Psychology Wiki
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{{Biopsy}}
[[Image:Medulla spinalis - Section - Latin.png|thumb|right||Cross-section through cervical spinal cord.]]
 
The '''spinal cord''' is a part of the [[vertebrate]] [[nervous system]] that is enclosed in and protected by the [[spine (anatomy)|vertebral column]] (it passes through the [[spinal canal]]). It consists of [[neuron|nerve cells]]. The cord conveys the 31 [[spinal nerve]] pairs of the [[peripheral nervous system]], as well as [[central nervous system]] pathways that innervate [[skeletal muscle]]s.
 
   
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[[Image:Gray90.png|right|thumb|300px|A diagram of a thoracic vertebra. Notice the [[Bone#Terminology|articulation]]s for the [[rib]]s]]
The vertebral column consists of [[vertebra]]e described as belonging to 5 groups (called segments). These segments are (in order from top to bottom): the [[cervical]], thoracic, and lumbar vertebrae, and the [[sacrum]] and [[coccyx]].
 
   
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A '''vertebra''' (plural: ''vertebrae'') is an individual [[irregular bone]] in the spinal or [[vertebral column]] (''aka'' ischis), a flexuous and flexible column that is a defining characteristic of [[vertebrate]] animals.
== Embryology ==
 
In the human [[fetus]], the spinal cord extends all the way down to the [[sacral vertebrae]]. As a person matures, the spinal cord shortens relative to the rest of the body, so at adulthood, the spinal cord only reaches down to around the level of L1 (the first, i.e. highest, [[lumbar vertebrae|lumbar vertebra]]), where it terminates and the [[cauda equina]] begin - this is why [[lumbar puncture]]s are usually carried out on an adult at the (lower) level of L3/L4.
 
   
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There are normally thirty-three (33) vertebrae in humans, including the five that are fused to form the [[sacrum]] (the others are separated by [[intervertebral disc]]s) and the four [[coccyx|coccygeal]] [[bone]]s which form the ''tailbone''.
== Anatomy ==
 
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The upper three regions comprise the remaining 24, and are grouped under the names ''cervical'' (7 vertebrae), ''thoracic'' (12 vertebrae) and ''lumbar'' (5 vertebrae), according to the regions they occupy.
The spinal cord originates inside the [[brain]] at the inferior end of the [[medulla oblongata]], exiting the [[skull]] via the [[foramen magnum]]. It is wrapped in three layers of membranes, called [[meninges]].
 
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This number is sometimes increased by an additional vertebra in one region, or it may be diminished in one region, the deficiency often being supplied by an additional vertebra in another.
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The number of cervical vertebrae is, however, very rarely increased or diminished.
   
  +
With the exception of the first and second cervical, the true or movable vertebrae (the upper three regions) present certain common characteristics which are best studied by examining one from the middle of the thoracic region.
The spinal cord carries [[sensory]] signals and [[motor]] innervation to most of the [[skeletal muscle]]s in the body. Just about every [[voluntary muscle]] in the body below the head depends on the spinal cord for control. Similarly, most cutaneous sensation below the neck is transmitted via the spinal cord. Most of the [[sympathetic nervous system|sympathetic pathway]]s and the lower (i.e. non-[[vagus nerve|vagal]]) [[parasympathetic nervous system|parasympathetic pathway]]s also go through the spinal cord.
 
   
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==General structure==
A cross-section through the spinal cord reveals that there is a [[central canal]] that carries [[cerebrospinal fluid]] (CSF) surrounded by [[grey matter]] on the inside, and this is surrounded by [[white matter]]. (This is the opposite to the [[brain]]'s [[cerebral cortex]].) A section of the cord can be divided into neat symmeterical halves by the dorsal median sulcus and ventral median fissure.
 
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[[Image:ACDF oblique annotated english.png|thumb|left|300px|Oblique view of cervical vertebrae]]
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A typical vertebra consists of two essential parts: an anterior (front) segment, which is the [[vertebral body]]; and a posterior part – the [[Vertebral arch|vertebral (neural) arch]] – which encloses the [[vertebral foramen]].
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The vertebral arch is formed by a pair of [[Pedicle of vertebral arch|pedicles]] and a pair of [[lamina of the vertebral arch|laminae]], and supports seven [[Process (anatomy)|processes]], four articular, two transverse, and one spinous, the latter also being known as the neural spine.
   
