Vertebral column

Source: Wikipedia, the free encyclopedia.
Vertebral column
The human vertebral column and its regions
Vertebral column of a goat
Details
Identifiers
Latincolumna vertebralis
Anatomical terminology

The vertebral column, also known as the backbone, spine, or spinal column, is the core part of the

neural arches that encloses and protects the spinal cord
.

There are around 50,000 species of animals that have a vertebral column.[2] The human vertebral column is one of the most-studied examples, as the general structure of human vertebrae is fairly typical (homologous) of that found in other mammals, reptiles and birds. The shape of the vertebral body does, however, vary somewhat between different groups of living species.

Individual vertebrae are named according to their corresponding

spinal anesthesia. There are also many different spinal diseases in humans that can affect both the bony vertebrae and the intervertebral discs, with kyphosis/scoliosis, ankylosing spondylitis, degenerative discs and spina bifida
being recognizable examples.

Structure

The number of vertebrae in a region can vary but overall the number remains the same. In a human vertebral column, there are normally 33 vertebrae.[3] The upper 24 pre-sacral vertebrae are articulating and separated from each other by intervertebral discs, and the lower nine are fused in adults, five in the sacrum and four in the coccyx, or tailbone. The articulating vertebrae are named according to their region of the spine. There are 7 cervical vertebrae, 12 thoracic vertebrae and 5 lumbar vertebrae. The number of those in the cervical region, however, is only rarely changed,[4] while that in the coccygeal region varies most.[5] Excluding rare deviations, the total number of vertebrae ranges from 32 to 35.[6] 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.[7][8][9] The most frequent deviations are: 11 (rarely 13) thoracic vertebrae, 4 or 6 lumbar vertebrae, 3 or 5 coccygeal vertebrae (rarely up to 7).[9]

There are

transverse processes
and the vertebral laminae.

Vertebrae

Numbering order of the vertebrae of the human spinal column

The vertebrae in the human vertebral column is divided into different regions, which correspond to the curves of the vertebral column. The articulating vertebrae are named according to their region of the spine. Vertebrae in these regions are essentially alike, with minor variation. These regions are called the cervical spine, thoracic spine, lumbar spine, sacrum, and coccyx. There are seven cervical vertebrae, twelve thoracic vertebrae, and five lumbar vertebrae.

The number of vertebrae in a region can vary but overall the number remains the same. The number of those in the cervical region, however, is only rarely changed.[4] The vertebrae of the cervical, thoracic, and lumbar spines are independent bones and generally quite similar. The vertebrae of the sacrum and coccyx are usually fused and unable to move independently. Two special vertebrae are the atlas and axis, on which the head rests.

Anatomy of a vertebra

A typical vertebra consists of two parts: the

spinous process
are posterior to (behind) the vertebral body. The spinous process comes out the back, one transverse process comes out the left, and one on the right. The spinous processes of the cervical and lumbar regions can be felt through the skin.

Above and below each vertebra are joints called

intervertebral foramina. The spinal nerves leave the spinal cord
through these holes.

Individual vertebrae are named according to their region and position. From top to bottom, the vertebrae are:

Combined vertebral regions

For some medical purposes, adjacent vertebral regions may be considered together:

  • Cervicothoracic spine (or region or division): the combined region of the cervical vertebrae and the thoracic vertebrae
  • Thoracolumbar spine (or region or division): the combined region of the thoracic vertebrae and the lumbar vertebrae[10]
  • Lumbosacral spine (or region or division): the combined region of the lumbar vertebrae and the
    sacral vertebrae

Shape

The vertebral column is curved in several places, a result of human bipedal evolution. These curves increase the vertebral column's strength, flexibility, and ability to absorb shock, stabilising the body in upright position. When the load on the spine is increased, the curvatures increase in depth (become more curved) to accommodate the extra weight. They then spring back when the weight is removed.[11]

The upper cervical spine has a curve, convex forward, that begins at the axis (second cervical vertebra) at the apex of the odontoid process or dens and ends at the middle of the second thoracic vertebra; it is the least marked of all the curves. This inward curve is known as a lordotic curve.

A thoracic spine X-ray of a 57-year-old male.

The thoracic curve, concave forward, begins at the middle of the second and ends at the middle of the twelfth thoracic vertebra. Its most prominent point behind corresponds to the spinous process of the seventh thoracic vertebra. This curve is known as a kyphotic curve.

