chp 14 the Spinal Cord

Overview of the Spinal Cord Structure

• The spinal cord is part of the Central Nervous System (CNS) and can be divided into sections:

  • Cervical: 8 sections (C1-C8)

  • Thoracic: 12 sections (T1-T12)

  • Lumbar: 5 sections (L1-L5)

  • Sacral: 5 sections (S1-S5)

• The spinal cord ends at approximately the L1 vertebra, leading to a shift where spinal nerves extend down further instead of being aligned with the vertebrae.

Spinal Cord and Vertebrae Relation

• There are discrepancies in spinal cord sections and their corresponding vertebrae:

  • Despite having 8 cervical spinal sections, there are only 7 cervical vertebrae (C1 to C7).

  • C8 spinal section is above the C7 vertebra, leading to an offset as the sections progress downwards.

• The spinal cord does not grow after adolescence, anchoring at the conus medullaris, located at L1.

Terminology and Components

• Conus Medullaris: The end of the spinal cord located at L1.

• Cauda Equina: Group of nerves extending from the conus medullaris, resembling a horsetail, that provides innervation to lower regions.

• CSF (Cerebrospinal Fluid): Surrounds the spinal cord, providing cushioning.

Anatomical Support

• Filum Terminale: Ligament anchoring the conus medullaris to the sacrum, preventing excessive movement.

• Denticulate Ligaments: Stabilize the spinal cord laterally.

• Meninges: Protective layers around the spinal cord, including dura mater, arachnoid mater, and pia mater. Dura mater continues down the spinal cord without folds.

Gray and White Matter in Spinal Cord

• Gray Matter: Located inside the spinal cord and structured in the shape of an “H.”

• White Matter: Surrounds the gray matter.

• Each spinal section features two primary roots:

  • Ventral Root: Motor output

  • Dorsal Root: Sensory input

• Dorsal Root Ganglia: Collections of sensory neuron cell bodies outside the spinal cord.

Neuronal Pathways

• Sensory Neurons:

  • Pseudo-unipolar, with cell bodies in the dorsal root ganglia.

• Motor Neurons: Multipolar, with cell bodies located in the anterior gray horn (somatic) and lateral gray horn (visceral).

• Interneurons: Connect sensory and motor pathways within the spinal cord.

Neural Mapping and Clinical Relevance

• Dermatomes: Regions of the body innervated by specific spinal nerves, important in diagnosing nerve injuries.

• Motor and sensory paths can help identify the level of spinal cord injury based on sensation loss or muscle control.

  • Example: Loss of sensation in a specific limb can indicate damage to the relevant spinal segments.

Spinal Nerve Branches

• The spinal nerves branch into:

  • Dorsal Rami: Innervate back muscles and skin of the posterior body wall.

  • Ventral Rami: Innervate anterior body wall and limbs.

  • Rami Communicantes: Connect to the autonomic nervous system.

Sensory and Motor Functions

• Sensory neurons receive input from the body’s receptors and send signals through the dorsal roots to the posterior gray horn.

• Motor commands exit the spinal cord via the ventral roots to activate effectors (muscles or glands).

Types of Receptors and Neurons

• Somatic Receptors: Involved with skeletal muscle and skin, requiring activation of somatic sensory neurons.

• Visceral Receptors: Associated with smooth muscle, cardiac muscle, and glands, activated by visceral sensory neurons.

Tracts in the White Matter

• Sensory tracts ascend to the brain, while motor tracts descend:

  • Ascending Tracts: Carry sensory information upward from the body to the brain.

  • Descending Tracts: Transmit motor commands from the brain to the body.

Summary of Functions

• Dorsal Root (Sensory Input) → Dorsal Root Ganglia (Cell Bodies) → Posterior Gray Horn (Interneurons) → Up to Brain.

• Brain processes information and sends commands via Descending Tracts → Anterior Gray Horn (Somatic) or Lateral Gray Horn (Visceral) → Ventral Root (Motor Output).

Conclusion

• Understanding the spinal cord’s structure, its anchoring, the relationship of spinal sections to vertebrae, and the path of sensory and motor neurons is crucial for both physiology and clinical applications. This knowledge assists in diagnosing conditions based on sensory and motor loss patterns.