Spinal cord
Encyclopedia
The spinal cord is a long, thin, tubular bundle of nervous tissue
and support cells that extends from the brain
(the medulla oblongata
specifically). The brain and spinal cord together make up the central nervous system
. The spinal cord begins at the occipital bone
and extends down to the space between the first and second lumbar vertebrae
; it does not extend the entire length of the vertebral column
. It is around 45 cm (17.7 in) in men and around 43 cm (16.9 in) long in women. Also, the spinal cord has a varying width, ranging from 1/2 inch thick in the cervical and lumbar regions to 1/4 inch thick in the thoracic area. The enclosing bony vertebral column protects the relatively shorter spinal cord. The spinal cord functions primarily in the transmission of neural signals between the brain
and the rest of the body but also contains neural circuits that can independently control numerous reflex
es and central pattern generator
s.
The spinal cord has three major functions:
A. Serve as a conduit for motor information, which travels down the spinal cord.
B. Serve as a conduit for sensory information, which travels up the spinal cord.
C. Serve as a center for coordinating certain reflexes.
near the second lumbar vertebra, terminating in a fibrous extension known as the filum terminale
.
It is about 45 cm (17.7 in) long in men and around 43 cm (16.9 in) in women, ovoid-shaped, and is enlarged in the cervical and lumbar regions. The cervical enlargement, located from C3 to T2 spinal segments, is where sensory input comes from and motor output goes to the arms. The lumbar enlargement, located between L1 and S3 spinal segments, handles sensory input and motor output coming from and going to the legs.
The spinal cord is protected by three layers of tissue, called spinal meninges
, that surround the canal. The dura mater
is the outermost layer, and it forms a tough protective coating. Between the dura mater and the surrounding bone of the vertebrae is a space called the epidural space
. The epidural space is filled with adipose tissue
, and it contains a network of blood vessel
s. The arachnoid mater
is the middle protective layer. Its name comes from the fact that the tissue has a spiderweb-like appearance. The space between the arachnoid and the underlyng pia mater
is called the subarachnoid space
. The subarachnoid space contains cerebrospinal fluid
(CSF). The medical procedure known as a lumbar puncture
(or spinal tap) involves use of a needle to withdraw cerebrospinal fluid from the subarachnoid space, usually from the lumbar
region of the spine. The pia mater is the innermost protective layer. It is very delicate and it is tightly associated with the surface of the spinal cord. The cord is stabilized within the dura mater by the connecting denticulate ligaments
, which extend from the enveloping pia mater laterally between the dorsal and ventral roots. The dural sac ends at the vertebral level of the second sacral
vertebra.
In cross-section, the peripheral region of the cord contains neuronal white matter
tracts containing sensory
and motor neuron
s. Internal to this peripheral region is the gray, butterfly-shaped central region made up of nerve cell bodies
. This central region surrounds the central canal
, which is an anatomic extension of the spaces in the brain known as the ventricles
and, like the ventricles, contains cerebrospinal fluid.
The spinal cord has a shape that is compressed dorso-ventrally, giving it an elliptical shape. The cord has grooves in the dorsal and ventral sides. The posterior median sulcus is the groove in the dorsal side, and the anterior median fissure is the groove in the ventral side.
s (mixed; motor and sensory), one on each side of the spinal cord. Spinal nerves, with the exception of C1 and C2, form inside intervertebral foramen (IVF). Note that at each spinal segment, the border between the central and peripheral nervous system can be observed. Rootlets are a part of the peripheral nervous system.
In the upper part of the vertebral column, spinal nerves exit directly from the spinal cord, whereas in the lower part of the vertebral column nerves pass further down the column before exiting. The terminal portion of the spinal cord is called the conus medullaris
. The dura mater
continues as an extension called the filum terminale
, which anchors the spinal cord to the coccyx. The cauda equina
(“horse’s tail”) is the name for the collection of nerves in the vertebral column that continue to travel through the vertebral column below the conus medullaris
. The cauda equina forms as a result of the fact that the spinal cord stops growing in length at about age four, even though the vertebral column continues to lengthen until adulthood. This results in the fact that sacral spinal nerves actually originate in the upper lumbar region. The spinal cord can be anatomically divided into 31 spinal segments based on the origins of the spinal nerves.
Each segment of the spinal cord is associated with a pair of ganglia, called dorsal root ganglia, which are situated just outside of the spinal cord. These ganglia contain cell bodies of sensory neurons. Axons of these sensory neurons travel into the spinal cord via the dorsal roots.
Ventral roots consist of axons from motor neurons, which bring information to the periphery from cell
bodies within the CNS. Dorsal roots and ventral roots come together and exit the intervertebral
foramina as they become spinal nerves.
The gray matter, in the center of the cord, is shaped like a butterfly and consists of cell bodies of
interneurons and motor neurons. It also consists of neuroglia cells and unmyelinated axons.
Projections of the gray matter (the “wings”) are called horns. Together, the gray horns and the gray
commissure
form the “gray H.”
The white matter is located outside of the gray matter and consists almost totally of myelinated motor
and sensory axons. “Columns” of white matter carry information either up or down the
spinal cord.
Within the CNS, nerve cell bodies are generally organized into functional clusters, called nuclei. Axons
within the CNS are grouped into tracts.
There are 33 spinal cord nerve segments in a human spinal cord:
Because the vertebral column grows longer than the spinal cord, spinal cord segments do not correspond to vertebral segments in adults, especially in the lower spinal cord. In the fetus, vertebral segments do correspond with spinal cord segments. In the adult, however, the spinal cord ends around the L1/L2 vertebral level, forming a structure known as the conus medullaris
. For example, lumbar and sacral spinal cord segments are found between vertebral levels T9 and L2.
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:
during development. As the neural tube begins to develop, the notochord
begins to secrete a factor known as Sonic hedgehog
or SHH. As a result, the floor plate
then also begins to secrete SHH, and this will induce the basal plate to develop motor neurons. Meanwhile, the overlying ectoderm
secretes bone morphogenetic protein
(BMP). This induces the roof plate to begin to secrete BMP, which will induce the alar plate
to develop sensory neurons. The alar plate and the basal plate are separated by the sulcus limitans
.
Additionally, the floor plate also secretes netrin
s. The netrins act as chemoattractants to decussation
of pain and temperature sensory neurons in the alar plate across the anterior white commissure, where they then ascend towards the thalamus
.
Lastly, it is important to note that the past studies of Viktor Hamburger and Rita Levi-Montalcini in the chick embryo have been further proven by more recent studies which demonstrated that the elimination of neuronal cells by programmed cell death
(PCD) is necessary for the correct assembly of the nervous system.
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.
