Habituation (Edexcel A (SNAB) A Level Biology)

• Animals must respond to changes in their external and internal environments in order to survive
  • Changes in the environment, or stimuli (singular stimulus) are detected by specialised receptor cells 
  • Receptor cells send signals via either the nervous system or the hormonal system to the body's co-ordination centres in the brain or spinal cord
  • Signals are then sent on to the parts of the body which respond, known as the effectors
• The process of detecting and responding to stimuli requires energy, so it is important that animals don't waste energy responding to non-threatening stimuli
  • Animals need to conserve energy for essential processes that increase their survival chances
• If a stimulus is repeated many times with no negative outcome, then an animal will learn not to respond to it; this process is known as habituation
  • An animal that doesn't respond to a stimulus is said to be habituated to that stimulus
• Examples of habituation include
  • Humans no longer noticing a new smell or sound after a period of exposure
  • Wild animals losing their fear of humans after regular non-harmful contact
  • Animals learning not to be alarmed by the presence of non-predatory species
• If a stimulus to which an animal has become habituated changes, then the nervous system will respond to it again
  • E.g. a constant low-level sound that suddenly becomes louder

The process of habituation

• Animals become habituated due to changes in the transmission of nerve impulses from one neurone to the next
  • Nerve impulses are transmitted across synapses by the diffusion of chemical neurotransmitters
  • Neurotransmitters are released at the presynaptic membrane in response to an influx of calcium ions
• When habituation has taken place fewer calcium ions move into the presynaptic neurone on arrival of a nerve impulse
• As a result, less neurotransmitter is released and an action potential is less likely to be generated in the postsynaptic neurone
  • Fewer molecules of neurotransmitter bind to receptors on the postsynaptic membrane
  • Fewer sodium ion channels open
  • Fewer sodium ions move into the axon and the charge inside the axon remains negative
  • Threshold potential is not reached
• The nerve impulse therefore does not reach the effector organ and the animal does not respond to the stimulus

• Habituation to a stimulus can be studied by measuring the changes in an animal's response to a non-harmful stimulus e.g.
  • Snails often respond to a stimulus by withdrawing into their shell, waiting to emerge again until the harmful stimulus is likely to be gone
  • As snails become habituated to a stimulus the time taken for them to re-emerge from their shells after a stimulus gets shorter

Apparatus

• Snail
• A soft object with which to provide a stimulus e.g. a damp cotton bud or a blade of grass
• Stopwatch

Method

1. Place a snail on a clean, flat surface and give it time to emerge from its shell
   • The same surface should be used throughout the experiment
   • Ensure that humidity remains the same throughout as snails will withdraw in a dry environment
2. Gently brush the snail's head with a damp cotton bud or blade of grass
   • It is expected that the snail will withdraw into its shell in response to the touch
3. Start the stopwatch and measure the time taken until the snail re-emerges from the shell and fully extends its eye-stalks again
4. Repeat steps 2 and 3 10-15 times, recording the time taken until full re-emergence each time
   • Ensure that the same soft object is used throughout and that the location of the touch on the snail's body remains the same
   • Waiting for full extension of the eye stalks ensures that the same end-point is used each time
5. Plot a graph of touch number against time taken for full re-emergence
   • The graph would be expected to show a gradual decrease in the time taken for full re-emergence as the snail becomes habituated to the stimulus

• Note that snails are living organisms and so welfare considerations should be taken into account when using them for experimental purposes
  • Snails should be returned to a suitable environment that replicates their natural habitat at the end of the experiment
    • If snails were taken from a garden or the school grounds then they should be returned to the exact location from which they were removed
  • Any handling and transfer of snails should be carried out gently and quickly
  • Snails should not be exposed to high temperatures or an overly dry environment