Unit 3
Topic 3 Control & Communication
Pupil Notes
Nervous Control
Animals have a system of neurons (nerve cells) which allows for rapid communication and
control of the organism.
Our nervous system is divided into two parts
1. The Central Nervous System (CNS), which consists of the brain and spinal cord
2. The Peripheral Nervous System which consists of all other nerves in the body.
The Brain
The diagram shows the brain
The functions of these parts of the brain are
Medulla:
The medulla is located at the top of the spinal cord.
It controls various essential involuntary functions such as heart rate and breathing. These
functions are described as involuntary as they occur without your conscious thought.
Cerebellum:
The cerebellum is located behind the medulla and below the cerebrum and controls functions
such as balance and movement coordination.
Cerebrum:
The cerebrum is the largest part of our brains and controls higher order functions such as
thought, perception, memory and personality.
The cerebrum also receives information from the sense organs and sends information to
muscles responsible for conscious movements of the different parts of the body
Spinal Cord
The spinal cord carries nerve impulses between the brain and the peripheral nervous system,
but as part of the CNS it also carries out a control function. The spinal cord controls quite
basic and primitive responses such as the reflex action.
A reflex action is an automatic and almost instantaneous response to a stimulus. A reflex
response normally occurs as a result of a stimulus which indicates potential harm to the
organism.
The response is a result of the spinal cord, not the brain, which makes it both
involuntary and quick.
The rapid reflex action occurs as a result of the reflex arc. The reflex arc consists of a series
of neurons which pass through the spinal cord. The three neurons in the reflex arc are the
sensory neuron, the relay neuron and the motor neuron as shown in the diagram below.
So, if you touch something very hot this stimulus is detected by sensory receptors in your
skin.
This triggers a nerve impulse which travels along the sensory neuron to the spinal cord.
The impulse then passes along the relay neuron to the motor neuron.
The motor neuron carries the impulse to an effector which brings about the response.
In this example the effector would be the muscles in your arm which would contract to move
your arm away from the hot object.
All of this occurs in a fraction of a second and without your conscious thought. The reflex arc
is demonstrated in the diagram below.
Synapse
Synapses are tiny gaps between neurons.
The electrical impulse which travels along a neuron cannot jump this gap.
When the impulse reaches the end of one neuron it causes the release of a chemical which
diffuses across the gap between the two neurons.
When the chemical reaches the second neuron it triggers a new nerve impulse which then
travels along this neuron.
The processes of the synapse are shown in the diagram below.
First neuron
Synapse
Second neuron
Nerve impulse reaches
the end of the first neuron
Chemical is released and
passes across the synapse
Chemical triggers a
nerve impulse in
the second neuron
Hormonal Control
Not all of the control and communication which occurs between tissues and cells in an animal
are by nerve impulses.
Some signals are sent using chemical messengers in the blood stream.
These chemical messengers are called hormones. Tissues which produce hormones and
release them into the blood are called endocrine glands.
Each hormone molecule carries out a particular function in the body. But if they travel in
the blood stream, which obviously goes to all tissues, how do they only bring about a
response in their target tissues?
The answer is that the cells of these tissues have receptor molecules in their cell membranes
that are specific to the hormones which act on that tissue.
So, if hormone molecules are present in the blood stream which a particular tissue doesn't
have the receptors for, nothing will happen at that tissue. But when the hormone does appear
that it does have the receptors for, it will detect its presence and bring about the response
Control of blood glucose
A good example of a hormone system in our bodies is the way that we control the
concentration of the sugar glucose in our blood stream.
Why does the concentration of glucose in our blood stream matter?
Too little glucose and our cells won't be able to respire, too much glucose in our blood
stream and our cells will begin to lose water by osmosis.
So, our bodies aim to have a relatively constant concentration of glucose in our blood stream
by adding more glucose if it goes down and removing some if it goes up...this process is
controlled by the hormones insulin and glucagon.
Glucose molecules are single sugars.
Glucose can be stored by joining glucose molecules together to form glycogen – each
glycogen molecule consists of long chains of glucose molecules.
Glycogen is stored in the liver.
When an animal cell needs more glucose it can get it from breaking down glycogen.
The diagram below summarises the relationship between glucose and glycogen.
Glucose is stored as glycogen in the liver.
Changes in blood glucose concentration are detected in the pancreas.
The two hormones insulin and glucagon are released from the pancreas into the bloodstream
and both are detected by the cells of the liver but they bring about different responses.
Insulin
Insulin is released by the pancreas into the bloodstream when the concentration of glucose in
the blood rises, for example after eating a meal.
Liver cells detect the presence of insulin in the blood which causes the conversion of glucose
to glycogen.
This lowers the concentration of glucose in the blood returning it to normal.
Glucagon
Glucagon is also a hormone released by the pancreas.
Glucagon is released when the concentration of glucose in the blood falls, for example after
exercise.
When glucagon is detected by the liver cells they convert stored glycogen to glucose which
raises the blood glucose concentration back to normal