, and the right and left posterior spinal arteries
. These travel in the subarachnoid space and send branches into the spinal cord. They form anastamoses (connections) via the anterior and posterior segmental medullary arteries
, which enter the spinal cord at various points along its length. The actual blood flow caudally through these arteries, derived from the posterior cerebral circulation, is inadequate to maintain the spinal cord beyond the cervical segments.
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. 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. 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 aneursym repair, can result in spinal cord infarction and paraplegia.
/vibration sensory pathway) and the anterolateral system
, or ALS (the pain/temperature sensory pathway). Both sensory pathways use three different neurons to get information from sensory receptors at the periphery to the cerebral cortex
. These neurons are designated primary, secondary and tertiary sensory neurons. In both pathways, primary sensory neuron cell bodies are found in the dorsal root ganglia, and their central axons project into the spinal cord.
In the dorsal column-medial leminiscus tract, a primary neuron's axon enters the spinal cord and then enters the dorsal column. If the primary axon enters below spinal level T6, the axon travels in the fasciculus gracilis
, the medial part of the column. If the axon enters above level T6, then it travels in the fasciculus cuneatus
, which is lateral to the fasiculus gracilis. Either way, the primary axon ascends to the lower medulla
, where it leaves its fasiculus and synapses with a secondary neuron in one of the dorsal column nuclei: either the nucleus gracilis or the nucleus cuneatus, depending on the pathway it took. At this point, the secondary axon leaves its nucleus and passes anteriorly and medially. The collection of secondary axons that do this are known as internal arcuate fibers
. The internal arcuate fibers decussate and continue ascending as the contralateral medial lemniscus
. Secondary axons from the medial lemniscus finally terminate in the ventral posterolateral nucleus
(VPL) of the thalamus
, where they synapse with tertiary neurons. From there, tertiary neurons ascend via the posterior limb of the internal capsule
and end in the primary sensory cortex.
The anterolateral system works somewhat differently. Its primary neurons enter the spinal cord and then ascend one to two levels before synapsing in the substantia gelatinosa
. The tract that ascends before synapsing is known as Lissauer's tract. After synapsing, secondary axons decussate and ascend in the anterior lateral portion of the spinal cord as the spinothalamic tract
. This tract ascends all the way to the VPL, where it synapses on tertiary neurons. Tertiary neuronal axons then travel to the primary sensory cortex via the posterior limb of the internal capsule.
It should be noted that some of the "pain fibers" in the ALS deviate from their pathway towards the VPL. In one such deviation, axons travel towards the reticular formation
in the midbrain. The reticular formation then projects to a number of places including the hippocampus
(to create memories about the pain), the centromedian nucleus
(to cause diffuse, non-specific pain) and various parts of the cortex. Additionally, some ALS axons project to the periaqueductal gray
in the pons, and the axons forming the periaqueductal gray then project to the nucleus raphe magnus
, which projects back down to where the pain signal is coming from and inhibits it. This helps control the sensation of pain to some degree.
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 Brodmann area
s 1, 2, 3, 4, and 6 and then descend in the posterior limb of the internal capsule
, through the crus cerebri, down through the pons, and to the medullary pyramids, where about 90% of the axons cross to the contralateral side at the decussation of the pyramids. They then descend as the lateral corticospinal tract. These axons synapse with lower motor neurons in the ventral horns
of all levels of the spinal cord. The remaining 10% of axons descend on the ipsilateral side as the ventral corticospinal tract. These axons also synapse with lower motor neurons in the ventral horns. Most of them will cross to the contralateral side of the cord (via the anterior white commissure
) right before synapsing.
The midbrain nuclei include four motor tracts that send upper motor neuronal axons down the spinal cord to lower motor neurons. These are the rubrospinal tract
, the vestibulospinal tract
, the tectospinal tract
and the reticulospinal tract
. The rubrospinal tract descends with the lateral corticospinal tract, and the remaining three descend with the anterior corticospinal tract.
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 axons which synapse on dorsal lateral (DL) lower motor neurons. The DL neurons are involved in distal limb control. Therefore, these DL neurons are found specifically only in the cervical and lumbosaccral enlargements within the spinal cord. There is no decussation in the lateral corticospinal tract after the decussation at the medullary pyramids.
The proprioception of the lower limbs differs from the upper limbs & upper trunk. There is a 4 neuron pathway for lower limbs 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 -> clarks column -> 2nd order neuron -> medulla oblogata (neucleus z of broadal) -> 3rd order neuron -> VPL of thalamus -> 4th order neuron -> posterior limb of internal capsule -> corona radiata -> sensory area of cerebrum.
The anterior corticospinal tract descends ipsilaterally in the anterior column, where the axons emerge and either synapse on lower ventromedial (VM) motor neurons in the ventral horn ipsilaterally or descussate at the anterior white commissure
where they synapse on VM lower motor neurons contralaterally . The tectospinal, vestibulospinal and reticulospinal descend ipsilaterally in the anterior column but do not synapse across the anterior white commissure. Rather, they only synapse on VM lower motor neurons ipsilaterally. The VM lower motor neurons control the large, postural muscles of the axial skeleton
. These lower motor neurons, unlike those of the DL, are located in the ventral horn all the way throughout the spinal cord.
. Also known as the anterior spinocerebellar tract, sensory receptors take in the information and travel into the spinal cord. The cell bodies of these primary neurons are located in the dorsal root ganglia. In the spinal cord, the axons synapse and the secondary neuronal axons decussates and then travel up to the superior cerebellar peduncle where they decussate again. From here, the information is brought to deep nuclei of the cerebellum including the fastigial and interposed nuclei
.
From the levels of L2 to T1, proprioceptive information enters the spinal cord and ascends ipsilaterally, where it synapses in Clarke's nucleus. The secondary neuronal axons continue to ascend ipsilaterally and then pass into the cerebellum via the inferior cerebellar peduncle
. This tract is known as the dorsal spinocerebellar tract.
From above T1, proprioceptive primary axons enter the spinal cord and ascend ipsilaterally until reaching the accessory cuneate nucleus
, where they synapse. The secondary axons pass into the cerebellum via the inferior cerebellar peduncle where again, these axons synapse on cerebellar deep nuclei. This tract is known as the cuneocerebellar tract.
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). 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.
. Usually, victims of spinal cord injuries will suffer loss of feeling in certain parts of their body. In milder cases, a victim might only suffer loss of hand
or foot function. More severe injuries may result in paraplegia
, tetraplegia (also known as quadriplegia), or full body paralysis
below the site of injury to the spinal cord.
