Overview
This lecture explains the four main types of mechanoreceptors in the skin specialized for sensing touch, pressure, vibration, and stretch, focusing on their structure, function, and significance.
Major Types of Mechanoreceptors
- Four encapsulated mechanoreceptors: Meissner's corpuscles, Pacinian corpuscles, Merkel's disks, and Ruffini's corpuscles detect tactile stimuli.
- These are low-threshold (high-sensitivity) receptors, activated by weak mechanical stimulation.
- All are innervated by large, myelinated axons for rapid signal transmission.
Meissner's Corpuscles
- Located just beneath the epidermis in fingers, palms, and soles, especially in glabrous (hairless) skin.
- Made of connective tissue and Schwann cell lamellae, with one or more afferent nerve fibers.
- Rapidly adapting; respond to low-frequency vibrations (30-50 Hz), efficient for detecting texture when objects move across the skin.
- Account for about 40% of sensory innervation in the human hand.
Pacinian Corpuscles
- Large, onionlike encapsulated endings in subcutaneous tissue and deeper body regions.
- Central rapidly adapting axons within a fluid-filled capsule.
- Very sensitive to high-frequency vibrations (250-350 Hz); adapt faster and have lower threshold than Meissner's.
- Involved in sensing fine textures and vibrations; make up 10-15% of hand cutaneous receptors.
Merkel's Disks
- Located in the epidermis, aligned with dermal papillae, dense in fingertips, lips, and genitalia.
- Each has a slowly adapting nerve ending connected to a specialized cell.
- Detect static pressure, shapes, edges, and rough textures.
- Account for about 25% of hand mechanoreceptors.
Ruffini's Corpuscles
- Spindle-shaped, found deep in skin, ligaments, and tendons, aligned parallel to skin stretch lines.
- Sensitive to skin stretching from digit or limb movements.
- Slowly adapting; about 20% of hand receptors.
- Likely respond to internally generated stretch, but specific tactile sensation remains unclear.
Key Terms & Definitions
- Mechanoreceptor — sensory receptor specialized to detect mechanical changes (touch, pressure, vibration).
- Low-threshold receptor — receptor activated by small or weak stimuli.
- Glabrous skin — smooth, hairless skin found on palms and soles.
- Rapidly adapting — receptor that quickly stops responding to a constant stimulus.
- Slowly adapting — receptor that continues to respond during a sustained stimulus.
Action Items / Next Steps
- Review the structures and functions of each mechanoreceptor.
- Study Table 9.1 and Figure 9.3 to reinforce location and features of each receptor type.