Lateral lumbar X-ray of a 34-year-old male

The lumbar curve is more marked in the female than in the male; it begins at the middle of the last thoracic vertebra, and ends at the sacrovertebral angle. It is convex anteriorly, the convexity of the lower three vertebrae being much greater than that of the upper two. This curve is described as a lordotic curve.

The sacral curve begins at the sacrovertebral articulation, and ends at the point of the coccyx; its concavity is directed downward and forward as a kyphotic curve.

The thoracic and sacral kyphotic curves are termed primary curves, because they are present in the fetus. The cervical and lumbar curves are compensatory, or secondary, and are developed after birth. The cervical curve forms when the infant is able to hold up its head (at three or four months) and sit upright (at nine months). The lumbar curve forms later from twelve to eighteen months, when the child begins to walk.

Surfaces

Anterior surface

When viewed from in front, the width of the bodies of the vertebrae is seen to increase from the second cervical to the first thoracic; there is then a slight diminution in the next three vertebrae. Below this, there is again a gradual and progressive increase in width as low as the sacrovertebral angle. From this point there is a rapid diminution, to the apex of the coccyx.[12]

Posterior surface

From behind, the vertebral column presents in the median line the spinous processes. In the cervical region (with the exception of the second and seventh vertebrae), these are short, horizontal, and bifid. In the upper part of the thoracic region they are directed obliquely downward; in the middle they are almost vertical, and in the lower part they are nearly horizontal. In the lumbar region they are nearly horizontal. The spinous processes are separated by considerable intervals in the lumbar region, by narrower intervals in the neck, and are closely approximated in the middle of the thoracic region. Occasionally one of these processes deviates a little from the median line — which can sometimes be indicative of a fracture or a displacement of the spine. On either side of the spinous processes is the vertebral groove formed by the laminae in the cervical and lumbar regions, where it is shallow, and by the laminae and transverse processes in the thoracic region, where it is deep and broad; these grooves lodge the deep muscles of the back. Lateral to the spinous processes are the articular processes, and still more laterally the transverse processes. In the thoracic region, the transverse processes stand backward, on a plane considerably behind that of the same processes in the cervical and lumbar regions. In the cervical region, the transverse processes are placed in front of the articular processes, lateral to the pedicles and between the intervertebral foramina. In the thoracic region they are posterior to the pedicles, intervertebral foramina, and articular processes. In the lumbar region they are in front of the articular processes, but behind the intervertebral foramina.[12]

Lateral surfaces

The sides of the vertebral column are separated from the posterior surface by the articular processes in the cervical and thoracic regions and by the transverse processes in the lumbar region. In the thoracic region, the sides of the bodies of the vertebrae are marked in the back by the facets for articulation with the heads of the ribs. More posteriorly are the intervertebral foramina, formed by the juxtaposition of the vertebral notches, oval in shape, smallest in the cervical and upper part of the thoracic regions and gradually increasing in size to the last lumbar. They transmit the special spinal nerves and are situated between the transverse processes in the cervical region and in front of them, in the thoracic and lumbar regions.[12]

Ligaments

There are different ligaments involved in the holding together of the vertebrae in the column, and in the column's movement. The

cervical vertebra.[15] From there it is continuous with the nuchal ligament
.

Development

The striking

sclerotomes shift their position to surround the spinal cord and the notochord
. This column of tissue has a segmented appearance, with alternating areas of dense and less dense areas.

As the sclerotome develops, it condenses further eventually developing into the

HOX genes
.

The less dense tissue that separates the sclerotome segments develop into the intervertebral discs.

The notochord disappears in the sclerotome (vertebral body) segments but persists in the region of the intervertebral discs as the

anulus fibrosus
make up the intervertebral disc.

The primary curves (thoracic and sacral curvatures) form during fetal development. The secondary curves develop after birth. The cervical curvature forms as a result of lifting the head and the lumbar curvature forms as a result of walking.

Function

Spinal cord

The spinal cord nested in the vertebral column.

The vertebral column surrounds the spinal cord which travels within the

splanchnic nerves
.

The spinal canal follows the different curves of the column; it is large and triangular in those parts of the column that enjoy the greatest freedom of movement, such as the cervical and lumbar regions, and is small and rounded in the thoracic region, where motion is more limited.[17] The spinal cord terminates in the conus medullaris and cauda equina.

