118. Sensory Communication to Spinal Cord – Quiz

📌 Professor’s Notes: Quick Revision

High-yield revision notes for rapid review before university examinations.

• Sensory pathways carry information from receptors to the spinal cord and higher centers for perception, integration, and reflex activity. Sensory fibers enter the spinal cord through the dorsal roots and ascend in organized pathways to the brain.
• Peripheral nerves contain the axons of pseudounipolar sensory neurons, whose cell bodies lie in the dorsal root ganglia (DRG). Each spinal nerve supplies a specific area of skin called a dermatome, which helps localize neurological lesions. Damage to a spinal nerve, dorsal root, or spinal cord segment produces sensory loss in the corresponding dermatome.
• Sensory (afferent) fibers are classified according to diameter, myelination, and conduction velocity:
  • Aα and Aβ fibers: Carry touch, pressure, vibration, and proprioception.
  • Aδ fibers: Carry fast pain and cold sensation.
  • C fibers: Carry slow pain, warmth, and some mechanical sensations.
• Clinical disorders of peripheral nerves:
  • Axotomy: Complete nerve transection causing Wallerian degeneration in the distal segment and chromatolysis in the proximal neuron. Peripheral nerves can regenerate if the pathway is preserved.
  • Demyelination: Slows or blocks saltatory conduction, producing sensory and motor deficits.
  • Ischemia: Myelinated fibers are affected first, so touch is lost before slow pain.
  • Compression neuropathy (e.g., carpal tunnel syndrome) mainly affects large myelinated fibers, causing early loss of touch, vibration, and proprioception.
  • Local anesthetics block small pain fibers before larger touch fibers, producing pain relief first.
• The dorsal root carries sensory fibers into the spinal cord, whereas the ventral root mainly carries motor fibers. This arrangement is described by the Bell–Magendie law. Dorsal root compression, commonly due to intervertebral disc prolapse, produces radicular pain, paresthesia, hyperesthesia, and later sensory loss in the affected dermatome.
• After entering the spinal cord, primary sensory fibers follow different pathways:
  • Some ascend in the dorsal columns to the medulla.
  • Others synapse in the dorsal horn, cross to the opposite side, and ascend in the anterolateral system.
  • Some fibers connect directly or indirectly with anterior horn motor neurons, forming the basis of spinal reflexes.
  • The spinal cord has a precise somatotopic organization, allowing accurate localization of sensory deficits.
• The gray matter of the spinal cord is arranged into 10 Rexed laminae:
  • Laminae I–VI: Sensory laminae of the dorsal horn.
  • Lamina I: Receives Aδ and C fibers carrying pain.
  • Lamina II (Substantia gelatinosa): Receives mainly C fibers and plays a major role in pain modulation.
  • Laminae III–VI: Receive Aβ fibers carrying touch, pressure, vibration, and proprioception.
  • Lamina VII–VIII: Mainly interneurons.
  • Lamina IX: Contains motor neurons.
  • Lamina X: Surrounds the central canal.
• Descending pathways from the cerebral cortex and brainstem terminate in both sensory and motor laminae, where they regulate pain transmission, spinal reflexes, posture, motor coordination, and sensory processing.