Know your Spine

The vertebral column is also called a spinal column, backbone, or spine. It is consists of 33 bones and each bone is called a vertebra. The vertebral column is divided into 5 segments and vertebrae in each segment have different structures.

Figure 1: Vertebral Column and its structure.

General Vertebra Structure

Figure 2: General Vertebra Structure 

As shown in Figure 2 above, Every vertebra consists of:

1. Body
2. Vertebral arch(foramen): It consists of Body -> Pedicle ->Transverse Process -> Lamina ->Spinous Process -> Lamina ->Transverse Process ->Pedicle -> Body

A) Vertebral Segments

1. Cervical Segment (7 Vertebrae - C1 to C7):

This segment has 7 vertebrae, the first 2 are atypical I.e.this 2 vertebrae have a different structure than the other vertebrae of the same group.

Figure 3: Atlas and Dens

1. Atlas(C1): This is the 1st vertebra, it is a ring-like structure of bones and it supports the skull.
• It doesn’t have a Spinous process and body. 
• Transverse processes are having a hole called transverse Foremen as shown in Figure 3 through which vertebral arteries are passes. 
• Neural arch/ Vertebral foramen is big in size.
2. Axis(C2): This is the 2nd vertebra, it does have a body and Spinous process.
• It has an upward-pointing tooth-like structure in a superior view called a Dens or Odontoid process. This articulates with Atlas and helps in head rotation.
3. Remaining 4 vertebrae(C3- C6) are the same and have the following Features:
• Bodies are small because they only have to bear the weight of the head. 
• The vertebral foramen is big. 
• Transverse processes are having a hole called transverse foramen. 
• The spinous process is short and split or bifurcated(bifid). 
• The vertebral foramen is big in size. 
4. Last cervical vertebra C7 has combined features of cervical and thoracic vertebrae. 
• Transverse processes are having a hole called transverse foramen. 
• The spinous process is long and not bifurcated(bifid)

2. Thoracic Segment (12 Vertebrae - T1 to T12):

• This segment consists of 12 vertebrae and forms the Upper back or thoracic spine and plays roles in forming a thoracic cage.
• They are increasing in size from T1 to T12 (T1 is the smallest and T12 is the biggest).
• Thoracic vertebrae have a facet to the body for ribs articulation.
• Spinous processes are long and turned downward overlapping on another vertebra.
• Transverse processes are thick and have facets called as coastal facets for ribs articulation.
• Bodies are big and have a heart-like shape.
• The vertebral foramen is small.
• Figure 4 shows the structure of the general Thoracic Vertebra.

Figure 4: Structure of Thoracic Vertebra

3. Lumber Segment (5 Vertebrae - L1 to L5):

This 5 vertebrae form the lumbar spine or the lower back. It has the following characteristics:

• Large bodies.
• The spinous process is short and broad.
• Transverse processes are thin.
• L5 is the largest vertebra in the human body and helps to support and transmit body weight to the base of Sacrum.
• L5 have a join with the sacrum and it called a Lumbo-Sacrum joint.
• Vertebral Foremen is triangular.
• The lumbar spine also contains accessory and mammillary processes.
• The spinal cord terminates as the conus medullaris (medullary cone) at the level of the L1/L2 vertebra.

Figure 5: Structure of Lumbar Vertebra and its articulation

4. Sacral Segment (5 Vertebrae):

In Adults, Sacrum is a collection of 5 fused vertebrae and it looks like an inverted triangle with the apex pointing in a downward direction (As shown in Figure 6). On lateral walls of scrum are facets for articulation with pelvic at the sacroiliac joint. It also has a joint with L5.

Figure 6: Sacrum, Coccyx and Pelvic girdle

5. Coccygeal Segment (4 Vertebrae):

The coccyx is small bone consists of 4 fused vertebrae and it articulates with the apex of the sacrum (as shown in above Figure 6).

B) Curvatures

We can notice curvatures of the vertebral column only when we look from either side i.e laterally. Please see Figure 1.
Adult’s vertebral column has 4 anteroposterior curvatures :

• Thoracic and Sacral - both are concave anteriorly. 
• Cervical and Lumber- both are concave posteriorly.
• Thoracic and Sacral curvature are called Primary curvatures as they are present before birth.
• Cervical and Lumbar curvature are called Secondary curvatures as they are developed later when the baby can get support to the head and can get an upright posture.

