10. A series of beakers of water of different temperatures are placed in front of you. You
place your finger in each one, and select the beaker containing water of the highest
temperature. How did your brain know?
a. The brain receives action potentials of different intensities that are
proportional to the temperature.
b. Action potentials are only sent when the temperature of the water exceeds a
certain threshold
c. The number of action potentials per second increases as temperature
increases.
d. Action potentials are received in a different part of the brain when
temperatures are higher
The brain distinguishes the highest temperature water beaker by interpreting the firing rate of sensory neurons, where hotter stimuli trigger more action potentials per second from thermoreceptors. This multiple-choice question tests neural coding in thermosensation, a key topic in CSIR NET Life Sciences under sensory physiology.
Correct Answer
Option c. The number of action potentials per second increases as temperature increases is correct. Thermoreceptors in the skin, such as warm-sensitive TRPV channels, increase their discharge frequency proportionally with rising temperature, allowing the brain to compare rates across fingers and identify the hottest beaker.
Option Analysis
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a. The brain receives action potentials of different intensities that are proportional to the temperature: Incorrect, as all action potentials have uniform amplitude and duration (“all-or-none” principle); intensity is encoded by frequency, not size.
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b. Action potentials are only sent when the temperature of the water exceeds a certain threshold: Incorrect, since thermoreceptors fire continuously at baseline skin temperature (~32-34°C) and modulate rate with changes, not just above a strict threshold.
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c. The number of action potentials per second increases as temperature increases: Correct, as frequency coding (higher firing rate for hotter stimuli) enables precise discrimination via rate comparison in somatosensory pathways.
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d. Action potentials are received in a different part of the brain when temperatures are higher: Incorrect; temperature signals converge to the same insular cortex regions, distinguished by firing patterns, not location.
Neural Mechanism
Skin thermoreceptors detect temperature via TRP channels (e.g., TRPV1 for warmth >43°C), generating graded receptor potentials that trigger action potentials in afferent fibers. These project via spinal dorsal horn to the thalamus and posterior insular cortex (“thermal cortex”), where frequency differences signal relative hotness. In the beaker experiment, the highest rate from one finger dominates perception. For CSIR NET, note frequency (rate) coding vs. population coding in sensory neurons.