Damage to upper motor neuron axons in the spinal cord results in a characteristic pattern of ipsilateral deficits. These include hyperreflexia
, hypertonia
and muscle weakness. Lower motor neuronal damage results in its own characteristic pattern of deficits. Rather than an entire side of deficits, there is a pattern relating to the myotome
affected by the damage. Additionally, lower motor neurons are characterized by muscle weakness, hypotonia
, hyporeflexia
and muscle atrophy
.
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 cervical spine (C1-C7) and the lumbar spine (L1-L5). (The notation C1, C7, L1, L5 refer to the location of a specific vertebra in either the cervical, thoracic, or lumbar region of the spine.)
, on July 16, 2008, launched the online "Allen Spinal Cord Atlas" (backed by Paul Allen
). Its first release included 4000 sets of digital images, showing spatial expression patterns for various genes. When complete, it is planned to map 20,000 genes in adult and juvenile mouse spinal cords. The spinal cord atlas is organized like the Allen Institute's earlier atlas of the mouse brain.
Nervous tissue
Nervous tissue is one of four major classes of vertebrate tissue.Nervous tissue is the main component of the nervous system - the brain, spinal cord, and nerves-which regulates and controls body functions...
and support cells that extends from the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
(the medulla oblongata
Medulla oblongata
The medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla...
specifically). The brain and spinal cord together make up the central nervous system
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...
. The spinal cord begins at the occipital bone
Occipital bone
The occipital bone, a saucer-shaped membrane bone situated at the back and lower part of the cranium, is trapezoidal in shape and curved on itself...
and extends down to the space between the first and second lumbar vertebrae
Lumbar vertebrae
The lumbar vertebrae are the largest segments of the movable part of the vertebral column, and are characterized by the absence of the foramen transversarium within the transverse process, and by the absence of facets on the sides of the body...
; it does not extend the entire length of the vertebral column
Vertebral column
In human anatomy, the vertebral column is a column usually consisting of 24 articulating vertebrae, and 9 fused vertebrae in the sacrum and the coccyx. It is situated in the dorsal aspect of the torso, separated by intervertebral discs...
. It is around 45 cm (17.7 in) in men and around 43 cm (16.9 in) long in women. Also, the spinal cord has a varying width, ranging from 1/2 inch thick in the cervical and lumbar regions to 1/4 inch thick in the thoracic area. The enclosing bony vertebral column protects the relatively shorter spinal cord. The spinal cord functions primarily in the transmission of neural signals between the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
and the rest of the body but also contains neural circuits that can independently control numerous reflex
Reflex
A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus. A true reflex is a behavior which is mediated via the reflex arc; this does not apply to casual uses of the term 'reflex'.-See also:...
es and central pattern generator
Central pattern generator
Central pattern generators are neural networks that produce rhythmic patterned outputs without sensory feedback. CPGs have been shown to produce rhythmic outputs resembling normal "rhythmic motor pattern production" even in isolation from motor and sensory feedback from limbs and other muscle...
s.
The spinal cord has three major functions:
A. Serve as a conduit for motor information, which travels down the spinal cord.
B. Serve as a conduit for sensory information, which travels up the spinal cord.
C. Serve as a center for coordinating certain reflexes.
Structure
The spinal cord is the main pathway for information connecting the brain and peripheral nervous system. The length of the spinal cord is much shorter than the length of the bony spinal column. The human spinal cord extends from the foramen magnum and continues through to the conus medullarisConus medullaris
The conus medullaris is the terminal end of the spinal cord. It occurs near lumbar vertebral levels 1 and 2 . After the spinal cord tapers out, the spinal nerves continue as dangling nerve roots called cauda equina. This terminal nerve root tail is referred to as the filum terminale...
near the second lumbar vertebra, terminating in a fibrous extension known as the filum terminale
Filum terminale
The filum terminale , is a delicate strand of fibrous tissue, about 20 cm. in length, proceeding downward from the apex of the conus medullaris. It gives longitudinal support to the spinal cord and consists of two parts:...
.
It is about 45 cm (17.7 in) long in men and around 43 cm (16.9 in) in women, ovoid-shaped, and is enlarged in the cervical and lumbar regions. The cervical enlargement, located from C3 to T2 spinal segments, is where sensory input comes from and motor output goes to the arms. The lumbar enlargement, located between L1 and S3 spinal segments, handles sensory input and motor output coming from and going to the legs.
The spinal cord is protected by three layers of tissue, called spinal meninges
Meninges
The meninges is the system of membranes which envelopes the central nervous system. The meninges consist of three layers: the dura mater, the arachnoid mater, and the pia mater. The primary function of the meninges and of the cerebrospinal fluid is to protect the central nervous system.-Dura...
, that surround the canal. The dura mater
Dura mater
The dura mater , or dura, is the outermost of the three layers of the meninges surrounding the brain and spinal cord. It is derived from Mesoderm. The other two meningeal layers are the pia mater and the arachnoid mater. The dura surrounds the brain and the spinal cord and is responsible for...
is the outermost layer, and it forms a tough protective coating. Between the dura mater and the surrounding bone of the vertebrae is a space called the epidural space
Epidural space
In the spine, the epidural space is the outermost part of the spinal canal. It is the space within the canal lying outside the dura mater...
. The epidural space is filled with adipose tissue
Adipose tissue
In histology, adipose tissue or body fat or fat depot or just fat is loose connective tissue composed of adipocytes. It is technically composed of roughly only 80% fat; fat in its solitary state exists in the liver and muscles. Adipose tissue is derived from lipoblasts...
, and it contains a network of blood vessel
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...
s. The arachnoid mater
Arachnoid mater
The arachnoid mater, literally from Latin "spider -like mother", is one of the three meninges, the membranes that cover the brain and spinal cord...
is the middle protective layer. Its name comes from the fact that the tissue has a spiderweb-like appearance. The space between the arachnoid and the underlyng pia mater
Pia mater
Pia mater often referred to as simply the pia, is the delicate innermost layer of the meninges, the membranes surrounding the brain and spinal cord. The word finds its roots in Latin, meaning literally "tender mother." The other two meningeal membranes are the dura mater and the arachnoid mater....
is called the subarachnoid space
Subarachnoid space
In the central nervous system, the subarachnoid cavity is the interval between the arachnoid membrane and pia mater....
. The subarachnoid space contains cerebrospinal fluid
Cerebrospinal fluid
Cerebrospinal fluid , Liquor cerebrospinalis, is a clear, colorless, bodily fluid, that occupies the subarachnoid space and the ventricular system around and inside the brain and spinal cord...
(CSF). The medical procedure known as a lumbar puncture
Lumbar puncture
A lumbar puncture is a diagnostic and at times therapeutic procedure that is performed in order to collect a sample of cerebrospinal fluid for biochemical, microbiological, and cytological analysis, or very rarely as a treatment to relieve increased intracranial pressure.-Indications:The...