C) Intervertebral disc (IVD)

• It is a cushion of fibrocartilage and principal joint between 2 adjacent vertebrae that fills the gap between 2 vertebrae. Each disk consists of 2 parts- Annulus fibrosis is tough and it forms the outer layer of the disc, the inside layer is of Nucleus pulposus is soft and more gel-like material.
• Intervertebral discs provide padding during weight-bearing.
• IVD is also flexible and can change shape so it allows movement of the vertebral column.
• It also acts as shock absorbers.
• Herniation of a disc occurs when the nucleus pulposus comes out of Annulus fibrosis, Annulus fibrosis usually breaks at the posterior-lateral side, and after that nucleus pulposus irritates surrounding spinal nerves which result in a variety of neurological and muscular symptoms.

Figure 7: Intervertebral Disc

D) Joints

1. Intervertebral joint:
1. Inter body joint: The joint of vertebra bodies are a secondary cartilaginous joint designed for weight-bearing and strength. The articulating surface of 2 adjacent bodies are connected through an Inter Vertebral Disc(IVD).
2. Inter Facet Joint: These articulations are plane synovial joints between superior articulating processes and inferior articulating processes of 2 adjacent vertebrae. This joint allows movement of the vertebral column.
2. Craniovertebral Joint:
1. Atlantooccipital joint: Located at the superior articulating surface of atlas and occipital condyles, permits nodding of the head in ‘Yes’(Neck flexion and extension). It also permits sideways tilting of the head.
2. Atlanto-axial joint: There are three atlantoaxial joints, two lateral (right and left atlantoaxial joints, lying between the inferior facets of the lateral masses of Atlas(C1) and the superior facets of Axis(C2)), and one median atlantoaxial joint (between the dens of the axis and the anterior arch of the atlas). The lateral atlantoaxial joints are a gliding type of synovial joints, whereas the median atlantoaxial joint is a pivot joint. Movement at all three atlantoaxial joint types permits the head to be turned from side to side to indicate ‘No’.
3. Costovertebral Joint: This joint is present between the head of the ribs and the adjacent vertebral bodies. The joint between tubercle of Rib and transverse process facet is called a costotransverse joint.
4. Lumbo-Sacral Joint: This is a join between L5 and Superior facet of Sacrum. This is responsible for the lumbosacral angle.
5. Sacro-iliac Joint: This is a join between the sacrum(Facets at the transverse wall) and iliac bone or pubic bones. This is also a synovial joint with limited mobility and its role to transmit most of the bodyweight to the hip bones.
6. Sacro-Coccygeal Joint: This is joint is between the apex of the sacrum and superior facet of Coccyx.

E) Vertebral Column Function

1. Protection to the spinal cord
2. Main axial support
3. Movements
4. Shock absorption

Some interesting facts about the Human Spine

Now, we'll see some fascinating, fun facts about our Spine.

• Giraffe and Camel have the same number of cervical vertebrae (vertebrae at the neck) as humans.
• Isn't this funny? Giraffe and Camel's neck looks too tall than ours but still, strangely this thing is common between Giraffe, Camel, and Humans.
• You are a little taller in the morning than you are at night.
•  Do you believe it! but yes, it's true. During the course of each day, due to gravitational force, our body puts increasing pressure on Intervertebral disc causing compressing of the disc so slightly it reduces our height and at night when we sleep again our intervertebral discs gets expanded. 
• Babies have more vertebrae than adults.
• Yes, we all start with 33 vertebrae when we born but then after some years(approx. after  25 years, our 5 sacrum vertebrae get fused together and form 1 triangular bone and 4 coccyx vertebrae get fused and form 1 conical bone.

References

• Gnosis Medical Yoga Teacher textbook
• https://upload.wikimedia.org/wikipedia/commons/0/0f/716_Intervertebral_Disk.jpg
• https://www.hss.edu/condition-list_arthritis-back-spine-neck.asp