Biology 1a 2

the nervous system allows you to react to what is going on around you

HideShow resource information

Nervous system

A stimulus is a change in your environment. Your five sense organs contain receptors. Receptors  are a group of cells which are sensitive to a stimulus. They change stimulus energy into electrical impulses.

The central nervous system consists of you brain and spinal cord only. Neurones are nerve cells which transmit electrical impulses to and from the central nervous system.

Sensory neurone - the nerve cells that carry electrical implses from the receptors in the sense organs to the central nervous system.

Relay neurone - the nerve cells that carry signals from the sensory neurone to the motor neurone.

Motor neurone - the nerve cells carry signals from the central nervous system to the effector muscles or glands.

Effectors - muscles contract in response to nervous impulses, whereas glands secrete hormones.

1 of 11

Reflexes and Synapses

The connection between two neurone 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.

Reflexes are automatic responses to certain stimuli. The passage of information in a reflex is called a reflex arc.

2 of 11

Reflex Arc

The neurones in reflex arcs go through the central nervous system.

When a stimulus is detected by receptors, impulses are sent along a sensory neurone to the CNS.

When the impulses reacha synapse they trigger chemicals to be releases, which cause impulses to be sent along a relay neurone.

When the impulses reach a synapse the same thing happens. Chemicals are released and impulses are sent down a motor neurone.

The impulses then travel to the effector.

The muscle then contracts and moves and moves your hand away.

Because you don't have to think about the response, it's quicker than normal responses.

3 of 11


Hormones are chemical messengers which travel in the blood to activate target cells.

Hormones are produced and secreted by various glands, they travel at the speed of your blood.

Nerves and hormones:

-nerves have very fast messages, hormones have slower messages.

-nerves act for a very short time, hormones act for a long time.

-nerves act on a precise area, hormones act in a more general way.

4 of 11

The Menstrual cycle

The pituitary gland - produces FSH and LH which are involved in the menstrual cycle. Ovaries produce oestrogen.

Day 1 - the bleeding starts (uterus lining breaks down)

Day 4-14 - the lining of the uterus builds up again ready to recieve a fertilised egg.

Day 14 - an egg is developed and then released from the ovary

Day 14-28 - the wall is maintained for 14 days, f there is no fertilised egg the lining of the uterus breaks down and the whole cycle starts again

5 of 11

Menstrual cycle - 3 hormones

There are 3 main hormones involved:

1. FSH (follicle stimulating hormone) - causes an egg to mature in one of the ovaries, stimulates the ovaries to produce oestrogen. Produced by the pituitary gland.

2. Oestrogen - causes pituitary to produce LH, inhibits the further release of FSH. Produced in the ovaries.

3. LH (lutenising hormone) - stimulates the release of an egg at around the middle of the menstrual cycle. Produced by the pituitary gland.

6 of 11

Controlloing Fertility

Oestrogen can be used to prevent the release of an egg, so it can be used as a method of contraception. If oestrogen is taken everyday to keep the level permanently high, it inhibits the production of FSH, production will stop and stay stopped.

Progesterone also reduces fertility, it prevents any sperm from getting through and reaching the egg.

The pill is an oral contraceptive which contains levels of oestrogen and progesterone. It can cause side effects and is 99% effective.

Some women have levels of FSH increased by injection to make their eggs mature so that they can get pregnant, it stimulates an egg release in the ovaries. It can be very expensive and too many eggs could be released causing multiple pregnancies.

7 of 11

Plant hormones

Auxin is a plant hormone that controls growth near the tips, shoots and roots, in respone to light which is called Phototropism, in response to gravity Gravitropism, and moisture Geotropism.

When a shoot tip is exposed to light, more auxin accumulates on the side thats in the shade, this makes the cells grow faster on the shaded side making the shoot bend towards the light.

When a shoot is growing sideways, gravity produces an unequal distribution of auxin, with more auxin on the lower side, causing the lower side to grow faster, bending upwards.

A root growing sideways has more auxin on the lower side, but in a root extra auxin inhibits growth, the cells on top grow quicker so the root bends downwards.

Roots grow towards moisture, uneven amount of moisture either side of a root produces more auxin on the side with the most moisture, it inhibits growth that side causing it to bend towards the moisture.

8 of 11


Some of the chemical changes caused by drugs can lead to the body becoming addicted. Of a drug isn't taken, an addict can suffer withdrawal symtoms.

There are 3 types of drugs:



-performance enhancing

9 of 11

Drugs in sport

Some athletes take performance enhancing drugs, there are several different types including anabolic steroids(increase muscle size) and stimulants(increase heart rate).

Steroids can cause high blood pressure.

For drugs: athletes have a right to make thier own decision about the risk of taking drugs. Drug free sport isn't really fair, different athletes have different training facilities, coaches equipment etc.

Against drugs: it's unfair if people gain an advantage by taking drugs. Athletes may not be fully informed of the serious health risks of the drugs they take.

10 of 11

Testing medical drugs

There are 3 main stages in drug testing:

1. Human cells and tissues

2. Live animals

3. Human volunteers in a clinical trial - blind trials are when a patient doesn't know if they are getting a drug or a placebo. Double blind trials are when neither the doctor nor the patient knows until the results are gathered.

Drug testing gone wrong:

Thalidomide was an intended sleeping pill, but it was found to be effective in relieving morning sickness. It hadn't been tested for morning sickness, it wasn't known that it could be passed through the placenta, so babies were born with limb abnormalities. 10,000 babies were affected and only half survived. It was banned, now it is only used for the treatment of leprosy.

11 of 11


No comments have yet been made

Similar Biology resources:

See all Biology resources »See all Nervous system, hormones and behaviour resources »