The Nervous System and Reflexes
The nervous system is the body's fast electrical communication system, made up of the central nervous system, the brain and spinal cord, and the peripheral nervous system, the neurons connecting it to the rest of the body, and it detects stimuli and coordinates rapid responses such as reflex actions. A reflex is a rapid, automatic response to a stimulus that does not involve conscious thought, passing along a fixed reflex arc: receptor, sensory neuron, relay neuron, motor neuron, effector.
Method
- Learn the reflex arc pathway in strict order and be ready to label or describe it: stimulus, then receptor, sensory neuron, relay neuron (in the spinal cord or CNS), motor neuron, effector (a muscle, which contracts, or a gland, which secretes a hormone), then the response.
- Explain why reflexes are fast: the pathway does not route through the parts of the brain responsible for conscious thought, so the response happens before the person is aware of the stimulus, protecting the body from harm.
- Learn what happens at a synapse: the electrical impulse cannot cross the gap directly, so it triggers the release of a neurotransmitter chemical, which diffuses across the synapse and binds to receptor molecules on the next neuron, starting a new electrical impulse there.
- For the required practical measuring reaction time, know the key variables: independent = the factor being tested, for example using the dominant hand versus the non-dominant hand; dependent = reaction time in seconds; control variables = the same participant, the same test method, and the same starting position each time.
- For a ruler-drop reaction time test, use the physics link: the ruler falls from rest under gravity, so the distance it falls before being caught can be converted into a reaction time using distance = 0.5 x g x time^2, rearranged to time = square root of (2 x distance / g).
- Distinguish a reflex (involuntary, does not involve the brain's conscious pathways, always the same response to the same stimulus) from a voluntary response (consciously decided by the cerebral cortex, and able to vary).
Worked example
In the ruler-drop reaction time test, a student named Priya holds her fingers ready to catch a ruler that her partner releases without warning. The ruler falls 18 cm before Priya catches it. Using distance = 0.5 x g x time^2, with g = 9.8 m/s^2, calculate Priya's reaction time, giving your answer to 3 significant figures.
- Convert the distance to metres and rearrange the equation to make time the subject: time = square root of (2 x distance / g).
- Substitute the values: time = square root of (2 x 0.18 / 9.8).
- Calculate inside the root: 2 x 0.18 = 0.36, and 0.36 / 9.8 = 0.0367 (3 s.f.).
- Take the square root: square root of 0.0367 = 0.192 (3 s.f.).
- Final answer: Priya's reaction time is approximately 0.192 seconds.
Practice questions
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Q1Name the two parts of the central nervous system (CNS).Show answer
Answer: The brain and the spinal cord.
Q2Put these into the correct order for a reflex arc: motor neuron, receptor, effector, relay neuron, sensory neuron.Show answer
Answer: Receptor, sensory neuron, relay neuron, motor neuron, effector.
Q3Define a synapse.Show answer
Answer: The junction, a small gap, between two neurons, across which a neurotransmitter chemical diffuses to pass on an impulse.
Q4Explain why reflex responses are faster than voluntary responses.Show answer
Answer: Reflexes do not pass through the conscious-thought pathways of the brain, so the pathway is shorter and the person reacts before consciously thinking about it, unlike a voluntary response.
Q5State the type of effector that produces a physical response by contracting.Show answer
Answer: A muscle.
Q6A ruler is released and falls 20 cm before being caught. Using distance = 0.5 x g x time^2 with g = 9.8 m/s^2, calculate the reaction time, to 3 significant figures.Show answer
Answer: 0.202 seconds (time = square root of (2 x 0.20 / 9.8) = square root of 0.0408 = 0.202).
Q7State why a reflex action is described as involuntary.Show answer
Answer: It happens automatically, without conscious control or decision by the brain.
Q8Name the chemical released at a synapse that carries the signal across the gap.Show answer
Answer: A neurotransmitter.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
Describe the pathway of a reflex arc when a person touches a sharp pin with their finger, from the stimulus to the response.
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A student carries out the ruler-drop required practical to investigate the effect of practice on reaction time. On the first attempt, the ruler falls 24 cm before being caught. (a) Using distance = 0.5 x g x time^2 with g = 9.8 m/s^2, calculate the reaction time on the first attempt, to 3 significant figures. (b) Suggest one way the student could improve the reliability of this investigation.
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Explain what happens in the nervous system when a person accidentally touches a hot saucepan, and explain why this reflex response is important for survival.
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Free printable worksheet
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This topic is chapter 21 of GCSE Biology Workbook, the whole course as one free printable PDF.
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