The Human Body Background notes 10
10. The nervous system: making sense
Our body is made of many components. For our body to function, flesh, blood and bone must work together.
Our nervous system, made up of a complex brain and 75 kms of nerves, orchestrates everything that goes
on inside us. Each of us has a unique personality. We have feelings, memories and thoughts. Our nervous
system organises and coordinates our mental, emotional and physical processes. It organises what we think
and how we behave.
Our nervous system is a massive communication network divided into the central nervous system (CNS) –
the brain and spinal cord and the peripheral nervous system. The job of the CNS is to receive, analyse and
store information from sensory nerve cells located all over the body. It also transmits relevant information
back to the body through the nerves of the peripheral nervous system. These messages may cause muscles
to move or glands to release hormones, for example. Each nerve contains thousands of axons, which are
long projections from nerve cells called neurons.
The Nervous System
Brain
Our brain is protected by our skull and is surrounded by a
liquid called cerebrospinal fluid (CSF). CSF protects the
brain from sudden jarring against the skull. It is divided
lengthways into halves; each half controls the opposite
side of the body. Inside the brain is a series of spaces
called ventricles, which produce CSF.
Spinal cord
The spinal cord extends from the base of the brain along
the whole length of the spine. The bony spine protects the
spinal cord from damage. Our spinal cord is bathed in
cerebrospinal fluid, which not only nourishes it, but also
acts as a shock absorber. Spinal nerves connect with
nerves of the peripheral nervous system, which enter the
spinal cord through gaps in the vertebrae. This set-up
allows nervous messages to be carried from the body to
the brain, and vice versa.
Peripheral nerves
The peripheral nerves deliver messages from the central
nervous system to every part of the body. Our peripheral
nerves also receive and relay messages from our sensory
receptors – our eyes, nose, mouth, ears, skin and internal
organs – to our brain, via the spinal cord. Some peripheral
nerves, such as those that cause our digestive system or
heart muscles to contract, carry messages to or from our
body parts without our conscious awareness. These
nerves are part of the autonomic nervous system. Other
messages, such as those that cause our skeletal muscles
to move, are consciously generated.
(Left to right) : Multipolar motor neurons. Source: Monash University;
Motor end plate. Source: University of Melbourne.
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The Human Body Background notes 10
The brain
Our brain has hundreds of billions of neurons, each connected to hundreds of other neurons. With trillions of
neural connections, our brain is able to process an enormous amount of information very quickly. As we
experience life, our brain stores information as memories. Each person has unique experiences and
memories. Our brain’s short-term memory, like holding such information as a phone numbers, soon fades.
Our long-term memory may hold information, like a popular tune or how to ride a bike, for a lifetime.
Memories are not stored in one location in the brain. Our memory is found in various locations scattered all
over the surface of our brain.
The thinking brain: the cerebral cortex This is
the grey outer layer of the brain, often referred to
as grey matter. It stores memories, solves
problems, and tells muscles when to contract. It is
the part of our brain associated with intelligence
and complex behaviour.
The sensitive brain: the somatosensory
cortex
This part of the brain receives nerve impulses
from sensory neurons in our body, especially the
skin. It receives and monitors information about
our environment and relays this information to
other parts of the brain.
The balanced brain: the cerebellum This part
controls the body’s balance and coordination. It
receives information from sensory nerves that tell
it what the body is doing and it sends messages
back to control the muscles in response to this
information.
The perceptive brain: the thalamus
All of our sensory information, from our sensory
nerves and receptors, comes into the brain via
this part of the brain. It filters out the information
that is important from that which is not. Pain is
detected by this part of the brain.
The developmental brain: the hypothalamus
This part regulates many important body
functions such as our temperature, salt and water
input and output, our emotions and hormones
and when we reach puberty.
The remembering brain: the hippocampus
This part of our brain allows us to remember
things from our past, to learn and be motivated. It
allows us to express our feelings and emotions.
The regulatory brain: the brain stem
This is the basic control centre of the brain. It
controls sleep and arousal, our heart rate
respiration and digestive processes. It is also
responsible for our basic body reflexes.
