Spinal cord
Spinal cord | |
---|---|
Details | |
Part of | Central nervous system |
Artery | spinal artery |
Vein | spinal vein |
Identifiers | |
Latin | medulla spinalis |
MeSH | D013116 |
NeuroNames | 22 |
TA98 | A14.1.02.001 |
TA2 | 6049 |
FMA | 7647 |
Anatomical terminology |
The spinal cord is a long, thin, tubular structure made up of
In humans, the spinal cord is a continuation of the brainstem and anatomically begins at the occipital bone, passing out of the foramen magnum and then enters the spinal canal at the beginning of the cervical vertebrae. The spinal cord extends down to between the first and second lumbar vertebrae, where it tapers to become the caudal equina. The enclosing bony vertebral column protects the relatively shorter spinal cord. It is around 45 cm (18 in) long in adult men and around 43 cm (17 in) long in adult women. The diameter of the spinal cord ranges from 13 mm (1⁄2 in) in the cervical and lumbar regions to 6.4 mm (1⁄4 in) in the thoracic area.
The spinal cord functions primarily in the transmission of nerve signals from the motor cortex to the body, and from the afferent fibers of the sensory neurons to the sensory cortex. It is also a center for coordinating many reflexes and contains reflex arcs that can independently control reflexes.[1] It is also the location of groups of spinal interneurons that make up the neural circuits known as central pattern generators. These circuits are responsible for controlling motor instructions for rhythmic movements such as walking.[2]
Structure
1 | central canal |
2 | posterior median sulcus |
3 | gray matter |
4 | white matter |
5 | dorsal root (left), dorsal root ganglion (right) |
6 | ventral root |
7 | fascicles |
8 | anterior spinal artery |
9 | arachnoid mater |
10 | dura mater |
The spinal cord is the main pathway for information connecting the brain and
It is about 45 centimetres (18 inches) long in males and about 43 cm (17 in) in females, ovoid-shaped, and is enlarged in the cervical and lumbar regions. The cervical enlargement, stretching from the C5 to T1 vertebrae, is where sensory input comes from and motor output goes to the arms and trunk. The lumbar enlargement, located between L1 and S3, handles sensory input and motor output coming from and going to the legs.
The spinal cord is continuous with the caudal portion of the medulla, running from the base of the skull to the body of the first lumbar vertebra. It does not run the full length of the vertebral column in adults. It is made of 31 segments from which branch one pair of sensory nerve roots and one pair of motor nerve roots. The nerve roots then merge into bilaterally symmetrical pairs of spinal nerves. The peripheral nervous system is made up of these spinal roots, nerves, and ganglia.
The dorsal roots are afferent
The spinal cord (and brain) are protected by three layers of tissue or membranes called
In cross-section, the peripheral region of the cord contains neuronal white matter tracts containing sensory and motor axons. Internal to this peripheral region is the grey matter, which contains the nerve cell bodies arranged in the three grey columns that give the region its butterfly-shape. This central region surrounds the central canal, which is an extension of the fourth ventricle and contains cerebrospinal fluid.
The spinal cord is elliptical in cross section, being compressed dorsolaterally. Two prominent grooves, or sulci, run along its length. The posterior median sulcus is the groove in the dorsal side, and the anterior median fissure is the groove in the ventral side.
Segments
The human spinal cord is divided into segments where pairs of spinal nerves (mixed; sensory and motor) form. Six to eight motor nerve rootlets branch out of right and left ventralateral sulci in a very orderly manner. Nerve rootlets combine to form nerve roots. Likewise, sensory nerve rootlets form off right and left dorsal lateral sulci and form sensory nerve roots. The ventral (motor) and dorsal (sensory) roots combine to form spinal nerves (mixed; motor and sensory), one on each side of the spinal cord. Spinal nerves, with the exception of C1 and C2, form inside the intervertebral foramen. These rootlets form the demarcation between the central and peripheral nervous systems.[citation needed]
Generally, the spinal cord segments do not correspond to bony vertebra levels. As the spinal cord terminates at the L1-L2 level, other segments of the spinal cord would be positioned superior to their corresponding bony vertebral body. For example, the T11 spinal segment is located higher than the T11 bony vertebra, and the sacral spinal cord segment is higher than the L1 vertebral body.[6]
The
The white matter is located outside of the grey matter and consists almost totally of
The spinal cord proper terminates in a region called the conus medullaris, while the pia mater continues as an extension called the filum terminale, which anchors the spinal cord to the coccyx. The cauda equina ("horse's tail") is a collection of nerves inferior to the conus medullaris that continue to travel through the vertebral column to the coccyx. The cauda equina forms because the spinal cord stops growing in length at about age four, even though the vertebral column continues to lengthen until adulthood. This results in sacral spinal nerves originating in the upper lumbar region. For that reason, the spinal cord occupies only two-thirds of the vertebral canal. The inferior part of the vertebral canal is filled with cerebrospinal fluid and the space is called the lumbar cistern.[7]
Within the central nervous system (CNS), nerve cell bodies are generally organized into functional clusters, called nuclei. Axons within the CNS are grouped into tracts.
