Biology: Coordination and Control

reflexes etc

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  • Created by: Phoebe
  • Created on: 12-05-12 12:45

Responding to Change

  • Your nervous system uses electrical impulses to enable you to react quickly to your surroundings and coordinate whatever you do
  • Neurons are nerve cells found in nerves - they carry electrical impulses
  • Cells called receptors detect stimuli (changes in the environment)
  • Receptor cells (e.g. the light receptor cells in the eyes) are like most animal cells - they have a nucleus, a cytoplasm and a membrane
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The Nervous System (In Depth)

  • A receptor detects a stimulus
  • An electrical impulse passes along the sensory neurons
  • A nerve is made up of thousands of neurons bundled together
  • The impulse travels along the sensory neurons until it reaches the CNS
  • The CNS is made up of the brain and the spinal cord
  • The brain sends impulses along the motor neurons
  • The motor neurons carry impulses from the CNS to the effector organs
  • The effector organs are muscles and glands
  • Muscles respond to the arrival of impulses by contracting
  • Glands respond to the arrival of impulses by secreting chemical substances
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Basic Nervous System

Stimulus ~~~~> Receptor ~~~~>     Sensory Neurons ~~~~> CNS ~~~~> Motor Neurons ~~~~> Effector

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Reflex Actions

  • Some responses to a stimuli are automatic and rapid
  • These are called reflex actions
  • Reflex actions run everyday bodily functions such as breathing
  • They also help us to avoid danger
  • They are essential similar to the basic nervous system, except that in a reflex actions the coordinator is a relay neuron either in the spinal cord or an unconscious part of the brain
  • This means the brain is not aware of the action until after it has occured
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A Reflex Arc (basic)

  • A receptor detects a stimulus (e.g. a sharp pain)
  • An electrical impulse travels along a sensory neuron to the CNS
  • The impulse then passes along a relay neuron (usually in the spinal cord) and stimulates a motor neuron
  • It then travels straight back along a motor neuron
  • The impulse then travels to an effector

The key point in a reflex action is that the impulse bypasses and brain, so that the time between the stimulus and the reflex action is as short as possible.

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  • The nerves are not joined up directly to each other
  • There are junctions between them called synapses
  • The electrical impulses travelling along the neurons have to cross the synapse
  • They cannot leap the gap
  • When the impulse arrives in the neuron, chemicals from sacs in the neuron are secreted into the synapse
  • They cross the synapse and stimulate the receptor cells on the next neuron
  • This starts up an electrical impulse in the next neuron
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The Reflex Arc in Detail

  • Receptors are stimulated by a stimulus
  • An electrical impulse passes along a sensory neuron to the CNS 
  • When the impulse arrives at the synapse with a relay neuron, chemicals are secreted, cross the synapse and set off the electrical impulse that passes along the relay neuron
  • When the impulse reaches the synapse between the relay neuron and the motor neuron, a chemical is released again
  • The chemical crosses the synapse and sets off the electrical impulse the the motor neuron
  • When the impulse reaches the effector organ, it is stimulated to respond

An impulse is also sent up the spinal cord to the brain, so the brain is aware of the reflex action, but only after it has occured.

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A Reflex Arc - Basic

Stimulus ~~~~>Receptor ~~~~>             Sensory neuron ~~~~> Coordinator (relay neurone in CNS) ~~~~>Motor neuron ~~~~> Effector ~~~~> Response

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  • Hormones are chemical substances that coordinate processes


  • Made and secreted by the pituitary gland
  • It causes eggs to mature
  • It stimulates the ovaries to produce oestrogen


  • Made and secreted by the ovaries
  • Stimulates the lining of the womb to to develop for pregnancy
  • Inhibits (slows down) FSH production
  • Stimulates the production of LH
  • Stimulates womb lining to develop to receive fertilised egg


  • Made and secreted by the pituitary gland
  • Stimulates release of a mature egg from the ovary (ovulation)
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The Menstrual Cycle

Day 1-5: Old egg leaves body in menstrual flow

Day 1-12: New egg maturing in ovary

Day 12-16: Egg released

Day 15-23: New egg travelling to womb

Day 20-28: New egg in womb

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The artificial control of fertility

  • Hormones can be used to control fertility
  • Oral contraceptives, such as the pill, contain hormones which stop FSH production so no eggs can mature
  • The contraceptive pill may conatin oestrogen and progesterone
  • Progesterone-only pills have fewer side effects
  • FSH can be used as a fertility drug for women to stimulate eggs to mature in their ovaries
  • These eggs may be used in IVF treatments
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Advantages and Disadvantages


  • The contraceptive pill has helped to reduce family sizes which has reduced poverty in some areas
  • It allows women to plan their pregnancies
  • Fertility drugs can help infertile couples who are having IVF


  • The pill can cause side effects
  • Some people object to its use for ethical/religious reasons
  • IVF is expensive
  • Some people think it is unethical for an older woman to have a baby by IVF
  • Extra embryos produced by be stored or destroyed
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Controlling Conditions

  • The conditions inside the body are known as its internal environment
  • Humans need to maintain a constant internal environment
  • Processes in the body control levels of water, ions, and blood sugar as well as temperature
  • Homeostasis is the result of the coordination of your nervous system, your hormones and your body organs
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Controlling Water and Ions

  • Water can move in and out of the body
  • How much it moves depends on the concentration of mineral ions and amount of water
  • If too much or too little water moves in or out of the body, cells can be damaged or destroyed
  • We take water and ions into our body as we eat and drink
  • We lose water and ions in sweat and urine produced by the kidneys
  • Sweating causes the body to cool - the body uses energy to evaporate water from the sweat
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Controlling Temperature

  • Core body temperature must be kept at 37C
  • At 37C, enzymes work best
  • A few degrees above or below core body temperature causes the reaction in your cells to stop, and you die
  • The body control temperature
  • You sweat to cool down
  • You shiver to warm up
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Controlling Blood Glucose

  • When you digest food, lots of glucose passes into the blood
  • Left alone, your blood glucose levels would keep changing
  • The levels would be very high after eating, but very low a few hours later
  • This would cause chaos in the body
  • However, the concentration of glucose in the blood is kept constant by hormones made in the pancreas
  • This means your body cells are provided with the constant supply of energy they need
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Hormones and the control of plant growth

  • Plants are sensitive to light, gravity and moisture
  • Plant shoots grow towards light - this is called phototropism
  • Roots grow down towards gravity - this is called gravitropism
  • Plant responses are brought about by plant hormones (auxin)
  • The responses of roots and shoots to stimuli such as light and gravity are the result of the unequal distribution of plant hormones
  • We can use plant growth hormones as weedkillers and as rooting hormones on cuttings
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Gravitropism and Auxin

  • A normal young bean plant is laid on its side in the dark
  • Auxin is equally spread through the tissues
  • In the root, more auxin gathers on the lower side
  • In the shoot, more auxin gathers on the lower side
  • The root grows more on the side with the least auxin, making it bend and grow down towards the force of gravity. This is called positive gravitropism
  • The shoot grows more on the side with the most auxin, making it bend and grow upwards away from the force of gravity. This is called negative gravitropism
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