Somatosensory Pathways Lecture

Overview

• Focus on somatosensory pathways, specifically the dorsal column (medial lemniscal pathway).
• Pathways vary for different sensations.
• Main focus: pathways carrying touch, vibratory sense, and proprioception to the cerebral cortex.

Spinal Cord Anatomy

• Dorsal root: carries sensory neurons from the periphery to the CNS.
• Ventral roots: carry motor neurons from the CNS to the PNS.
• Dorsal column tract: located at the back of the spinal cord.
  • Divided into two parts:
    • Fasciculus cuneatus (lateral)
    • Fasciculus gracilis (medial)

Function of Dorsal Column Tracts

• Carry sensory information:
  • Touch
  • Pressure
  • Vibration
  • Proprioception
• Relay sensory information to the somatosensory cortex in the brain.

Somatosensory Cortex

• Location where somatic sensory information is perceived.

Thalamus

• Acts as an important relay station in the ascending pathway.

Brain Anatomy

• Brainstem consists of:
  • Midbrain
  • Pons
  • Medulla
• Focus on cervical and lumbar spinal cord cross-sections.

Mechanoreceptors

• Merkel cells: detect touch and pressure (superficial skin).
• Ruffini endings: stimulated by sustained pressure.
• Meissner corpuscles: stimulated by tap and flutter sensations.
• Pacinian corpuscles: detect deep pressure and vibration.

Neuron Pathways in Dorsal Column

• First-order neuron: carries sensory info to the dorsal column.
• Fasciculus cuneatus: upper body sensory info.
• Fasciculus gracilis: lower body sensory info.
• Second-order neuron: crosses over at medulla and travels up medial lemniscal tract.
• Third-order neuron: carries info to primary somatosensory cortex.

Lateralization

• Sensation on the left side of the body is perceived by the right side of the brain and vice versa.

Proprioception

• Provides sense of equilibrium and balance.
• Proprioceptors located in muscles, tendons, and joints.

Types of Proprioceptors

• Muscle spindles: provide info about muscle length changes.
• Golgi tendon organs: detect changes in muscle tension.

Recap of Pathway

• Proprioceptive information travels up the dorsal column:
  • In the upper body, via fasciculus cuneatus.
  • Synapses at the nucleus cuneatus in the medulla.
  • Ascends to the contralateral side of the brain, reaching the primary somatosensory cortex.