(or spinal tap) involves use of a needle to withdraw cerebrospinal fluid from the subarachnoid space, usually from the lumbar
Lumbar
In tetrapod anatomy, lumbar is an adjective that means of or pertaining to the abdominal segment of the torso, between the diaphragm and the sacrum ...
region of the spine. The pia mater is the innermost protective layer. It is very delicate and it is tightly associated with the surface of the spinal cord. The cord is stabilized within the dura mater by the connecting denticulate ligaments
Denticulate ligaments
The pia mater of the spinal cord has a pair of denticulate ligaments with 21 attachments per side which attach it to the arachnoid and dura maters. Named for their tooth-like appearance, the denticulate ligaments are traditionally believed to provide stability for the spinal cord against motion...
, which extend from the enveloping pia mater laterally between the dorsal and ventral roots. The dural sac ends at the vertebral level of the second sacral
Sacrum
In vertebrate anatomy the sacrum is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx...
vertebra.
In cross-section, the peripheral region of the cord contains neuronal white matter
White matter
White matter is one of the two components of the central nervous system and consists mostly of myelinated axons. White matter tissue of the freshly cut brain appears pinkish white to the naked eye because myelin is composed largely of lipid tissue veined with capillaries. Its white color is due to...
tracts containing sensory
Sensory neuron
Sensory neurons are typically classified as the neurons responsible for converting external stimuli from the environment into internal stimuli. They are activated by sensory input , and send projections into the central nervous system that convey sensory information to the brain or spinal cord...
and motor neuron
Motor neuron
In vertebrates, the term motor neuron classically applies to neurons located in the central nervous system that project their axons outside the CNS and directly or indirectly control muscles...
s. Internal to this peripheral region is the gray, butterfly-shaped central region made up of nerve cell bodies
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
. This central region surrounds the central canal
Central canal
For the engineering project, see Indiana Central Canal.The central canal is the cerebrospinal fluid-filled space that runs longitudinally through the length of the entire spinal cord. The central canal is contiguous with the ventricular system of the brain...
, which is an anatomic extension of the spaces in the brain known as the ventricles
Ventricular system
The ventricular system is a set of structures containing cerebrospinal fluid in the brain. It is continuous with the central canal of the spinal cord.-Components:The system comprises four ventricles:* right and left lateral ventricles* third ventricle...
and, like the ventricles, contains cerebrospinal fluid.
The spinal cord has a shape that is compressed dorso-ventrally, giving it an elliptical shape. The cord has grooves in the dorsal and ventral sides. The posterior median sulcus is the groove in the dorsal side, and the anterior median fissure is the groove in the ventral side.
Spinal cord segments
The human spinal cord is divided into 31 different segments. At every segment, right and left pairs of spinal nerves (mixed; sensory and motor) form. Six to eight motor nerve rootlets branch out of right and left ventro lateral 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 nerveSpinal nerve
The term spinal nerve generally refers to a mixed spinal nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body...
s (mixed; motor and sensory), one on each side of the spinal cord. Spinal nerves, with the exception of C1 and C2, form inside intervertebral foramen (IVF). Note that at each spinal segment, the border between the central and peripheral nervous system can be observed. Rootlets are a part of the peripheral nervous system.
In the upper part of the vertebral column, spinal nerves exit directly from the spinal cord, whereas in the lower part of the vertebral column nerves pass further down the column before exiting. The terminal portion of the spinal cord is called the conus medullaris
Conus medullaris
The conus medullaris is the terminal end of the spinal cord. It occurs near lumbar vertebral levels 1 and 2 . After the spinal cord tapers out, the spinal nerves continue as dangling nerve roots called cauda equina. This terminal nerve root tail is referred to as the filum terminale...
. The dura mater
Dura mater
The dura mater , or dura, is the outermost of the three layers of the meninges surrounding the brain and spinal cord. It is derived from Mesoderm. The other two meningeal layers are the pia mater and the arachnoid mater. The dura surrounds the brain and the spinal cord and is responsible for...
continues as an extension called the filum terminale
Filum terminale
The filum terminale , is a delicate strand of fibrous tissue, about 20 cm. in length, proceeding downward from the apex of the conus medullaris. It gives longitudinal support to the spinal cord and consists of two parts:...
, which anchors the spinal cord to the coccyx. The cauda equina
Cauda equina
The cauda equina is a structure within the lower end of the spinal column of most vertebrates, that consists of nerve roots and rootlets from above...
(“horse’s tail”) is the name for the collection of nerves in the vertebral column that continue to travel through the vertebral column below the conus medullaris
Conus medullaris
The conus medullaris is the terminal end of the spinal cord. It occurs near lumbar vertebral levels 1 and 2 . After the spinal cord tapers out, the spinal nerves continue as dangling nerve roots called cauda equina. This terminal nerve root tail is referred to as the filum terminale...
. The cauda equina forms as a result of the fact that the spinal cord stops growing in length at about age four, even though the vertebral column continues to lengthen until adulthood. This results in the fact that sacral spinal nerves actually originate in the upper lumbar region. The spinal cord can be anatomically divided into 31 spinal segments based on the origins of the spinal nerves.
Each segment of the spinal cord is associated with a pair of ganglia, called dorsal root ganglia, which are situated just outside of the spinal cord. These ganglia contain cell bodies of sensory neurons. Axons of these sensory neurons travel into the spinal cord via the dorsal roots.
Ventral roots consist of axons from motor neurons, which bring information to the periphery from cell
bodies within the CNS. Dorsal roots and ventral roots come together and exit the intervertebral
foramina as they become spinal nerves.
The gray matter, in the center of the cord, is shaped like a butterfly and consists of cell bodies of
interneurons and motor neurons. It also consists of neuroglia cells and unmyelinated axons.
Projections of the gray matter (the “wings”) are called horns. Together, the gray horns and the gray
commissure
Commissure
A commissure is the place where two things are joined. The term is used especially in the fields of anatomy and biology.In anatomy, commissure refers to a bundle of nerve fibers that cross the midline at their level of origin or entry .* The most common usage of the term refers to the brain's...
form the “gray H.”
The white matter is located outside of the gray matter and consists almost totally of myelinated motor
and sensory axons. “Columns” of white matter carry information either up or down the
spinal cord.
Within the CNS, nerve cell bodies are generally organized into functional clusters, called nuclei. Axons
within the CNS are grouped into tracts.
There are 33 spinal cord nerve segments in a human spinal cord:
- 8 cervical segments forming 8 pairs of cervical nervesCervical nervesThe cervical nerves are the spinal nerves from the cervical vertebrae.Although there are seven cervical vertebrae , there are eight cervical nerves . All nerves except C8 emerge above their corresponding vertebrae, while the C8 nerve emerges below the C7 vertebra...