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Nerve cells are called neurons
Nerve cells are called neurons. They are the cells that relay messages in the brain and the nervous system.
Neurons have a large star-shaped cell body with a nucleus inside. Dendrites are small finger-like extensions
that extend from the cell body, making connections and receiving input messages from surrounding nerve
cells and tissue. An axon is a long process, that may extend to a metre in length that carries electrical
messages from the neuron to other cells, tissues and organs of the body. These are the signals that tell the
body what to do and how to respond to the environment. Neurons have different shapes depending on
where they are, where they send messages to and what messages they are conveying.
Sensory neurons detect stimuli from internal tissues and organs and external stimuli that affect the body.
They allow us to detect smell, light (sight), taste, sound (hearing), and changes in our body position due to
gravity (balance). There are also general sensory nerve receptors, such as those for pressure (touch),
temperature and pain. Motor neurons conduct impulses from the central nervous system, through myelinated
axons that travel to muscle tissue in the body to make them contract
(Left to right) : Brain neuron, motor neuron, sensory neuron. A neuron stained with a fluorescent dye. Like many neurons this cell has
lacy dendrites that receive impulses and the main axon that carries impulses to other cells. Source: John Wiley & Sons, STM Journal
Production.
Alive with electricity
Nerves are like cables, with nerve axons as the
individual strands. Messages flash along axons at
400kph, as tiny electrical impulses. Each impulse is
triggered by a stimulus, such as heat or a chemical.
Impulses are transmitted from one neuron to the next
across a small gap, called a synapse. When an
impulse reaches the end of a neuron, a chemical is
released into the synapse. This chemical, called a
neurotransmitter, triggers an electrical impulse in the
next neuron, and so the message continues. Nerve
impulses travel in only one direction along a neuron
otherwise the nervous system would collapse in a
confusion of messages.
This illustration shows a cut away of a nerve ending releasing
neurotransmitters that diffuse across the synapse and bind to the
next cell. This may initiate an electrical event in another neuron or
cell.
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The Human Body Background notes 10
Neurotransmitters: mind-altering chemicals
Neurotransmitters are chemicals that control how nerve cells communicate with each other to process
information and send messages to the rest of the body. They can stimulate or reduce the nerve impulse that
is travelling along a certain nerve path.
Neurotransmitters are central to memory, learning, mood, behaviour, sleep, pain perception and sexual urge.
There are many different neurotransmitters. Serotonin and dopamine affect our mood, and endorphins
reduce our perception of pain. The neurotransmitters, glutamate and GABA are important to many brain
functions, from memory to sleep. Many drugs act by mimicking or interrupting the effect of neurotransmitters
in the brain.
Neurons need care and defence
Nerve cells can live for over 100 years. A network of
supporting cells ensures that nerve cells receive
nutrients and are protected from infection.
Astrocytes are cells found in large numbers
in brain tissue. They exchange nutrients between
the blood and brain tissue, and service neurons
so they remain strong and healthy. Astrocytes
are star shaped and have long branching extensions
that wrap around blood vessels and the nerve
cells that they are servicing.
Myelin sheaths insulate nerve bundle and allow
electrical nerve messages to travel along nerve axons
very quickly.
The thick myelin sheath that surrounds nerve axons is shown in the
image above. Source: University of Melbourne
Microglial cells are small cells that occur in the brain
in relatively small numbers. They have a tiny nucleus inside a very small cell body. They also have many
highly branched feathery processes that extend from the cell body and connect to surrounding tissue. They
respond to tissue damage by transforming into large defender cells that engulf and destroy invading cells
and damaged tissue.
When things go wrong:
Alzheimer’s Disease
Alzheimer’s Disease causes irreversible degeneration of the brain. Over time, people with Alzheimer’s suffer
loss of memory, confusion and a reduced attention span. As the disease progresses, loved ones are no
longer recognised, and even language is lost. These symptoms usually appear over several years and are
due to changes in parts of the brain that deal with thinking and memory. Examination of affected brains
shows that neurons become tangled into clumps and eventually die and the protective myelin sheath may
also deteriorate. Although usually seen in elderly people, Alzheimer’s disease may begin in middle age.