There are 31 spinal cord nerve segments in a human spinal cord:
- 8 cervical segments forming 8 pairs of cervical nerves(C1 spinal nerves exit the spinal column between the foramen magnum and the C1 vertebra; C2 nerves exit between the posterior arch of the C1 vertebra and the lamina of C2; C3–C8 spinal nerves pass through the intervertebral foramen above their corresponding cervical vertebrae, with the exception of the C8 pair which exit between the C7 and T1 vertebrae)
- 12 thoracic segments forming 12 pairs of thoracic nerves
- 5 lumbar segments forming 5 pairs of lumbar nerves
- 5 sacral segments forming 5 pairs of sacral nerves
- 1 coccygeal segment
Species | Cervical | Thoracic | Lumbar | Sacral | Caudal/Coccygeal | Total |
---|---|---|---|---|---|---|
Dog | 8 | 13 | 7 | 3 | 5 | 36 |
Cat | 8 | 13 | 7 | 3 | 5 | 36 |
Cow | 8 | 13 | 6 | 5 | 5 | 37 |
Horse | 8 | 18 | 6 | 5 | 5 | 42 |
Pig | 8 | 15/14 | 6/7 | 4 | 5 | 38 |
Human | 8 | 12 | 5 | 5 | 1 | 31 |
Mouse[9] | 8 | 13 | 6 | 4 | 3 | 35 |
In the fetus, vertebral segments correspond with spinal cord segments. However, because the vertebral column grows longer than the spinal cord, spinal cord segments do not correspond to vertebral segments in the adult, particularly in the lower spinal cord. For example, lumbar and sacral spinal cord segments are found between vertebral levels T9 and L2, and the spinal cord ends around the L1/L2 vertebral level, forming a structure known as the conus medullaris.
Although the spinal cord cell bodies end around the L1/L2 vertebral level, the spinal nerves for each segment exit at the level of the corresponding vertebra. For the nerves of the lower spinal cord, this means that they exit the vertebral column much lower (more caudally) than their roots. As these nerves travel from their respective roots to their point of exit from the vertebral column, the nerves of the lower spinal segments form a bundle called the cauda equina.
There are two regions where the spinal cord enlarges:
- Cervical enlargement – corresponds roughly to the brachial plexus nerves, which innervate the upper limb. It includes spinal cord segments from about C4 to T1. The vertebral levels of the enlargement are roughly the same (C4 to T1).
- lower limb. It comprises the spinal cord segments from L2 to S3 and is found about the vertebral levels of T9 to T12.
Development
The spinal cord is made from part of the
Earlier findings by Viktor Hamburger and Rita Levi-Montalcini in the chick embryo have been confirmed by more recent studies which have demonstrated that the elimination of neuronal cells by programmed cell death is necessary for the correct assembly of the nervous system.[14]
Overall, spontaneous embryonic activity has been shown to play a role in neuron and muscle development but is probably not involved in the initial formation of connections between spinal neurons.
