Spinal Anatomy
By Dr. Saleha Jabali 2023
The structure of the spine provides stability, protection of the spinal cord, and weight-bearing movement in the upright posture. The spine consists of vertebrae, which are 33 in number, divided by structural similarity into cervical (7), thoracic (12), lumbar (5), sacral (5), and coccygeal regions (4).
Vertebral Structure
Each vertebra consists of the same structure:
- Body
- Transverse process
- Spinous process
- Lamina
- Pedicles
Spinal Cord Structure
The spinal cord is located within the spinal canal, which is created by bony vertebral structures and a ligamentous network. This canal provides stability and protection for the spinal cord, which extends from the foramen magnum to approximately the 1st or 2nd lumbar (L1-L2) vertebral interspace in adults. It ends at the conus medullaris.
Fibrous bands extend from the conus medullaris to fix the spinal cord to the coccyx. The tissues surrounding the spinal cord are called meninges. The outermost area is called the epidural space, followed by the dura mater, which is a tube that contains cerebrospinal fluid (CSF) and may extend up to the S1 level in adults and S3 in infants.
Termination of Spinal Cord
- Adults: usually ends at L1
- Infants: ends at L3
There are anatomical variations. For most adults, it is generally safe to place a spinal needle below L2.
Spinal Nerves
There are 31 pairs of spinal nerves, named and numbered according to the region and level of the spinal cord from which they emerge. All spinal nerves are mixed (motor and sensory) except C1. At each vertebral body, nerve roots exit from the spinal cord bilaterally, and specific skin areas (dermatomes) are innervated by single or groups of spinal nerves.
Assessing the dermatomes assists in determining the region of the block (numbness).
Important Dermatomes and Anatomical Landmarks
- L1-L2: front of thigh
- T10: Umbilicus
- T6: Xyphisternum
- T4: Nipple
- C6: thumb
- C7: middle finger
- C8: little finger
Note: To avoid complications, the level of the block should be at T4 with no numbness in the upper limbs or fingers.
