Biology Unit 2 - The Human Reactive System and Drugs

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Biology Revision - The Human Reactive
System and Drugs (Unit 2)
The Human Reactive System
1. The Nervous System
The nervous system consists of the brain, spinal cord, and a complex network of neurons.
This system is responsible for sending, receiving, and interpreting information from all parts
of the body. The nervous system monitors and coordinates internal organ function and
responds to changes in the external environment. This system can be divided into two parts:
the central nervous system and the peripheral nervous system. There are different parts of
the nervous system:
The stimulus (or stimuli) is something that you can sense - these triggers or stimulates the
nervous system.
The receptor is the part of the system that detects the stimulus and creates and electrical
impulse to be sent to the brain or a default system, carrying the message.
The sensory neurone is what the impulse initially travels along when first activated. It is the
first neurone to receive the impulse. It carries the impulse from the receptor to the spinal
The co-ordinator is the part of the body that receives the impulse and decides the action that
needs to take place before sending out another impulse describing that action. A common
example of a co-ordinator is the brain.
The motor neurone sends the impulse to the part of the body carrying out the action i.e. to
the muscle groups. It takes the impulses from the spinal cord to an effector.
The effector is the part of the body that carries out the response to the stimulus e.g. the
muscle groups.
The relay neurone links two different neurones together. They are commonly found in the
CNS. They carry impulses through the spinal cord and up to the brain, and then from the brain
back along the spinal cord.
2. The CNS (or Central Nervous System)
The CNS is made up of two main parts: the brain and the spinal cord. Together, they can send
out signals to the body through the nerves and then back to the brain, which can then act
accordingly to the stimuli.
3. The PNS (or Peripheral Nervous System)
The PNS made up of two different parts: the cranial nerves and the spinal nerves. Again, they
have a similar job to the CNS, detecting stimuli and sending signals back to the brain.
4. Neurones and how they work
There are three main types of neurones: sensory neurones, relay neurones and motor
neurones. Their main function is to carry messages in the form of electrical impulses around
the body. A nerve is a bundle of neurones.

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The motor neurone goes from the Co-ordinator to the effector, in the CNS. The direction of
the impulse goes towards the motor end plates on the muscle fibres at approximately
100m/s. This is due to the myelin sheath - the fat insulates the electricity, allowing the
impulse to speed up more than without the sheath.
5. Receptors
A receptor is an organ or a cell that is sensitive to external stimuli or changes in the
environment.…read more

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When pain is detected by a sensory receptor in your finger, it sends an impulse along a
sensory neurone to a relay neurone in your spinal cord. The relay neurone sends an impulse in
an arc via a motor neurone to an effector, a muscle, which contracts, pulling your hand away.
This is called a reflex action. It is automatic and very quick. There are other types of reflex
actions such as coughing and blinking and our pupils contracting and retracting.…read more

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Addiction is a physical or psychological need for a habit-forming substance, such as a drug or
an alcohol. In physical addiction, the body adapts to the substance being used and gradually
requires increased amounts to reproduce the effects originally produced by smaller doses.
You cannot manage without the drug when you are addicted. Dependence is a compulsive or
chronic deed; in other words, an addiction.…read more

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Damage or failure of the kidneys, resulting in the need for transplant and/or dialysis
o Damage or failure of the liver, requiring transplant and/or interferon treatment
o It causes alterations of circuitry in the brain that may create permanent impairment)
o The reproductive system (especially prenatal effects in pregnant women)
It can also cause rheumatologic problems in the body, such as arthritis. Heroin is an extremely
addictive drug meaning that people can quickly become dependent on it.…read more

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Definition: Any of a class of drugs that lower the levels of low-density lipoproteins (LDL)
cholesterol in the blood by inhibiting the activity of an enzyme involved in the production of
cholesterol in the liver.
What are their uses?
Statins are medicines, so they are good for you. They are usually prescribed to healthy
people, but who are at risk from high cholesterol levels. These people have a risk of heart
disease, a heart attack, a stroke, or peripheral artery disease.…read more

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Smoking kills. In Britain alone, nearly 100,000 people die each year from diseases caused by
smoking. This is twelve times the number killed in road accidents.
Tobacco smoke contains about 1000 chemicals, many of which are extremely harmful to the
body. Here are some of the poisonous ones:
Nicotine is a poisonous drug. It damages the heart, blood vessels and nerves. Smokers get
addicted to it, which is why they find it hard to give up.…read more

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Pain lung action can be a common symptom of lung cancer
because it is a bi-product of the restricted airflow in the lungs.
Other diseases include peripheral vascular disease, which can lead to limb loss, ischaemic
heart disease, strokes, bladder cancer, memory loss, exacerbation of asthma, and tongue
and lip cancer.
Many non-smokers find smoking highly unpleasant, as it makes the eyes sting, and can cause
sore throats and headaches. It can irritate babies and people with hay fever and asthma.…read more

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If a woman smokes tobacco whilst pregnant, miscarriage or stillbirth is likely. If the baby is
born, it is likely to be a premature birth and the baby is likely to be underweight or to have
various types of birth defects.
A lot of smoker's babies die for unknown reasons. This is called sudden infant death
syndrome or "crib death".
Menstrual disorders and early menopause are both common in women users.
Short-term effects of Tobacco
Nicotine reaches the brain in a very short time.…read more

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It can also lead to an increased risk of a variety of cancers, particularly breast cancer and
cancer of the gullet. It is also frequently associated with mental health problems, such as
depression and anxiety.…read more



This is an all you need to know set of notes with diagrams on the nervous system and drugs. The notes are detailed and explain the key points of the topics well. They would be suitable for any candidate needing to know about the nervous system.

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