Blood supply
The spinal cord is supplied with blood by three arteries that run along its length starting in the brain, and many arteries that approach it through the sides of the spinal column. The three longitudinal arteries are the
The major contribution to the arterial blood supply of the spinal cord below the cervical region comes from the radially arranged posterior and anterior radicular arteries, which run into the spinal cord alongside the dorsal and ventral nerve roots, but with one exception do not connect directly with any of the three longitudinal arteries.[15] These intercostal and lumbar radicular arteries arise from the aorta, provide major anastomoses and supplement the blood flow to the spinal cord. In humans the largest of the anterior radicular arteries is known as the artery of Adamkiewicz, or anterior radicularis magna (ARM) artery, which usually arises between L1 and L2, but can arise anywhere from T9 to L5.[16] Impaired blood flow through these critical radicular arteries, especially during surgical procedures that involve abrupt disruption of blood flow through the aorta for example during aortic aneurysm repair, can result in spinal cord infarction and paraplegia.
Function
This section needs additional citations for verification. (January 2024) |
Somatosensory organization
In the
The proprioception of the lower limbs differs from the upper limbs and upper trunk. There is a four-neuron pathway for lower limb proprioception. This pathway initially follows the dorsal spino-cerebellar pathway. It is arranged as follows: proprioceptive receptors of lower limb → peripheral process → dorsal root ganglion → central process →
The anterolateral system works somewhat differently. Its primary neurons axons enter the spinal cord and then ascend one to two levels before synapsing in the
Some of the "pain fibers" in the ALS deviate from their pathway towards the VPLN. In one such deviation, axons travel towards the
Motor organization
Level | Motor function |
---|---|
C1–C6 | flexors
|
C1–T1 | extensors
|
C3, C4, C5 | Supply diaphragm (mostly C4) |
C5, C6 | Move shoulder, raise arm (deltoid); flex elbow (biceps) |
C6 | externally rotate ( supinate ) the arm
|
C6, C7 | pronate wrist
|
C7, C8 | Flex wrist; supply small muscles of the hand |
T1–T6 | |
L1
|
Abdominal muscles |
L4
|
Flex hip joint
|
L4
|
quadriceps femoris )
|
L5, S1
|
tibialis anterior); Extend toes
|
Extend leg at the hip (gluteus maximus); flex foot and flex toes |
The corticospinal tract serves as the motor pathway for upper motor neuronal signals coming from the cerebral cortex and from primitive brainstem motor nuclei.
Cortical upper motor neurons originate from
The midbrain nuclei include four motor tracts that send upper motor neuronal axons down the spinal cord to lower motor neurons. These are the
The function of lower motor neurons can be divided into two different groups: the lateral corticospinal tract and the anterior cortical spinal tract. The lateral tract contains upper motor neuronal
The anterior corticospinal tract descends
Spinocerebellar tracts
From the levels of L2 to T1, proprioceptive information enters the spinal cord and ascends ipsilaterally, where it synapses in
From above T1, proprioceptive primary axons enter the spinal cord and ascend ipsilaterally until reaching the
Motor information travels from the brain down the spinal cord via descending spinal cord tracts. Descending tracts involve two neurons: the upper motor neuron (UMN) and lower motor neuron (LMN).[17] A nerve signal travels down the upper motor neuron until it synapses with the lower motor neuron in the spinal cord. Then, the lower motor neuron conducts the nerve signal to the spinal root where efferent nerve fibers carry the motor signal toward the target muscle. The descending tracts are composed of white matter. There are several descending tracts serving different functions. The corticospinal tracts (lateral and anterior) are responsible for coordinated limb movements.[17]
Clinical significance
A
Injury
Spinal cord injuries can be caused by trauma to the spinal column (stretching, bruising, applying pressure, severing, laceration, etc.). The vertebral bones or
Damage to upper motor neuron axons in the spinal cord results in a characteristic pattern of ipsilateral deficits. These include
Spinal shock and neurogenic shock can occur from a spinal injury. Spinal shock is usually temporary, lasting only for 24–48 hours, and is a temporary absence of sensory and motor functions. Neurogenic shock lasts for weeks and can lead to a loss of muscle tone due to disuse of the muscles below the injured site.
The two areas of the spinal cord most commonly injured are the
Globally, it is expected there are around 40 to 80 cases of spinal cord injury per million population, and approximately 90% of these cases result from traumatic events.[19]
Real or suspected spinal cord injuries need immediate immobilisation including that of the head. Scans will be needed to assess the injury. A steroid, methylprednisolone, can be of help as can physical therapy and possibly antioxidants.[citation needed] Treatments need to focus on limiting post-injury cell death, promoting cell regeneration, and replacing lost cells. Regeneration is facilitated by maintaining electric transmission in neural elements.
