The spinal cord is a long, thin, tubular bundle of nervous tissue A neuron (pronounced /ˈnjʊərɒn/ NOOR-on, also known as a neurone or nerve cell) 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 and support cells Glial cells, commonly called neuroglia or simply glia , are non-neuronal cells that maintain homeostasis, form myelin, and provide support and protection for the brain's neurons. In the human brain, there is roughly one glia for every neuron with a ratio of about two neurons for every three glia in the cerebral gray matter that extends from the brain The brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all. In vertebrates, the brain is located in the head, protected by the skull and close to the primary (the medulla 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. The medulla contains the cardiac, respiratory, vomiting and vasomotor centers and deals with autonomic functions, such as breathing, heart rate and blood pressure specifically). The brain and spinal cord together make up the central nervous system The central nervous system is the part of the nervous system that 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. It contains the majority of the nervous system and consists of the brain and the spinal cord, as well as. The spinal cord extends down to the space between the first and second 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. They are designated L1 to L5, starting at the top. This area of the spine is the source of much body; it does not extend the entire length of the vertebral column. It is around 45 cm (18 in) in men and around 43 cm (17 in) long in women. The enclosing bony vertebral column In human anatomy, the vertebral column is a column usually consisting of 33 vertebrae, the sacrum, intervertebral discs, and the coccyx situated in the dorsal aspect of the torso, separated by spinal discs. It houses the spinal cord in its spinal canal protects the relatively shorter spinal cord. The spinal cord functions primarily in the transmission of neural signals Neurotransmission , also called synaptic transmission, is an electrical movement within synapses caused by a propagation of nerve impulses. As each nerve cell receives neurotransmitter from the presynaptic neuron, or terminal button, to the postsynaptic neuron, or dendrite, of the second neuron, it sends it back out to several neurons, and they do between the brain The brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all. In vertebrates, the brain is located in the head, protected by the skull and close to the primary and the rest of the body but also contains neural circuits that can independently control numerous reflexes A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus. In most contexts, in particular those involving humans, reflex actions are mediated via the reflex arc; this is not always true in other animals, nor does it apply to casual uses of the term 'reflex' and central pattern generators "Central pattern generators can be defined as neural networks that can endogenously (i.e. without rhythmic sensory or central input) produce rhythmic patterned outputs" or as "neural circuits that generate periodic motor commands for rhythmic movements such as locomotion." CPGs have been shown to produce rhythmic outputs. 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. ^[1]

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 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. The medulla contains the cardiac, respiratory, vomiting and vasomotor centers and deals with autonomic functions, such as breathing, heart rate and blood pressure and continues through the conus medullaris The conus medullaris is the terminal end of the spinal cord. It occurs near lumbar vertebral levels 1 and 2 (L2). 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 upper end of the conus medullaris is usually not well near the first or second lumbar vertebra 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. They are designated L1 to L5, starting at the top. This area of the spine is the source of much body, terminating in a fibrous extension known as the filum terminale.

It is about 45 cm (18 in) long in men and around 43 cm (17 in) in women, ovoid-shaped, and is enlarged in the cervical and lumbar regions. The cervical enlargement, located from C4 to T1, is where sensory input comes from and motor output goes to the arms. The lumbar enlargement, located between T9 and T12, handles sensory input and motor output coming from and going to the legs. You should notice that the name is somewhat misleading. However, this region of the cord does indeed have branches that extend to the lumbar region.

In cross-section, the peripheral region of the cord contains neuronal 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 its usual preservation in formaldehyde. A 20 year- tracts containing sensory 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. Unlike neurons of the central nervous system, whose and motor neurons In vertebrates, the term motor neuron classically applies to neurons located in the central nervous system (or CNS) that project their axons outside the CNS and directly or indirectly control muscles. The motor neuron is often associated with efferent neuron, primary neuron, or alpha motor neurons. Internal to this peripheral region is the gray, butterfly-shaped central region made up of nerve cell bodies A neuron (pronounced /ˈnjʊərɒn/ NOOR-on, also known as a neurone or nerve cell) 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. This central region surrounds the central canal, which is an anatomic extension of the spaces in the brain known as the ventricles 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 and, like the ventricles, contains cerebrospinal fluid Cerebrospinal fluid , Liquor cerebrospinalis, sometimes called brain juice, is a clear bodily fluid that occupies the subarachnoid space and the ventricular system around and inside the brain and spinal cord. In essence, the brain "floats" in it.

