The Nervous System
A stimulus is a change in your environment which you may need to react.
A stimulus can be light, sounds, touch, pressure, pain, chemical or a change in temperature.
Receptors are a group of cells which are sensitive to a stimulus. They change stimulus energy (e.g. light energy into electrical impulses).
The Central Nervous System
The Central Nervous System (CNS) is where all the information from the sense organs is sent, and where the reflexes and actions are coordinated.
The CNS consists of the brain and spinal cord only.
Neurones (nerve cells) transmit the infromation (as electrical impulses) very quickly to and from the CNS.
"Instructions" are then sent from the CNS to the effectors (muscles and glands), which respond quickly.
Muscles and glands are known as effectors - they respond in different ways. Muscles contract in response to a nervous impulse, whereas glands secrete hormones.
The nerves cells that carry signals to the effector muscles or glands.
The nerve cells that carry signals as electrical impulses from the receptors in the sense organs to the CNS.
Reflexes are automatic responses to certain stimuli - they can reduce the chances of being injured.
For example, if someone shines a bright light in your eyes, your pupils automatically get smaller so that less light gets into the eye - this prevents them being damaged.
The passage of infromation in a reflex (from receptor to effector) is called a reflex arc.
The Reflex Arc
The Reflex Arc goes through the CNS
1) The neurones in reflex arcs go through the spinal cord or through the unconscious part of the brain.
2) When a stimulus (e.g. a painful bee sting) is detected by receptors, an impulse is sent along a sensory neurone to the CNS.
3) In the CNS the sensory neurone passes on the message to another type of neurone - a relay neurone.
4) Relay Neurones relay the impulse to a motor neurone.
5) The impulse then travels along the motor neurone to the effector.
6) The muscle then contracts and moves your hand away from the bee.
7) Because you do not have to think about the response it is quicker than normal responses.
The connection between two neurones (nerve cells) is called a synapse.
The nerve signal is transferred by chemicals which diffuse (move) across the gap.
These chemicals then set off a new electrical signal in the next neurone.
Hormones are chemical messengers sent in the blood to activate target cells.
They are carried in the blood plasma to other parts of the body, but only affect particular cells (called target cells).
The Pituitary Gland - this produces many important hormones including: LH, FSH and ADH (which controls water content).
Pancreas - produce insulin for the control of blood sugar.
Ovaries - produce oestrogen, which controls the menstrual cycle and promotes all female secondary sexual characteristics during puberty, e.g. extra body hair, breasts develop.
Testes - produce testosterone, which promotes all male secondary at puberty, e.g. extra hair and males voice tend to be deeper.
Hormones control the different stages of the menstrual cycle:
FSH (Follicle- Stimulating Hormone):
- produced by the pituitary gland.
- causes an egg to mature in one of the ovaries.
- stimulates the ovaries to produce oestrogen.
- produced in the ovaries.
- causes pituitary to produce LH.
- inhibits the futher release of FSH.
LH (Luteinising Hormone):
- produced by the pituitary gland.
- stimulates the release of an egg at around the middle of the menstrual cycle (day 14).
The hormones oestrogen can be used to prevent the release of an egg - so oestrogen can be used as a method of contraception. The pill is an oral contraceptive that contains oestrogen.
- the pill's over 99% effective at preventing pregnancy.
- it reduces the risk of getting some types of cancer.
- It is not 100% effective - there is still a very slight chance of getting pregnant.
- It can cause side effects like headaches, nausea, irregular menstrual bleeding, and fluid retention.
- It does not protect against sexually transmitted diseases (STDs).
However, it can increase fertility also..
Some women have levels of FSH that are too low to cause their eggs to mature. This means that no eggs are released and that the women is unable to get pregnant.
The hormones FSH can be taken by these women to stimulate egg production in their ovaries.
(FSH stimulates the ovaries to produce oestrogen, which stimulates the pituitary gland to produce LH, which stimulates the release of an egg).