Dorsal Column Medial Leminiscus Pathway Lecture Notes

Overview

• Purpose: Controls touch, proprioception, pressure, vibration, and stretch.
• Pathway: Called the dorsal column medial leminiscus pathway.

Key Functions

• Discriminative Touch: Ability to distinguish fine touch and two-point contact.
• Pressure and Stretch: Detects pressure and stretch on the skin.
• Vibrations: Special receptors detect vibrations.
• Proprioception: Awareness of body position in space, crucial for cerebellum and central nervous system.

Pathway Characteristics

• Ascending Tract: Pathway is a bundle of axons in the CNS, moving upwards.
• Types of Tracts: Discussed other tracts like the anterolateral system and spinocerebellar tract.

Receptors and Their Functions

• Meissner's Corpuscles: Found in dermal papilla, detect fine touch.
• Merkel's Discs: Located at the stratum basale, detect fine touch and superficial pressure.
• Pacinian Corpuscles: Found in deeper layers, detect deep pressure and vibrations.
• Peritrichial Nerve Endings: Located around hair follicles, detect hair movement.
• Ruffini Corpuscles: Found in the dermis/joint capsules, detect skin stretch and vibrations.

Proprioceptive System

• Muscle Fibers:
  • Intrafusal Fibers: Detect muscle stretch.
  • Nuclear Chain and Bag Fibers: Respond to muscle stretch (onset and progression).
  • Golgi Tendon Organs (GTOs): Detect tendon stretch, prevent over-stretching.

Nerve Fibers

• Types of Nerve Fibers:
  • A, B, C fibers: Differ in myelination.
  • A-alpha fibers: Fast, for muscle spindles and GTOs.
  • A-beta fibers: For pressure, touch, and vibration sensations.

Pathway Process

• Entry: Fibers enter the spinal cord at the dorsal root ganglion.
• Cross Sections: Cervical and lumbar sections identified.
• Somatotopic Arrangement: Medial to lateral organization, impacts lesion effects.
• Ipsilateral Pathway: Remains on the same side of the spinal cord initially.

Pathway Components

• Fasciculus Gracilis & Cuneatus:
  • Gracilis: Below T6, lower limbs.
  • Cuneatus: Above T6, upper limbs.
• Second Order Neurons: Located in medulla (nucleus gracilis and cuneatus).
• Crossing Over: Internal arcuate fibers cross to contralateral side.
• Medial Leminiscus: Travels to the thalamus.
• Third Order Neurons: Located in thalamus (ventral posterior lateral nucleus).

Convergence to Cortex

• Posterior Limb of Internal Capsule: Pathway travels through here.
• Projection Fibers: Includes coronaradiata, connects to somatosensory cortex.
• Somatosensory Cortex: Primary (S1) and secondary (S2) regions.

Somatotopic Organization

• Sensory Homunculus: Specific areas of the cortex correspond to body areas.

Additional Fiber Types

• Projection Fibers: Allow for sensory and motor information transfer.
• Commissural Fibers: Connect hemispheres (e.g., corpus callosum).
• Association Fibers: Connect different parts of the cortex (e.g., arcuate fasciculus).

Closing

• Emphasized importance of understanding these pathways for clinical applications.
• Encouraged engagement with content and resources.