Anatomy of the Vertebral Column, Spinal Cord and Meninges
- The spine consists of 33 vertebrae:
- 7 cervical, 12 thoracic, 5 lumbar, 5 that are fused to form the sacrum, and 4 fused to form the coccyx.
- The spine has four curves:
- The thoracic and sacral curves have concave forward curvature and are primary curvatures formed at birth.
- The cervical and lumbar curves have forward convex curvatures and are secondary curvatures developed after birth.
- When the patient is supine, the highest point is in L3 and the lowest point is in T5
- The vertebrae consist of two essential parts: an anterior solid segment or body and a posterior segment or arch.
- The arch is divided on each side into a pedicle attached to the body and a lamina at the back.
- The spinous process extends backward from the junction of the two laminae.
- The junction of the pedicles and laminae forms the transverse process, which extends outward from each side of the arch.
- The pedicles of each vertebral arch are notched forming an incomplete ring, the intervertebral foramen. The spinal nerves enter and exit through these holes from each side of the vertebral canal
- The intervertebral discs - a pad of fibrous cartilage between adjacent surfaces of the vertebral bodies. They provide mobility and shock absorption to the spine.
- Five ligaments connect the vertebral processes:
- The supraspinous ligament connects the tips of the spinous processes
- The interspinous ligament connects the spinous processes.
- The ligamentum flavum connects the laminae of adjacent vertebrae and consists of elastic fibers. It becomes progressively thicker from front to back, and it is easily recognized by the increased resistance to the passage of the needle. Actually composed of 2 ligaments - right and left, which join in the middle forming a ventral opening.
- The other two ligaments are the posterior longitudinal ligament and the anterior longitudinal ligament, which run connecting their respective faces of the vertebral bodies
- The spinal cord originates in the brainstem and continues through the occipital foramen magnum ending in the conus medullaris. This distal end ranges from L3 in infants up to the bottom of L1 in adults due to differences in growth between the bony spinal canal and central nervous system. It ends at the conus medullaris from where the lumbar nerve, sacral and coccygeal roots emerge to form the cauda equina (horse tail). It is in this area (below L2) that spinal needles are inserted.
Meninges and Spaces
- Meninges and spaces include the pia mater, which is the innermost layer closely attached to the spinal cord and brain. It ends as terminal filum and is highly vascularized. The arachnoid is an avascular membrane tightly attached to the outermost layer, the dura mater. The arachnoid acts as a major barrier to the flow of drugs from the cerebrospinal fluid (CSF).
- The dura mater is the third and outermost membrane of the spinal canal. It is the continuation of the cranial dura mater, extending from foramen magnum to S2.
- The subarachnoid space lies between the pia mater and the arachnoid. In it is found the CSF, the spinal nerves, a network of trabeculae between the two membranes, and blood vessels supplying the spinal cord. It extends from S2 to the cerebral ventricles.
- The subdural space is a virtual space between the dura and the arachnoid that contains small amounts of serous fluid that allows the membranes to move past each other.
- The epidural space extends from the foramen magnum to the sacral hiatus. It is bounded by the posterior longitudinal ligament at the sides by the pedicles and the intervertebral foramen and posteriorly by the ligamentum flavum. It contains nerve roots, venous plexuses, arteries, and fat.
- The amount of adipose tissue in the epidural space diminishes with age, which may cause much of the variation in epidural blocks with age
- The iliac crests intersect the vertebral column just above the L4 spine, forming the intercristal line
- Sacral hiatus - the failure of laminae of S5 and sometimes S4 to fuse in the midline, producing a V-shaped boney defect
- Highly variable, from slit-like to wide-based and absent in 1 in 20 patients
- Contains the terminal portion of the dural sac, which ends cephalad to a line joining the posterior superior iliac spines - S2
- Clear, Liquid, colorless.
- Specific Gravity: 1,003–1,009 at 37oC
- Total volume: 120–150 ml
- Spinal CSF volume: 25–30 ml
- Ventricular CSF volume: 60–75 ml
- Average pressure: 100–150 cm of water
- pH: 7.6
- Formed continuously at a rate of 450 ml/day by way of the secretion or plasma ultrafiltration from the choroidal arterial plexus located in the lateral ventricle and the third and fourth ventricle.
- The CSF is reabsorbed into the bloodstream through the arachnoid villi and granulations, which protrude from the dura mater to be in contact with the endothelium of the cerebral venous sinuses.
- The CSF is a determinant of the effects of intrathecally administered substances because all the drugs injected into the subarachnoid space are diluted in the CSF before reaching their target in the spinal cord.
- Volume of CSF is one of the most important factors affecting the level of sensory block and duration of spinal anesthesia. The volume of CSF varies from one individual to another and, with the exception of weight, is not related to the anthropometric values clinically available.
Thirty-One Pairs of Spinal Nerves
- Thirty-one pairs of spinal nerves emerge from the spinal cord by the anterior and posterior roots. Each spinal nerve innervates a specific area of skin or dermatome and skeletal muscles.
- The anterior spinal root is efferent and contains:
- Motor fibers (voluntary muscles).
- Preganglionic sympathetic fibers (T1–L2) join spinal nerves to form the sympathetic chain. The sympathetic chain extends along the entire column (anterolateral side of the vertebral bodies). It gives rise to the stellate ganglion, splanchnic nerves, and celiac plexus.
- The posterior spinal root is afferent.
- All afferent impulses from the body, including viscera, pass through the posterior roots. Each has a dorsal root ganglion.