Habituation and the nervous systemEdexcel International A Level Biology: Revision notes
Section 1
What is habituation?
Habituation is a decrease in the response to a stimulus that is repeated and is harmless. It is a simple form of learning.
It is useful because the animal does not waste energy responding to stimuli that do not matter, and it can pay attention to new stimuli that might signal food or danger. For example, a snail that withdraws into its shell when tapped stops doing so if the tapping is repeated with no harm.
Habituation is specific to one stimulus. A different stimulus still produces a response, which shows that the animal is not simply tired.
Habituation is not muscle fatigue or receptor adaptation. A different stimulus still gets a normal response.
Section 2
How habituation happens
Habituation involves changes at synapses in the pathway. With repeated stimulation, fewer calcium ion channels open in the presynaptic membrane, so less calcium enters the presynaptic knob. Less neurotransmitter is released, so the postsynaptic neurone is depolarised less and may not reach threshold. The response becomes weaker, then disappears.
Section 3
The mammalian nervous system
The mammalian nervous system has two parts:
- The central nervous system (CNS): the brain and spinal cord
- The peripheral nervous system (PNS): all the nerves outside the CNS, containing sensory neurones (from receptors to the CNS) and motor neurones (from the CNS to effectors)
The motor neurones include the somatic neurones to skeletal muscles (voluntary) and the autonomic neurones to involuntary effectors such as the heart and the iris.
Section 4
Nervous and hormonal coordination
Animals are coordinated by two systems:
- Nervous: electrical impulses along neurones; very fast, short-lived, usually to a precise target
- Hormonal: hormones are chemicals secreted by glands into the blood and act on target cells with specific receptors; slower, longer lasting, may affect many organs
The two often work together. In a fright response, the nervous system causes a rapid reaction and the adrenal glands release adrenaline, which keeps the heart rate high for longer.
Section 5
Practical: investigating habituation
A suitable investigation uses a harmless stimulus on a small animal such as a snail or earthworm.
- Apply the same stimulus (for example a tap on the shell) at regular intervals.
- Measure the response each time, such as the time withdrawn or the number of taps until there is no response.
- Control variables: strength of stimulus, interval, temperature, light, species and size.
- Repeat with several animals and calculate a mean.
- Apply a different stimulus at the end to show the animal is not tired.
- Treat the animals ethically: handle gently and return them to their habitat.
Must know
- Habituation is a decreased response to a repeated harmless stimulus
- It saves energy and lets the animal respond to new stimuli
- Less calcium enters the presynaptic knob, so less neurotransmitter is released
- CNS: brain and spinal cord; PNS: sensory and motor neurones outside the CNS
- Nervous coordination is fast and short-lived; hormonal coordination is slower and longer lasting
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Habituation and the nervous system
- A student taps the shell of a pond snail once every 20 seconds. At first the snail withdraws into its shell each time. After about a dozen taps it no longer withdraws, although the taps are exactly the same.Suggest how the student could show that the loss of response is due to habituation and not to the muscles of the snail becoming tired.2 marks
- A barefoot walker treads on a sharp stone and pulls her foot away immediately. A few minutes later she notices that her heart is still beating fast and she feels shaky.Compare the speed and duration of her foot withdrawal with those of the response that kept her heart rate high, and explain the differences.2 marks
- A woman walking in a park hears a loud bang. She jumps and her heart rate stays high for several minutes. Two weeks later she lives next to a building site where similar bangs happen every day, and by the end of the fortnight she no longer jumps at them.Explain how both the nervous system and the hormonal system contribute to her response to the first bang.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).