The basic configuration of a vertebra varies; the vertebral body (alsocentrum) is of bone and bears the load of the vertebral column. The upper and lower surfaces of the vertebra body give attachment to theintervertebral discs. The posterior part of a vertebra forms avertebral arch, in eleven parts, consisting of two pedicles (pedicle of vertebral arch), two laminae, and sevenprocesses. The laminae give attachment to theligamenta flava (ligaments of the spine). There are vertebral notches formed from the shape of the pedicles, which form theintervertebral foramina when the vertebraearticulate. These foramina are the entry and exit conduits for thespinal nerves. The body of the vertebra and the vertebral arch form thevertebral foramen; the larger, central opening that accommodates thespinal canal, which encloses and protects thespinal cord.
Vertebrae articulate with each other to give strength and flexibility to the spinal column, and the shape at their back and front aspects determines the range of movement. Structurally, vertebrae are essentially alike across the vertebrate species, with the greatest difference seen between anaquatic animal and other vertebrate animals. As such, vertebrates take their name from the vertebrae that compose the vertebral column.
In the humanvertebral column, the size of the vertebrae varies according to placement in the vertebral column, spinal loading, posture and pathology. Along the length of thespine, the vertebrae change to accommodate different needs related to stress and mobility.[1] Each vertebra is an irregular bone.
Side view of vertebrae
A typical vertebra has abody (vertebral body), also known as thecentrum, which consists of a large anterior middle portion, and a posteriorvertebral arch,[2] also called aneural arch.[3] The body is composed ofcancellous bone, which is the spongy type ofosseous tissue, whose microanatomy has been specifically studied within the pedicle bones.[4] This cancellous bone is in turn, covered by a thin coating ofcortical bone (or compact bone), the hard and dense type of osseous tissue. The vertebral arch and processes have thicker coverings of cortical bone. The upper and lower surfaces of the body of the vertebra are flattened and rough in order to give attachment to theintervertebral discs. These surfaces are the vertebralendplates which are in direct contact with the intervertebral discs and form the joint. The endplates are formed from a thickened layer of the cancellous bone of the vertebral body, the top layer being more dense. The endplates function to contain the adjacent discs, to evenly spread the applied loads, and to provide anchorage for thecollagen fibers of the disc. They also act as a semi-permeable interface for the exchange of water and solutes.[5]
Anatomy of a vertebra
The vertebral arch is formed by pedicles and laminae. Two pedicles extend from the sides of the vertebral body to join the body to the arch. The pedicles are short thickprocesses that extend, one from each side, posteriorly, from the junctions of the posteriolateral surfaces of the centrum, on its upper surface.From each pedicle a broad plate, alamina, projects backward and medially to join and complete the vertebral arch and form the posterior border of the vertebral foramen, which completes the triangle of the vertebral foramen.[6] The upper surfaces of the laminae are rough to give attachment to theligamenta flava. These ligaments connect the laminae of adjacent vertebra along the length of the spine from the level of thesecond cervical vertebra. Above and below the pedicles are shallow depressions calledvertebral notches (superior andinferior). When the vertebraearticulate the notches align with those on adjacent vertebrae and these form the openings of theintervertebral foramina. The foramina allow the entry and exit of the spinal nerves from each vertebra, together with associated blood vessels. The articulating vertebrae provide a strong pillar of support for the body.
A major part of a vertebra is a backward extending spinous process (sometimes called the neural spine) which projects centrally.[7] This process pointsdorsally andcaudally from the junction of the laminae.[7] The spinous process serves to attachmuscles andligaments.
The two transverse processes, one on each side of the vertebral body, project laterally from either side at the point where the lamina joins thepedicle, between the superior and inferior articular processes.[7] They also serve for the attachment of muscles and ligaments, in particular theintertransverse ligaments. There is a facet on each of the transverse processes of thoracic vertebrae which articulates with thetubercle of the rib.[8] A facet on each side of the thoracic vertebral body articulates with thehead of the rib. The transverse process of alumbar vertebra is also sometimes called thecostal[9][10] orcostiform process[11] because it corresponds to a rudimentary rib (costa) which, as opposed to the thorax, is not developed in the lumbar region.[11][12]
There are superior and inferiorarticularfacet joints on each side of the vertebra, which serve to restrict the range of movement possible. These facets are joined by a thin portion of the vertebral arch called thepars interarticularis.
