Habituation

what does habituation do?

• Habituation is part of learning process

• All animals including human beings learn not to react to world around them.

• Habituation may be short or long term and response can be lost permanently.

• Scientists believe habituation involved in brain development as memories form and permanent changes occur at synapses.

why is it important that animals don’t respond to every stimuli

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

what is habituation

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

If a stimulus to which an animal has become habituated changes, then the nervous system will respond to it again

what nerve change causes habituation to occur?

• 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

habituaiton experiment on Aplysia

Aplysia grown in lab not habituated to waves or water movement.

Eric Kandel studied this phenomenon by performing classic experiments.

He stimulated the siphon with a jet of water the gill is withdrawn.

He repeatedly applied the stimulation.

The response became less and less until eventually squirting water on the siphon had no effect on the gill.

The animal was habituated and had learnt to ignore the stimulus.

how cna habituation be investigated

• 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

what welfare considerations need to be taken into account when using snails 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