nerves + hormones

• enables humans to react to their surroundings and coordinate their behaviour
• receptors detect stimuli (changes in the environment)
  eye receptors - sensitive to light
  ear receptors - sensitive to sound
                           sensitive to changes in position and enable us to keep           balance  
  tongue + nose receptors  - sensitive to chemicals, enable us to taste and smell
  skin receptors - sensitive to touch, pressure, pain + temp. changes
• light receptor cells have nucleus, cytoplasm + cell membrane
• information from receptors passes along neurones in nerves to the brain. the brain coordinates the response. reflex actions are automatic and rapid.

1) impulses from a receptor pass along the sensory neurone to the central nervous system

2) at a synapse between the sensory neurone and the relay neurone in the central nervous system, a chemical is released that causes an impulse to be sent along the relay neurone

3) a chemical is then released at the synapse between a relay and motor neurone in the cns, causing impulses to be sent along the motor neurone to the organ (effector) that brings about the response

4) effector = muscle/gland
muscle responds by contracting
gland responds by secreting chemical substances 

internal conditions that are controlled include:

• water content - leaves body through respiration, through sweating + lost through the kidneys in urine 
• ion content - lost through the skin when we sweat, excess urine lost through kidneys in urine
• temperature - to maintain the temperature at which the enzymes work best
• blood sugar levels - to provide the cells with a constant supply of energy

hormones: chemical substances which are transported to their target organs through the bloodstream, secreted by the glands, regulate the functions of many organs and cells
many processes within the body are coordinated using hormones

FSH - secreted in pituitary gland, causes egg maturation in the ovaries, stimulates oestrogen production
LH - stimulates release of eggs from ovary
Oestrogen - secreted by ovaries, inhibits further production of FSH 

Use of hormones in controlling fertility:

• oral contraceptives containing oestrogen/progesterone inhibits egg maturation 
• giving FSH and LH in a fertility drug to a woman whose own level of FSH is too low to stimulate eggs to mature

IVF
mother is given FSH and LH to stimulate maturation of several eggs
eggs are collected from mother and fertilised by sperm from father
fertilised eggs develop into embryos
at the stage when they are tiny balls of cells, one or two embryos are inserted into the mother's womb 

• plants are sensitive to moisture, light and gravity
  - shoots grow towards light and against force of gravity
  - roots grow towards moisture and in the direction of the force of gravity
  - in roots auxins inhibit growth 
• plant growth hormones are used in agriculture and horticulture as weed killers and as rooting hormones