Vertebrae take their names from the regions of the vertebral column that they occupy. There are usually thirty-three vertebrae in the human vertebral column — sevencervical vertebrae, twelvethoracic vertebrae, fivelumbar vertebrae, five fusedsacral vertebrae forming thesacrum and fourcoccygeal vertebrae, forming thecoccyx. Excluding rare deviations, the total number of vertebrae ranges from 32 to 35.[13] In about 10% of people, both the total number of pre-sacral vertebrae and the number of vertebrae in individual parts of the spine can vary.[14][15] The most frequent deviations are eleven (rarely thirteen) thoracic vertebrae, four or six lumbar vertebrae and three or five coccygeal vertebrae (rarely up to seven).[15]
The regional vertebrae increase in size as they progress downward but become smaller in the coccyx.
There are sevencervical vertebrae (but eight cervicalspinal nerves), designated C1 through C7. These bones are, in general, small and delicate. Their spinous processes are short (with the exception of C2 and C7, which have palpable spinous processes). C1 is also called theatlas, and C2 is also called theaxis. The structure of these vertebrae is the reason why theneck andhead have a large range of motion. Theatlanto-occipital joint allows theskull to move up and down, while theatlanto-axial joint allows the upper neck to twist left and right. The axis also sits upon the first intervertebral disc of the spinal column.
Cervical vertebrae possess transverse foramina to allow for the vertebral arteries to pass through on their way to theforamen magnum to end in thecircle of Willis. These are the smallest, lightest vertebrae and the vertebral foramina are triangular in shape. The spinous processes are short and oftenbifurcated (the spinous process of C7 is not bifurcated, and is substantially longer than that of the other cervical spinous processes).[16]
The atlas differs from the other vertebrae in that it has no body and no spinous process. It has instead a ring-like form, having an anterior and a posterior arch and two lateral masses. At the outside centre points of both arches there is a tubercle, ananterior tubercle and aposterior tubercle, for the attachment of muscles. The front surface of the anterior arch is convex and its anterior tubercle gives attachment to thelongus colli muscle. The posterior tubercle is arudimentary spinous process and gives attachment to therectus capitis posterior minor muscle. The spinous process is small so as not to interfere with the movement between the atlas and theskull. On the under surface is a facet for articulation with thedens of the axis.
Specific to the cervical vertebra is thetransverse foramen (also known asforamen transversarium). This is an opening on each of the transverse processes which gives passage to thevertebral artery andvein and asympathetic nerve plexus. On the cervical vertebrae other than the atlas, the anterior and posterior tubercles are on either side of the transverse foramen on each transverse process. The anterior tubercle on the sixth cervical vertebra is called thecarotid tubercle because it separates thecarotid artery from thevertebral artery.
There is a hook-shapeduncinate process on the side edges of the top surface of the bodies of the third to the seventh cervical vertebrae and of the first thoracic vertebra. Together with the vertebral disc, this uncinate process prevents a vertebra from sliding backward off the vertebra below it and limits lateral flexion (side-bending).Luschka's joints involve the vertebral uncinate processes.
The twelvethoracic vertebrae and their transverse processes have surfaces thatarticulate with theribs. Some rotation can occur between the thoracic vertebrae, but their connection with therib cage prevents muchflexion or other movement. They may also be known as "dorsal vertebrae" in the human context.
The vertebral bodies are roughly heart-shaped and are about as wide anterio-posteriorly as they are in the transverse dimension. Vertebral foramina are roughly circular in shape.
The top surface of the first thoracic vertebra has a hook-shaped uncinate process, just like the cervical vertebrae.
Thethoracolumbar spine or thoracolumbar division refers to the thoracic and lumbar vertebrae together, and sometimes also their surrounding areas.
