1.1 The nervous system and hormones allow us to respond to external changes.
1.1.1 They also help us control the conditions inside our bodies.
22.214.171.124 Hormones are used in some types of
contraception and fertility treatment.
2 The Nervous system
2.1 The nervous system enables humans to react to their
surroundings and to coordinate their behaviour.
2.1.1 Cells called receptors detect stimuli (changes in the environment)
126.96.36.199 Receptors and the stimuli they can detect include:
188.8.131.52.1 Receptors in the eye which are sensitive to light
184.108.40.206.1.1 Receptors in the ears which are sensitive to sound.
220.127.116.11.1.1.1 Receptors on the tongue and in the nose which are sensitive to chemicals and
enable us to taste and smell
18.104.22.168.22.214.171.124 Receptors in the ears which are sensitive to changes in position and enable us to balance
126.96.36.199.188.8.131.52.1 Receptors in the skin which are sensitive to touch, pressure, pain and temperature changes.
184.108.40.206 Light receptor cells, like most animal cells have
a nucelous, cytoplasm and cell membrane.
220.127.116.11 The brain and the spinal cord form the central nervous system (CNS)
18.104.22.168.1 Nerves contain nuerons.
22.214.171.124.1.1 Sensory nuerons carry impulses from receptors to the CNS
126.96.36.199.1.1.1 Motor nuerons carry impulses from the CNS to the effector organs which may be muscles or
188.8.131.52.184.108.40.206 The muscles repsond by contracting.
220.127.116.11.18.104.22.168 The glands respond by secreting chemicals.
3 Reflex actions
3.1 Reflex actions are automatic and rapid
3.1.1 They often involve sensory, relay and motor nuerons.
22.214.171.124 In a simple reflex action:
126.96.36.199.1 A receptor detects a stimulus (e.g a sharp pain)
188.8.131.52.1.1 Electrical impulses are transmitted from the receptor, along the sensory neuron to the CNS
184.108.40.206.1.1.1 At the synapse (a junction between 2 neurons) between the sensory and the relay neuron in the CNS, a chemical diffuses across the gap which causes the impulse to be sent along the
220.127.116.11.18.104.22.168 A chemical is then released at the synapse between the relay neuron and the motor neuron in the CNS which causes the impulses to be transmitted along the motor neuron to the effector
that brings about the response.
22.214.171.124.126.96.36.199.1 The effector is either a muscle or a gland; if it is a muscle is the impulse causes it to contract but if it is a gland the response is to secretes chemicals.
188.8.131.52.2 The sequence from receptor to effector is called a reflex arc
3.2 REFLEX ARC
4 Control in the human body
4.1 internal conditions that are controlled include:
4.1.1 The water content of the body ~ water leaves the body via the lungs when we
breathe out and via the skin when we sweat to cool us down, the excess water is
lost via the kidneys in urine
184.108.40.206 The ion content of the body ~ ions are lost via the skin when we sweat and via the kidneys in urine
220.127.116.11.1 Temperature ~ to maintain the temperature at which enzymes work best
18.104.22.168.1.1 Blood sugar levels ~ to provide cells with constant energy, controlled by the pancreas
4.2.1 Many processes within the body are controlled by chemical substances called hormones
22.214.171.124 Hormones are secreted by glands and are usually transported to their target organs by the bloodstream
126.96.36.199 Hormones regulate the functions of many organs and cells
188.8.131.52.1 For example the monthly release of an egg from a woman's ovaries and the changes in the
thickness of the lining of her womb are controlled by hormones secreted by the pituitary gland
and by the ovaries.
184.108.40.206.1.1 Several hormones are involved in the menstrual cycle of a woman
220.127.116.11.1.1.1 Hormones are involved in promoting the release of an egg:
18.104.22.168.22.214.171.124 Follicle Stimulating Hormone (FSH) is secreted by the pituitary gland and causes the egg to mature in the ovaries.
