Spinal Anatomy Study Guide: Chapters 6-10

Neuroanatomy and Spinal Structures

• Spinal Cord and Nerve Root Pathways within the Vertebral Column

  • Vertebral Foramen: This is the large opening in the center of the vertebra through which the spinal cord travels. In the cervical, thoracic, and upper lumbar regions, this foramen houses the spinal cord itself, protected by the vertebral arch and body.

  • Intervertebral Foramen: These are the lateral openings between adjacent vertebrae. These foramina serve as the exit points for the spinal nerve roots as they leave the spinal canal to travel to the rest of the body.

• Termination of the Spinal Cord

  • Level of Termination: The adult spinal cord typically ends at the level of the $L1$ or $L2$ vertebrae.

  • Conus Medullaris: This represents the terminal, cone-shaped portion of the spinal cord located at the aforementioned level. Below this point, the spinal canal contains the cauda equina, a collection of nerve roots.

• The Spinal Meninges and Cerebrospinal Fluid

  • Layers of the Spinal Meninges: The spinal cord is protected by three distinct layers of connective tissue known as meninges:

    1. Dura Mater: The outermost, toughest, and most fibrous layer.

    2. Arachnoid Mater: The middle layer, characterized by its spider-web-like appearance.

    3. Pia Mater: The innermost layer, which is thin and adheres directly to the surface of the spinal cord and nerve roots.

  • Cerebrospinal Fluid (CSF) Location: CSF is found within the subarachnoid space. This space is located between the arachnoid mater and the pia mater.

Vascular Anatomy of the Spine

• The Great Vessels

  • Identity of the Vessels: The "great vessels" relative to the spinal and abdominal anatomy are the Abdominal Aorta and the Inferior Vena Cava ($IVC$).

  • Bifurcation Levels: These vessels divide into the common iliac vessels at specific vertebral levels:

    • Abdominal Aorta: This vessel bifurcates into the left and right common iliac arteries at the level of the $L4$ vertebra.

    • Inferior Vena Cava ($IVC$): This vessel bifurcates (or more accurately, is formed by the junction of the iliac veins) at the level of the $L5$ vertebra.

• Blood Oxygenation Status

  • Oxygenated Blood: Vessels that carry oxygenated blood away from the heart to the body tissues are arteries, such as the Aorta.

  • Deoxygenated Blood: Vessels that carry deoxygenated blood back toward the heart from the body tissues are veins, such as the Inferior Vena Cava ($IVC$).

• Transverse Foramen (Foramen Transversarium)

  • Structure Contained: The vertebral artery is the primary vascular structure that travels through the transverse foramina found in the cervical vertebrae ($C1$ through $C6$).

Vertebral Mechanics and Regional Characteristics

• Joint Actions of the Upper Cervical Spine

  • Atlanto-occipital Joint ($C0-C1$): The primary action at this joint is flexion and extension of the head, often referred to as the "Yes" motion.

  • Atlanto-axial Joint ($C1-C2$): The primary action at this joint is rotation of the head, often referred to as the "No" motion.

• Regional Anatomy and Ribs

  • Bifid Spinous Processes: This anatomical feature, where the spinous process is divided into two clefts or tips, is characteristic of the cervical spinal region (specifically vertebrae $C2$ through $C6$).

  • Human Rib Pairs: Humans typically possess $12$ pairs of ribs, which articulate with the thoracic vertebrae.

Peripheral and Central Nervous System Organization

• Components of the Central Nervous System (CNS)

  • The CNS is composed of two primary structures: the Brain and the Spinal Cord.

• Spinal Nerve Exit Patterms and Count

  • Total Spinal Nerves: There are $31$ pairs of spinal nerves that exit the spinal column.

  • Cervical Nerve Exit ($C4$): In the cervical spine, nerve roots exit above their corresponding vertebrae. Therefore, the $C4$ nerve root exits the vertebral column through the intervertebral foramen between the $C3$ and $C4$ vertebrae.

  • Lumbar Nerve Exit ($L4$): Below the cervical region (starting from the $T1$ level downward), nerve roots exit below their corresponding vertebrae. Therefore, the $L4$ nerve root exits the vertebral column through the intervertebral foramen between the $L4$ and $L5$ vertebrae.

• The Psoas Muscle

  • Location: The psoas major muscle is located in the lower lumbar region of the spine. It originates from the transverse processes and lateral aspects of the vertebral bodies of $T12$ through $L5$ and inserts into the lesser trochanter of the femur.

• Dermatomes vs. Myotomes

  • Dermatome: A specific area of skin that is primarily supplied by a single spinal nerve root. It is used to test sensory function.

  • Myotome: A group of muscles that are primarily innervated by a single spinal nerve root. It is used to test motor function.

Radiographic and Advanced Medical Imaging

• X-Ray Imaging and Radiopacity

  • Radiopaque Objects: Dense objects, such as bone and metal (implants, hardware), appear radiopaque (white) on X-rays.

  • Scientific Basis: These objects appear brighter because they have a higher physical density and atomic number, causing them to absorb or attenuate more X-ray photons, which prevents them from reaching the film or digital detector.

• Fluoroscopy

  • Benefits in Surgery: Fluoroscopy provides real-time, continuous X-ray imaging. This allows surgeons to visualize the placement of instruments, needles, and hardware (like pedicle screws) during a procedure, ensuring accuracy and safety without the need for static films.

• Cross-Sectional (Axial) Imaging

  • Advantages: The primary advantage of an axial view is the ability to visualize structures without the superimposition of other tissues. This is crucial for evaluating the dimensions of the spinal canal, identifying disc herniations, and assessing foramen narrowing (stenosis).

• MRI Sequence Characteristics (T1 vs. T2)

  • Fat Visualization: Fat appears brighter (high signal intensity) on T1-weighted imaging.

  • Water/Fluid Visualization: Water and fluids (such as cerebrospinal fluid) appear bright (high signal intensity) on T2-weighted imaging.

• Diagnostic Utility: MRI vs. CT

  • MRI (Magnetic Resonance Imaging): Generally best for visualizing soft tissue structures, including the spinal cord, nerve roots, intervertebral discs, and ligaments.

  • CT (Computed Tomography): Generally best for visualizing bony structures, allowing for the assessment of complex fractures, bone density, osteophytes, and ossification of ligaments.

• Myelography

  • Injection Site: In a myelogram, contrast dye (radiopaque medium) is injected into the subarachnoid space (within the thecal sac).

  • Purpose: The dye is injected to outline the spinal cord and nerve roots. Under fluoroscopy or CT, this allows clinicians to see if anything (like a disc or bone spur) is compressing the neural structures by observing indentations in the dye-filled space.