The Cervical Spine and Health
The cervical spine consists of seven vertebrae in humans. In fact, almost all mammals have seven bones in the neck, even the giraffe. The names of each vertebra are the same in all species. That is, they are numbered from the top down C1 to C7.
The first cervical vertebra, which is located beneath the skull, is also known as atlas. It was so named because it holds up the globe of the head. The second cervical vertebra is known as axis. It has an odontoid process, odontoid meaning toothlike. The toothlike process projects upward and sits in a slight recess near the backside of atlas. The purpose of the odontoid process is pivoting, hence the name axis. Approximately fifty percent of left and right rotation of the neck occurs around the odontoid process. The seventh cervical vertebra is called vertebra prominens and is unique due to its long spinous process. The remaining vertebra are numbered in sequence C3 through C6. The upper cervical spine and upper cervical angle is complex and will be discussed separately. Because of its unique location and design the upper cervical spine can cause many problems in humans.
Except for atlas all of the cervical vertebra, like the rest of the spine, have a body on the front or belly side, and a motor unit on the back side. The body in front is basically for bearing the weight of the head. Again, atlas is unlike the remaining cervical vertebra in that it is not connected to it's neighboring segments by cartilage called discs.
The picture to the left is of the atlas/C1 vertebra, the picture below to the right is axis/C2. The picture below axis represents vertebrae from C3 to C7, keeping in mind C7 has an elongated spinous process.
The motor unit of all cervical vertebrae is the area that allows for movement. It includes lateral processes called transverse processes on the left and right sides and the spinous process at the posterior (back). The motor unit attaches to the body of the vertebra and encloses the spinal cord in a protective arch. The transverse and spinous processes are for the attachment of muscles (motors) that move the spine. Ligaments also attach to these structures to help keep the spine in line during movement. The motor units also contain special gliding joints called facets located on each side that allow the segments to move in relation to their neighboring segments when the spine is in motion.
The foramen transversarium is the hole seen in each of the transverse processes and transmits the vertebral artery, vertebral vein and a plexus of sympathetic nerves. The vertebral arteries supply blood to the brain and the vertebral veins allow for brain drainage.
A notch or opening called the intervertebral foramen, created by two vertebrae, is formed and located between the body of the vertebra and disc on their backside and the facets (articular surface) of the vertebral segments on their front side. The intervertebral foramen allow the cervical nerves an exit from the spine and are best viewed from the side. Among other things the upper cervical nerves go to the diaphragm to sustain breathing. The lower cervical nerves go to the neck, shoulders and arms, as well as the accessory muscles used for respiration that lift the upper ribs and help to expand the ribcage.
The neural (spinal) canal is located between and formed by the body of the vertebra on the frontside and the arch formed by the transverse and spinous processes of the motor unit in the rear. The neural canal contains the spinal cord. It also contains the vertebral venous plexus (VVP) which is important to drainage of the brain during upright posture.
The overall shape of the cervical spine is a gentle arch that curves forward toward the chin. The arch in the neck makes it stronger and easier to hold up the head. It also distributes compressive forces down through the arch. The curve gives the head and neck spring-like action. The normal forward curve is called a lordosis. A loss of lordosis results in a straightening of the neck known as a military neck. A reversal in the normal lordotic curve is called a kyphosis. In addition to the shape of the spine, military and kyphotic neck curves change the shape of the spinal canal, as well as the load bearing structures of the spine. Both conditions can cause problems.
As for load bearing, as in the rest of the spine, except for momentary changes that occur briefly during upright posture and movement, the facet joints of the cervical spine are not meant to bear weight. Chronic compression of the facets can irritate them causing significant swelling and pain. In practice, cervical compression tests are used by physicians to check for bad discs. Bad discs give way more easily under compressive loads, which puts pressure on the highly sensitive facets. As you can see in the picture, the facets are shaped liked kidney beans but their shape and the direction they face is determined by their position in the spine. In the cervical spine they are basically aligned in a sloping fashion facing backwards. The joints of the upper cervical spine are much more complex in shape than the lower neck.
The support for the cervical spine, and the rest of the spine, for that matter, is different than in four legged creatures or other creatures with horizontal spines. The biggest difference is in tension and compression loads acting on the spine. In the example of the spinosaurus dinosaur below the large spinous processes located along the back are like the towers in a suspension bridge. In this case ligament and muscle cables in the spinosaurus run from the tall towers on the back out to the bones of the neck and tail to hold them up. In humans, upright posture changes everything.
The picture below is very telling when it comes to contrasting the neck in humans to apes. The human head and brain are relatively large compared to the ape. The neck bones on the other hand, especially in the upper cervical spine, are relatively small. The bones are much smaller because the design of the spine, due to upright posture, allows the large head to sit in near perfect balance on top of it like a waiter holding a tray. Consequently, it requires much less muscular energy and connective tissue support to hold the head upright. This resulted, over time, in a reduction in the size of the neck muscles and ligaments.
The downside is, it changes the loads acting on the spine. In the ape, the body, motor units and discs are mostly under tension like a suspension bridge. In humans, tension loads are decreased significantly while compression loads acting on the body and discs are significantly increased subjecting them to compression degeneration from upright posture. Additionally, the combination of large head design, smaller neck muscles and ligaments and greater range of motion makes humans especially vulnerable to whiplash injuries. What's more, whiplash injuries can result in damage to connective tissues and muscles, as well as misaligment and malfunction of the joints of the spine leading to cervical spondylosis. Misalignments and malfunction of the cervical spine, especially the upper cervical spine can have profound affects on health.
Aside from injuries, there are a host of design problems in the skull and upper cervical spine associated with upright posture that are congenital. These conditions include, Klipple-Feil syndrome, block vertebra, platybasia, craniosynostosis and Chiari malformations to name but a few. Many degenerative disease that affect connective tissues such as osteoarthritis, ankylosing spondylitis, rheumatoid arthritis, lupus erythematosis, Paget's disease and many others, alter the design of the spine.
Beause they carry most of the weight of the head and suffer the greatest strains during extreme ranges of motion and stress, the last several vertebra in the cervical spine and their cartilage are the most frequently involved in stenosis of the intertransverse foramen and subsequent pinching of the nerve roots. Stenosis means narrowing. The narrowing occurs as a result of enlargement of the facets or ligaments from arthritic degeneration or degeneration of the discs.
In addition to the upper cervical spine, problems also occur more frequently at the lower end of the neck where it joins the thoracic spine at the shoulders. In contrast to the cervical spine the thoracic spine is connected to the twelve sets of ribs. The strength and stiffness of the rib cage significantly limits the range of motion in the thoracic spine. The thoracic outlets contain the nerves and blood vessels to the arms. Thoracic outlet syndromes(TOS) can compress both nerves and blood vessels causing shoulder, arm and hand pain. Design flaws, such as an elongated transverse process of C7 vertebra can also cause TOS.
Lastly, degeneration of the ligaments and discs of the cervical spine can cause stenosis of the spinal canal. Some people inherit tight spinal canals that can result in stenosis and curvatures of the spine which can cause functional stenosis. Even mild stenosis (narrowing) of the spinal canal can result in compression of the thecal sac of the spinal cord. This is no small matter as the thecal sac contains the vertebral venous plexus of veins. In addition to providing venous drainage of the cord the VVP is also used by humans to drain the brain during upright posture. In this regard, the theory of chronic cerebrospinal venous insufficiency attributes the cause of multiple sclerosis to inadequate venous drainage of the brain. Stenosis can also cause compression of arterial blood supply routes to the cord. Finally, moderate to severe stenosis can cause compression of the spinal cord, which can have far more serious consequences.
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