Questions: Sensory Receptor Transduction and Adaptation

Question 1

A person grips a coffee cup. The pressure sensation on their palm is vivid at first, then fades over the next minute even though they continue holding the cup at the same pressure. Meanwhile, the heat from the cup remains noticeable throughout. What best explains the difference in these two sensations?

• A. Pressure receptors are damaged by sustained mechanical load while heat receptors are more structurally robust
• B. Phasic (rapidly adapting) mechanoreceptors in the palm undergo ion channel inactivation during sustained stimulation, reducing their receptor potential; tonic thermoreceptors maintain firing throughout a persistent temperature stimulus
• C. The brain filters out pressure signals after a few seconds but continues processing temperature as a safety signal
• D. Adaptation only occurs for mechanical stimuli — thermal receptors are physiologically incapable of adapting

Answer: (B)

Meissner's corpuscles and other rapidly adapting (phasic) mechanoreceptors in the skin signal changes in pressure rather than sustained pressure. At the cellular level, the sodium channels that initially opened to generate the receptor potential progressively inactivate even though mechanical deformation continues, reducing depolarization and firing rate. Nociceptors and some thermoreceptors are tonic — they maintain firing during a persistent stimulus. Both behaviors are functional adaptations, not limitations: phasic receptors free attention for new stimuli, while tonic receptors continuously report ongoing conditions like temperature that require sustained monitoring.

Question 2

A neurophysiologist records from a single sensory neuron while applying a constant pressure to the skin. The neuron fires a burst of action potentials when pressure is applied, falls silent within 2 seconds despite continued pressure, then fires another burst when pressure is released. This response pattern is characteristic of:

• A. A slowly adapting (tonic) receptor reporting the sustained state of the skin
• B. A damaged receptor undergoing receptor potential failure from mechanical fatigue
• C. A rapidly adapting (phasic) receptor signaling stimulus onset and offset rather than sustained presence
• D. Central adaptation — the spinal cord has gated the signal before it reaches the cortex

Answer: (C)

Phasic (rapidly adapting) receptors are tuned to detect change. They fire when a stimulus is first applied (onset) and when it is removed (offset), but fall silent during the sustained plateau. This is because ion channel inactivation reduces the receptor potential back toward resting potential even though the mechanical stimulus persists. Hair follicle receptors and Meissner's corpuscles behave this way. The burst at release (off-response) occurs because removing the stimulus is itself a change — the mechanical deformation reverses, and transiently activated channels fire again. Tonic receptors would maintain firing throughout.

Question 3

Sensory adaptation is a failure of receptor function — over time, receptors become fatigued or damaged by continuous stimulation and can no longer generate adequate receptor potentials.

• T. True
• F. False

Answer: False

Adaptation is an actively designed feature of sensory systems, not a failure or fatigue. Rapidly adapting receptors are specifically structured to inactivate their ion channels during sustained stimulation — this is the intended mechanism. The adaptive value is substantial: by filtering out constant background stimuli, the nervous system remains sensitive to changes in the environment, which are typically more behaviorally relevant than steady states. You stop noticing your clothes but immediately detect a fly landing on your arm. A system that could not adapt would be flooded with constant-stimulus signals, obscuring detection of genuinely new or threatening changes.

Question 4

A stronger stimulus produces a larger receptor potential because more transducer ion channels open, and stimulus intensity is subsequently encoded in the central nervous system as the frequency of action potentials in the sensory nerve.

• T. True
• F. False

Answer: True

Sensory transduction uses two sequential encoding steps. First, stimulus strength is encoded in the receptor potential amplitude (a graded potential): a more intense stimulus causes greater conformational change in transducer molecules, opening more ion channels, producing larger depolarization. This is analog, not digital. Second, if the receptor potential is large enough to reach threshold in the associated sensory neuron, it triggers action potentials. Since action potentials are all-or-nothing, intensity is re-encoded digitally as firing frequency — stronger stimulus, more frequent spikes. This frequency coding is how the nervous system distinguishes a gentle touch from a firm press over the digital action potential communication system.

Question 5

What is the cellular mechanism underlying rapid (phasic) adaptation, and why is this a functional advantage rather than a limitation of the sensory system?

Model answer: Rapid adaptation arises from ion channel inactivation. When a mechanical or other stimulus is first applied, transducer channels open and generate a receptor potential. However, these voltage-sensitive or mechanically-gated channels have an inactivation gate that closes within milliseconds to seconds even while the activating stimulus persists — the channel is open briefly, then enters an inactivated state that produces no current even though the stimulus is still present. The receptor potential declines, firing rate drops, and the sensation fades. The functional advantage is that it frees the sensory system to detect change rather than cataloguing steady states. The nervous system has limited processing capacity; a phasic receptor that signals only onset and offset is much more informative than one that generates constant noise during a constant background condition. This allows the system to be exquisitely sensitive to new stimuli while ignoring the chronic stimulation of everyday contact (clothing, gravity, ambient temperature).

Channel inactivation in sensory receptors is the same biophysical mechanism as voltage-gated sodium channel inactivation in action potential generation — the channel visits open, closed (resting), and inactivated states, and the inactivated state requires hyperpolarization to recover. In sensory receptors, the mechanical or chemical gate controls entry into the open state, but inactivation proceeds on its own timescale regardless. Understanding this mechanism connects sensory physiology to the ion channel selectivity and gating concepts that are prerequisites for this topic.