B1.2 - Nerves and hormones

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The nervous system

  • The nervous system uses electrical impulses to enable you to react quickly to your surroundings and coordinate what you do
  • Cells called receptors detect stimuli (changes in the environment)
  • Light receptor cells like most animal cells, have a nucleus, cytoplasm and cell membrane
  • Information from receptors passes along neurones in nerves to the brain which then coordinates the response
  • Reflex actions are automatic, rapid and many involve sensory, relay and motor neurones

In a simple reflex action:

  • Impulses from a receptor pass along a sensory neurone to the CNS
  • At a synapse between a sensory neurone and a relay neurone in the CNS, a chemical is released that causes an impulse to be sent along a relay neuron
  • A chemical is then realeased at the synapse between a relay neurone and a motor neurone in the CNS, causing impulses to be sent along a motor neurone to the organ (the effector) that brings about the response
  • The effector is either a muscle or a gland; a muscle respond by contracting and a gland responds by secreting chemical substances
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Control in the human body

Internal conditions that are controlled include:

  • Water content - water leaves the body via the lungs when we breathe out and via the skin when we sweat, and excess water is lost via the kidneys in urine
  • Ion content - ions are lost via the skin when we sweat and excess ions are lost via the kidneys in urine
  • Temperature - to maintain the temperature at which enzymes work best
  • Blood sugar levels - to provide the cells with a constant supply of energy
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  • Many processes within the body are coordinated by chemical substances called hormones which are secreted by glands and are usually transported to their target organs by the bloodstream
  • Hormones regualte the functions of many organs and cells

Several hormones are involved in the menstrual cycle such as promoting the release of an egg:

  • FSH is secreted by the pituitary gland and causes eggs to mature in the ovaries and also stimulates the ovaries to produce oestrogen
  • LH stimulates the release of eggs from the ovary
  • Oestrogen is secreted by the ovaries and inhibits further producion of FSH

The use of hormones in controlling fertility include:

  • Oral contraceptive that contain hormones to inhibit FSH production so that no eggs mature
  • Giving FSH and LH in a 'fertility drug' to a woman whose own lever of FSH is too low to stimulate eggs to mature e.g. IVF
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Control in plants

  • Plants are sensitive to light, moisture and gravity:
    • Their shoots grow towards light and against the force of gravity
    • Their roots grow towards moisture and and in the direction of the force of gravity
  • Plants produce hormones to coordinate and control growth such as auxin which controls phototropism (response to light) and gravitropism (response to gravity)
  • The responses of roots and shoots to light, gravity and moisture are the result of unequal distribution of hormones, causing unequal growth rates
  • Plant growth hormones are used in agriculture and horticulture as weed killers and as rooting hormones
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