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8.1.9Unit 6: Practical Skills in Biology II

Habituation — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 6 parts Lesson 43 of 48 Unit quiz

📚 IAL Edexcel Linear Hub / Topical Revision
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Part 1 of 6What Habituation Is

Habituation is a simple form of learning in which an animal stops responding to a repeated, harmless stimulus.

Definition
  • Definition An animal learns to ignore a stimulus that is repeated many times with no harmful result.
Saves energy
  • Saves energy Detecting and responding to stimuli costs energy, which is better kept for essential and threatening events.
Human example
  • Human example People stop noticing a new smell or a constant background noise after a period of exposure.
Animal example
  • Animal example Wild animals can lose their fear of humans after regular contact that does no harm.
Not permanent
  • Not permanent If the stimulus changes, for example a steady sound becomes louder, the nervous system responds again.

Part 2 of 6The Mechanism at the Synapse

Habituation results from reduced transmission at synapses along the response pathway.

1
When habituation has occurred, fewer calcium ions enter the presynaptic knob when a nerve impulse arrives.
2
As a result less neurotransmitter is released from vesicles into the synaptic cleft.
3
Fewer neurotransmitter molecules bind to receptors, so fewer sodium ion channels open in the postsynaptic membrane.
4
Fewer sodium ions enter, the threshold potential is not reached and no action potential is generated.
5
The impulse does not reach the effector, so the animal does not respond to the stimulus.

Part 3 of 6Investigating Habituation in Snails

A snail withdraws into its shell when touched, and the time it takes to re-emerge can be measured.

1
Place a snail on a clean, flat surface and wait until it has emerged and extended its eye-stalks.
2
Gently touch the snail's head with a damp cotton bud and note that it withdraws into the shell.
3
Time with a stopwatch until the snail has re-emerged and fully extended its eye-stalks again.
4
Repeat the touch and timing 10 to 15 times, using the same object, the same place on the snail and the same surface.
5
Plot touch number against time to re-emerge; a falling trend shows habituation.

Part 4 of 6Control Variables and Welfare

Control Variables and Welfare
Constant humidity
  • Constant humidity Snails withdraw in a dry atmosphere, so the humidity must stay the same throughout the experiment.
Same end-point
  • Same end-point Always wait for full extension of the eye-stalks so each timing is measured to the same point.
Gentle handling
  • Gentle handling Snails are living organisms and should be handled gently and quickly, and kept away from heat and dry air.
Return to habitat
  • Return to habitat After the investigation the snails should be returned to the place from which they were collected.

Part 5 of 6Worked Example

Question

  • A snail was touched five times.
  • Times to re-emerge were 62 s, 48 s, 35 s, 27 s and 20 s.
  • Calculate the percentage decrease from the first to the fifth touch and say what it shows.

  1. Decrease in time = 62 - 20 = 42 s.
  2. Percentage decrease = 42 / 62 × 100.
  3. 42 / 62 = 0.677, so the decrease is 67.7 %.

Answer The time fell by about 68 % (67.7 %), which shows the snail became habituated to the repeated touch.

Part 6 of 6Exam Checklist & Tips

8.1.9 – Key Points for the Exam

  • Habituation A reduced response to a stimulus that is repeated many times without a harmful outcome.
  • Stimulus A change in the internal or external environment that is detected by a receptor.
  • Effector A muscle or gland that produces a response to a stimulus.
  • Threshold potential The potential that must be reached for an action potential to be generated.

Common Mistakes

  • Saying habituation is caused by receptor cells wearing out; it is caused by changes at synapses.
  • Stating that more calcium ions enter the presynaptic knob; fewer enter in a habituated animal.
  • Describing habituation as permanent; the response returns if the stimulus changes.
  • Timing to different end-points in the snail experiment; always use full extension of the eye-stalks.

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