Touch and Proprioception

Touch and Proprioception

Introduction to Neural Communication

  • Examination of communication mechanisms between two neurons, including:

    • Neuron structure, membrane and membrane potential.

    • Graded potentials reception.

    • Action potential triggering and ionic basis.

    • Presynaptic terminal neurotransmitter release.

    • Post-synaptic terminal information reception and neurotransmitter receptor interactions.

    • Synaptic plasticity governing communication increase or decrease.

  • Transition to functional neural circuitry, focusing on somatization and pain.

Sensation Overview

  • Definition: Sensation refers to the conversion of sensory stimuli (light, sound, touch, taste, smell) into a neural signal.

    • Information conveyed to the brain through AFFERENT signaling.

    • Brain interprets both QUALITATIVE and QUANTITATIVE characteristics of the stimulus.

    • Focus on the somatosensory system, which includes:

    • Touch

    • Pressure

    • Vibration

    • Limb position

    • Temperature

    • Itch

Definitions of Key Concepts

  • Somatosensation:

    • A diverse sensory system linked to afferent neurons in the skin, muscles, and joints.

    • Pertains to touch, pressure, pain, temperature, position, movement, and vibration perception.

  • Proprioception:

    • Also referred to as kinaesthesia.

    • Ability to sense the position and movement of body limbs and parts.

    • Information sourced from specialized receptors in muscles, tendons, and joints.

    • Includes pain, temperature, and non-discriminate stimuli.

Peripheral Somatosensation Resources

Mechanical Forces in Sensory Activation

  • Activation Mechanism: Sensory neurons in skin, muscles, and joints respond to mechanical forces.

  • Neuron terminals interact with various cell types including glia, with information relayed to the CNS.

Types of Skin

  1. Glabrous Skin (hair-less):

    • Example areas: palms, soles, lips.

    • Specialization for discriminative touch.

  2. Hairy Skin:

    • Over 90% of human body, provides both discriminative and non-discriminative (emotional response evoking) touch.

Sensory Afferent Fibers and Mechanoreceptors

  • Mechanoreceptors: Specialized receptors for sensing mechanical forces.

    • Respond to innocuous mechanical stimuli via low-threshold mechanoreceptors (LTMRs).

  • Two primary roles of somatosensory afferent fibers:

    1. Transduce mechanical force into neural signals.

    2. Provide the pathway for neural signals to reach the CNS.

Somatosensory Afferent Variables

  1. Cell Body Size

  2. Conduction Velocity:

    • Influenced by axon diameter and degree of myelination.

      • Largest and most heavily myelinated: Muscle sensory receptors.

      • Slightly smaller but heavily myelinated: Cutaneous sensory afferents.

      • Smaller or unmyelinated: Pain, temperature, and certain light touch sensations.

  3. Stimuli-Specific Responses:

    • Different receptors for touch, stretch, pain, temperature, etc.

    • Parallel pathways manage various information types about identical stimuli.

  4. Response Properties to Stimulation:

    • Adaptation Mechanisms:

      • Slow adaptation: Responses persist for the entire duration of stimulus.

      • Fast adaptation: Responses activate at stimulus onset and sometimes at offset.

Receptive Fields of Cutaneous Sensory Afferents

  • Receptive Field (RF): The area of skin surface where stimulation elicits action potentials; stronger responses occur centrally.

    • RF size determined by:

    • Axonal arborization: Increased branching leads to larger RF.

    • Density of innervating fibers.

    • RFs tend to overlap:

    • This can hinder tactile discrimination spatial accuracy but allow compensation for afferent damage.

    • Concept of two-point discrimination explained.

Classes of Mechanosensory Cells in Glabrous Skin

  • Cutaneous afferent fibers possess specialized receptor cells for refined somatic stimulation transmission.

  • Free nerve endings: No specialized receptors, primarily for pain sensation.

  • Four classes of specialized mechanosensory cells:

    1. Merkel Cells

    2. Meissner Corpuscles

    3. Pacinian Corpuscles

    4. Ruffini Corpuscles

Detailed Descriptions of Mechanosensory Cells

  • Merkel Cells:

    • Anatomy: Oval-shaped, located predominantly in areas with high spatial discrimination (e.g., fingertips).

    • Mechanism of Action: Connect to skin cells via adhesion proteins; activation of Merkel cells occurs through skin movement, sensitive to mechanical changes.

    • Function: Detects tactile stimuli with the highest spatial resolution among sensory afferents; connected to Aβ-SAI LTMRs with slow adaptation characteristics.

  • Meissner Corpuscles:

    • Anatomy: Composed of connective tissue and capsule with flattened lamellar cells, located at the tips of dermal papillae.

    • Mechanism of Action: Changes in collagen fibers from skin indentation signal sensory response. 4x more sensitive to skin indentations compared to Merkel cells.

    • Function: Detects changes in tactile stimulation and low-frequency vibrations (e.g., hand grip); fast adaptation with large, heavily myelinated axons connected to Aβ-RAI LTMRs.

  • Pacinian Corpuscles:

    • Anatomy: Onion-like structure with concentric membranes surrounding a single afferent fiber, located deep in the dermis.

    • Mechanism of Action: Sensitive to high-frequency disturbances; dampens low-frequency vibrations.

    • Function: Detects vibration changes, sensitive with large receptive fields; rapid adaptation and connected to Aβ-RAII LTMRs.

  • Ruffini Corpuscles:

    • Anatomy: Elongated spindle-shaped capsules found deep in the dermis.

    • Function: Sensitive to skin stretching, providing feedback for finger positions and hand conformation; connections to Aβ-SAII LTMRs with slow adaptation.

Adaptation Properties of Cutaneous Receptors

  • Adaptation Overview:

    • Rapid adaptation (e.g., Meissner and Pacinian corpuscles) versus slow adaptation (e.g., Merkel and Ruffini corpuscles).

    • Correlation of adaptation speed with receptive field sizes (small RF for Meissner and large for Pacinian).

Response Evaluation for Braille Letters

  • Mechanoreceptors' response to raised Braille letters depicted:

    • Progression of receptor activation as the skin traverses Braille letters displaying spatial resolution and adaptation.

Mechanoreceptors in Hairy Skin

  • Mechanoreceptors interact with hair follicles, involving:

    • Ruffini (stretch) and Pacinian (tactile) corpuscles present in hairy skin.

    • Merkel cells involved as Aβ-SAI LTMRs afferents in touch domes associated with hair follicles.

    • Additional receptor types: Circumferential endings and longitudinal lanceolate endings.

Specifics on Circumferential and Longitudinal Endings

  • Circumferential Endings:

    • Location: Surround hair follicles (zigzag, Awl/Auchene types).

    • Function: Sensitive to skin stroking; slower conduction.

    • Afferents: Aβ Field-LTMRs.

  • Longitudinal Lanceolate Endings:

    • Location: Surrounds guard cells of hair follicles.

    • Function: Sensitive to hair deflection and skin stroking; slower conduction compared to glabrous receptors; can also be associated with nociception.

Physiological Response Table

  • Table 12.1: Summary of physiological subtype characteristics, encompassing axon conduction velocities, skin diameters, optimal stimuli, afferent connections, and associated response properties.

Sensory Transduction

  • Definition: The process in which sensory stimuli energy is converted into an electrical signal.

    • Involves alterations in cation channel permeability, leading to depolarization, and generating receptor potentials (EPSP/IPSP).

    • Magnitude of Graded Potentials: Correlates with amplitude; action potential intensity aligns with frequency.

Topic Coverage Moving Forward

  • Upcoming class focus: Proprioception and its implications in somatosensory feedback intricacies.