A student argues that a very loud sound produces larger action potentials in the auditory nerve than a quiet sound — that's how the brain tells them apart. What is wrong with this reasoning?
AAuditory nerve fibers do not conduct action potentials; they use graded potentials throughout
BAction potentials are all-or-none events with fixed amplitude; loudness is encoded by firing frequency, not spike size
CThe brain cannot distinguish loudness at all — it only detects presence or absence of sound
DLouder sounds actually produce fewer action potentials because adaptation rapidly silences the auditory nerve
Action potentials are all-or-none: once threshold is reached, the spike fires at a fixed amplitude regardless of how strong the stimulus is. There is no mechanism to make an individual spike bigger. Instead, a stronger receptor potential brings the neuron back to threshold more quickly after each spike, increasing firing rate. Loudness is coded in the frequency of spikes, not their size — this is the fundamental principle of frequency coding.
Question 2 Multiple Choice
A constant weight is placed on a subject's palm. They feel it initially, stop noticing it after a few seconds, then notice again when it is removed. Which receptor type best explains this pattern?
ASlowly adapting tonic receptors, which sustain their firing as long as a stimulus is present
BRapidly adapting phasic receptors, which fire at stimulus onset and offset but go silent during sustained stimulation
CNociceptors, which only activate when the stimulus crosses a tissue-damage threshold
DThermoreceptors, which respond to temperature changes in the skin caused by the weight
Rapidly adapting (phasic) receptors signal change, not maintained conditions. They fire briskly when the weight is applied (onset), fall silent during constant pressure, and fire again when the weight is removed (offset). Meissner's and Pacinian corpuscles work this way. Slowly adapting (tonic) receptors would maintain firing throughout and would not explain why the subject stops noticing the constant weight.
Question 3 True / False
A stronger stimulus produces larger-amplitude action potentials in sensory neurons, which is how the nervous system encodes stimulus intensity.
TTrue
FFalse
Answer: False
This is the most common misconception about neural coding. Action potentials are all-or-none — once the membrane reaches threshold, the spike fires at a fixed, stereotyped amplitude. A stronger stimulus produces a larger receptor potential, but this translates into a higher action potential firing rate, not bigger spikes. Stimulus intensity is encoded in frequency (rate coding), not spike amplitude.
Question 4 True / False
Sensory adaptation reduces the nervous system's responsiveness to constant stimuli, which allows it to remain sensitive to changes in the environment.
TTrue
FFalse
Answer: True
Sensory adaptation is a design feature, not a flaw. By reducing firing in response to sustained, unchanging stimuli, the nervous system filters out background 'noise' and keeps higher brain centers available to respond to new, potentially significant events. The trade-off is reduced sensitivity to slowly-building threats, but the benefit is that the system remains tuned to what is novel and therefore more likely to require action.
Question 5 Short Answer
Why must stimulus intensity be encoded in the frequency of action potentials rather than in their amplitude?
Think about your answer, then reveal below.
Model answer: Because action potentials are all-or-none events. Once the membrane depolarizes to threshold, the spike fires at a fixed amplitude regardless of how much the threshold was exceeded. There is no way to make a single action potential larger. Instead, a larger receptor potential (caused by a stronger stimulus) depolarizes the neuron more strongly after each spike, reducing the interspike interval and increasing firing rate. Frequency is the only variable available to carry graded intensity information forward from the receptor potential to the central nervous system.
This is the core logic of frequency coding. The conversion from graded receptor potential to all-or-none action potentials could have been a lossy bottleneck — but evolution solved the problem by using firing rate as the channel for intensity information. Understanding this also explains why sensory pathways have upper limits (refractory periods set a maximum firing rate) and why adaptation matters (reducing background firing preserves the dynamic range for detecting changes).