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When the vertebrae are articulated with each other, the bodies form a strong pillar for the support of the [[head (anatomy)|head]] and [[Torso|trunk]], and the vertebral foramina constitute a canal for the protection of the ''medulla spinalis'' ([[spinal cord]]).
The dorsal (towards the back) side of the spinal cord carries sensory information. The neurons that bring somatosensory information to the spinal cord reside in the [[dorsal root ganglion]]. Sensation from the lower body travels up the ''gracile tract'', while sensation from the upper body and arms travels up the ''cuneate tract'', which lies lateral to the gracile tract. There is no cuneate tract in the lumbar part of the [[spinal cord]] as sensory information from the arms does not travel through this area.
 
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In between every pair of vertebrae are two apertures, the [[intervertebral foramina]], one on either side, for the transmission of the spinal nerves and vessels.
   
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Two [[butt hole process]] 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.
Motor information (signals coming from the brain to move the muscles) travels down the ventral (toward the front) half of the spinal cord. [[Motor neuron]]s are located in the anterior (this means close to the front, in humans it means the same as ventral) horn of the grey matter. There are two main columns of neurons in the anterior horn, the ''medial'' and ''lateral motor columns''.
 
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Superior and inferior [[Bone#Terminology|articular]] [[Zygapophyseal joint|facets]] on each vertebra act to restrict the range of movement possible. These facets are joined by a thin portion of the neural arch called the ''[[pars interarticularis]]''.
   
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==Classification==
The spinal cord proper ends at the level of L1. It terminates at a conical point known as the [[conus medullaris]], from which the pia mater extends caudally as the '''''filum terminale''''' and attaches to the dorsal surface of the first cocygeal vertebra. After the termination of the cord, the spinal nerves continue as dangling nerves called the [[cauda equina]] (literally "horse's tail").
 
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The centra of the vertebra can be classified based upon the fusion of its elements. In [[aspidospondyly]], bones such as the [[neural spine]], the [[pleurocentrum]] and the [[intercentrum]] are separate ossifications. Fused elements however, classify a vertebra as having [[holospondyly]].
   
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A vertebra can also be described in terms of the shape of the ends of the centra. Humans are said to be [[acoelous]], or with flat ends. These flat ends of the centra are especially good at supporting and distributing compressive forces. [[Amphicoelus]] vertebra is represented by both ends of the centra being concave. This shape is common in fish, where most motion is limited. Amphicoelus centra often are integrated with a full [[notochord]]. Procoelus vertebra are anteriorly concave, and posteriorly convex. A opisthocoelus vertebra however, is quite the opposite, where the vertebra displays posterior convexity , and anterior concavity. [[Heterocoelous]] vertebrae are saddle shaped at each end of the centra. 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.
The actual cord is approxiamately [[cylinder (geometry)|cylindrical]] in shape, but the diameter varies at different vetebral levels. There are two enlargements, cervical and lumbar. The ''cervical enlargement'' is due to the cord segments from C5 to T1 which innervates the upper limb via the [[brachial plexus]]. The ''[[lumbar]] enlargement'' arise from segments L1 to S3 (only the region around L1/L2 is part of the spinal cord proper) and innervates the lower limbs via the [[lumbar plexus|lumbar]] and [[sacral plexus|sacral]] plexi.
 
   
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==Regions==
There is a higher proportion of white matter in the [[cervical]] (neck) part of the spinal cord. This is because information to and from the whole body (such as the feet) must pass through here. In contrast, the lumbar and sacral areas do not carry information from anywhere above them, so have less white matter.
 
