Spine anatomy can be understood by dividing the spine into bony and soft tissue components. Spine can be divided into cervical, thoracic, lumbar, sacral and coccygeal regions. There are anatomical variations both in skeletal and nonskeletal components in all these regions.
Spinal cord and the nerves coming out of spine are the neural tissue contained in the spine.
Vertebral Column
The skeletal part of the spine is formed by the vertebral column.
The vertebral column is composed of 33 individual bones [vertebra] and extends from just below the base of the skull to a point just below the beginning of gluteal cleft. You can palpate the lower part by following the spine till its lower end. or vertebra stacked one on top of the other.
There are seven cervical vertebrae, twelve thoracic vertebrae, and typically five lumbar vertebrae.
The function of the vertebral column or spinal column is to provide support so that erect posture is possible. At the same time, it also allows the body to bend and twist.
The vertebral column is stabilized by contributions from individual vertebral joints, muscles, and ligaments.
A vertebra is the smallest unit of the vertebral column. Plural term is vertebrae. Vertebrae have been modified according to the function they are expected to serve in different parts of the spine.
Spinal Curvature and Curves
To a naked and unaccustomed eye, the spine appears as a straight column. But that is not the case. It appears straight in anteroposterior view (That means looking from front or back) but in lateral view or looking from the side, we can see multiple curves in different regions giving the spine an S shape.
These curves are called kyphotic if they are concave in anterior(front) and convex on posterior (back) and lordotic if they are convex in anterior and concave on posterior.
Cervical and lumbar curves are lordotic whereas the thoracic curve is kyphotic. Given that spine anatomy has no abnormality, these curves normally balance out each other when the patient stands erect with their straight head.
This standing posture minimizes the effect of gravity and consumes the minimum energy when moving or walking.
Regions of Spine
It is divided into five regions
Cervical
This part is present in the neck and consists of first seven vertebrae. Each vertebra is known by C followed by
the number of the vertebra. Thus the first vertebra is called C1 and the last vertebra is called C7. It has a normal lordotic curve.
Thoracic
This is also called the dorsal spine. It is below cervical and spans upper trunk or thorax or area corresponding to the chest. It contains a total of 12 vertebrae designated as T1 to T12 in a fashion similar to we discussed for cervical vertebrae. It has a normal kyphotic curve.
Lumbar
It follows Thoracic and consists of 5 vertebrae, L1 to L5. Some of the people might have anatomical variations in which 4 or 6 vertebrae might be present in Lumbar spine. In both cases, the extra vertebra is added to or taken from Sacral spine. Accordingly,if there are 4 lumbar vertebrae, it is called sacralization of lumbar vertebra and if there are 6 lumbar vertebrae, it is called lumbarization of sacral vertebra.
Sacral
It is composed of five sacral vertebrae S1 to S5. In contrast to other parts we just discussed, vertebrae in the sacral spine are fused to each other. This part of the spine follows the lumbar spine and is present in the pelvic area.
Coccyx or Coccygeal
This is also known as the tailbone. It is the final segment of the human vertebral column, of four fused vertebrae (the coccygeal vertebrae) below the sacrum.
The function of the spine is to support and transmit the weight of trunk to lower limbs, to aid in the motion of the trunk and to support and protect the spinal cord & other vital organs. Spinal Cord is the structure that transmits neural fibers. These fibers further supply the organs in the form of nerves.
Structure of Spine
Vertebrae are the structural units of the spine. They are stacked together to form the entire vertebral column. Between each vertebra, are cushion-like structures called intervertebral discs which act as shock absorbers and also permit some movement between the vertebral bodies. Various ligaments and muscles stabilize the spine and also allow movements.
Each vertebra contains a foramen which together forms the vertebral canal.
Movements of the spine occur at special joints called facet joints that are formed between two adjacent vertebrae. Various muscles carry out the movements of the spine. These muscles are important for maintaining posture and transmission of loads created during normal activities, work, and play. The strength of these muscles is important and lack of strength would lead to various backache problems.
