Spinal Cord

Physical dimensions and characteristics

• Length: The spinal cord measures approximately $42-45\,cm$ in length.
• Vertebral Column Comparison: The total length of the vertebral column is approximately $70\,cm$.
• Weight: The spinal cord weighs approximately $35\,gms$.
• Asymmetry: In approximately $75\%$ of the population, the spinal cord is asymmetrical, typically with the right side being larger than the left.

Anatomy and Segmentation

• Extent: The spinal cord extends from the decussation of the pyramids (at the level of the foramen magnum) and terminates at the level of the $L2$ vertebra, known as the conus medullaris.
• Regional Divisions:
  • Cervical Region: Includes segments $C1$ through $C8$.
  • Thoracic Region: Includes segments $T1$ through $T12$.
  • Lumbar Region: Includes segments $L1$ through $L5$.
  • Sacral Region: Includes segments $S1$ through $S5$.
• Spinal Enlargements: There are two distinct regions where the cord is thickened to accommodate the innervation of the limbs:
  • Cervical Enlargement: Corresponds to segments $C5-T1$.
  • Lumbar Enlargement: Corresponds to segments $L3-S2$.
• Inferior Structures:
  • Conus Medullaris: The tapered, lower end of the spinal cord reaching the $L2$ level.
  • Cauda Equina: A collection of spinal nerves and rootlets (arranged like a horse's tail) descended past the conus medullaris.
  • Filum Terminale: A strand of fibrous tissue extending from the conus medullaris to the coccyx. It is divided into the internal filum terminalis (within the dural sac) and the external filum terminalis (extending to the tip of the coccyx).

Spinal Meninges and Protective Spaces

• Meningeal Layers (Superficial to Deep):
  • Dura Mater: The outermost, toughest protective layer. The dural sac typically ends at the level of the $S2$ vertebra.
  • Arachnoid Mater: The middle layer, appearing like a cobweb.
  • Pia Mater: The innermost layer, tightly adherent to the surface of the spinal cord. It includes specific structures like the epipial layer and the pia intima.
• Meningeal Spaces:
  • Epidural Space: The space external to the dura mater.
  • Subdural Space: The potential space between the dura and the arachnoid mater.
  • Subarachnoid Space: The space between the arachnoid and the pia mater, which contains the cerebrospinal fluid (CSF).
• Supporting Structures:
  • Dentate Ligament: Flat, triangular sheets of pia mater that extend laterally to attach the spinal cord to the dura mater, providing stability.

Internal Structure and Gray Matter Organization

• Neurotome: Defined as the specific part of the spinal cord that gives rise to a single spinal nerve.
• Cross-Sectional Features:
  • Dorsal Median Sulcus (DMS): A shallow groove on the posterior midline.
  • Anterior Median Fissure (AMF): A deeper groove on the anterior midline.
  • Central Canal: A small, fluid-filled space in the center of the cord.
  • Gray Commissure: The bridge of gray matter surrounding the central canal.
  • Lissauer’s Tract: Located at the tip of the dorsal horn.
• Functional Horns:
  • Dorsal Horn (DH): Primarily sensory functions.
  • Ventral Horn (VH): Primarily motor functions.
  • Lateral Horn: Contains pre-ganglionic sympathetic neurons; specifically found between segments $T1$ and $L3$.

Rexed Laminae and Nuclei (1952)

The gray matter is organized into ten specific functional layers or nuclei:

• General Sensory Nuclei:
  1. Posterior Marginal Nucleus (Lamina I): Appears dark; processes sensory input.
  2. Substantia Gelatinosa (Lamina II): Appears as a light jelly-like band; involved in pain and temperature processing.
  3. Proper Sensory Nucleus (Laminae III and IV): Composed of tightly packed cells.
• Autonomic Nuclei:
  4. Intermediolateral Nucleus: Responsible for sympathetic motor control ($T1-L3$).
  5. Intermediomedial Nucleus: Processes sympathetic sensory information.
• Proprioceptive Nucleus:
  6. Dorsal Nucleus of Clark (Clark's Column): Dedicated to proprioception.
• Motor Nuclei (Ventral Horn):
  7. Axial Motor Neurons (Trunk Muscles): Innervation of the muscles of the trunk.
  8. Accessory Nucleus: Located between $C1-C5$; serves the spinal portion of Cranial Nerve XI (Spinal Accessory Nerve).
  9. Phrenic Nucleus: Located between $C3-C5$; innervates the diaphragm.
  10. Limb Muscle Innervation: Organized topographically for the limbs:
      • Flexors vs. Extensors: Categorized by muscle action.
      • Proximal vs. Distal: Categorized by muscle location on the limb.

White Matter Tracts

Descending (Motor) Tracts

• Lateral Corticospinal Tract: Controls voluntary muscle movement, primarily for the limbs.
• Rubrospinal Tract: Controls the limb motor system.
• Vestibulospinal Tract: Responsible for axial muscle control and unconscious proprioception (e.g., managing swaying).
• Anterior Corticospinal Tract: Provides conscious control of axial muscles.
• Olivospinal Tract: Aids in axial motor control.
• Tectospinal Tract: Controls posture and reflexive movements in response to stimuli.

Ascending (Sensory) Tracts

• Gracile Fasciculus: Carries information regarding fine touch, vibration, and conscious proprioception for the lower body (below $T6$).
• Cuneate Fasciculus: Carries fine touch, vibration, and conscious proprioception for the upper body (above $T6$).
• Posterior Spinocerebellar Tract: Carries unconscious proprioception (spatial orientation) to the cerebellum.
• Anterior Spinocerebellar Tract: Carries unconscious proprioception.
• Lateral Spinothalamic Tract: The main pathway for pain and temperature.
• Anterior Spinothalamic Tract: Carries sensations of crude touch and pressure.
• Spino-olivary Tract: Acts as a sensory pathway for motor control; works in conjunction with the Olivospinal tract.
• Spinotectal Tract: Involved in posture; works in conjunction with the Tectospinal tract.

Blood Supply of the Spinal Cord

• Arterial Supply:
  • Anterior Spinal Artery: Supplies the anterior two-thirds of the spinal cord.
  • Posterior Spinal Arteries: A pair of arteries supplying the posterior third.
  • Arterial Vasocorona: An anastomotic network surrounding the spinal cord.
  • Radicular Arteries: Branches that follow the nerve roots (anterior and posterior) to supply the cord.
  • Segmental Arteries: Source vessels originating from the Aorta, including Lumbar arteries and Posterior Intercostal arteries. Other sources include the Vertebral artery, Deep Cervical artery, Ascending Cervical artery, and Suboccipital artery.
  • Spinal Artery: Enters through the Intervertebral Foramen (IVF) and splits into Dorsal, Intermediate, and Ventral branches.
• Venous Drainage:
  • Sulcal Vein: Drains internal structures.
  • Posterolateral Spinal Vein: Drains the posterolateral aspects.
  • Posterior Spinal Vein: Located on the posterior surface.

Nerve Fiber Classifications

Sensory Nerve Classification (Erlanger-Gasser)

Motor Nerve Classification