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[[Image:Orientation.PNG|thumb|right|300px|Orientation of vertebral column on surface. T3 is at level of medial part of [[spine of scapula]]. T7 is at [[inferior angle of the scapula]]. L4 is at highest point of [[iliac crest]]. S2 is at the level of [[posterior superior iliac spine]]. Furthermore, C7 is easily localized as a prominence at the lower part of the neck.<ref>Anatomy Compendium (Godfried Roomans and Anca Dragomir)</ref>]]
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=== Cervical ===
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{{main|Cervical vertebrae}}
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These are generally small and delicate. Their spinous processes are short (with the exception of C2 and C7, which have palpable spinous processes). Numbered top-to-bottom from C1-C7, [[atlas (anatomy)|atlas]] (C1) and [[axis (anatomy)|axis]] (C2), are the vertebrae that allow the [[neck]] and [[head]] so much movement. For the most part, the [[atlanto-occipital joint]] allows the [[skull]] to move up and down, while the [[atlanto-axial joint]] allows the upper neck to twist left and right. The axis also sits upon the first intervertebral disk of the spinal column. All [[mammal]]s except [[manatee]]s and [[sloth]]s have seven cervical vertebrae, whatever the length of the neck.
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Cervical vertebrae possess transverse foramina to allow for the vertebral arteries to pass through on their way to the [[foramen magnum]] to end in the [[circle of Willis]]. These are the smallest, lightest vertebrae and the vertebral foramina are triangular in shape. The spinous processes are short and often [[bifurcated]] (the spinous process of C7, however, is not bifurcated, and is substantially longer than that of the other cervical spinous processes).
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=== Thoracic ===
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{{main|Thoracic vertebrae}}
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Their transverse processes have surfaces that [[Bone#Terminology|articulate]] with the [[rib]]s. Some rotation can occur between the thoracic vertebrae, but their connection with the rib cage prevents much ''flexion'' or other excursion. They may also be known as 'dorsal vertebrae', in the human context.
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Bodies are roughly heart-shaped and are about as wide anterio-posterioly as they are in the transverse dimension. Vertebral foramina are roughly circular in shape.
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=== Lumbar ===
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{{main|Lumbar vertebrae}}
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These vertebrae are very robust in construction, as they must support more weight than other vertebrae. They allow significant ''flexion'' and ''extension'', moderate lateral flexion (sidebending), and a small degree of rotation. The discs between these vertebrae create a [[lumbar]] lordosis (curvature that is concave posteriorly) in the human spine.
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[[Image:Illu vertebral column.jpg|thumb|right|250px]]
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=== Sacral ===
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{{main|Sacral vertebrae}}
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There are 5 vertebrae (S1-S5). They are fused in maturity, with no [[intervertebral disc]]s.
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=== Coccygeal ===
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{{main|Coccygeal vertebrae}}
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There are 3-5 vertebrae (Co1-Co5), with no [[intervertebral disc]]s. Many animals have a greater number of "tail vertebrae" and, in animals, they are more commonly known as "caudal vertebrae." Pain at the [[coccyx|coccyx (tailbone)]] is known as [[coccydynia]].
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== Development ==
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The striking segmented pattern of the human spine is established during embryogenesis when the precursor of the vertebrae, the somites, are rhythmically added to the forming posterior part of the embryo. In humans, somite formation begins around the third week post-fertilization and continues until a total of around 52 somites are formed. The somites are epithelial spheres that contain the precursors of the vertebrae, the ribs, the skeletal muscles of the body wall and limbs, and the dermis of the back. The periodicity of somite distribution and production is thought to be imposed by a molecular oscillator or clock acting in cells of the presomitic mesoderm (PSM). Somites form soon after the beginning of gastrulation, on both sides of the neural tube from a tissue called the presomitic mesoderm (PSM). The PSM is part of the paraxial mesoderm and is generated caudally by gastrulation when cells ingress through the primitive streak, and later, through the tail bud. Soon after their formation, somites become subdivided into the dermomyotome dorsally, which gives rise to the muscles and dermis, and the sclerotome ventrally which will form the spine components. Sclerotomes become subvidided into an anterior and a posterior compartment. This subdivision plays a key role in the definitive patterning of vertebrae which 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 human patients with human congenital scoliosis suggesting that the mechanisms involved in vertebral segmentation are conserved across vertebrates. In humans the first four somites are incoporated in the basi-occipital bone of the skull and the next 33 somites will form the vertebrae. The remaining posterior somites degenerate. During the fourth week of [[embryonic development]], the [[sclerotome]]s shift their position to surround the [[spinal cord]] and the [[notochord]]. The sclerotome is made of [[germ layer#Mesoderm|mesoderm]] and originates from the ventromedial part of the [[somite]]s. This column of tissue has a segmented appearance, with alternating areas of dense and less dense areas.
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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'' [[gene]]s.
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The less dense tissue that separates the sclerotome segments develop into the [[intervertebral disc]]s.
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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 [[annulus fibrosus disci intervertebralis|annulus fibrosus]] make up the intervertebral disc.
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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.
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There are various defects associated with vertebral development. [[Scoliosis]] can result from improper fusion of the vertebrae. In [[Klippel-Feil anomaly]] patients have two or more cervical vertebrae that are fused together, along with other associated birth defects. One of the most serious defects is failure of the vertebral arches to fuse. This results in a condition called [[spina bifida]]. There are several variations of spina bifida that reflect the severity of the defect.
   