(C1 spinal nerves exit spinal column between occiput and C1 vertebra; C2 nerves exit between posterior arch of C1 vertebra and lamina of C2 vertebra; C3-C8 spinal nerves through IVF above corresponding cervica vertebra, with the exception of C8 pair which exit via IVF between C7 and T1 vertebra) - 12 thoracic segments forming 12 pairs of thoracic nervesThoracic nervesThe thoracic nerves are the spinal nerves emerging from the thoracic vertebrae. Branches also exit the spine and go directly to the Sympathetic Chain Ganglia of the Autonomic Nervous System where they are involved in the functions of organs and glands in the head, neck, thorax and abdomen.-Anterior...
(exit spinal column through IVF below corresponding vertebra T1-T12) - 5 lumbar segments forming 5 pairs of lumbar nervesLumbar nervesThe lumbar nerves are the five spinal nerves emerging from the lumbar vertebrae. They are divided into posterior and anterior divisions.-Posterior divisions:...
(exit spinal column through IVF, below corresponding vertebra L1-L5) - 5 sacral segments forming 5 pairs of sacral nervesSacral nervesThe five sacral nerves emerge from the sacrum. Although the vertebral components of the sacrum are fused into a single bone, the sacral vertebrae are still used to number the sacral nerves....
(exit spinal column through IVF, below corresponding vertebra S1-S5) - 3 coccygeal segments joined up becoming a single segment forming 1 pair of coccygeal nerves (exit spinal column through the sacral hiatus).
Because the vertebral column grows longer than the spinal cord, spinal cord segments do not correspond to vertebral segments in adults, especially in the lower spinal cord. In the fetus, vertebral segments do correspond with spinal cord segments. In the adult, however, the spinal cord ends around the L1/L2 vertebral level, forming a structure known as the conus medullaris
Conus medullaris
The conus medullaris is the terminal end of the spinal cord. It occurs near lumbar vertebral levels 1 and 2 . After the spinal cord tapers out, the spinal nerves continue as dangling nerve roots called cauda equina. This terminal nerve root tail is referred to as the filum terminale...
. For example, lumbar and sacral spinal cord segments are found between vertebral levels T9 and L2.
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
Cauda equina
The cauda equina is a structure within the lower end of the spinal column of most vertebrates, that consists of nerve roots and rootlets from above...
.
There are two regions where the spinal cord enlarges:
- Cervical enlargementCervical enlargementThe cervical enlargement corresponds with the attachments of the large nerves which supply the upper limbs.It extends from about the third cervical to the second thoracic vertebra, its maximum circumference being on a level with the attachment of the sixth pair of cervical nerves.The reason behind...
- corresponds roughly to the brachial plexusBrachial plexusThe brachial plexus is a network of nerve fibers, running from the spine, formed by the ventral rami of the lower four cervical and first thoracic nerve roots...
nerves, which innervate the upper limbUpper limbThe upper limb or upper extremity is the region in an animal extending from the deltoid region to the hand, including the arm, axilla and shoulder.-Definition:...
. It includes spinal cord segments from about C4 to T1. The vertebral levels of the enlargement are roughly the same (C4 to T1).
- Lumbosacral enlargement - corresponds to the lumbosacral plexusLumbosacral plexusThe anterior divisions of the lumbar nerves, sacral nerves, and coccygeal nerve form the lumbosacral plexus, the first lumbar nerve being frequently joined by a branch from the twelfth thoracic. For descriptive purposes this plexus is usually divided into three parts:* lumbar plexus* sacral plexus*...
nerves, which innervate the lower limbLower limbThe lower limb is a limb of the body.According to Terminologia Anatomica, it includes the pelvic girdle, buttocks, hip, and thigh, as well as the components distal to the knee.-References:...
. It comprises the spinal cord segments from L2 to S3 and is found about the vertebral levels of T9 to T12.
Embryology
The spinal cord is made from part of the neural tubeNeural tube
In the developing vertebrate, the neural tube is the embryo's precursor to the central nervous system, which comprises the brain and spinal cord...
during development. As the neural tube begins to develop, the notochord
Notochord
The notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In some chordates, it persists throughout life as the main axial support of the body, while in most vertebrates it becomes...
begins to secrete a factor known as Sonic hedgehog
Sonic hedgehog
Sonic hedgehog homolog is one of three proteins in the mammalian signaling pathway family called hedgehog, the others being desert hedgehog and Indian hedgehog . SHH is the best studied ligand of the hedgehog signaling pathway. It plays a key role in regulating vertebrate organogenesis, such as...
or SHH. As a result, the floor plate
Floor plate
The floor plate is a structure integral to the developing nervous system of vertebrate organisms. Located on the ventral midline of the embryonic neural tube, the floor plate is a specialized glial structure that spans the anteroposterior axis from the midbrain to the tail regions...
then also begins to secrete SHH, and this will induce the basal plate to develop motor neurons. Meanwhile, the overlying ectoderm
Ectoderm
The "ectoderm" is one of the three primary germ cell layers in the very early embryo. The other two layers are the mesoderm and endoderm , with the ectoderm as the most exterior layer...
secretes bone morphogenetic protein
Bone morphogenetic protein
Bone morphogenetic proteins are a group of growth factors also known as cytokines and as metabologens . Originally discovered by their ability to induce the formation of bone and cartilage, BMPs are now considered to constitute a group of pivotal morphogenetic signals, orchestrating tissue...
(BMP). This induces the roof plate to begin to secrete BMP, which will induce the alar plate
Alar plate
The alar plate is a neural structure in the embryonic nervous system, part of the dorsal side of neural tube, that involves the communication of general somatic and general visceral sensory impulses. The caudal part later becomes sensory axon part of the spinal cord.-External links:* *...
to develop sensory neurons. The alar plate and the basal plate are separated by the sulcus limitans
Sulcus limitans
In the floor of the fourth ventricle, the sulcus limitans separates the cranial nerve motor nuclei from the sensory nuclei . In the superior part of the rhomboid fossa, it corresponds with the lateral limit of the fossa and presents a bluish-gray area, the locus ceruleus In the floor of the fourth...
.
Additionally, the floor plate also secretes netrin
Netrin
Netrins are a class of proteins involved in axon guidance. They are named after the Sanskrit word "netr", which means "one who guides." Netrins are genetically conserved across nematode worms, fruitflies, frogs, and mice. Structurally, netrin resembles laminin.Netrins are chemotropic; a growing...
s. The netrins act as chemoattractants to decussation
Decussation
Decussation is used in biological contexts to describe a crossing.Examples include:* In the brain, where nerve fibers obliquely cross from one lateral part to the other, that is to say they cross at a level other than their origin...
of pain and temperature sensory neurons in the alar plate across the anterior white commissure, where they then ascend towards the thalamus
Thalamus
The thalamus is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections...
