The vertebral column is part of the axial skeleton and functions to support the body’s weight and encloses and protects the spinal cord within the spinal canal. It provides a framework the attachment of several muscles; the extrinsic back muscles are involved in moving the upper limbs and ribcage, whereas the intrinsic back muscles are important in maintaining posture and in moving the vertebral column itself.
The vertebral column consists of 24 articulating vertebrae, and the sacrum and coccyx which consist of 9 fused vertebrae.
There are 33 vertebrae: 24 of these are separate vertebrae, 9 are fused (sacrum and coccyx)
- 7 cervical
- 12 thoracic – these articulate with the ribs
- 5 lumbar
- 5 sacral (fused)
- 3-4 coccygeal (fused)
In between each vertebrae there is an intervertebral disc which forms a fibrocartilaginous joint allowing slight movement between the vertebrae and functioning as a shock absorber for the spine.
The intervertebral disc consists of a tough outer fibrous ring, called the annulus fibrosis and a gelatinous centre – the nucleus pulposus.
Degenerative changes or mechanical trauma can cause tears in the outer annulus fibrosis, squeezing out the nucleus pulposus which can exert direct pressure on the spinal cord itself or more commonly on the nerve roots as they leave the spinal cord – this is known as a disc herniation.
The vertebrae are identified by a letter and a number, so for the cervical vertebrae you have vertebrae C1-C7; for the thoracic vertebrae you have T1-T12, and for the lumbar vertebrae you have L1-L5. This naming convention is useful when referring to anatomical land marks and when referring to spinal nerves.
With regard to the spinal nerves, these are named according to the level at which they exit the vertebral column. Within the cervical spine, although there are only 7 vertebrae, there are actually 8 spinal nerves – the first cervical spinal nerve exits above the first cervical vertebra – C1.
In terms of using spinal levels for reference points to anatomical landmarks, a clinical example would be in the context of performing a lumbar puncture. In adults we know that the spinal cord terminates at approximately the L1 level.
We can palpate the iliac crests, the top of which we know lie at approximately the L4 level – from here we can identify the spinous process of L4, and a safe entry point for the needle to avoid damage to the spinal cord.
With regard to other examples of anatomical landmarks, an example would include the sternal Angle of Louis, or the manubriosternal junction, which lies at the T4/5 level.
This level determines the thoracic plane – an artificial horizontal plane which divides the superior mediastinum from the inferior mediastinum and at which there are a few key anatomical landmarks, such as the start and end of the aortic arch, and the bifurcation of the trachea.
The adult vertebral column is curved , with 4 specific curvatures:
- The cervical lordosis – convex anteriorly
- Thoracic kyphosis – convex dorsally
- Lumbar lordosis – convex anteriorly
- Pelvic curvature – concave anteriorly, starts at the lumbosacral junction and ends at the coccyx.
In neonates, the vertebral column just has one fixed kyphotic curvature, concave anteriorly, known as the primary curvature; the other lordotic curvatures are developed later in life and are known as secondary curvatures.
Movements of the Vertebral Column
In terms of movements of the vertebral column, these vary according to the spinal level, but there are four movements which are possible:
- lateral flexion.
These movements are limited by the morphology of the joints between the vertebrae, the presence of ribs in the thoracic region and the ligamentous structures, which we will look at in one of the following tutorials.
Specialised Joints of the Spine
The joint between the skull and the first cervical vertebrae is the atlanto-occipital joint. Its given this name because the first vertebra is also referred to as the atlas, and the second vertebra is known as the axis. The part of the skull it articulates with is the occipital bone, hence the atlanto-occipital joint. This joint consists of 2 synovial condyloid joints. At this joint we get flexion and extension, and a small amount of lateral flexion.
The joint between the first and second vertebrae is known as the atlanto-axial joint. C1 is the atlas and C2 is the axis, hence the name. This is a complex joint, which consists of a specialised synovial pivot joint, formed between the odontoid peg of C2 and the anterior arch of C1. This unique anatomtical configuration enables rotational movement.