 
== Pathology ==
 
== Pathology ==
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[[ca:Vèrtebra]]
[[fr:Moelle épinière]]
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[[cs:Obratel]]
[[is:Mæna]]
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[[cy:Fertibra]]
[[he:חוט השדרה]]
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[[de:Wirbel (Anatomie)]]
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[[eo:Vertebro]]
[[ja:脊髄]]
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[[pl:Rdzeń kręgowy]]
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[[pt:Medula espinal]]
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[[uk:Спинний мозок]]
 
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[[hu:Gerinc]]
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[[nl:Wervel (anatomie)]]
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{{enWP| Spinal_cord}}
 
{{enWP| Spinal_cord}}

Revision as of 08:16, 7 November 2008

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File:Gray90.png

A diagram of a thoracic vertebra. Notice the articulations for the ribs

A vertebra (plural: vertebrae) is an individual irregular bone in the spinal or vertebral column (aka ischis), a flexuous and flexible column that is a defining characteristic of vertebrate animals.

There are normally thirty-three (33) vertebrae in humans, including the five that are fused to form the sacrum (the others are separated by intervertebral discs) and the four coccygeal bones which form the tailbone. The upper three regions comprise the remaining 24, and are grouped under the names cervical (7 vertebrae), thoracic (12 vertebrae) and lumbar (5 vertebrae), according to the regions they occupy. This number is sometimes increased by an additional vertebra in one region, or it may be diminished in one region, the deficiency often being supplied by an additional vertebra in another. The number of cervical vertebrae is, however, very rarely increased or diminished.

With the exception of the first and second cervical, the true or movable vertebrae (the upper three regions) present certain common characteristics which are best studied by examining one from the middle of the thoracic region.

General structure

File:ACDF oblique annotated english.png

Oblique view of cervical vertebrae

A typical vertebra consists of two essential parts: an anterior (front) segment, which is the vertebral body; and a posterior part – the vertebral (neural) arch – which encloses the vertebral 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.

When the vertebrae are articulated with each other, the bodies form a strong pillar for the support of the head and trunk, and the vertebral foramina constitute a canal for the protection of the medulla spinalis (spinal cord). In between every pair of vertebrae are two apertures, the intervertebral foramina, one on either side, for the transmission of the spinal nerves and vessels.

Two butt hole process 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. Superior and inferior articular facets on each vertebra act to restrict the range of movement possible. These facets are joined by a thin portion of the neural arch called the pars interarticularis.

Classification

The centra of the vertebra can be classified based upon the fusion of its elements. In aspidospondyly, bones such as the neural spine, 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 centra. Humans are said to be acoelous, or with flat ends. These flat ends of the centra are especially good at supporting and distributing compressive forces. Amphicoelus vertebra is represented by both ends of the centra being concave. This shape is common in fish, where most motion is limited. Amphicoelus centra often are integrated with a full notochord. Procoelus vertebra are anteriorly concave, and posteriorly convex. A opisthocoelus vertebra however, is quite the opposite, where the vertebra displays posterior convexity , and anterior concavity. Heterocoelous vertebrae are saddle shaped at each end of the centra. 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.

Regions

File:Orientation.PNG

Orientation of vertebral column on surface. T3 is at level of medial part of spine of scapula. T7 is at inferior angle of the scapula. L4 is at highest point of iliac crest. S2 is at the level of posterior superior iliac spine. Furthermore, C7 is easily localized as a prominence at the lower part of the neck.[1]

Cervical

Main article: Cervical vertebrae

These are generally small and delicate. Their spinous processes are short (with the exception of C2 and C7, which have palpable spinous processes). Numbered top-to-bottom from C1-C7, atlas (C1) and axis (C2), are the vertebrae that allow the neck and head so much movement. For the most part, the atlanto-occipital joint allows the skull to move up and down, while the atlanto-axial joint allows the upper neck to twist left and right. The axis also sits upon the first intervertebral disk of the spinal column. All mammals except manatees and sloths have seven cervical vertebrae, whatever the length of the neck.

