Adaptation of Sensory Receptors

Adaptation refers to the reduction of receptor sensitivity in the presence of a constant stimulus. This process allows the nervous system to filter out unchanging background stimuli and focus on new or changing information.

Types of Adaptation

• Phasic Receptors (Fast-Adapting):

  • Respond rapidly to changes in the environment.

  • Report the rate of change of a stimulus.

  • Adapt quickly and stop responding if the stimulus remains constant.

  • Example: Olfactory receptors (smell), which quickly adapt to persistent odors.

• Tonic Receptors (Slow-Adapting):

  • Respond slowly and continue to generate action potentials as long as the stimulus is present.

  • Provide a sustained response to a constant stimulus.

  • Example: Pain receptors (nociceptors), which continue to signal as long as the painful stimulus persists.

Adaptation as a Continuum

Adaptation can be viewed as a spectrum, with phasic and tonic receptors at opposite ends:

• Phasic receptors (e.g., thermoreceptors): Adapt quickly, signaling changes in temperature but not constant temperature.

• Mechanoreceptors: Can be either phasic or tonic, depending on the specific type and function.

• Tonic receptors (e.g., nociceptors): Adapt slowly, signaling ongoing pain or pressure.

Examples and Applications

• Example of a tonic receptor: A receptor in the eye that detects rapid changes in light intensity (slow to adapt, continues to respond to constant stimulus).

• Example of a phasic receptor: A receptor that adapts quickly to the sound of a ticking clock (responds to change, not constant stimulus).

Practice Questions and Key Points

• Phasic receptors:

  • Do not respond to sustained stimuli with a continuous firing rate.

  • Respond with a burst of action potentials when a stimulus is first applied and quickly adapt to the stimulus.

  • Adapt slowly to changes in stimulus intensity (incorrect; they adapt quickly).

  • Do not exhibit continuous, constant firing in response to a stimulus.

• Thermoreceptors tend to be phasic receptors because they:

  • Take a long time to respond to a temperature change (incorrect; they respond quickly).

  • Produce a generator potential (correct; all sensory receptors do this).

  • Produce a receptor potential (correct; this is the graded potential generated in response to a stimulus).

Key Terms and Definitions

• Receptor Potential: A graded electrical response produced by a sensory receptor cell in response to a stimulus.

• Generator Potential: A type of receptor potential that can initiate an action potential in a sensory neuron.

• Phasic Receptor: A receptor that adapts rapidly to a stimulus and stops responding if the stimulus remains constant.

• Tonic Receptor: A receptor that adapts slowly and continues to respond to a constant stimulus.

Comparison Table: Phasic vs. Tonic Receptors

Summary

• Adaptation is a key property of sensory receptors, allowing the nervous system to prioritize new or changing stimuli.

• Phasic receptors adapt quickly and are best for detecting changes, while tonic receptors adapt slowly and are suited for monitoring persistent stimuli.