.
Lastly, it is important to note that the past studies of Viktor Hamburger and Rita Levi-Montalcini in the chick embryo have been further proven by more recent studies which demonstrated that the elimination of neuronal cells by programmed cell death
Programmed cell death
Programmed cell-death is death of a cell in any form, mediated by an intracellular program. PCD is carried out in a regulated process which generally confers advantage during an organism's life-cycle...
(PCD) is necessary for the correct assembly of the nervous system.
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 called the anterior spinal arteryAnterior spinal artery
In human anatomy, the anterior spinal artery is the blood vessel that supplies the anterior portion of the spinal cord. It arises from branches of the vertebral arteries and is supplied by the anterior segmental medullary arteries, including the artery of Adamkiewicz, and courses along the anterior...
, and the right and left posterior spinal arteries
Posterior spinal artery
The posterior spinal artery arises from the vertebral artery, adjacent to the medulla oblongata.-Path:It passes posteriorly to descend the medulla passing in front of the posterior roots of the spinal nerves...
. These travel in the subarachnoid space and send branches into the spinal cord. They form anastamoses (connections) via the anterior and posterior segmental medullary arteries
Segmental medullary artery
Each segmental medullary artery is a branch of the cervical part of the vertebral artery. These small branches penetrate into the vertebral bone through small openings such as the intervertebral foramina...
, which enter the spinal cord at various points along its length. The actual blood flow caudally through these arteries, derived from the posterior cerebral circulation, is inadequate to maintain the spinal cord beyond the cervical segments.
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
Radicular artery
The posterior and anterior radicular arteries run along the posterior and anterior roots of the spinal nerves and supply them with blood.-Relationship to Segmental Medullary 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. 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. 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 aneursym repair, can result in spinal cord infarction and paraplegia.
Somatosensory organization
Somatosensory organization is divided into the dorsal column-medial lemniscus tract (the touch/proprioceptionProprioception
Proprioception , from Latin proprius, meaning "one's own" and perception, is the sense of the relative position of neighbouring parts of the body and strength of effort being employed in movement...
/vibration sensory pathway) and the anterolateral system
Anterolateral system
In the nervous system, the anterolateral system is an ascending pathway that conveys pain, temperature , and crude touch from the periphery to the brain. It comprises three main pathways:-External links:*...
, or ALS (the pain/temperature sensory pathway). Both sensory pathways use three different neurons to get information from sensory receptors at the periphery to the cerebral cortex
Cerebral cortex
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...
. These neurons are designated primary, secondary and tertiary sensory neurons. In both pathways, primary sensory neuron cell bodies are found in the dorsal root ganglia, and their central axons project into the spinal cord.
In the dorsal column-medial leminiscus tract, a primary neuron's axon enters the spinal cord and then enters the dorsal column. If the primary axon enters below spinal level T6, the axon travels in the fasciculus gracilis
Fasciculus gracilis
The fasciculus gracilis is a bundle of axon fibres in the posterior column of the spinal cord and carries information from the middle thoracic and lower limbs of the body...
, the medial part of the column. If the axon enters above level T6, then it travels in the fasciculus cuneatus
Fasciculus cuneatus
The fasciculus cuneatus is a tract of nerves in the spinal cord which primarily transmits information from the arms...
, which is lateral to the fasiculus gracilis. Either way, the primary axon ascends to the lower medulla
Medulla oblongata
The medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla...
, where it leaves its fasiculus and synapses with a secondary neuron in one of the dorsal column nuclei: either the nucleus gracilis or the nucleus cuneatus, depending on the pathway it took. At this point, the secondary axon leaves its nucleus and passes anteriorly and medially. The collection of secondary axons that do this are known as internal arcuate fibers
Internal arcuate fibers
Internal arcuate fibers are the axons of second-order neurons contained within the gracile and cuneate nuclei of the medulla oblongata.These fibers cross from one side of the medulla to the other to form the medial lemniscus....
. The internal arcuate fibers decussate and continue ascending as the contralateral medial lemniscus
Medial lemniscus
The medial lemniscus, also known as Reil's band or Reil's ribbon, is a pathway in the brainstem that carries sensory information from the gracile and cuneate nuclei to the thalamus.-Path:...
. Secondary axons from the medial lemniscus finally terminate in the ventral posterolateral nucleus
Ventral posterolateral nucleus
The ventral posterolateral nucleus is a nucleus of the thalamus. Together with the ventral posteromedial nucleus , VPI and VMpo, it constitutes the ventral posterior nucleus.-Input and output:...
(VPL) of the thalamus
Thalamus
The thalamus is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections...
, where they synapse with tertiary neurons. From there, tertiary neurons ascend via the posterior limb of the internal capsule
Internal capsule
The internal capsule is an area of white matter in the brain that separates the caudate nucleus and the thalamus from the lenticular nucleus. The internal capsule contains both ascending and descending axons....
and end in the primary sensory cortex.
The anterolateral system works somewhat differently. Its primary neurons enter the spinal cord and then ascend one to two levels before synapsing in the substantia gelatinosa
Substantia gelatinosa
Substantia gelatinosa can refer to:* Substantia gelatinosa of Rolando * Substantia gelatinosa centralisSubstantia gelatinosa is a structure involved in pain transmission....
. The tract that ascends before synapsing is known as Lissauer's tract. After synapsing, secondary axons decussate and ascend in the anterior lateral portion of the spinal cord as the spinothalamic tract
Spinothalamic tract
The spinothalamic tract is a sensory pathway originating in the spinal cord. It transmits information to the thalamus about pain, temperature, itch and crude touch...
. This tract ascends all the way to the VPL, where it synapses on tertiary neurons. Tertiary neuronal axons then travel to the primary sensory cortex via the posterior limb of the internal capsule.
It should be noted that some of the "pain fibers" in the ALS deviate from their pathway towards the VPL. In one such deviation, axons travel towards the reticular formation
Reticular formation
The reticular formation is a part of the brain that is involved in actions such as awaking/sleeping cycle, and filtering incoming stimuli to discriminate irrelevant background stimuli...
in the midbrain. The reticular formation then projects to a number of places including the hippocampus
Hippocampus
The hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...
(to create memories about the pain), the centromedian nucleus
Centromedian nucleus
In the anatomy of the brain, the centromedian nucleus, also known as the centrum medianum, is a part of the intralaminar nucleus of the thalamus...
