File Spine

[Telly Piatt]

Thevertebral column, also known as the spinal column, spine or backbone, is the core part of the axial skeleton in vertebrate animals. The vertebral column is the defining and eponymous characteristic of the vertebrate endoskeleton, where the notochord (an elastic collagen-wrapped glycoprotein rod) found in all chordates has been replaced by a segmented series of mineralized irregular bones (or sometimes, cartilages) called vertebrae, separated by fibrocartilaginous intervertebral discs (the center of which is a notochord remnant).[1] The dorsal portion of the vertebral column houses the spinal canal, an elongated cavity formed by alignment of the vertebral neural arches that encloses and protects the spinal cord, with spinal nerves exiting via the intervertebral foramina to innervate each body segments.

There are around 50,000 species of animals that have a vertebral column.[2] The human spine 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 body region (neck, thorax, abdomen, pelvis or tail). In clinical medicine, features on vertebrae (particularly the spinous process) can be used as surface landmarks to guide medical procedures such as lumbar punctures and 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.

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(Top to bottom or head to pelvis). 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 numerous ligaments extending the length of the column, which include the anterior and posterior longitudinal ligaments at the front and back of the vertebral bodies, the ligamentum flavum in deep to the laminae, the interspinous and supraspinous ligaments between spinous processes, and the intertransverse ligaments between the transverse processes.

The vertebrae in the human vertebral column is divided into different body regions, which correspond to the curvatures 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.

A typical vertebra consists of two parts: the vertebral body (or centrum), which is ventral (or anterior, in the standard anatomical position) and withstands axial structural load; and the vertebral arch (also known as neural arch), which is dorsal (or posterior) and provides articulations and anchorages for ribs and core skeletal muscles. Together, these enclose the vertebral foramen, the series of which align to form the spinal canal, a body cavity that contains the spinal cord. Because the vertebral column will outgrow the spinal cord during child development, by adulthood the spinal cord often ends at the upper lumbar spine (at around L1/L2 level), the lower (caudal) end of the spinal canal is occupied by a ponytail-like bundle of spinal nerves descriptively called cauda equina (from Latin "horse's tail"), and the sacrum and coccyx are fused without a central foramen.

The vertebral arch is formed by a ventral pair of pedicles and a dorsal pair of laminae, and supports seven processes, four articular, two transverse and one spinous, the latter also being known as the neural spine. The transverse and spinous processes and their associated ligaments serve as important attachment sites for back and paraspinal muscles and the thoracolumbar fasciae. The spinous processes of the cervical and lumbar regions can be felt through the skin, and are important surface landmarks in clinical medicine.

The four articular processes for two pairs of plane facet joints above and below each vertebra, articulating with those of the adjacent vertebrae and are joined by a thin portion of the neural arch called the pars interarticularis. The orientation of the facet joints restricts the range of motion between the vertebrae. Underneath each pedicle is a small hole (enclosed by the pedicle of the vertebral below) called intervertebral foramen, which transmit the corresponding spinal nerve and dorsal root ganglion that exit the spinal canal.

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.

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.

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 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.

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]

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]

There are different ligaments involved in the holding together of the vertebrae in the column, and in the column's movement. The anterior and posterior longitudinal ligaments extend the length of the vertebral column along the front and back of the vertebral bodies.[13] The interspinous ligaments connect the adjoining spinous processes of the vertebrae.[14][better source needed] The supraspinous ligament extends the length of the spine running along the back of the spinous processes, from the sacrum to the seventh cervical vertebra.[15] From there it is continuous with the nuchal ligament.

The striking segmented pattern of the spine is established during embryogenesis when somites are rhythmically added to the posterior of the embryo. Somite formation begins around the third week when the embryo begins gastrulation and continues until all somites are formed. Their number varies between species: there are 42 to 44 somites in the human embryo and around 52 in the chick embryo. The somites are spheres, formed from the paraxial mesoderm that lies at the sides of the neural tube and they contain the precursors of spinal bone, the vertebrae ribs and some of the skull, as well as muscle, ligaments and skin. Somitogenesis and the subsequent distribution of somites is controlled by a clock and wavefront model acting in cells of the paraxial mesoderm. Soon after their formation, sclerotomes, which give rise to some of the bone of the skull, the vertebrae and ribs, migrate, leaving the remainder of the somite now termed a dermamyotome behind. This then splits to give the myotomes which will form the muscles and dermatomes which will form the skin of the back. Sclerotomes become subdivided into an anterior and a posterior compartment. This subdivision plays a key role in the definitive patterning of vertebrae that form when the posterior part of one somite fuses to the anterior part of the consecutive somite during a process termed resegmentation. Disruption of the somitogenesis process in humans results in diseases such as congenital scoliosis. So far, the human homologues of three genes associated to the mouse segmentation clock, (MESP2, DLL3 and LFNG), have been shown to be mutated in cases of congenital scoliosis, suggesting that the mechanisms involved in vertebral segmentation are conserved across vertebrates. In humans the first four somites are incorporated in the base of the occipital bone of the skull and the next 33 somites will form the vertebrae, ribs, muscles, ligaments and skin.[16] The remaining posterior somites degenerate. During the fourth week of embryogenesis, the 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.

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