The thoracic vertebrae attach to ribs and so have articular facets specific to them; these are thesuperior,transverse andinferior costal facets. As the vertebrae progress down the spine they increase in size to match up with the adjoining lumbar section.
The fivelumbar vertebrae are the largest of the vertebrae, their robust construction being necessary for supporting greater weight than the other vertebrae. They allow significantflexion,extension and moderate lateral flexion (side-bending). The discs between these vertebrae create a naturallumbarlordosis (a spinal curvature that is concave posteriorly).[citation needed] This is due to the difference in thickness between the front and back parts of the intervertebral discs.
The lumbar vertebrae are located between the ribcage and the pelvis and are the largest of the vertebrae. The pedicles are strong, as are the laminae, and the spinous process is thick and broad. The vertebral foramen is large and triangular. The transverse processes are long and narrow and three tubercles can be seen on them. These are a lateralcostiform process, amammillary process and anaccessory process.[17] The superior, or upper tubercle is the mammillary process which connects with the superior articular process. Themultifidus muscle attaches to the mammillary process and this muscle extends through the length of the vertebral column, giving support. The inferior, or lower tubercle is the accessory process and this is found at the back part of the base of the transverse process. The termlumbosacral is often used to refer to the lumbar and sacral vertebrae together, and sometimes includes their surrounding areas.
Somites form in the earlyembryo and some of these develop into sclerotomes. The sclerotomes form the vertebrae as well as the rib cartilage and part of theoccipital bone. From their initial location within the somite, the sclerotome cells migrate medially toward thenotochord. These cells meet the sclerotome cells from the other side of theparaxial mesoderm. The lower half of one sclerotome fuses with the upper half of the adjacent one to form each vertebral body.[20] From this vertebral body, sclerotome cells move dorsally and surround the developingspinal cord, forming thevertebral arch. Other cells move distally to the costal processes ofthoracic vertebrae to form the ribs.[20]
Support of the vertebrae function in the skeletomuscular system by forming the vertebral column to support the body
Protection. Vertebrae contain avertebral foramen for the passage of the spinal canal and its enclosedspinal cord and coveringmeninges. They also afford sturdy protection for the spinal cord. The upper and lower surfaces of the centrum are flattened and rough in order to give attachment to the intervertebral discs.
Movement. The vertebrae also provide the openings, the intervertebral foramina which allow the entry and exit of thespinal nerves. Similarly to the surfaces of the centrum, the upper and lower surfaces of the fronts of the laminae are flattened and rough to give attachment to theligamenta flava. Working together in the vertebral column their sections provide controlled movement and flexibility.
Feeding of the intervertebral discs through the reflex (hyaline ligament) plate that separates the cancellous bone of the vertebral body from each disk
There are a number ofcongenital vertebral anomalies, mostly involving variations in the shape or number of vertebrae, and many of which are unproblematic. Others though can cause compression of the spinal cord. Wedge-shaped vertebrae, calledhemivertebrae can cause an angle to form in the spine which can result in the spinal curvature diseases ofkyphosis,scoliosis andlordosis. Severe cases can cause spinal cord compression.Block vertebrae where some vertebrae have become fused can cause problems.Spina bifida can result from the incomplete formation of the vertebral arch.
Spondylolysis is a defect in thepars interarticularis of the vertebral arch. In most cases this occurs in the lowest of the lumbar vertebrae (L5), but may also occur in the other lumbar vertebrae, as well as in the thoracic vertebrae.
Alaminectomy is a surgical operation to remove the laminae in order to access the spinal canal.[21] The removal of just part of a lamina is called alaminotomy.
Apinched nerve caused by pressure from a disc, vertebra or scar tissue might be remedied by aforaminotomy to broaden the intervertebral foramina and relieve pressure. It can also be caused by a foraminastenosis, a narrowing of the nerve opening, as a result ofarthritis.
Another condition isspondylolisthesis when one vertebra slips forward onto another. The reverse of this condition isretrolisthesis where one vertebra slips backward onto another.