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The Human Body Background notes 11
11. The hormonal system: the messengers
Hormones have powerful control over our bodies. They are invisible chemical messengers made by our
endocrine glands, which secrete their content into the blood stream. Hormones keep us alive by maintaining
the balance of nutrients in our blood, as well as regulating our energy levels. They influence the way our
bodies develop and grow. They may be tiny, but hormones have a huge influence in the way our bodies
work.
Our bodies produce many different hormones. A hormone influences whether a particular cell will produce a
substance or not. It can control whether our cells will grow and divide, or move around the body, or when
they might die. Different hormones affect different target cells. Some hormones, called growth factors, target
cells that are very close to where they are released. Others might have an effect at remote sites around the
body.
Hormone secreting glands and organs
Glands are organs of the body that are designed specifically to produce hormones. Some of these glands
are quite small. Some organs produce hormones as well as perform other functions in the body.
The Pituitary gland is a pea-sized gland attached to the
brain by a stalk and it sits in a bony cavity. The pituitary
gland releases at least eight different hormones. One of
these is growth hormone, which is important for our growth.
The Pineal gland is a tiny gland located deep within the
brain. It produces melatonin, which helps regulate our body
clock and body rhythms.
The Thyroid gland is shaped like a butterfly and wraps
around the trachea. It produces thyroid hormone, which
regulates our energy levels.
The Parathyroid glands are so small we can hardly see
them. They are attached to the back of the thyroid and
secrete parathyroid hormone, which controls the levels of
calcium in our bodies.
The Adrenal glands are attached to the top of each of our
two kidneys. These glands release adrenaline to increase
our heart rate and blood pressure and enable us to deal
with physical, emotional and mental stress.
The Hypothalamus is a region of the brain that produces
a variety of hormones, including those responsible for the
onset of puberty such as gonadotrophin-releasing
hormone, which controls the production of sex hormones.
The Pancreas produces substances important for the
digestion of our food together with the hormones insulin
and glucagon which together regulate our blood sugar
levels.
The Testes produce sperm in men, as well as the male
sex hormone testosterone, which promotes sexual
development.
Gland tissue has ducts into which hormones are
secreted into.
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The Ovaries are responsible for the production of eggs in
women, but they also produce the female sex hormones
oestrogen and progesterone crucial for the sexual
development of women.
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The Human Body Background notes 11
Reaching their target
To have an effect, hormones must reach their target cells. A target cell has receptors on its surface, or
inside it, to which a hormone attaches.
Hormones travel in the blood until they bind to the receptors of their target cells. Some receptors are located on the cell surface.
When a hormone binds to its receptor it sets off a series of responses within the target cell. Other hormones are able to slip
through the cell membrane and bind to receptors within the target cell.
Many drugs act by blocking or mimicking the binding of hormones to receptors. This is because they are
similar in shape to the hormone. The drug ephedrine affects our bodies very much like the naturally occurring
hormone adrenaline. Ephedrine binds to the adrenaline receptor and mimics its actions. Ephedrine is used
as a nasal decongestant.
When things go wrong
Not enough hormone – Insulin and diabetes
After a meal, when blood sugar levels are high, the pancreas releases the hormone insulin. Insulin
stimulates our fat and muscle cells to absorb glucose from the blood. Our pancreas releases about 2
milligrams of insulin into our bloodstream daily. This means that a teaspoonful of insulin (about 15 grams)
would be enough to regulate glucose levels for a period of 25 years.
Diabetes mellitus type 1 is a hormonal disorder caused when the pancreas does not produce insulin. People
that have diabetes mellitus type 1 must inject insulin regularly to ensure that their cells get enough glucose to
function properly.
Too much hormone – Testosterone and anabolic steroids
Anabolic steroids are drugs that act like the male hormone, testosterone. One effect of testosterone is to
promote muscle growth. The side effects to steroid use include aggressive behaviour, high blood pressure,
liver damage, menstrual problems in women, and impotence in men.
