Sensitivity and coordination

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Sensitivity and coordination

• Organisms detect changes around them

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• All living organisms are sensitive to their
  environment. This means that they can detect
  changes in their environments.
• The changes they detect are called stimuli and
  they respond to these stimuli in various ways
  which have the effect of helping them to survive.
• This capacity of living protoplasm to respond to
  stimuli is known as irritability.

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Response and coordination in animals

• The response of many simple multi-cellular
  invertebrates, such as millipedes, woodlice,
  insect larvae and adult ants, to stimuli, is a
  movement of the whole organism towards or away
  from the particular stimulus. ( I like to Move it
  , move it)

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• In small, simple, unicellular organisms like
  Amoeba there are usually no specialised
  structures set aside for receiving, passing on
  and responding to stimuli.
• Whole or parts of these organisms may respond in
  definite ways to certain stimuli.
• Amoeba, for example, responds to contact with
  food by enclosing it with the nearest pseudopodia
  (false feet) to form a food vacuole

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Nervous system

• As organisms get larger and more complex, the
  need arises for some means of carrying sensations
  from one part of the organism to another, so that
  it can act as a unit. This is done by means of a
  nervous system.

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Simple nervous system (nerve net)

• Found in the sea anemone and other members of the
  jellyfish or cnidarian group of animals
• Sense cells receive stimuli, and pass them on
  through special conducting cells or neurones
  which link up with each other to form a nerve
  net.
• Eventually the stimulus reaches muscle cells
  which respond by contracting.

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Receptors Effectors

• Cells that receive stimuli are known as
  receptors.
• Those which respond are known as effectors.
• The conducting cells or neurones have a number of
  thin fibres leading out from their cell bodies
  along which sensations or messages can pass.

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Any direction

• In the sea anemone, and its relatives, messages
  can travel in any direction along these fibres

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Larger active animals need a more efficient
nervous system.

• Mammalian neurons contain the same basic parts as
  any animal cell
• Each has a nucleus, cytoplasm and a cell
  membrane. Their structure is specially adapted to
  be able to carry messages very quickly.

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Nerve fibers

• They have long, thin fibers of cytoplasm
  stretching out from the cell body.
• Nerves fibers carrying impulses into the cell
  body are called dendrons or dendrites
• Usually there is one nerve fibre taking impulses
  away from the cell body. This is called the axon

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Structure

• In many neurones, the axon is the longest fibre.
  Axons may be more than a metre long.
• The dendrites pick up messages from other
  neurones lying nearby. They pass the message to
  the cell body, and then along the axon.

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Messages

• The axon might then pass it on to another
  neurone.
• The messages can pass in one direction only
• This helps to make the system more precise.
• Unlike what happens in the sea anemone

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