Degenerative disc disease is a condition usually associated with ageing in which one or more discs degenerate. This can often be a painfree condition but can also be very painful.
In other animals, the vertebrae take the same regional names except for the coccygeal – in animals with tails, the separate vertebrae are usually called thecaudal vertebrae.[19] Because of the different types of locomotion and support needed between the aquatic and other vertebrates, the vertebrae between them show the most variation, though basic features are shared. The spinous processes which are backward extending are directed upward in animals without an erect stance. These processes can be very large in the larger animals since they attach to the muscles and ligaments of the body. In theelephant, the vertebrae are connected by tight joints, which limit the backbone's flexibility. Spinous processes are exaggerated in some animals, such as theextinctDimetrodon andSpinosaurus, where they form a sailback or finback.
Vertebrae with saddle-shaped articular surfaces on their bodies, called "heterocoelous", allow vertebrae to flex both vertically and horizontally while preventing twisting motions. Such vertebrae are found in the necks of birds and some turtles.[23]
An example of procoelous vertebrae dissected from a rattlesnake.
"Procoelous" vertebrae feature a spherical protrusion extending from the caudal end of the centrum of one vertebra that fits into a concave socket on the cranial end of the centrum of an adjacent vertebra.[24] These vertebrae are most often found inreptiles,[25][26] but are found in someamphibians such as frogs.[27] The vertebrae fit together in a ball-and-socket articulation, in which the convex articular feature of ananterior vertebra acts as the ball to the socket of a caudal vertebra.[25] This type of connection permits a wide range of motion in most directions, while still protecting the underlying nerve cord. The central point of rotation is located at the midline of each centrum, and therefore flexion of themuscle surrounding thevertebral column does not lead to an opening between vertebrae.[27]
In many species, though not in mammals, thecervical vertebrae bear ribs. In many groups, such aslizards andsaurischian dinosaurs, the cervical ribs are large; in birds, they are small and completely fused to the vertebrae. Thetransverse processes of mammals are homologous to the cervical ribs of otheramniotes. In the whale, the cervical vertebrae are typically fused, an adaptation trading flexibility for stability during swimming.[28][29] Allmammals exceptmanatees andsloths have seven cervical vertebrae, whatever the length of the neck.[30] This includes seemingly unlikely animals such as the giraffe, the camel, and the blue whale, for example.Birds usually have more cervical vertebrae with most having a highly flexible neck consisting of 13–25 vertebrae.
In all mammals, thethoracic vertebrae are connected toribs and their bodies differ from the other regional vertebrae due to the presence of facets. Each vertebra has a facet on each side of the vertebral body, which articulates with thehead of a rib. There is also a facet on each of the transverse processes which articulates with thetubercle of a rib. The number of thoracic vertebrae varies considerably across the species.[31] Mostmarsupials have thirteen, butkoalas only have eleven.[32] The usual number is twelve to fifteen inmammals, (twelve in thehuman), though there are from eighteen to twenty in thehorse,tapir,rhinoceros andelephant. In certain sloths, there is an extreme number of twenty-five and at the other end only nine in thecetacean.[33]
There are fewerlumbar vertebrae inchimpanzees andgorillas, which have three in contrast to the five in the genusHomo. This reduction in number gives an inability of the lumbar spine tolordose but gives an anatomy that favours vertical climbing, and hanging ability more suited to feeding locations in high-canopied regions.[34] Thebonobo differs by having four lumbar vertebrae.
Caudal vertebrae are the bones that make up the tails of vertebrates.[35] They range in number from a few to fifty, depending on the length of the animal's tail.In humans and other taillessprimates, they are called thecoccygeal vertebrae, number from three to five and are fused into thecoccyx.[36]
^Romer, Alfred (1956).Osteology of the Reptiles. Malabar, Florida: Krieger Publishing Company. pp. 0–89464–985–X.
^abKardong, Kenneth V. (2015).Vertebrates: Comparative Anatomy, Function, Evolution (7 ed.). New York, NY: McGraw-Hill Education.ISBN978-0-07-802302-6.
