Nerves & Hormones

Our nerve and hormone systems help us to control and coordinate our bodies. Usually receptors in sense organs pick up changes inside the surroundings of our bodies. One of these systems coordinates an appropriate response to the change.

Sense organs:

The eyes are receptors for sight. The nose is a receptor for smell. Skin is a receptor for temperature, touch and pain. The mouth is a receptor for taste. Ears are receptors for sound.

Receptors are cells which recieve imformation or stimuli. They are found all over the body. E.g. the eye has receptors called rods and cones, found on the retina.

A nervous response consists of: Stimulus, Receptor, Sensory Neuron, Brain, Motor Neuron, Effector, Response.

Hormones are chemicals produced by glands. They are carried around the body in the blood and affect their target organs.

The reflex ark is the pathway taken by a nerve impulse in a reflex response. It happens very quickly because the brain is missed out. Instead, the spinal chord co-ordinates the response.

The main steps to a reflex action:

• A receptor detects a stimulus (e.g. a sharp pain)
• A sensory neuron transmits the impulse to the CNS (central nervous system)
• A relay neuron passes the impulse on.
• A motor neuron is stimulated.
• The impulse passes to an effector (muscle or gland)
• Action is taken.

A synapse is a gap between two neurones. The electrical impulse cannot cross it. Instead the end of the neurone releases a chemical which diffuses the synapse and causes an electrical impulse in the next neurone.

The body carefully controls its internal environment. Controlled internal conditions include:

• Water Content
• Ion Content
• Temperature
• Blood Sugar Level

Water is constantly leaving the body as we breathe out and sweat. We lose other excess water in the urine. We also lose ions by sweating and in the urine. We must keep our temperature level constant otherwise the enzymes in the body will not work properly. Sugar in the blood is the energy source for cells. The level of sugar in our blood is controlled by the pancreas.

The process which keeps everything constant is called homeostasis.

If the body has too much sugar some is converted into glycogen, for storage. If the body is too cold, you will shiver or develop hypothermia. If the body has too mcuh water, you produce more urine. If the body has insufficient levels of salt, you will crave salty foods.

FSH is made by the pituitary gland. It cause the egg to mature and oestrogen to be produced.

Oestrogen is produced by the ovaries and stops further FSH production. It stimulates the production of LH and also stimulates the womb lining to develop to recieve the fertilised egg.

LH is made by the pituitary gland and stimulate sthe mature egg to be released from the ovary, also known as ovulation.

The menstrual cycle takes approximatley 28 days and ovulation happens 14 days into the cycle.

Progesterone is a steroid hormone which is released by the corpus luteurn that stimulate the uterus to prepare for pregnancy. This will protect the child and stops the release of LH and FSH.

A high concentration of oestrogen will stop further FSH production and release LH which the egg needs to be released. If the concentration is not high enough the FSH will not of had enough time for the egg to mature and the woman may not get pregnant. A few days before LH releases the egg, oestrogen levels will start to decrease.

The oral contraceptive or the contraceptive pill greatly reduces the chances of mature eggs being produced. The pill contains oestrogen, or oestrogen and progesterone. These hormones inhibit the production of FSH, which in turn stops eggs maturing in the ovaries.

The advantages of this are that it helps to reduce family size which has stopped poverty in many areas, it also allows women to plan their pregnancies. Disadvantages are that it can cause side effects and some people object to use it for religious and ethical reasons.

Some women have difficulty becoming pregnant because they don't produce enough FSH to allow their eggs to mature. 'Fertility drugs' contain FSH, which stimulates eggs to mature in the ovary.

The advantages of this are that they can help infertile couples having IVF and IVF helps couples to have a baby. The disadvantages are that IVF is an expensive process. Some people believe it is unethical to have babies by IVF and extra embroyos may be stored or destroyed

A tropism is a growth in response to a stimulus. Plants grow towards sources of water and light, which they need to survive and grow.

Posotive Tropism - Towards the stimulus.

Negative Tropism - Away from the simulus.

Phototropism - Growth in response to the direction of light.

Gravitropism - Growth in response to the direction of gravity.

Hydrotropism - Growth in response to the direction of moisture.

Auxin is the hormone which controls phototropism and gravitropism. Unequal distribution of this can lead to bending of the shoot or root.

Plant hormones can be used by farmers and gardeners. Weedkillers are used to kill unwanted plants on lawns. If plant hormones are used incorrectly they can cause damage o the environment.

Name the two main types of effector in the human body.

Name three stimuli which affect humans.

What effect does oestrogen have on FSH production?

Sate two internal conditions which need to be controlled.

How are impulses transmitted across a synapse?

Name a plant hormone.

What is gravitropism?

Why do plant shoots bend towards light?

How does the contraceptive pill work?

Why are FSH and LH given in fertility treatment?