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Anatomy of the Spine


Overview

The spine is divided into 4 segments
Cervical: 7 vertebral segments, nerve roots
Thoracic:12 vertebral segments,12 nerve roots
Lumbar: 5 vertebral segments,5 nerve roots
Sacral: 5 fused segments,5 nerve roots 

The spinal cord is connected to the brain stem and carries a number of motor and sensory tracts.  The position of the white matter and gray matter is opposite to the brain.  The white matter is in the periphery and the gray matter is central and has a butterfly shape. At each level a spinal nerve root exits the spinal cord and has specific motor and sensory functions.  The spinal cord typically terminates between T12 and L2, which explains why we perform lumbar punctures below L2. 

Spinal Nerve Roots
Each corresponding nerve roots exits the spine in a specific pattern and this pattern differs between the cervical and the thoracic/lumbar regions.  The spinal nerves exit the cervical spine above their corresponding vertebral body level.  For example, the C7 nerve root exits above C7 through the C6-C7 neural foramen.  C8 exits in between T1 and C7, since there is no C8 vertebral body level.  

This orientation is reversed in the thoracic and lumbar spine.  The T and L spinal nerve roots exit below their corresponding vertebral body level.  For example the L3 nerve root exits below L3 through the L3-L4 foramen.  It is important to know which nerve root exits which foramen in order to understand if a patient's radiculopathy symptoms can be explained by a specific anatomic derangement.         

Vertebral Segments
There are features that are common to all vertebral segments and others that are unique to each level.  With the exception of C1, each segment has a vertebral body, which is the anterior portion of the vertebral segment.  The superior and inferior portions of the vertebral body are referred to as the end plates which provide nutrition to the adjacent disk.  The body is connected to the  posterior elements by bilateral pedicles which are linear bony struts. The posterior elements consist of the pedicles, lamina, facets (articular process), transverse process and spinous process.     

The Disk
Each vertebral body segment is attached to the level above and below by an intervertebral disk.   

The disk has several functions:
    1)  It serves as a connection between the vertebral bodies
    2)  It acts as a pivot point   
    3)  Distribute compressive forces

The disk is made of the Nucleus Pulposus and the Annulus Fibrosis:
Nucleus Pulposus: Central portion of the disk, has a gelatinous consistency and is made of H20 and proteoglycan
Annulus Fibrosis: Outer layer of the disk, has a fibrous consistency and is made of concentric collagen rings

Ligaments

The spinal segments are also connected by a number of ligaments. The anterior longitudinal ligament connects the anterior portion of the vertebral bodies with one another.  The posterior longitudinal ligament connects the posterior portion of the vertebral bodies.  The ligamentum flavum connects the lamina of adjoining levels.  The interspinous ligaments connect the neighboring spinous processes. Additional ligaments are present at the C1-C2 articulation.  

In trauma patients, ligamentous injury is best evaluated with MRI.