Touch Receptors and Tactile Encoding

Overview of Lecture Topics

• Touch receptor morphologies, tactile encoding, tactile sensitivity, and tactile perception are explored

Microneurography and FA Afferents

• Microneurography can identify quickly adapting (FA) afferents
• FA afferents respond to on/off stimuli, slips, flutters, and blowing across the skin
• FAs do not respond to sustained stimulation

Microneurography and SA Afferents

• Microneurography can identify slowly adapting (SA) afferents
• SA afferents respond to sustained stimulation and stretch (SA2)

Cutaneous Receptor Morphology

• There are 4 types of cutaneous receptors

Mechanoreceptor Morphologies: SA1 (Merkel Cells)

• SA1 mechanoreceptors are located within the epidermis, specifically at fingerprint ridges.
• PIEZO2 channels are critical for skin deformation response.
• Deleting PIEZO2 reduces response to static indentation in studies using mouse models
• Merkel cells encode pressure
• Vesicles release norepinephrine
• Detect points, edges, and curvature
• SA1 mechanoreceptors crucial for shape detection

Firing Characteristics: SA1 (Merkel Cells)

• SA1 Merkel cells exhibit fundamental responses to sustained indentation due to norepinephrine release from vesicles.
• Firing pattern is irregular
• Sensitive to points, edges and shapes
• Sensitive to low frequency vibrations (2-32Hz)

Mechanoreceptor Morphologies: SA2 (Ruffini Organs)

• SA2 mechanoreceptors have a spindle shape.
• Located in the dermis, tendons, and ligaments.
• 1 afferent innervates each Ruffini organ.
• These have diffuse receptive fields.

Firing Characteristics: SA2 (Ruffini Organs)

• SA2 Ruffini organs have a high threshold for indentation and a regular firing pattern.
• Sensitive to skin stretch and force
• Responds to skin deformation without direct pressure
• SA2s continue to respond even after a stimulus is removed and the skin remains deformed.
• Sensitive to low vibration frequencies (<8Hz).

Mechanoreceptor Morphologies: FA1 (Meissner Corpuscles, RAs)

• FA1 receptors are located withing dermal papillae, close to the skin cells
• Elongated receptors containing flattened lamella cells
• They express PIEZO2 channels
• 1 afferent terminates in 2-6 corpuscles
• Movement of ridges and valleys activates receptors

Response Characteristics: FA1 (Meissner Corpuscles, RAs)

• FA1 receptors show distinct on-off responses and respond to flutters.
• Codes rate and shear velocity of skin indentation
• Respond to slip and tangential forces
• Useful for grip control and sensitive to edges.
• Respond to low-mid frequencies (8-64Hz).

Mechanoreceptor Morphologies: FA2 (Pacinian Corpuscles, PCs)

• FA2 mechanoreceptors are located deep in the dermis.
• Onion-like structures with multiple layers of membranes.
• Act as a filter for low frequencies
• Innervated by only 1 afferent fiber and respond to high frequencies

Response Characteristics: FA2 (Pacinian Corpuscles, PCs)

• FA2 Pacinian corpuscles show distinct on-off responses and are activated by blowing across the skin
• Code acceleration and vibrations
• Respond to high frequencies. (>64Hz, favor 250Hz)

Responses During Lifting

• Multiple receptors are engaged during lifting, from initial contact to release.
• SA2 receptors do not require as much force

Perception Modalities

• Cutaneous and subcutaneous mechanoreceptors: Alpha-beta Fibers
• Meissner corpuscle: RA1 for stroking/flutter
• Merkel disk receptor: SA1 for pressure/texture
• Pacinian corpuscle: RA2 for vibration
• Ruffini ending: SA2 for skin stretch

Texture Encoding

• The size and shape of objects touching the hand can be coded for by SAI's
• A smaller probe area results in higher discharge rates in fewer receptors
• A larger probe results in lower discharge rates, but engages more afferents.

Texture Encoding and Receptor Resolution

• SA1s most closely resemble braille since they have a very high spacial resolution
• FA1s present a blurred image because RFs are slightly larger than the spacing
• SA2 and FA2 do not capture the pattern at all because RFs are too large

Touch Sensitivity: Receptive Field Sizes

• Touch sensitivity varies across regions due to receptive field sizes and innervation density.
• Higher sensitivity is associated with small receptive fields.
• Lower sensitivity is associated with large receptive fields.

Touch Sensitivity: Innervation Density

• More dense innervation leads to more sensitivity
• Less dense innervation leads to less sensitivity

Two-Point Discrimination

• Two-point discrimination depends on receptive field size, convergence, and receptor density
• Higher sensitivity is associated with small receptive fields and greater receptor density