Types of Receptors (Physiology) - Long Answer Question

Introduction

Classification of Receptors

A. By Type of Stimulus (Primary Classification)

B. By Adaptation (Phasic vs. Tonic)

1. Phasic (Rapidly Adapting) Receptors

• Respond at the onset and offset of a stimulus
• Detect change in stimulus (velocity/movement), NOT sustained intensity
• Fire a brief burst of action potentials at onset, then go silent even if stimulus continues; fire again at offset
• Examples: Pacinian corpuscle (very rapidly - detects vibration/tapping), Meissner corpuscle (rapidly - flutter, tapping), Hair follicle receptors (rapidly - velocity, direction of movement)

2. Tonic (Slowly Adapting) Receptors

• Respond throughout the duration of a sustained stimulus
• Encode both intensity and duration
• Fire continuously as long as stimulus persists; rate proportional to stimulus strength
• Examples: Merkel receptors / Tactile discs (vertical indentation), Ruffini corpuscle (stretch, joint rotation), Muscle spindles, Nociceptors

Detailed Properties of Each Receptor Type

1. Mechanoreceptors

• Pacinian corpuscles are the largest and most readily identifiable mechanoreceptors
• Meissner corpuscles are most prominent at fingertips and enable fine two-point discrimination
• Ruffini corpuscles have large receptive fields - stimuli at a distance can activate them
• Merkel receptors and Meissner corpuscles have small receptive fields - good for fine spatial discrimination

2. Photoreceptors

• Activated by light energy (electromagnetic spectrum)
• Located in the retina of the eye
• Types: Rods (dim light/night vision, no color) and Cones (bright light, color vision - 3 subtypes for red, green, blue)

3. Chemoreceptors

• Activated by chemicals
• Include:
  • Olfactory receptors (smell) - olfactory mucosa
  • Taste buds (taste) - tongue
  • Peripheral chemoreceptors at carotid and aortic bodies (detect arterial PO2, PCO2, pH)
  • Central chemoreceptors in ventrolateral medulla (detect pH of CSF/PCO2)

4. Thermoreceptors

• Slowly adapting receptors that detect skin temperature changes
• Two classes:
  • Cold receptors - active below 36°C (become quiescent when skin warms above 36°C)
  • Warm receptors - active above 36°C (become quiescent when skin cools below 36°C)
• Both types overlap activity in the moderate range (~36°C)
• At temperatures above 45°C, warm receptors become inactive; polymodal nociceptors take over
• Transduction mechanisms:
  • Warm: TRPV channels (transient receptor potential - vanilloid family); also activated by capsaicin
  • Cold: TRPM8 channels; also activated by menthol

5. Nociceptors

• Respond to noxious/damaging stimuli
• Slowly adapting - prolonged signaling warns of ongoing injury
• Two major classes:
  1. Thermal/Mechanical nociceptors - respond to sharp pricking pain; supplied by Aδ myelinated fibers (fast, sharp pain)
  2. Polymodal nociceptors - respond to high-intensity mechanical, chemical, hot AND cold stimuli; supplied by unmyelinated C fibers (slow, burning/aching pain)
• Sensitization: damaged tissue releases bradykinin, prostaglandins, substance P, K+, H+, histamine → hyperalgesia (reduced threshold for pain)

General Properties of Sensory Receptors

1. Differential (Adequate) Sensitivity

2. Sensory Transduction

1. Stimulus interacts with receptor, causing structural/chemical change
2. Ion channels open/close → change in membrane ion current
3. Change in membrane potential = receptor potential (= generator potential)
   • Depolarizing receptor potential → increases likelihood of action potentials
   • Hyperpolarizing receptor potential → decreases likelihood
4. Receptor potential is graded (proportional to stimulus strength) - unlike action potentials which are all-or-none

3. Receptive Field

• Area of the body that when stimulated results in a change in firing rate of a sensory neuron
• Small receptive fields = high spatial resolution (fingertips, lips) - Meissner, Merkel
• Large receptive fields = lower resolution (back) - Ruffini

4. Receptor Adaptation

• When a constant stimulus is applied, firing frequency initially high → gradually declines
• Phasic receptors: adapt rapidly, detect stimulus onset/offset and velocity of change (e.g., Pacinian, Meissner, hair follicle)
• Tonic receptors: adapt slowly, detect stimulus intensity and duration (e.g., Merkel, Ruffini, nociceptors, muscle spindles)

5. Labeled Line Coding

• Each sensory modality is carried by a specific nerve tract to a specific brain area
• The type of sensation felt depends on which brain region is activated, NOT the type of stimulus
• Example: stimulating a pain fiber electrically still produces pain (labeled line principle)

6. Population Coding

• Some modalities (e.g., color vision) require input from a population of receptors; no single receptor uniquely codes for the sensation

Autonomic Receptor Types (ANS Context)

Summary Table: Receptor Types at a Glance