Lecture 17 - Somatosensory System: Touch and Proprioception

Somatosensory System: Touch and Proprioception

Overview of the Somatosensory System

• Sensory Transduction: The process of converting energy from a stimulus (e.g., touch, pressure, temperature) into an electrical signal.

• Mechanoreceptors: Afferent fiber terminals that detect and transmit touch sensory stimuli.

  • Pseudounipolar neurons in Dorsal Root Ganglia (DRG) ramify in skin, muscles, or joints and synapse onto spinal cord neurons.

• Free Nerve Endings: Afferent fiber terminals that detect pain and temperature sensations.

Key Concepts and Terms

• Two-Point Discrimination: Minimum distance required to distinguish between two separate stimuli applied to the skin.

• Somatosensory Receptive Field: The area of skin surface that alters the action potential (AP) of a sensory neuron; larger receptive fields correspond to less sensitivity.

• Proprioception: The body’s ability to sense its position and movement in space.

Function of Somatosensory Afferents

• Parallel Pathways: Different somatosensory afferents vary in conduction velocity, receptive field size, and response to stimuli.

  • Myelinated axons have a lower action potential generation threshold and are more sensitive than free nerve endings.

• Receptive Fields:

  • Larger fields with more arborizations result in less fine discrimination between stimuli.

  • Smaller fields with dense innervation allow for better two-point discrimination.

Somatosensory Afferents Types

• Slowly Adapting Mechanoreceptors: Respond continuously to a sustained stimulus.

• Rapidly Adapting Mechanoreceptors: Respond briefly to changes in stimulus.

• High-Fidelity Representation of Braille: Different mechanoreceptors provide varying information about texture and patterns.

Central Pathways Conveying Tactile Information

• Dorsal Column Medial Lemniscal Pathway: Carries mechanosensory information from the body to the brain with topographical organization.

• Trigeminothalamic Pathway: Transmits tactile sensation from the face, utilizing trigeminal nerve pathways.

Somatosensory Cortex Features

• Functional Representation: The somatosensory cortex has a larger representation for the hands and face compared to other body regions.

• Somatotopic Organization: Organization based on body layout; areas representing face and hands are disproportionately larger.

• Neuronal Columns: Neurons form functionally distinct columns that contribute to sensation processing.

Functional Changes in the Somatosensory Cortex

• Cortical Representation Changes: Following amputation or skill acquisition, the remapping of the somatosensory cortex affects sensory input.

  • Reorganization can occur at any age and is influenced by experience, training, and injury.

Clinical Applications

• Dermatomes: Areas of skin innervated by specific spinal nerves, important for diagnosing nerve injuries and conditions like shingles.

• Two-Point Discrimination Variability: Demonstrates differences in sensory acuity across body parts (e.g., fingertips vs. thighs).

Two-Point Discrimination of the Skin

• Minimum distance required to distinguish between two separate stimuli applied to the skin.

Somatosensory Receptive Field

• The area of skin surface that alters the action potential (AP) of a sensory neuron; larger receptive fields correspond to less sensitivity.

Influence of Somatosensory Afferents on Two-Point Discrimination

• Different somatosensory afferents vary in conduction velocity, receptive field size, and response to stimuli. Myelinated axons have a lower action potential generation threshold and are more sensitive than free nerve endings. Larger fields with more arborizations result in less fine discrimination between stimuli; smaller fields with dense innervation allow for better two-point discrimination.

Proprioception

• The body’s ability to sense its position and movement in space.

Central Pathways Conveying Tactile Information

• Dorsal Column Medial Lemniscal Pathway: Carries mechanosensory information from the body to the brain with topographical organization.

• Trigeminothalamic Pathway: Transmits tactile sensation from the face, utilizing trigeminal nerve pathways.

Somatosensory Cortex Function

• Functional Representation: The somatosensory cortex has a larger representation for the hands and face compared to other body regions.

• Somatotopic Organization: Organization based on body layout; areas representing face and hands are disproportionately larger.

• Neuronal Columns: Neurons form functionally distinct columns that contribute to sensation processing. Changes in the somatosensory cortex can occur following amputation or skill acquisition, affecting sensory input. Reorganization may occur at any age, influenced by experience, training, and injury.