Cervical vertebrae possess transverse foramina to allow for the vertebral arteries to pass through on their way to the foramen magnum to end in the circle of Willis. These are the smallest, lightest vertebrae and the vertebral foramina are triangular in shape. The spinous processes are short and often bifurcated (the spinous process of C7, however, is not bifurcated, and is substantially longer than that of the other cervical spinous processes).

Thoracic

Main article: Thoracic vertebrae

Their transverse processes have surfaces that articulate with the ribs. Some rotation can occur between the thoracic vertebrae, but their connection with the rib cage prevents much flexion or other excursion. They may also be known as 'dorsal vertebrae', in the human context.

Bodies are roughly heart-shaped and are about as wide anterio-posterioly as they are in the transverse dimension. Vertebral foramina are roughly circular in shape.

Lumbar

Main article: Lumbar vertebrae

These vertebrae are very robust in construction, as they must support more weight than other vertebrae. They allow significant flexion and extension, moderate lateral flexion (sidebending), and a small degree of rotation. The discs between these vertebrae create a lumbar lordosis (curvature that is concave posteriorly) in the human spine.

File:Illu vertebral column.jpg

250px

Sacral

Main article: Sacral vertebrae

There are 5 vertebrae (S1-S5). They are fused in maturity, with no intervertebral discs.

Coccygeal

Main article: Coccygeal vertebrae

There are 3-5 vertebrae (Co1-Co5), with no intervertebral discs. Many animals have a greater number of "tail vertebrae" and, in animals, they are more commonly known as "caudal vertebrae." Pain at the coccyx (tailbone) is known as coccydynia.

Development

The striking segmented pattern of the human spine is established during embryogenesis when the precursor of the vertebrae, the somites, are rhythmically added to the forming posterior part of the embryo. In humans, somite formation begins around the third week post-fertilization and continues until a total of around 52 somites are formed. The somites are epithelial spheres that contain the precursors of the vertebrae, the ribs, the skeletal muscles of the body wall and limbs, and the dermis of the back. The periodicity of somite distribution and production is thought to be imposed by a molecular oscillator or clock acting in cells of the presomitic mesoderm (PSM). Somites form soon after the beginning of gastrulation, on both sides of the neural tube from a tissue called the presomitic mesoderm (PSM). The PSM is part of the paraxial mesoderm and is generated caudally by gastrulation when cells ingress through the primitive streak, and later, through the tail bud. Soon after their formation, somites become subdivided into the dermomyotome dorsally, which gives rise to the muscles and dermis, and the sclerotome ventrally which will form the spine components. Sclerotomes become subvidided into an anterior and a posterior compartment. This subdivision plays a key role in the definitive patterning of vertebrae which 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 human patients with human congenital scoliosis suggesting that the mechanisms involved in vertebral segmentation are conserved across vertebrates. In humans the first four somites are incoporated in the basi-occipital bone of the skull and the next 33 somites will form the vertebrae. The remaining posterior somites degenerate. During the fourth week of embryonic development, the sclerotomes shift their position to surround the spinal cord and the notochord. The sclerotome is made of mesoderm and originates from the ventromedial part of the somites. 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 annulus 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.

There are various defects associated with vertebral development. Scoliosis can result from improper fusion of the vertebrae. In Klippel-Feil anomaly patients have two or more cervical vertebrae that are fused together, along with other associated birth defects. One of the most serious defects is failure of the vertebral arches to fuse. This results in a condition called spina bifida. There are several variations of spina bifida that reflect the severity of the defect.

Pathology

See also


Nervous system

Brain - Spinal cord - Central nervous system - Peripheral nervous system - Somatic nervous system - Autonomic nervous system - Sympathetic nervous system - Parasympathetic nervous system

External links

This page uses Creative Commons Licensed content from Wikipedia (view authors).
  1. Anatomy Compendium (Godfried Roomans and Anca Dragomir)