| Vertebral column | |
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The human vertebral column and its regions
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Vertebral column of a goat
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The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordates has been replaced by a segmented series of bone: vertebrae separated by intervertebral discs. The vertebral column houses the spinal canal, a cavity that encloses and protects the spinal cord.
There are about 50,000 species of animals that have a vertebral column. The human vertebral column is one of the most-studied examples.
Structure
In a human's vertebral column there are normally thirty-three vertebrae; the upper twenty-four 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 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.
There are ligaments extending the length of the column at the front and the back, and in between the vertebrae joining the spinous processes, the transverse processes and the vertebral laminae.
Vertebrae
Numbering order of the vertebrae of the human spinal column
The vertebrae in the human vertebral column are divided into
different regions, which correspond to the curves of the spinal 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.
Anatomy of a vertebra
A typical vertebra consists of two parts: the vertebral body and the vertebral arch. The vertebral arch is posterior, meaning it faces the back of a person. Together, these enclose the vertebral foramen, which contains the spinal cord.
Because the spinal cord ends in the lumbar spine, and the sacrum and
coccyx are fused, they do not contain a central foramen. The vertebral
arch is formed by a pair of pedicles and a pair of laminae, and supports seven processes, four articular, two transverse, and one spinous, the latter also being known as the neural spine. Two transverse processes and one 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 facet joints. These restrict the range of movement possible, and are joined by a thin portion of the neural arch called the pars interarticularis. In between each pair of vertebrae are two small holes 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:
- Cervical spine: 7 vertebrae (C1–C7)
- Thoracic spine: 12 vertebrae (T1–T12)
- Lumbar spine: 5 vertebrae (L1–L5)
- Sacrum: 5 (fused) vertebrae (S1–S5)
- Coccyx: 4 (3–5) (fused) vertebrae (Tailbone)
Shape
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 to 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.
- 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.
- 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.
Ligaments
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. The interspinous ligaments connect the adjoining spinous processes of the vertebrae. 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. From there it is continuous with the nuchal ligament.
Development
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 around 52 somites are formed. 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. 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.
As the sclerotome develops, it condenses further eventually developing into the vertebral body. Development of the appropriate shapes of the vertebral bodies is regulated by 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 nucleus pulposus. The nucleus pulposus and the fibers of 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 spinal canal, formed from a central hole within each vertebra. The spinal cord is part of the central nervous system that supplies nerves and receives information from the peripheral nervous system within the body. The spinal cord consists of grey and white matter and a central cavity, the central canal. Adjacent to each vertebra emerge spinal nerves. The spinal nerves provide sympathetic nervous supply to the body, with nerves emerging forming the sympathetic trunk and the splanchnic nerves.
The spinal canal
follows the different curves of the column; it is large and triangular
in those parts of the column which 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.
The spinal cord terminates in the conus medullaris and cauda equina.
Clinical significance
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.
Another, though rare, congenital disease is Klippel-Feil syndrome which is the fusion of any two of the cervical vertebrae.
Spondylolisthesis is the forward displacement of a vertebra and retrolisthesis is a posterior displacement of one vertebral body with respect to the adjacent vertebra to a degree less than a dislocation.
Spinal disc herniation, more commonly called a "slipped disc", is the result of a tear in the outer ring (anulus fibrosus) of the intervertebral disc, which lets some of the soft gel-like material, the nucleus pulposus, bulge out in a hernia.
Spinal stenosis
is a narrowing of the spinal canal which can occur in any region of
the spine though less commonly in the thoracic region. The stenosis can
constrict the spinal canal giving rise to a neurological deficit.
Pain at the coccyx (tailbone) is known as coccydynia.
Spinal cord injury is damage to the spinal cord that causes changes in its function, either temporary or permanent.
Curvature
Excessive or abnormal spinal curvature is classed as a spinal disease or dorsopathy and includes the following abnormal curvatures:
- Kyphosis is an exaggerated kyphotic (concave) curvature in the thoracic region, also called hyperkyphosis. This produces the so-called "humpback" or "dowager's hump", a condition commonly resulting from osteoporosis.
- Lordosis as an exaggerated lordotic (convex) curvature of the lumbar region, is known as lumbar hyperlordosis and also as "swayback". Temporary lordosis is common during pregnancy.
- Scoliosis, lateral curvature, is the most common abnormal curvature, occurring in 0.5% of the population. It is more common among females and may result from unequal growth of the two sides of one or more vertebrae, 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
Surface projections of organs of the torso. The transpyloric line is seen at L1
Individual vertebrae of the human vertebral column can be felt and used as surface anatomy, with reference points are taken from the middle of the vertebral body. This provides anatomical landmarks that can be used to guide procedures such as a lumbar puncture and also as vertical reference points to describe the locations of other parts of human anatomy, such as the positions of 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 haemal arch or chevron is found underneath the centrum in the caudal (tail) vertebrae of fish, most reptiles, some birds, some dinosaurs and some mammals
with long tails. The vertebral processes can either give the structure
rigidity, help them articulate with ribs, or serve as muscle attachment
points. Common types are transverse process, diapophyses, parapophyses,
and zygapophyses (both the cranial zygapophyses and the caudal
zygapophyses). The centrum of the vertebra can be classified based on
the fusion of its elements. In temnospondyls, bones such as 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.
Regional vertebrae
Vertebrae
are defined by the regions of the vertebral column that they occur in,
as in humans. Cervical vertebrae are those in the neck area. With the exception of the two sloth genera (Choloepus and Bradypus) and the manatee genus, (Trichechus), all mammals have seven cervical vertebrae. In other vertebrates, the number of cervical vertebrae can range from a single vertebra in amphibians, to as many as 25 in swans or 76 in the extinct plesiosaur Elasmosaurus. The dorsal vertebrae range from the bottom of the neck to the top of the pelvis. Dorsal vertebrae attached to the ribs
are called thoracic vertebrae, while those without ribs are called
lumbar vertebrae. The sacral vertebrae are those in the pelvic region,
and range from one in amphibians, to two in most birds and modern
reptiles, or up to three to five in mammals. When multiple sacral
vertebrae are fused into a single structure, it is called the sacrum.
The synsacrum
is a similar fused structure found in birds that is composed of the
sacral, lumbar, and some of the thoracic and caudal vertebra, as well as
the pelvic girdle. Caudal vertebrae compose the tail, and the final few can be fused into the pygostyle in birds, or into the coccygeal or tail bone in 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.
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.
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.
Lampreys have vertebral arches, but nothing resembling the vertebral bodies found in all 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.
Other vertebrates
Vertebral anatomy of a human spine
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.
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.
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.
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.
Dinosaurs
The vertebral column in dinosaurs consists of the cervical (neck), dorsal (back), sacral (hips), and caudal (tail) vertebrae. Saurischian
dinosaur vertebrae sometimes possess features known as pleurocoels,
which are hollow depressions on the lateral portions of the vertebrae,
perforated to create an entrance into the air chambers within the
vertebrae, which served to decrease the weight of these bones without
sacrificing strength. These pleurocoels were filled with air sacs, which
would have further decreased weight. In sauropod
dinosaurs, the largest known land vertebrates, pleurocoels and air sacs
may have reduced the animal's weight by over a ton in some instances, a
handy evolutionary adaption in animals that grew to over 30 metres in
length. In many hadrosaur and theropod
dinosaurs, the caudal vertebrae were reinforced by ossified tendons.
The presence of three or more sacral vertebrae, in association with the
hip bones, is one of the defining characteristics of dinosaurs. The occipital condyle is a structure on the posterior part of a dinosaur's skull which articulates with the first cervical vertebra.
