The Neck - Bones
What is the cervical spine?
The cervical spine is the most superior portion of the vertebral column, lying between the cranium and the thoracic vertebrae.
The cervical spine performs several crucial roles, including:
-
The spinal cord is a bundle of nerves that extends from the brain and runs through the cervical spine and thoracic spine (upper and middle back) prior to ending just before the lumbar spine (lower back).
Each vertebra has a large hole (vertebral foramen) for the spinal cord to pass through. Together, these vertebrae keep the spinal cord shielded inside a bony tunnel called the spinal canal.
-
The cervical spine handles a heavy load, as the head weighs on average between 10 and 13 pounds.
In addition to supporting the head, the cervical spine allows for the neck’s flexibility and head’s range of motion.
-
Small holes (foramina in the transverse processes) in the cervical spine provide a passageway for vertebral arteries to carry blood to the brain.
These openings for the blood vessels are present only in the vertebrae of the cervical spine from C1 down to C6 (not in C7 or lower).
With so many critical nerves, blood vessels, and joints in such a relatively small space, the cervical spine is one of the body’s most complicated regions.
“Typical” vertebrae?
Cervical vertebrae C3 through C6 are known as typical vertebrae because they share the same basic characteristics with most of the vertebrae throughout the rest of the spine.
Typical vertebrae have:
The cervical vertebrae have three main features which distinguish them from other vertebrae:
Triangular vertebral foramen.
Bifid spinous process – this is where the spinous process splits into two distally.
Transverse foramina – holes in the transverse processes. They give passage to the vertebral artery, vein and sympathetic nerves.
-
This thick bone is cylindrical-shaped and located at the front of the vertebra. The vertebral body carries most of the load for a vertebra.
At most levels of the spine, an intervertebral disc sits between 2 vertebral bodies to provide cushioning and help absorb the shock of everyday movements.
-
This bony arch wraps around the spinal cord toward the back of the spine and consists of 2 pedicles and 2 laminae.
The pedicles connect with the vertebral body in the front, and the laminae transition into the spinous process (a bony hump) in the back of the vertebra.
-
Each vertebra has a pair of facet joints, also known as zygapophysial joints.
These joints, located between the pedicle and lamina on each side of the vertebral arch, are lined with smooth cartilage to enable limited movement between 2 vertebrae.
Spinal degeneration or injury to the facet joints are among the most common causes of chronic neck pain.
The Atlas and the Axis
The atlas and axis have additional features that mark them apart from the other cervical vertebrae.
The atlas (C1) is the first cervical vertebra and articulates with the occiput of the head and the axis (C2).
It differs from the other cervical vertebrae in that it has no vertebral body and no spinous process.
Instead, the atlas has lateral masses which are connected by an anterior and posterior arch. Each lateral mass contains a superior articular facet (for articulation with occipital condyles), and an inferior articular facet (for articulation with C2).
The anterior arch contains a facet for articulation with the dens of the axis. This is secured by the transverse ligament of the atlas – which attaches to the lateral masses.
The posterior arch has a groove for the vertebral artery and C1 spinal nerve.
The axis (C2) is easily identifiable due to its dens (odontoid process) which extends superiorly from the anterior portion of the vertebra.
The dens articulates with the anterior arch of the atlas, in doing so creating the medial atlanto-axial joint. This allows for rotation of the head independently of the torso.
The axis also contains superior articular facets, which articulate with the inferior articular facets of the atlas to form the two lateral atlanto-axial joints.
The Vertebra Prominens
The seventh cervical vertebra, also called the vertebra prominens, is commonly considered a unique vertebra and has the most prominent spinous process.
When feeling the back of the neck, the C7 vertebra’s spinous process (bony hump) sticks out more than the other cervical vertebrae.
C7 is the bottom of the cervical spine and connects with the top of the thoracic spine, T1, to form the cervicothoracic junction—also referred to as C7-T1.