Stenosis
Spinal stenosis at the lumbar region are usually due to
Tumours
Spinal tumours can occur in the spinal cord and these can be either inside (intradural) or outside (extradural) the dura mater.
Procedures
The spinal cord ends at the level of vertebrae L1–L2, while the
Additional images
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Spinal Cord Sectional Anatomy. Animation in the reference.
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Diagrams of the spinal cord
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Cross-section through the spinal cord at the mid-thoracic level
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Cross-sections of the spinal cord at varying levels
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Cervical vertebra
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A portion of the spinal cord, showing its right lateral surface. The dura is opened and arranged to show the nerve roots.
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The spinal cord with dura cut open, showing the exits of the spinal nerves
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The spinal cord showing how the anterior and posterior roots join in the spinal nerves
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The spinal cord showing how the anterior and posterior roots join in the spinal nerves
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A longer view of the spinal cord
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Projections of the spinal cord into the nerves (red motor, blue sensory)
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Projections of the spinal cord into the nerves (red motor, blue sensory)
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Cross-section of rabbit spinal cord
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Cross section of adult rat spinal cord stained using Cajal method
Dissection images |
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See also
- Brown-Séquard syndrome
- Hereditary spastic paraplegia (HSP, or familial spastic paraplegia – FSP, Strümpell–Lorrain syndrome)
- Myelomere
- Neutral spine
- Poliomyelitis
- Post-polio syndrome
- Redlich–Obersteiner's zone
- Subacute combined degeneration of spinal cord
- Tethered spinal cord syndrome
- Upper-limb surgery in tetraplegia
References
- ISBN 978-0-13-981176-0.
- PMID 23403923.
- ISBN 978-1429216357.
- ISBN 978-0-12-385-870-2.
- ^ Purves, D; Augustine, GJ; Fitzpatrick, D (2001). "The Internal Anatomy of the Spinal Cord". Neuroscience (Second ed.). Sunderland, UK: Sinauer Associates. Archived from the original on 5 October 2019. Retrieved 20 March 2022.
- S2CID 21809666.
- ISBN 978-0-12-385158-1, archivedfrom the original on 2021-04-25, retrieved 2020-10-21
- ^ "Spinal Cord Gross Anatomy". Archived from the original on December 21, 2015. Retrieved December 27, 2015.
- from the original on 21 June 2022. Retrieved 21 June 2022.
- ^ Kaufman, Bard. "Spinal Cord – Development and Stem Cells". Life Map Discovery Compendium. Archived from the original on 29 June 2020. Retrieved 12 Dec 2015.
- ^ Kaufman, Bard. "Spinal Cord-Development and Stem Cells". Stem Cell Development Compendium. Archived from the original on 29 June 2020. Retrieved 2 Dec 2015.
- S2CID 37162863.
- ISBN 9780072907865.
- S2CID 6747529.
- ^ ISBN 978-0-7817-6274-8.
- (PDF) from the original on 2023-03-05. Retrieved 2019-09-03.
- ^ a b Saladin. Anatomy and Physiology, 5th Ed.
- ^ ISBN 9780071831420.
- ^ "Spinal cord injury". www.who.int. Archived from the original on 2022-03-25. Retrieved 2022-03-25.
- from the original on 2022-08-01. Retrieved 2022-06-21.
External links
- Spinal Cord Histology – A multitude of great images from the University of Cincinnati
- "The Nervous System: Sensory and Motor Tracts of the Spinal Cord" (PDF). Napa Valley College / Southeast Community College Lincoln, Nebraska. Archived from the original (PDF) on 3 May 2021. Retrieved 20 May 2013.
- eMedicine: Spinal Cord, Topographical and Functional Anatomy
- WebMD. May 17, 2005. Spina Bifida – Topic Overview Information about spina bifida in fetuses and throughout adulthood. WebMD children's health. Retrieved March 19, 2007.
- Potential for spinal injury repair Retrieved February 6, 2008.
- 4000 sets of digital images, showing spatial expression patterns for various genes in adult and juvenile mouse spinal cords from the Allen Institute for Brain Science
- Spinal cord photomicrographs