The spinal cord is a cylindrical bundle of nervous tissue and supporting cells, extending from the brainstem's medulla oblongata through the vertebral column to the L1-L2 vertebrae. It is a key component of the central nervous system, characterized by central gray matter (cell bodies) surrounded by white matter (myelinated nerve fibers), and divided into 31 segments that send out paired spinal nerves for movement and sensation.
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Gross Anatomy and Structure
- Extent: The cord extends from the foramen magnum (base of the skull) to approximately the L1-L2 vertebra in adults, ending in a tapered cone called the conus medullaris.
- Segments: The spinal cord is organized into 31 segments, with 31 corresponding pairs of spinal nerves: 8 cervical (C), 12 thoracic (T), 5 lumbar (L), 5 sacral (S), and 1 coccygeal (Co).
- Enlargements: Two major enlargements exist, housing neurons for limb innervation: the cervical enlargement (innervates arms) and the lumbar enlargement (innervates legs).
- Cauda Equina: Below the L1-L2 level, the cord tapers, but nerve roots continue downward, forming the "horse's tail" known as the cauda equina.
Anchoring: The cord is anchored at the bottom by the filum terminale, a strand of pia mater.
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Cross-Sectional Anatomy
- Gray Matter (Inner H-shape): Contains neuronal cell bodies, dendrites, and glial cells. It is divided into the dorsal horn (sensory processing), ventral horn (motor neuron cell bodies), and lateral horn (autonomic neurons in thoracic/lumbar areas).
- White Matter (Outer region): Composed of myelinated axons (nerve tracts) that ascend (sensory) or descend (motor) between the brain and body.
Central Canal: A small, hollow space in the center containing cerebrospinal fluid (CSF).
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Protection and Covering
- Vertebral Column: The bony spine (cervical, thoracic, lumbar, sacral vertebrae) protects the cord.
- Meninges: Three protective layers surround the cord: the outer dura mater, middle arachnoid mater, and inner pia mater.
CSF: The subarachnoid space, filled with CSF, cushions the spinal cord.
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Major Tracts (White Matter)
- Ascending Tracts: Carry sensory information (pain, temperature, touch, proprioception) up to the brain (e.g., dorsal columns, spinothalamic tracts).
Descending Tracts: Carry motor instructions from the brain down to muscles (e.g., corticospinal tracts).
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Spinal Nerve Roots
- Each segment has a pair of dorsal roots (sensory) and ventral roots (motor) that merge to form a spinal nerve, which exits the vertebral column