Vertebrae are shaped in such a way that the spinal cord is protected from damage by the bones of the entire spinal column.
Vertebrae
The vertebrae are responsible for the transmission of the weight. Each vertebra consists of an anterior body which is attached to a posterior ring called the posterior neural arch.
Two struts of bones called pedicles to arise from the body and join two converging struts called laminae. Pedicles, laminae and posterior surface of the body form boundaries of spinal canal. This spinal canal is the space where the spinal cord passes.
There is a transverse process on either side of the arch which serves as an attachment to various muscles and ligaments. Dead posteriorly is the posterior spinous process that can be felt on our back as bumps. Posterior spinous processes also serve as an attachment to ligaments and muscles of the spine.
Adjacent to the base of transverse processes are bony projections that go superior and inferior. These are called superior and inferior articular facets and take part in the formation of facet joints. There are two facet joints between each pair of vertebrae one on each side. They are primarily designed to allow the vertebral bodies to rotate with respect to each other.
The vertebral anatomy may vary in different regions.
Cervical spine vertebrae
These are designed for flexibility and movement. These are denoted by C followed by number of the vertebra. The first two cervical vertebrae Atlas and Axis [C1 and C2] have special shapes to stabilize the head on the neck and to allow neck movements from side to side.
Read more| Anatomy of Atlas or C1 Vertebra
Read more| Anatomy of Axis or C2 vertebra
Cervical vertebrae are smaller as compared to thoracic and lumbar vertebrae. One of the unique features of the cervical vertebra is the presence of foramen transversarium, a foramen on the transverse process for passage of vertebral artery.
Thoracic Vertebrae
The thoracic region contains 12 thoracic vertebrae named T1-T12. Thoracic spine functions to keep the body upright and also protects vital chest organs like heart and lungs along with the thoracic cage.
Thoracic vertebrae are stouter than the cervical vertebrae. On each side it bears 2 costal facets for articulation with ribs.
The thoracic vertebrae towards the junction with cervical or lumbar vertebrae vary in anatomy. Those near the lumbar vertebrae are more like lumbar vertebrae and those towards the cervical spine would be similar to cervical vertebrae.
Read more|Typical and Atypical Thoracic Vertebra
Lumbar Vertebrae
There are usually five lumbar vertebrae, named L1-L5 and are the stoutest vertebrae designed for load-bearing and other forces that this region is subjected to.
In some people, there may be only 4 lumbar vertebrae with fifth becoming the sacral vertebra. Similarly, there may be six lumbar vertebrae when one of the sacral vertebrae becomes lumbar. These variations are called as sacralization and lumbarization of vertebrae.
Apart from being large-sized, the lumbar vertebrae also have the largest lumbar canal.
Sacral and Coccygeal Vertebrae
The sacrum is made of five fused vertebrae and acts as a wedge between the two iliac pelvic bones. It articulates with ilium on either side through the sacroiliac joints.
Coccyx is also called tailbone and is formed by the 4 fused vertebrae. Coccygeal vertebrae are kind of rudimentary and coccyx is also called the tailbone.
Joints
The vertebral column has multiple joint at each level.
The occipital-cervical joint is formed between C1 and skull.
Atlantoaxial joint is a pivot joint between the first and second cervical vertebrae.
The first two cervical vertebrae are an exception and do not have discs.
For rest of the spine, each level has a three-joint complex consisting of two facet joints in the back and one that between vertebral bodies with the intervening disc.
A facet joint is formed between inferior facets of the superior vertebra with superior facets of the inferior vertebra. Thus each vertebra forms facet joints with the superior and inferior vertebra.
For example, superior facets of T10 vertebra form facet joints with inferior facets of T9. T10 also participates in facet joint formation with T11 where inferior facets of T10 form joint with superior facets of T10. Thus each vertebra contributes to 4 facet joints.
