13_TSH. Mechano Chemoreceptors.2025

Sense Organs: Mechanoreceptors and Chemoreceptors

Course Information

  • Module Code: M4.36

  • Title: Sense organs: Mechanoreceptors and Chemoreceptors

  • Lecturers: Dr. Jacqueline Daly, Dr. Adrian Dervan

  • Dates: 3rd March 2014, 13th February 2025

  • Institution: RCSI - Royal College of Surgeons in Ireland

Learning Objectives

  • Define the five functional types of sensory receptors.

  • Describe three types of encapsulated end organs and one free nerve mechanoreceptors:

    • Pacinian corpuscles

    • Meissner’s corpuscles

    • Ruffini’s end organs

    • Merkel’s discs

  • Discuss muscle spindles and Golgi tendon organs as proprioceptors.

  • Recall the structure and function of taste bud chemoreceptors.

  • Discuss the olfactory sense, including structure and function of the olfactory membrane.

Sensory Receptors

  • Definition: Specialized cells or nerve endings that convert stimuli from the environment into afferent nerve impulses for the CNS.

  • Major Sense Organs:

    • Eyes (optic)

    • Ears (auditory)

  • Other Senses: Taste, Smell, and General Senses (light touch, pressure, temperature, pain, and position awareness) are important for homeostasis.

Classification of Sensory Receptors

  • Five functional categories:

    1. Mechanoreceptors: Detect mechanical stimulation.

    2. Thermoreceptors: Sense temperature changes (Krause’s end bulbs/corpuscles).

    3. Nociceptors: Free nerve endings that respond to pain (e.g., pinching, tearing).

    4. Photoreceptors: Rods and cones of the retina.

    5. Chemoreceptors: Senses of taste and smell.

Mechanoreceptors

  • Overview: Most varied of all receptors.

  • Types: Free nerve endings, tactile hair cells, and encapsulated end-organs.

  • Functions in various parts of the body including skin, muscles, and gut.

Types of Tactile Receptors

  • Pacinian Corpuscles: Deep pressure and vibration detection.

  • Meissner’s Corpuscles: Light touch and vibration.

  • Ruffini Endings: Detect pressure and stretching.

  • Merkel’s Discs: Light touch sensation.

Pacinian Corpuscles

  • Structure resembles a small onion pierced by a fiber.

  • Largest encapsulated nerve endings (1-2 mm long, 0.5 mm diameter).

  • Found in:

    • Subdermal skin layers

    • Abdomen

    • External genitalia

    • Tendons, ligaments, and joints.

  • Respond to strong or rapid movement of tissues, excellent for detecting vibrations.

Ruffini's End Organs

  • Loosely encapsulated nerve endings, medium size between Pacinian and Krause’s end bulbs.

  • Located in dermis and deeper organs (such as joints).

  • Considered slowly adapting mechanical receptors that respond to continuous deformation and distortion. **Ruffini's End Organs** Ruffini's End Organs are loosely encapsulated nerve endings that are intermediate in size between the larger Pacinian corpuscles and the smaller Krause’s end bulbs. They are primarily located in the dermis layer of the skin as well as in deeper structures, including joints and the connective tissue of various organs. These receptors are classified as slowly adapting mechanoreceptors, meaning they respond mainly to sustained pressure and stretch rather than to transient stimuli. They play a critical role in providing the CNS with information about joint movement and position (proprioception). This continuous feedback is essential for activities that require awareness of limb position and movement. Ruffini's End Organs are especially

Meissner’s Corpuscles

  • Smaller than Pacinian corpuscles, oval shaped.

  • Concentrated in dermal papillae of sensitive skin areas (e.g., fingertips, lips).

  • Important for detecting precise touch and light sensations.

Merkel’s Discs

  • Located in epidermis near the stratum basale.

  • Detect light touch sensations.

  • Composed of small cup-shaped cells and naked nerve endings making contact with them.

Proprioceptors

  • Function: Sense position without adaptation.

  • Types: Muscle spindles (detect length) and Golgi tendon organs (detect muscle tension).

  • Muscle spindles continuously monitor muscle tone and provide sensory feedback during movement.

Golgi Tendon Organs

  • Located within muscle tendons, surrounded by connective tissue fibers.

  • Detect muscle tension and provide feedback for muscle control and coordination at a subconscious level.

Chemoreceptors

  • Two major types: those for taste and those for smell.

Taste (Gustation)

  • Taste Buds: Located in lingual papillae, composed of modified epithelial cells around a taste pore.

  • Number: Approximately 3,000 taste buds in humans.

  • Respond to chemicals in food and are crucial for taste sensation.

Olfactory Sense

  • Olfactory Epithelium: Responsible for detecting odours in the nasal cavity.

  • Contains about 100 million olfactory receptor cells, which are bipolar neurons.

  • Capable of detecting very small amounts of odours (e.g., 1 part in 30 billion for certain compounds).

^ so summarised!!

General Functionality

  • The combination of activated receptors allows detection of numerous scent types and taste sensations.

  • Adaptation: Olfactory receptors adapt quickly, often within the first second of stimulus.