Lecture Notes: Dorsal Column Medial Lemniscus Pathway

Introduction

• Focus Topic: Dorsal Column Medial Lemniscus Pathway (DCMLP)
• Importance: Controls touch, proprioception, pressure, vibration, stretch.

Sensations Carried by DCMLP

• Discriminative Touch:
  • Fine and discriminative touch (two-point touch).
• Pressure and Stretch:
  • Ability to detect pressure and stretch on the skin.
• Vibrations:
  • Detects vibrations through specialized receptors.
• Proprioception:
  • Awareness of joint, ligament, muscle positions (e.g., knowing arm/hand position without seeing).

Pathway Characteristics

• Ascending Tract:
  • Tract is a bundle of axons in the CNS, specifically ascending = going up.

Receptors Involved

• Meissner's Corpuscles:
  • In dermal papilla, detect fine and discriminative touch.
• Merkel's Disks:
  • Located in stratum basale, detect fine touch and superficial pressure.
• Pacinian Corpuscles:
  • Found in dermis/hypodermis, detect deep pressure and vibrations.
• Peritrichial Nerve Endings:
  • Around hair follicles, detect hair movement.
• Ruffini Corpuscles:
  • In dermis/joint capsules, detect skin stretch and vibrations.
• Proprioceptive Receptors:
  • Muscle Spindles: Detect muscle stretch; nuclear chain and nuclear bag fibers.
  • Golgi Tendon Organs (GTO): Detect tendon stretch, 1b fibers.

Nerve Fibers

• A Fibers:
  • A Alpha: Fastest, for muscle spindles and GTO.
  • A Beta: Medium speed, for touch and pressure receptors.
  • A Delta: For fast pain and crude touch.
• B and C Fibers: Less myelinated, slower conduction.

Pathway Details

• Fasciculus Gracilis:
  • Carries input from lower limbs (below T6).
• Fasciculus Cuneatus:
  • Carries input from upper limbs (above T6).
• Somatotopic Organization:
  • Medial to lateral organization from sacral to cervical regions.

Pathway Neurons

• First-Order Neuron:
  • Located in dorsal root ganglion.
• Second-Order Neuron:
  • Located in medulla (nucleus gracilis/cuneatus).
  • Crosses midline as internal arcuate fibers.
• Third-Order Neuron:
  • Located in thalamus (ventral posterior lateral nucleus).
  • Projects to cerebral cortex via posterior limb of internal capsule.

Cortical Termination

• Primary and Secondary Somatosensory Cortex (S1, S2):
  • Located in post-central gyrus.
• Sensory Homunculus:
  • Represents body regions and their respective sensory cortex areas.

Additional Information

• Projection Fibers:
  • Include internal capsule and corona radiata, crucial for sensory and motor pathways.
• Commissural and Association Fibers:
  • Allow interhemispheric communication and intrahemispheric processing.

Closing Remarks

• Importance of understanding DCMLP for clinical implications and sensory processing.
• Encouragement to engage with additional resources such as social media and previous lectures for further learning.