Lower down, the sacroiliac is formed between sacrum and ilium, on each side of the sacrum.
However, both joints move together as single unit
Intervertebral Disc
Intervertebral discs are present between vertebral bodies and act as shock absorbers in addition to binding force between two adjacent vertebrae.
The discs are designed to absorb the stresses carried by the vertebral column while allowing the vertebral bodies to move with respect to each other. They made up of a strong outer ring of fibers called the annulus fibrosus that contains nucleus pulposus.
The annulus is comprised of layers of lamellae, a specialized connective tissue. The annulus is stiff as it contains substantial collagen.
Nucleus pulposus is soft and jelly-like contained in the center.
It provides hydration and carries pressure distribution.
The discs bond to the vertebrae by vertebral endplates present on either side of the disc. A vertebral endplate has two layers- a cartilaginous layer that fuses with the disc and thin layer of porous bone that attaches to the vertebra.
The size of the discs corresponds to the vertebral size. Therefore, the discs increase in size from the neck to the low back.
The discs are essential for spinal stability and alignment. Capillaries from the vertebral bodies supply the disc with nutrients but as such, it is an avascular tissue.
Neural Foramen
The neural foramen is the opening where the nerve roots exit the spine and travel to the rest of the body. There are two neural foramina located between each pair of vertebrae, one on each side.
Muscles
Various muscles and ligaments attach to the spine. They help the spine to stabilize and allow it to carry various movements. There are two main groups of muscles in the spine – flexors and extensors.
Flexors help to bend the spine forward whereas extensors help to extend the spine or erect it from bending position.
Different combinations of these muscles help to produce different types of moments including lateral bending and rotation.
The back muscles also stabilize the spine. Poor muscle tone can be associated with various back problems including back pain.
Ligaments
Various ligaments hold the vertebrae together and keep them in alignment.
There are three major ligaments of the spine.
Ligamentum flavum is the ligament is attached between adjacent laminae to form a continuous lamino-ligamentous covering on the posterior aspect of the spinal cord.
Anterior longitudinal ligament and posterior longitudinal ligaments are continuous bands that run from the top to the bottom of the spinal column along with the vertebral bodies. The anterior longitudinal ligament is anterior to the vertebral body and the posterior longitudinal ligament is posterior.
Interspinous ligament connect spinous processes and provides posterior support.
Spinal cord and Nerves
The spinal cord runs within the spinal canal from the brainstem to the 1st lumbar vertebra and is about 18 inches long. Spinal nerves come out of the spinal cord through neural foramina, roughly at each vertebral level to supply different sensory and motor areas. The spinal nerves are known by the name of the vertebra through which they come out.
For example C5 nerve, L1 nerve.
At L1 the cord fibers separate and form a tail-like structure called cauda equina which contains fibers for L2 levels downward carrying fibers for the spinal nerves L2 and below.
The spinal cord acts as a relaying organ between brain and periphery and spinal nerves serve a specific sensory and motor region.
Any damage to the spinal cord can result in a loss of sensory and motor function at and below the level of injury.
For example cervical injury can cause paralysis of all four limbs or quadriplegia. an injury to the lumbar spine can result in paraplegia or paralysis of both lower limbs.
The motor and sensory loss may be incomplete. Incomplete injuries can result in various patterns of spinal injuries.
There are 31 pairs of spinal nerves. They function to carry messages back and forth from spine to a specific motor and sensory region called dermatomes and myotomes.
Each spinal nerve is formed by ventral motor root and dorsal sensory root.
The spinal cord is covered with the same meninges as brain.
- Pia mater-innermost
- Arachnoid mater – middle
- Dura mater – Outermost
The space between the pia and arachnoid mater is called subarachnoid space and contains cerebrospinal fluid.
Further Reading on Spine Anatomy
For detailed anatomy of different regions of spine, please go to following articles