(to cause diffuse, non-specific pain) and various parts of the cortex. Additionally, some ALS axons project to the periaqueductal gray
Periaqueductal gray
Periaqueductal gray is the gray matter located around the cerebral aqueduct within the tegmentum of the midbrain. It plays a role in the descending modulation of pain and in defensive behaviour...
in the pons, and the axons forming the periaqueductal gray then project to the nucleus raphe magnus
Nucleus raphe magnus
The nucleus raphes magnus, located directly rostral to the nucleus raphes obscurus, is afferently stimulated from axons in the spinal cord and cerebellum....
, which projects back down to where the pain signal is coming from and inhibits it. This helps control the sensation of pain to some degree.
Motor organization
The corticospinal tractCorticospinal tract
The corticospinal or pyramidal tract is a collection of axons that travel between the cerebral cortex of the brain and the spinal cord....
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 Brodmann area
Brodmann area
A Brodmann area is a region of the cerebral cortex defined based on its cytoarchitectonics, or structure and organization of cells.-History:...
s 1, 2, 3, 4, and 6 and then descend in the posterior limb of the internal capsule
Internal capsule
The internal capsule is an area of white matter in the brain that separates the caudate nucleus and the thalamus from the lenticular nucleus. The internal capsule contains both ascending and descending axons....
, through the crus cerebri, down through the pons, and to the medullary pyramids, where about 90% of the axons cross to the contralateral side at the decussation of the pyramids. They then descend as the lateral corticospinal tract. These axons synapse with lower motor neurons in the ventral horns
Spinal cord horn
Three spinal cord horns are visible in cross-sectional views of the spinal cord: dorsal horn, ventral horn, and the lateral horn . Each is composed of gray matter, meaning that the horns comprise mostly neuronal and glial cell bodies...
of all levels of the spinal cord. The remaining 10% of axons descend on the ipsilateral side as the ventral corticospinal tract. These axons also synapse with lower motor neurons in the ventral horns. Most of them will cross to the contralateral side of the cord (via the anterior white commissure
Anterior white commissure
The anterior white commissure, also known as the alba anterior medullae spinalis, is a bundle of nerve fibers which cross the midline of the spinal cord just anterior to the gray commissure...
) right before synapsing.
The midbrain nuclei include four motor tracts that send upper motor neuronal axons down the spinal cord to lower motor neurons. These are the rubrospinal tract
Rubrospinal tract
The rubrospinal tract is a part of the nervous system. It is a part of the lateral indirect extra-pyramidal tract.-Function:In humans, the rubrospinal tract is one of several major motor control pathways. It is smaller and has fewer axons than the corticospinal tract, suggesting that it is less...
, the vestibulospinal tract
Vestibulospinal tract
The vestibulospinal tract is a component of the extrapyramidal system and also can be classified as a component of the medial pathway. The vestibulospinal fibers relay information from nuclei to motor neurons, like other descending motor pathways. Specifically, the vestibular nuclei receive...
, the tectospinal tract
Tectospinal tract
In humans, the tectospinal tract is a nerve pathway which coordinates head and eye movements. It is part of the indirect extrapyramidal tract...
and the reticulospinal tract
Reticulospinal tract
The reticulospinal tract is an extrapyramidal motor tract which travels from the reticular formation.-Functions:...
. The rubrospinal tract descends with the lateral corticospinal tract, and the remaining three descend with the anterior corticospinal tract.
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 axons which synapse on dorsal lateral (DL) lower motor neurons. The DL neurons are involved in distal limb control. Therefore, these DL neurons are found specifically only in the cervical and lumbosaccral enlargements within the spinal cord. There is no decussation in the lateral corticospinal tract after the decussation at the medullary pyramids.
The proprioception of the lower limbs differs from the upper limbs & upper trunk. There is a 4 neuron pathway for lower limbs 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 -> clarks column -> 2nd order neuron -> medulla oblogata (neucleus z of broadal) -> 3rd order neuron -> VPL of thalamus -> 4th order neuron -> posterior limb of internal capsule -> corona radiata -> sensory area of cerebrum.
The anterior corticospinal tract descends ipsilaterally in the anterior column, where the axons emerge and either synapse on lower ventromedial (VM) motor neurons in the ventral horn ipsilaterally or descussate at the anterior white commissure
Anterior white commissure
The anterior white commissure, also known as the alba anterior medullae spinalis, is a bundle of nerve fibers which cross the midline of the spinal cord just anterior to the gray commissure...
where they synapse on VM lower motor neurons contralaterally . The tectospinal, vestibulospinal and reticulospinal descend ipsilaterally in the anterior column but do not synapse across the anterior white commissure. Rather, they only synapse on VM lower motor neurons ipsilaterally. The VM lower motor neurons control the large, postural muscles of the axial skeleton
Axial skeleton
The axial skeleton consists of the 80 bones along the central axis of the human body. It is composed of six parts; the human skull, the ossicles of the middle ear, the hyoid bone of the throat, the rib cage, sternum and the vertebral column...
. These lower motor neurons, unlike those of the DL, are located in the ventral horn all the way throughout the spinal cord.
Spinocerebellar tracts
Proprioceptive information in the body travels up the spinal cord via three tracts. Below L2, the proprioceptive information travels up the spinal cord in the ventral spinocerebellar tractVentral spinocerebellar tract
The ventral spinocerebellar tract conveys proprioceptive information from the body to the cerebellum. It is part of the somatosensory system and runs in parallel with the dorsal spinocerebellar tract...
. Also known as the anterior spinocerebellar tract, sensory receptors take in the information and travel into the spinal cord. The cell bodies of these primary neurons are located in the dorsal root ganglia. In the spinal cord, the axons synapse and the secondary neuronal axons decussates and then travel up to the superior cerebellar peduncle where they decussate again. From here, the information is brought to deep nuclei of the cerebellum including the fastigial and interposed nuclei
Interposed nucleus
The interposed nucleus is a deep nucleus of the cerebellum and is composed of the globose nuclei and the emboliform nuclei. It is located in the roof of the fourth ventricle, lateral to the fastigial nucleus...
.
From the levels of L2 to T1, proprioceptive information enters the spinal cord and ascends ipsilaterally, where it synapses in Clarke's nucleus. The secondary neuronal axons continue to ascend ipsilaterally and then pass into the cerebellum via the inferior cerebellar peduncle
Inferior cerebellar peduncle
The upper part of the posterior district of the medulla oblongata is occupied by the inferior cerebellar peduncle , a thick rope-like strand situated between the lower part of the fourth ventricle and the roots of the glossopharyngeal and vagus nerves.Each cerebellar inferior peduncle connects the...
. This tract is known as the dorsal spinocerebellar tract.