Clinical significance

3D Medical Animation still shot of Spina Bifida
3D Medical Animation still shot of Spina Bifida

Disease

Spina bifida is a congenital disorder in which there is a defective closure of the vertebral arch. Sometimes the spinal meninges and also the spinal cord can protrude through this, and this is called spina bifida cystica. Where the condition does not involve this protrusion it is known as spina bifida occulta. Sometimes all of the vertebral arches may remain incomplete.[18]

Another, though rare, congenital disease is Klippel–Feil syndrome, which is the fusion of any two of the cervical vertebrae.

vertebral body
with respect to the adjacent vertebra to a degree less than a dislocation.

Spondylolysis, also known as a pars defect, is a defect or fracture at the pars interarticularis of the vertebral arch.

nucleus pulposus, bulge out in a hernia
.

neurological deficit
.

Pain at the coccyx (tailbone) is known as coccydynia.[19]

Spinal cord injury is damage to the spinal cord that causes changes in its function, either temporary or permanent. Spinal cord injuries can be divided into categories: complete transection, hemisection, central spinal cord lesions, posterior spinal cord lesions, and anterior spinal cord lesions.

Scalloping vertebrae is the increase in the concavity of the posterior vertebral body. It can be seen on lateral X-ray and sagittal views of CT and MRI scans. Its concavity is due to the increased pressure exerting on the vertebrae due to a mass. Internal spinal mass such as spinal astrocytoma, ependymoma, schwannoma, neurofibroma, and achondroplasia causes vertebrae scalloping.[20]

Curvature

Diagram showing normal curvature of the vertebrae from childhood to teenage

Excessive or abnormal spinal curvature is classed as a spinal disease or dorsopathy and includes the following abnormal curvatures:

  • Kyphosis is an exaggerated kyphotic (convex) curvature of the thoracic region in the sagittal plane, also called hyperkyphosis. This produces the so-called "humpback" or "dowager's hump", a condition commonly resulting from osteoporosis.
  • lumbar hyperlordosis and also as "swayback". Temporary lordosis is common during pregnancy
    .
  • unequal growth of the two sides of one or more vertebrae,[21][22] so that they do not fuse properly. It can also be caused by pulmonary atelectasis (partial or complete deflation of one or more lobes of the lungs) as observed in asthma or pneumothorax
    .
  • Kyphoscoliosis, a combination of kyphosis and scoliosis.

Anatomical landmarks

L1

Individual vertebrae of the human vertebral column can be felt and used as

organs
.

Other animals

Variations in vertebrae

The general structure of vertebrae in other animals is largely the same as in humans. Individual vertebrae are composed of a centrum (body), arches protruding from the top and bottom of the centrum, and various processes projecting from the centrum and/or arches. An arch extending from the top of the centrum is called a neural arch, while the

spinous process
, the pleurocentrum and the intercentrum are separate ossifications. Fused elements, however, classify a vertebra as having holospondyly.

A vertebra can also be described in terms of the shape of the ends of the centrum. Centra with flat ends are acoelous, like those in mammals. These flat ends of the centra are especially good at supporting and distributing compressive forces. Amphicoelous vertebra have centra with both ends concave. This shape is common in fish, where most motion is limited. Amphicoelous centra often are integrated with a full notochord. Procoelous vertebrae are anteriorly concave and posteriorly convex. They are found in frogs and modern reptiles. Opisthocoelous vertebrae are the opposite, possessing anterior convexity and posterior concavity. They are found in salamanders, and in some non-avian dinosaurs. Heterocoelous vertebrae have saddle-shaped articular surfaces. This type of configuration is seen in turtles that retract their necks, and birds, because it permits extensive lateral and vertical flexion motion without stretching the nerve cord too extensively or wringing it about its long axis.

In horses, the Arabian (breed) can have one less vertebrae and pair of ribs. This anomaly disappears in foals that are the product of an Arabian and another breed of horse.[23]

Regional vertebrae

Vertebrae are defined by their location in the vertebral column. Cervical vertebrae are those in the neck area. With the exception of the two

chimpanzees (and humans
).

