Nerves and Hormones

Notes on SECTION 1: Nerves and Hormones for edexel from the cgp books syllabus.

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The Nervous System
The nervous system allows the body to respond to changes in the environment. This process is
usually co-ordinated by the brain. Reflex actions are quick responses to stimuli (things that set off
reactions in the nervous system).
Receptors ­ this is a group of specialised cells which can detect changes in the environment (stimuli)
and turn these changes into electrical impulses. Receptors are located in the SENSE organs (eye, ear,
skin etc.)
skin Touch, pressure, pain, temperature
tongue Chemicals in food
nose Chemicals in air
Eyes Light
Ears Sound and position of head
CNS ­ Central Nervous System is made up of the spinal cord and the brain. When a receptor is
stimulated, a signal is sent along the nerve cells (neurones) to the brain.
Effectors ­ This is any part of the body that can produce a response.
A muscle contracting to move the arm
A muscle squeezing saliva from the salivary gland
A gland releasing hormones into the blood
Neurones ­ Neurones are nerve cells that carry information as electrical signals. There are three
different types of neurone:
1. Sensory Neurone ­ carry signals from receptors to the spinal cord and brain.
2. Relay Neurones ­ carry messages from one part of the CNS to another.
3. Motor Neurones ­ carry signals from the CNS to effector.
The diagram above shows a typical neurone - in this case, a motor neurone. It has tiny branches at
each end and a long fibre carries the signals.
Synapses ­ This is the gap where two neurones meet. Signals cross through the gap using chemicals.
One neurone releases the chemical into the gap and the chemical diffuses across the gap and
makes the next neurone transmit an electrical signal.

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Reflex Actions
When you stimulate a receptor, a signal is sent to the CNS and the brain then co ordinates a
response, but sometimes a very quick response is needed that doesn't need the involvement of the
brain, this is a reflex action. They happen without us thinking.
E.g. If you put your hand in a hot flame, you would pull it away without thinking about it.
This is what happens:
1. Receptor detects a stimulus - change in the environment
2.…read more

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Radial muscles of the iris contract.
Circular muscles of the iris relax.
More light enters the eye through the dilated pupil.
One of the functions of the blood is to transport dissolved substances (including hormones) around
the body. Hormones are chemical substances that help regulate the many processes in the body.…read more

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Regulating Glucose in Blood
Glucose is needed by cells for respiration. It is essential that the concentration of glucose in blood
is maintained at a constant level. Insulin is a hormone produced by the pancreas that regulates
glucose levels in the blood.…read more

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Diabetes is a disorder in which the blood
glucose levels remain too high. The pancreas doesn't produce insulin, meaning the blood sugar
levels increase to levels where you could go into a coma. It can be treated by injecting insulin. The
extra insulin allows the glucose to be taken up by the liver and other tissues, so cells get the glucose
they need and blood-sugar levels stay normal.…read more

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The hormone oestrogen is secreted by the ovaries. Oestrogen makes two things happen:
1. it stops FSH being produced - so that only one egg matures in a cycle
2. it stimulates the pituitary gland to release the hormone LH
The hormone LH causes the mature egg to be released from the ovary.
Progesterone is a hormone secreted by ovaries. It maintains the lining of the uterus during the
middle part of the menstrual cycle and during pregnancy.…read more

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Enzymes are
happen too
slowly at
(37 degrees Celsius) if we did not have them
Temperature affects enzymes:
If the temperature decreases, the rate of reactions slows because the enzymes and other
molecules collide less often.
If the temperature is too hot, the enzymes become denatured. This means the active site
is changed so it no longer fits the molecule, and the lock-and-key model no longer works.
This change is irreversible.…read more

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Cooling down
Sweat glands produce more sweat when we are hot because it cools us down when it evaporates
off the skin. Water lost through sweating must be replaced otherwise it could lead to dehydration.
In humid climates, sweat does not evaporate, so it does not help cool you down.
Vasodilation is when the blood vessels near the skin fill with blood, so more heat energy is
transferred to the environment (opposite of vasoconstriction). Vasodilation and vasoconstriction
are antagonistic effectors.…read more


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