126.96.36.199.188.8.131.52.1 It also stimulates the ovaries to produce hormones including oestrogen.
184.108.40.206.220.127.116.11.1.1 Luteinising hormone (LH) is made by the puitary gland and stimulates the release of a mature egg from the ovaries (Ovulation)
18.104.22.168.22.214.171.124.1.1.1 Oestrogen is produced by the ovaries and inhibits the further production of FSH.
126.96.36.199.188.8.131.52.184.108.40.206 It also stimulates the production of LH and stimulates the womb lining to develop to receive the fertilised egg
220.127.116.11.1.1.2 The menstrual cycle takes around 28 days with ovulation happening
about 14 days into the cycle
4.2.2 The artificial control of fertility
18.104.22.168 Oral contraceptives contain hormones such as Oestrogen and progesterone to inhibit the production of FSH
22.214.171.124.1 This means that no eggs mature
126.96.36.199.1.1 The first birth control pills contained large amounts of oestrogen, which resulted in women suffering significant side effects.
188.8.131.52.1.1.1 Now, birth control pills contain a much lower dose of oestrogen, or are progesterone only.
184.108.40.206.220.127.116.11 Progesterone only pills have fewer side effects.
18.104.22.168.1 It stands for In Vitro Fertilisation
22.214.171.124.1.1 IVF involves giving a women FSH and LH to stimulate the maturation of several eggs.
126.96.36.199.1.1.1 The eggs are collected from the mother and fertilised with sperm from the father
188.8.131.52.184.108.40.206 The fertilised eggs develop into embryos and when they are at the stage of being tiny balls of cells, one or two are inserted into the woman's uterus (womb)
5 Control in plants
5.1 Plants are sensitive to light, moisture and gravity
5.1.1 Plant shoots grow TOWARDS light. This response is called PHOTOTROPISM
220.127.116.11 The shoot is positively phototropic and grows towards the light
enabling the plant to photosynthesize more.
18.104.22.168 The root is negatively phototropic and grows away from light
5.1.2 Plants grow down TOWARDS gravity. This is called GRAVITROPISM or GEOTROPSIM.
22.214.171.124 The shoots are negatively geotropic and so grow against the force of gravity.
126.96.36.199 The roots are positively geotropic and so grow in the direction of gravity.
5.1.3 Roots also grow TOWARDS water. This response is called HYDROTROPISM.
5.1.4 A chemical called Auxin controls a plant's responses to light and gravity.
188.8.131.52 Cells at the tips of stems and roots produce auxin.
184.108.40.206.1 It slows down growth in roots.
220.127.116.11.2 It speeds up growth in stems.
18.104.22.168.3 Unequal distribution of auxin causes unequal growth.
22.214.171.124.3.1 This results in bending of the shoot or root.
126.96.36.199.3.1.1 When light comes from above, auxin spreads out evenly down the stem. The stem grows straight.
188.8.131.52.184.108.40.206 When light comes from one side, the auxin spreads down the SHADED side of the stem
220.127.116.11.18.104.22.168.1 Auxin makes the cells grow FASTER. This causes the stem to bend towards the light.
22.214.171.124.3.1.2 If a plant is laid on its side, auxin gathers in the lower half of the stem and root.
126.96.36.199.188.8.131.52 Auxin in the roots SLOWS DOWN the growth of root cells
184.108.40.206.220.127.116.11.1 The root curves downwards.
18.104.22.168.22.214.171.124.1.1 Auxin that has spread the the side of the stem SPEEDS UP the growth of stem cells.
126.96.36.199.188.8.131.52.1.1.1 The stem curves upwards.
5.2 Using plant hormones
5.2.1 Plant hormones can be used by farmers and gardeners.
184.108.40.206 Weed killers are used to kill unwanted plants on lawns.
220.127.116.11.1 When cuttings are taken from plants, hormones are used to encourage the roots to grow before the cutting is planted.
18.104.22.168.1.1 Some hormones are used to encourage fruit to ripen.
22.214.171.124.1.1.1 If plant hormones are used incorrectly they can cause damage to the
environment, e.g. weedkillers may harm other more useful plants.