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The Human Body Background notes 12
12. The immune system: the defenders
The Immune System
Bacteria, viruses and parasites may cause disease if they enter our bodies. Our skin and the membranes
that line our body openings are excellent protective barriers. They are the first-line of defence against
invaders. However, sometimes germs get through and our immune system protects us against these
intruders. Our immune system also recognises and destroys sick or damaged cells of the body. It is primarily
made up of white blood cells, which recognise and protect healthy cells that belong to our body and wage
war against anything else.
White blood cells – Our protectors
Cells have distinctive membranes that are
recognised by our immune system. The cells in our
immune system can distinguish between foreign
cells, bacteria, viruses and those belonging to the
body. Foreign cells are attacked and body cells are
left alone. White blood cells play a major role in
immunity. They patrol our whole body, but are
concentrated in the lymph nodes and lymphatic
system.
Lymph, lymph vessels and lymph nodes
Some of the fluid that baths the tissues of the body
returns directly to the blood. The rest is collected
into open ended lymph vessels that drain this
lymph fluid through a network of progressively
larger vessels. The lymph is then filtered and
cleaned as it passes through lymph nodes Lymph
nodes are filled with immune cells (lymphocytes
and macrophage) that wait to destroy invaders.
Clusters of lymph nodes are located in the neck,
armpits, groin and gut. The clean lymph is
eventually drained back into the bloodstream.
The thymus
The thymus lies close to the heart. It is large in
childhood when it is most active however it stops
growing at adolescence and starts to shrink. By old
age it is hardly noticeable. The thymus is where
immune cells called T cells develop. It is within the
thymus that T cells learn to react to foreign
invaders and not to the body’s own cells.
The spleen
The spleen is covered in a thin membranous
capsule that can be ruptured by a sharp blow or a
severe infection.
It is responsible for cleaning the blood by removing
old blood cells, debris, bacteria, viruses and toxins.
The spleen also stores white blood cells
(lymphocytes).
The bone marrow
Blood cells don’t last forever. In time they have to
be replaced by new ones. Bone marrow is a major
site of white and red blood cell production. Growth
factors produced by cells in the marrow stimulate
the growth and activation of immune cells.
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The Human Body Background notes 12
The Non Specific Immune Response - fast and furious
A particular group of white blood cells and white cell
products respond immediately to injury and attack by a wide
range of invaders. Macrophages, neutrophils and killer cells
play a major role in the immediate non-specific immune
response to tissue injury, inflammation and infection.
Macrophages hunt down and engulf dead cells and foreign
invaders all over the body. This process is called
phagocytosis. Once inside the macrophage the particle is
digested by corrosive chemicals that are stored in
lysosomes. Macrophages also secrete substances called
cytokines, which attract other immune cells to the site of
infection.
Immunological cell products are also released in response to
tissue invasion and inflammation. Interferon is a group of
proteins that defend the body against viral invasion. The
Complement System is another group of proteins activated
by the immune system, to attack foreign cells by attaching to
their membrane.
Macrophage engulfing particles.
Source: California Institute of Technology
Platelets: sticking and plugging
Platelets are flattened cell fragments. They plug holes in
broken blood vessels and are very important in the
inflammatory and repair process of injured tissue.
What’s pus
Pus is a mixture of dead and dying neutrophils, dead tissue
cells, and living and dead infectious agents.
Inflammation
When body tissues are injured, a general (or non-specific)
immune response called inflammation occurs.
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Platelets are produced in bone marrow from large
cells called megakaryocytes.
Source: Peter MacCallum Cancer Institute
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The Specific immune response - slow and steady
The specific immune response refers to the activation of lymphocytes to attack specific invading cells that the
body has encountered before and is now primed to recognise and specifically attack and destroy. There are
two types of specific immune responses: the cell-mediated response is activated by T lymphocytes (T cells);
and the antibody-mediated response activated by B lymphocytes (B cells).