Fish and amphibians

A vertebra (diameter 5 mm) of a small ray-finned fish

The vertebrae of lobe-finned fishes consist of three discrete bony elements. The vertebral arch surrounds the spinal cord, and is of broadly similar form to that found in most other vertebrates. Just beneath the arch lies a small plate-like pleurocentrum, which protects the upper surface of the notochord, and below that, a larger arch-shaped intercentrum to protect the lower border. Both of these structures are embedded within a single cylindrical mass of cartilage. A similar arrangement was found in the primitive Labyrinthodonts, but in the evolutionary line that led to reptiles (and hence, also to mammals and birds), the intercentrum became partially or wholly replaced by an enlarged pleurocentrum, which in turn became the bony vertebral body.[26] In most ray-finned fishes, including all teleosts, these two structures are fused with, and embedded within, a solid piece of bone superficially resembling the vertebral body of mammals. In living amphibians, there is simply a cylindrical piece of bone below the vertebral arch, with no trace of the separate elements present in the early tetrapods.[26]

In cartilaginous fish, such as sharks, the vertebrae consist of two cartilaginous tubes. The upper tube is formed from the vertebral arches, but also includes additional cartilaginous structures filling in the gaps between the vertebrae, and so enclosing the spinal cord in an essentially continuous sheath. The lower tube surrounds the notochord, and has a complex structure, often including multiple layers of calcification.[26]

higher vertebrates. Even the arches are discontinuous, consisting of separate pieces of arch-shaped cartilage around the spinal cord in most parts of the body, changing to long strips of cartilage above and below in the tail region. Hagfishes lack a true vertebral column, and are therefore not properly considered vertebrates, but a few tiny neural arches are present in the tail.[26]

Other vertebrates

The general structure of human vertebrae is fairly typical of that found in mammals, reptiles, and birds. The shape of the vertebral body does, however, vary somewhat between different groups. In mammals, such as humans, it typically has flat upper and lower surfaces, while in reptiles the anterior surface commonly has a concave socket into which the expanded convex face of the next vertebral body fits. Even these patterns are only generalisations, however, and there may be variation in form of the vertebrae along the length of the spine even within a single species. Some unusual variations include the saddle-shaped sockets between the cervical vertebrae of birds and the presence of a narrow hollow canal running down the centre of the vertebral bodies of geckos and tuataras, containing a remnant of the notochord.[26]

Reptiles often retain the primitive intercentra, which are present as small crescent-shaped bony elements lying between the bodies of adjacent vertebrae; similar structures are often found in the caudal vertebrae of mammals. In the tail, these are attached to chevron-shaped bones called

haemal arches, which attach below the base of the spine, and help to support the musculature. These latter bones are probably homologous with the ventral ribs of fish. The number of vertebrae in the spines of reptiles is highly variable, and may be several hundred in some species of snake.[26]

In birds, there is a variable number of cervical vertebrae, which often form the only truly flexible part of the spine. The thoracic vertebrae are partially fused, providing a solid brace for the wings during flight. The sacral vertebrae are fused with the lumbar vertebrae, and some thoracic and caudal vertebrae, to form a single structure, the synsacrum, which is thus of greater relative length than the sacrum of mammals. In living birds, the remaining caudal vertebrae are fused into a further bone, the pygostyle, for attachment of the tail feathers.[26]

Aside from the tail, the number of vertebrae in mammals is generally fairly constant. There are almost always seven cervical vertebrae (sloths and manatees are among the few exceptions), followed by around twenty or so further vertebrae, divided between the thoracic and lumbar forms, depending on the number of ribs. There are generally three to five vertebrae with the sacrum, and anything up to fifty caudal vertebrae.[26]

Dinosaurs

The vertebral column in

occipital condyle is a structure on the posterior part of a dinosaur's skull that articulates with the first cervical vertebra.[27]

See also

References

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  8. ^ Bardeen, C.R. (1904). "Numerical vertebral variations in the human adult and embryo". Anatomischer Anzeiger. 25: 497–519. Archived from the original on 2021-04-10. Retrieved 2021-04-10 – via Biodiversity Heritage Library.
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  14. ^ "interspinal ligament". Merriam-Webster. Archived from the original on 13 July 2018. Retrieved 29 January 2016.
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  23. ^ Edwards, The Arabian, pp. 27–28
  24. ^ "Sticking Their Necks out for Evolution: Why Sloths and Manatees Have Unusually Long (or Short) Necks". May 6th 2011. Science Daily. Archived from the original on 26 April 2019. Retrieved 25 July 2013.
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External links