The Sensory Receptors Study Notes

The Sensory Receptors

Introduction

  • Course Modulated by: Prof. Dr. Magdi Ali El-Damarawi
  • Position: Professor of Medical Physiology, Faculty of Medicine

Learning Objectives

  • By the end of this lecture, you should be able to:
    1. Define sensory receptors.
    2. List the properties of sensory receptors and explain specificity.
    3. Define receptor potential and explain its mechanism.
    4. Define Weber-Fechner law.
    5. List the different classifications of receptors.

Definition of Receptors

  • Receptors are specialized structures located at the peripheral end of afferent neurons.
    • Sensitive to changes in the environment and capable of detecting them.
  • Function: Respond to different types of stimuli and transform these stimuli into electrical energy.

Classifications of Sensory Receptors

A. Histological Classification
  1. Free Nerve Endings:
    • Receptors for pain, temperature, and crude touch.
  2. Expanded Nerve Endings:
    • Merkel’s Disc: For fine touch.
    • Ruffini’s Endings: Respond to fine touch, pressure sense, and sense of position.
    • Krause’s End Bulb: Sensitive to cold sensation.
B. Physiological Classification

  1. Mechanoreceptors:

    • Stretch Receptors: Found in muscles and walls of blood vessels.
    • Tension Receptors: Located in tendons.
    • Touch and Pressure Receptors: Located in skin and subcutaneous tissues.
    • Joint Receptors: For sense of position and movement.
    • Auditory Receptors: Present in the organ of Corti.
    • Vestibular Receptors: Located in the macula and crista ampullaris.

  2. Chemoreceptors:

    • Receptors for taste and smell.
    • Glucoreceptors and Osmoreceptors: Located in the hypothalamus.
    • Arterial O2 Receptors: Present in aortic and carotid bodies.

  3. Thermoreceptors: Cold and warm receptors.

  4. Pain Receptors (Nociceptors): Respond to noxious stimuli which cause tissue damage.

  5. Electromagnetic Receptors: Includes photoreceptors (rods and cones of retina).

C. Classification According to Location

  1. Superficial (Cutaneous) Receptors:

    • Present in skin and subcutaneous tissues.
    • Free Nerve Endings: For pain, temperature, and crude touch.
    • Hair Follicle, Meissner’s Corpuscles, and Merkel’s Discs: For fine touch.

  2. Deep Receptors:

    • Free Nerve Endings: For pain.
    • Proprioceptors: Found in structures deep to the skin.
    1. Muscle Spindles: Located in skeletal muscle.
    2. Golgi Tendon Organ: Present in tendons.
    3. Pacinian Corpuscles: Found in the periosteum of bones and ligaments and tendons of skeletal muscles.

  3. Visceral Receptors:

    • Free Nerve Endings: For pain.
    • Stretch Receptors: Located in the walls of hollow viscera, e.g., urinary bladder.
    • Chemoreceptors: Found in carotid and aortic bodies.
    • Baroreceptors: Present in arterial walls.

Properties of Receptors

  1. Specificity:

    • Each receptor type is highly sensitive to a specific stimulus and nearly non-responsive to other types of sensory stimuli.
    • Example: Rods and cones in the eyes are responsive to light and not to heat, cold, or pressure.

  2. Excitability:

    • Receptors are excitable; the stimulation causes depolarization, referred to as receptor potential (also called generator potential).

  3. Weber-Fechner Law:

    • Describes how the frequency of action potentials in a nerve is directly proportional to the logarithm of the rate of increase in stimulus intensity.
    • Allows for a broad range of intensity detection from stimuli despite the limited number of action potentials reaching the CNS.

  4. Adaptation:

    • Receptors can decrease their response despite constant stimulation.
    • Classification of adapting receptors:
    1. Rapidly Adapting: Such as touch receptors.
    2. Slowly Adapting Receptors: Include mechanoreceptors and pain receptors.
    3. Moderately Adapting Receptors: Temperature, smell, taste, and pressure receptors.

Mechanism of Receptor Potential

  • Receptors produce local graded changes in membrane potential upon stimulation.
  • This change is termed receptor potential or generator potential:
    • Characterized as follows:
    1. A depolarization wave caused by sodium influx.
    2. Does not adhere to the all-or-none principle.
    3. Represents a graded response (magnitude increases with stimulus strength).
    4. Can be summated, meaning effects of multiple stimuli can combine to generate an action potential.
Example: Pacinian Corpuscle
  • Anatomy: Composed of a central nerve fiber surrounded by concentric capsule layers.
  • Central fiber becomes myelinated just before exiting into a sensory nerve.
  • Deformation of the capsule causes Na+ channels to open, resulting in receptor potential, which leads to action potentials transmitted to the CNS.

Mechanisms of Adaptation in Mechanoreceptors

  • Pacinian Corpuscle Example:
    • The viscoelastic nature of the corpuscle leads to immediate receptor potential upon sudden force application, but potential fades within a short duration due to fluid redistribution within the corpuscle.
    • Secondary mechanism involves membrane accommodation, where continuous stimulation leads to Na+ channel inactivation, resulting in decreased receptor potential over time.

Conclusion

  • The understanding of sensory receptors is crucial for understanding how we perceive different stimuli, process sensations, and respond to our environment.

Reference

  • Loëwenstein WR: Excitation and inactivation in a receptor membrane. Ann NY Acad Sci 94:510, 1961.