Between different neurones, there are small
gaps called synapses. The synapse ensures
that an impulse can only travel one way. The
impulse is transmitted across the synapse by a
chemical called a neurotransmitter. As an
impulse reaches a synapse it causes the
The neuron
5. secretion of a neurotransmitter, which diffuses across the synapse and attaches to the
next neurone. This causes an electrical impulse to be generated in the next neurone.
When the impulse reaches the effector, the muscle contracts. If the effector is a gland,
it responds by secreting a chemical.
1. This diagram of a neurone is incomplete. Drag the name of each part of the
diagram to the appropriate text box in the diagram.
2. Can you match these parts of the nervous system with their descriptions? Drag the
name of each part to its corresponding description.
Unit Review
6. Reflex Actions
Some responses to stimuli
are automatic and happen
very fast, for example
blinking or pulling away
from a hot object, or
withdrawing your finger
from a hot object. These
are called reflex actions
and often protect the body
from danger. They take
place very rapidly, before
you have time to make a
decision. This is because very few neurones are involved in the pathway. This
arrangement of neurones (three neurons: a sensory neurone, relay neurone and
motor neurone) is called a reflex arc. You register what has happened after it has
taken place.
The sensory neurone transmits the nerve impulse into the back of the spinal cord. The
nerve impulse is then transmitted through a small relay neurone to the motor neurone.
It then travels along the motor neurone to the arm muscle, causing the arm to move.
In a reflex response, the part of the CNS which is closest to the site of the receptor
will be used to co-ordinate the response. The coordinator in this example is the spinal
cord. For the change in pupil size of the eye, the brain is the coordinator.
For example, in the knee jerk reflex:
stimulus → receptor → sensory neurone →
coordinator → motor neurone → effector →
response
tap below knee → in leg → sensory neurone
→ spinal cord → motor neurone → muscle in thigh
→ leg jerks straight
Voluntary Actions
Voluntary actions involve the conscious part of the
brain and are brought about when a decision or
choice is made. For example you can choose to get up and walk away from the
computer at this moment, just because you want to! The response of moving away will
be brought about in a similar way to the reflex responses, where an impulse travels
from the co-ordinator, to an effecter, to cause a response. In the case of deciding to
stand up and walk somewhere, the co-coordinator is the CNS. The effectors are the leg
muscle cells, and the response is to move away. The difference between reflex
responses and voluntary actions is that a voluntary response has to be learned and
requires thought. The co-ordination is taking place within your brain in response to a
thought of your own.
1. Write the correct order for a reflex response using the following words:
Response, efector, receptor, coordinator, stimulus.
Voluntary and reflex actions