Not only is C7’s spinous process significantly bigger than those of the vertebrae above, it is also a different shape to better fit with T1 below.
C7 also lacks holes (foramina in its transverse processes) for vertebral arteries to pass, which are present in all of the other cervical vertebrae.
Due to its larger size and key location at the cervicothoracic junction, more muscles connect to C7’s spinous process compared to other cervical vertebrae
What are the joints of the cervical spine?
The joints of the cervical spine can be divided into two groups – those that are present throughout the vertebral column, and those unique to the cervical spine.
-
Between vertebral bodies – adjacent vertebral bodies are joined by intervertebral discs, made of fibrocartilage. This is a type of cartilaginous joint, known as a symphysis.
Between vertebral arches – formed by the articulation of superior and inferior articular processes from adjacent vertebrae. It is a synovial type joint.
-
The atlanto-axial joints are formed by the articulation between the atlas and the axis.
Lateral atlanto-axial joints (x2) – formed by the articulation between the inferior facets of the lateral masses of C1 and the superior facets of C2. These are plane type synovial joints.
Medial atlanto-axial joint – formed by the articulation of the dens of C2 with the articular facet of C1. This is a pivot type synovial joint.
The atlanto-occipital joints consist of an articulation between the spine and the cranium. They occur between then superior facets of the lateral masses of the atlas and the occipital condyles at the base of the cranium. These are condyloid type synovial joints, and permit flexion at the head i.e. nodding.
What are the cervical discs?
Cervical discs support the neck’s vertebral bones while also enabling flexibility for head movements.
Sitting between adjacent cervical vertebrae stacked atop each other, each cervical disc acts as a shock absorber to help the cervical spine handle various stresses and loads.
There are 6 intervertebral discs in the highly-mobile cervical spine.
These cervical discs tend to be thinner than the lumbar discs in the lower back but thicker than the thoracic discs in the less-mobile upper back.
Each cervical disc has 2 basic components:
-
This tough exterior is comprised of collagen fibres that surround and protect the inner core. The annulus fibrosus also distributes the forces placed on the structure.
-
This soft jelly interior is a loose, fibrous network suspended in mucoprotein gel that is sealed by the annulus fibrosus. The nucleus pulposus helps to provide cushioning and flexibility to the disc.
The discs need to be well-hydrated in order to maintain their strength and softness to serve as the spine’s major carrier of axial load.
What is the hyoid bone?
Even though the hyoid bone is an important structure, it has not received the attention that it deserves. It participates in the functions of:
speech
respiration
mastication (chewing)
swallowing
It also maintains the patency (openness) of the airway between the oropharynx and the tracheal rings.
The hyoid bone is a ‘U’ shaped structure located in the anterior neck.
It lies at the base of the mandible (approximately C3), where it acts as a site of attachment for the anterior neck muscles.
The hyoid bone is unique in the fact that it does not articulate with any other bones, and is suspended in place by the muscles and ligaments that attach to it.
The hyoid is composed of a body, two greater horns and two lesser horns:
Body – the central part of the bone. It has an anterior convex surface and a concave posterior surface.
Greater horn – projects from each end of the body in a posterior, superior and lateral direction. It acts as a site of attachment for numerous neck muscles.
Lesser horn – arises from the superior aspect of the hyoid bone, near the origin of the greater horn. It projects superoposteriorly (toward the styloid process of the temporal bone). The stylohyoid ligament attaches to the apex of the lesser horn.
There are three main ligaments that attach to the hyoid bone – stylohyoid, thyrohyoid and hyoepiglottic. They act to support the position of the hyoid in the neck.
Stylohyoid ligament – extends from the styloid process of the temporal bone to the lesser horn of the hyoid bone.
Thyrohyoid membrane – originates from the superior border of the thyroid cartilage and attaches to the posterior surface of the hyoid bone and the greater horns.
Hyoepiglottic ligament – connects the hyoid bone to the anterior aspect of the epiglottis.
