CNS II Notes

Learning Objectives

• Understand the structural and functional organization of the cerebral cortex and spinal cord.
• Describe the connections within and between cortical layers, projective sites, and the arrangement of spinal cord segments.
• Recognize the neurological implications of injuries at specific spinal levels.

Cerebral Cortex

• Composed of two hemispheres and five major lobes:
  • Frontal lobe
  • Parietal lobe
  • Temporal lobe
  • Occipital lobe
  • Insular cortex (hidden beneath others)
• Key structures include:
  • Precentral gyrus (primary motor cortex)
  • Postcentral gyrus (primary somatosensory cortex)
  • Calcarine fissure (primary visual cortex)
  • Superior temporal gyrus (primary auditory cortex)

Primary and Association Cortex

• Primary Cortex
  • Unimodal processing for a single sensory modality.
• Association Cortex
  • Unimodal: Processes a single modality
  • Heteromodal: Integrates multiple modalities across regions.

Cortical Layers

• The cortex has six distinct layers:
  1. Molecular layer
  2. External granular layer (II)
  3. External pyramidal layer (III)
  4. Internal granular layer (IV)
  5. Internal pyramidal layer (V)
  6. Multiform layer (VI, pleomorphic cells)
• Layers III and V serve different functions for connections and projections.

Cortical Connections

• Types of Fibers:
  • Association Fibers: Connect regions within the same hemisphere (short and long).
  • Commissural Fibers: Cross hemispheres (e.g., corpus callosum).
  • Projection Fibers: Convey information to/from deeper structures and spinal cord.
  • Cortical connections involve layers II, III, IV, V, and VI in interactions with thalamus and subcortical structures.

Spinal Segment Organization

• Dorsal Horn: Receives sensory information.
• Ventral Horn: Contains motor neurons that send out motor information.
• Dorsal/ventral roots: Carry sensory and motor signals between the spinal cord and periphery.

Rexed's Laminae

• Spinal cord organization into 10 laminae (I-X) based on functionality.
• Lamina II and IV: Involved in the processing of nociceptive signals (lateral spinothalamic tract).

Spinal Tracts

• Ascending Tracts (Sensory):
  • Dorsal Column-Medial Lemniscus: Perception of fine touch, vibration.
  • Lateral Spinothalamic: Pain and temperature sensation.
  • Dorsal Spinocerebellar: Unconscious proprioception.
• Descending Tracts (Motor):
  • Lateral Corticospinal: Voluntary motor control.
  • Rubrospinal: Facilitates flexor muscle tone.

Clinical Relevance: Brown-Sequard Syndrome

• Characterized by:
  • Ipsilateral loss of proprioception and fine touch below the injury
  • Contralateral loss of pain and temperature sensation below the injury
• Example case: Injury at spinal level T6.

Vasculature of the Spinal Cord

• Arterial Supply:
  • Anterior Spinal Artery: Supplies anterior 2/3 of the spinal cord.
  • Posterior Spinal Arteries: Supply posterior 1/3.
  • Regional supplies via radicular arteries (varies by region: cervical, thoracic, lumbar).

Thalamus

• Functions as the main relay station for sensory information (excluding olfaction).
• Involved in reciprocal connections to cerebral cortex, facilitating communication with VPL (ventral posterior lateral nucleus) for sensory processing.