Cell-mediated immune response
T cells recognise certain bacteria and viruses, and they kill cells that may be infected by them. They also
attack body cells that are cancerous or damaged. A particular group of T cells, called killer T cells, attach to
infected or foreign cells and releases substances that kill them.
Antibody-mediated (humeral) immune response
Any foreign body or particle that enters the body and can potentially cause harm, is referred to as an antigen
- bacteria, bacterial toxins, viruses and foreign tissues. When a specific antigen enters the body, B cells
respond by producing proteins called antibodies. These antibodies attach themselves to the antigen and
form an antibody-antigen complex.
Antibodies can cause antigen destruction in different ways:
1. An antibody may transform the shape of a toxic chemical in such a way that it cannot enter cells of the
body or cause detrimental affect to them. This is called antibody neutralisation.
2. Sometimes antibodies cross-link with many cellular antigens in a process called agglutination, to form
large clusters or clumps of antigens – for example bacterial cells or even foreign blood cells. Non-specific
immune cells are then relied upon to digest and remove these clusters from the body.
3. The most important role of antibodies however is to assist and enhance the role of the bodies non-specific
immune responses. This occurs when antibody-antigen complexes activate the complement system,
enhance phagocytosis by macrophage and neutrophils and activate cell lysis by natural killer cells.
(Left to right) : Magnification of a lymphocyte in amongst blood cells. Source: University of Melbourne; B cells secrete unique antibodies
that protect against specific foreign agents.
The DNA code for every new antibody is stored in the B cells genetic memory, its DNA. If the body has no
memory of a particular antigen it must make antibodies against it from scratch. This, however, can take
several days. In the case of first time infections, especially if the pathogen is virulent or if the individual’s nonspecific immune system is weak, the invading organisms may be able to multiply and cause disease
symptoms within the time it takes for the body to make specific antibodies against it.
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The Human Body Background notes 12
What invaders?
Bacteria
Bacteria are single cell organisms that divide rapidly. They are a vital part of our planet although a few kinds
of bacteria can be harmful if they enter our bodies. These bacteria can multiply rapidly inside the body and
release poisons, which affect the body in distinctive ways.
Viruses
Viruses are unique organisms. They do not consist of cells. Many scientists dispute whether or not they
should be classified as living things at all. This is because these tiny infective organisms – simply comprised
of a small strand of genetic material surrounded by layers of a protein capsule – can only reproduce and
complete their lifecycle when they inject their genetic material into a host cell.
Not all viruses are detrimental to human health.
Some viruses can lay dormant inside a cell and
have relatively no affect on our cells or our health.
Other viruses target specific cells of the body and
multiply by tricking the cells to replicate their
genetic material. The cell also, unknowingly,
makes new virus capsules. When the replicated
viral DNA is packaged into the new capsules, the
virus kills the cell by bursting out of it. The new
viruses infect new cells, and the process
continues, often resulting in specific disease
patterns that we are able to recognise.
Life cycle of a virus – infecting a cell, multiplication and cell lysis
(breakdown). Source: Janssen – Cilag Pty Ltd
When things go wrong
Cancer
Most cells are unable to divide more than a few times. Cancer cells, however, divide uncontrollably to form a
mass of cells that invades surrounding tissue. Cancer cells are often recognised by our immune system as
being foreign. However, sometimes these cells reproduce very rapidly and overwhelm our bodies’ defences.
Auto-immunity
The immune system can turn against itself. Our bodies can produce antibodies and sensitised T cells that
destroy our own tissues. This phenomenon is called an auto-immune disease.
Immunodeficiency
Immunodeficiency occurs when the immune system isn’t working properly. When this happens, the body's
defences against bacteria, viruses and parasites are weakened. There are many degrees of
immunodeficiency and many triggers.
The immunodeficiency of most concern today is AIDS (acquired immunodeficiency syndrome), which is
caused by a virus called HIV (human immunodeficiency virus). This virus enters the body by blood
transfusions, or blood-containing needles, or during intimate sexual contact that may allow the virus access
to blood. HIV destroys a particular group of immune cells called T cells. In time, AIDS makes the immune
system unable to protect the body against infection.
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