From above T1, proprioceptive primary axons enter the spinal cord and ascend ipsilaterally until reaching the accessory cuneate nucleus
Accessory cuneate nucleus
The accessory cuneate nucleus is located lateral to the cuneate nucleus in the medulla oblongata at the level of the sensory decussation ....
, where they synapse. The secondary axons pass into the cerebellum via the inferior cerebellar peduncle where again, these axons synapse on cerebellar deep nuclei. This tract is known as the cuneocerebellar tract.
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). 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.
Injury
Spinal cord injuries can be caused by trauma to the spinal column (stretching, bruising, applying pressure, severing, laceration, etc.). The vertebral bones or intervertebral disks can shatter, causing the spinal cord to be punctured by a sharp fragment of boneBone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
. Usually, victims of spinal cord injuries will suffer loss of feeling in certain parts of their body. In milder cases, a victim might only suffer loss of hand
Hand
A hand is a prehensile, multi-fingered extremity located at the end of an arm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs...
or foot function. More severe injuries may result in paraplegia
Paraplegia
Paraplegia is an impairment in motor or sensory function of the lower extremities. The word comes from Ionic Greek: παραπληγίη "half-striking". It is usually the result of spinal cord injury or a congenital condition such as spina bifida that affects the neural elements of the spinal canal...
, tetraplegia (also known as quadriplegia), or full body paralysis
Paralysis
Paralysis is loss of muscle function for one or more muscles. Paralysis can be accompanied by a loss of feeling in the affected area if there is sensory damage as well as motor. A study conducted by the Christopher & Dana Reeve Foundation, suggests that about 1 in 50 people have been diagnosed...
below the site of injury to the spinal cord.
Damage to upper motor neuron axons in the spinal cord results in a characteristic pattern of ipsilateral deficits. These include hyperreflexia
Hyperreflexia
Hyperreflexia is defined as overactive or overresponsive reflexes. Examples of this can include twitching or spastic tendencies, which are indicative of upper motor neuron disease as well as the lessening or loss of control ordinarily exerted by higher brain centers of lower neural pathways...
, hypertonia
Hypertonia
Hypertonia a condition marked by an abnormal increase in muscle tension and a reduced ability of a muscle to stretch. It is caused by lesions to upper motor neurons in the central nervous system, which carry information from the central nervous system to the muscles and control posture, muscle...
and muscle weakness. Lower motor neuronal damage results in its own characteristic pattern of deficits. Rather than an entire side of deficits, there is a pattern relating to the myotome
Myotome
In vertebrate embryonic development, a myotome is a group of tissues formed from somites that develop into the body wall muscle.Each myotome divides into a dorsal epaxial part and a ventral hypaxial part....
affected by the damage. Additionally, lower motor neurons are characterized by muscle weakness, hypotonia
Hypotonia
Hypotonia is a state of low muscle tone , often involving reduced muscle strength. Hypotonia is not a specific medical disorder, but a potential manifestation of many different diseases and disorders that affect motor nerve control by the brain or muscle strength...
, hyporeflexia
Hyporeflexia
Hyporeflexia is the condition of below normal or absent reflexes. It can be tested for by using a reflex hammer. It is the opposite of hyperreflexia....
and muscle atrophy
Muscle atrophy
Muscle atrophy, or disuse atrophy, is defined as a decrease in the mass of the muscle; it can be a partial or complete wasting away of muscle. When a muscle atrophies, this leads to muscle weakness, since the ability to exert force is related to mass...
.
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 cervical spine (C1-C7) and the lumbar spine (L1-L5). (The notation C1, C7, L1, L5 refer to the location of a specific vertebra in either the cervical, thoracic, or lumbar region of the spine.)
Spinal cord genomic map
The Allen Institute for Brain ScienceAllen Institute for Brain Science
The Allen Institute for Brain Science is a Seattle-based nonprofit medical research organization that was launched in 2003 with a $100 million seed donation from philanthropist, founder and former Microsoft executive Paul Allen.-Overview:...
, on July 16, 2008, launched the online "Allen Spinal Cord Atlas" (backed by Paul Allen
Paul Allen
Paul Gardner Allen is an American business magnate, investor, and philanthropist. Allen co-founded Microsoft with Bill Gates...
). Its first release included 4000 sets of digital images, showing spatial expression patterns for various genes. When complete, it is planned to map 20,000 genes in adult and juvenile mouse spinal cords. The spinal cord atlas is organized like the Allen Institute's earlier atlas of the mouse brain.
See also
- Cauda equinaCauda equinaThe cauda equina is a structure within the lower end of the spinal column of most vertebrates, that consists of nerve roots and rootlets from above...
- Conus medullarisConus medullarisThe conus medullaris is the terminal end of the spinal cord. It occurs near lumbar vertebral levels 1 and 2 . After the spinal cord tapers out, the spinal nerves continue as dangling nerve roots called cauda equina. This terminal nerve root tail is referred to as the filum terminale...
- MeningesMeningesThe meninges is the system of membranes which envelopes the central nervous system. The meninges consist of three layers: the dura mater, the arachnoid mater, and the pia mater. The primary function of the meninges and of the cerebrospinal fluid is to protect the central nervous system.-Dura...
- Spinal nerves
- Lumbar punctureLumbar punctureA lumbar puncture is a diagnostic and at times therapeutic procedure that is performed in order to collect a sample of cerebrospinal fluid for biochemical, microbiological, and cytological analysis, or very rarely as a treatment to relieve increased intracranial pressure.-Indications:The...
- Neutral spineNeutral spineA neutral spine or good posture refers to the "three natural curves [that] are present in a healthy spine."- Posture :The word "posture" comes from the Latin verb "ponere" which is defined as "to put or place." The general concept of human posture refers to "the carriage of the body as a whole, the...
- Brown-Sequard SyndromeBrown-Séquard syndromeBrown-Séquard syndrome, also known as Brown-Séquard's hemiplegia and Brown-Séquard's paralysis, is a loss of sensation and motor function that is caused by the lateral hemisection of the spinal cord...
- Tetraplegic upper limb surgery
- Redlich-Obersteiner's ZoneRedlich-Obersteiner's ZoneThe Redlich-Obersteiner's zone is a boundary between the central nervous system , the peripheral nervous system . The Redlich-Obersteiner's zone is located at the point of entry of either between cranial nerves and the brain or spinal nerves and the spinal cord...
External links
- Spinal Cord Histology - A multitude of great images from the University of CincinnatiUniversity of CincinnatiThe University of Cincinnati is a comprehensive public research university in Cincinnati, Ohio, and a part of the University System of Ohio....
- Spinal Cord Medical Notes - Online medical notes on the spinal cord
- 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.