Spinal column


The spinal column, also known as the vertebral column, spine or backbone, is the
part of the axial skeleton in vertebrates. The vertebral column is the defining and eponymous characteristic of the vertebrate. The spinal column is a segmented column of vertebrae that surrounds and protects the spinal cord. The vertebrae are separated by intervertebral discs in a series of cartilaginous joints. The dorsal portion of the spinal column houses the spinal canal, an elongated cavity formed by the 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 segment.
There are around 50,000 species of animals that have a vertebral column. The human spine is one of the most-studied examples, as the general structure of human vertebrae is fairly typical 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 region including the neck, thorax, abdomen, pelvis or tail. In clinical medicine, features on vertebrae such as 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, and degenerative discs being recognizable examples. Spina bifida is the most common birth defect that affects the spinal column.

Structure

The number of vertebrae in a region can vary but overall the number remains the same. In a human spinal column, there are normally 33 vertebrae. 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.
From top to bottom, 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, while that in the coccygeal region varies most. Excluding rare deviations, the total number of vertebrae ranges from 32 to 35. 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. The most frequent deviations are: 11 thoracic vertebrae, 4 or 6 lumbar vertebrae, 3 or 5 coccygeal vertebrae.
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 deep to the laminae, the interspinous and supraspinous ligaments between spinous processes, and the intertransverse ligaments between the transverse processes.

Vertebrae

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. 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, which is ventral and withstands axial structural load; and the vertebral arch, which is dorsal 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, the lower end of the spinal canal is occupied by a ponytail-like bundle of spinal nerves descriptively called cauda equina, 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 called intervertebral foramen, which transmit the corresponding spinal nerve and dorsal root ganglion that exit the spinal canal.
From top to bottom, the vertebrae are:
For some medical purposes, adjacent vertebral regions may be considered together:
  • Cervicothoracic spine : the combined region of the cervical vertebrae and the thoracic vertebrae
  • Thoracolumbar spine : the combined region of the thoracic vertebrae and the lumbar vertebrae
  • Lumbosacral spine : 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 to accommodate the extra weight. They then spring back when the weight is removed.
The upper cervical spine has a curve, convex forward, that begins at the axis 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 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 and sit upright. 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.
;Posterior surface
From behind, the vertebral column presents in the median line the spinous processes. In the cervical region, 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.
;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.