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. Running down the center of the spinal cord is a cavity, called the central canal.

The three meninges The meninges is the system of membranes which envelops 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 that cover the spinal cord—the outer dura mater The dura mater , or dura, is the outermost of the three layers of the meninges surrounding the brain and spinal cord. 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 keeping in the cerebrospinal fluid. The name "dura mater" is derived from, the arachnoid mater The arachnoid mater is one of the three meninges, the membranes that cover the brain and spinal cord. It is interposed between the two other meninges, the more superficial dura mater and the deeper pia mater, and is separated from the pia mater by the subarachnoid space, and the innermost pia mater The pia mater is the delicate innermost layer of the meninges—the membranes surrounding the brain and spinal cord—are continuous with that in the brainstem and cerebral hemispheres. Similarly, cerebrospinal fluid is found in the subarachnoid space In the central nervous system, the subarachnoid cavity is the interval between the arachnoid membrane and pia mater. The cord is stabilized within the dura mater by the connecting denticulate ligaments, which extend from the enveloping pia mater The pia mater is the delicate innermost layer of the meninges—the membranes surrounding the brain and spinal cord laterally between the dorsal and ventral roots. The dural sac ends at the vertebral level of the second sacral 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 (tailbone). In children, it consists of usually five unfused vertebra.

The spinal cord is protected by three layers of tissue, called spinal meninges, that surround the cord. 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 vessels. The arachnoid 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 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 , Liquor cerebrospinalis, sometimes called brain juice, is a clear bodily fluid that occupies the subarachnoid space and the ventricular system around and inside the brain and spinal cord. In essence, the brain "floats" in it (CSF). The medical procedure known as a “spinal tap” involves use of a needle to withdraw CSF 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.

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 nerves The term spinal nerve generally refers to the mixed spinal nerve, which is formed from the dorsal and ventral roots that come out of the spinal cord. A spinal nerve emerges at two points from the spinal cord, the ventral and dorsal roots. The ventral and dorsal roots merge to form the whole spinal nerve. The spinal nerve emerges from the spinal (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 conus medullaris is the terminal end of the spinal cord. It occurs near lumbar vertebral levels 1 and 2 (L2). 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 upper end of the conus medullaris is usually not well. The pia mater The pia mater is the delicate innermost layer of the meninges—the membranes surrounding the brain and spinal cord continues as an extension called the filum terminale, which anchors the spinal cord to the coccyx. The 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. The space in which the cerebrospinal fluid is present is actually an extension of the subarachnoid space (“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 conus medullaris is the terminal end of the spinal cord. It occurs near lumbar vertebral levels 1 and 2 (L2). 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 upper end of the conus medullaris is usually not well. 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 An interneuron is a multipolar neuron which connects afferent neurons and efferent neurons in neural pathways. Like motor neurons, interneuron cell bodies are always located in the central nervous system (CNS) 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 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 commissures, of which there are two—the anterior and posterior—and which consist of fibre tracts that connect the two cerebral hemispheres and span the longitudinal fissure form the “gray H.”

The white matter is located outside of the gray matter and consists almost totally of myelinated Myelin is a dielectric material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system. Myelin is an outgrowth of a glial cell: Schwann cells supply the myelin for peripheral neurons, whereas oligodendrocytes, specifically of the interfascicular type, 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 (some EMS text say 25, counting the sacral as one solid piece) spinal cord nerve segments in a human spinal cord:

• 8 cervical segments forming 8 pairs of cervical nerves Although there are seven cervical vertebrae , there are eight cervical nerves (C1-C8). All nerves except C8 emerge above their corresponding vertebrae, while the C8 nerve emerges below the C7 vertebra. (In the other portions of the spine, the nerve emerges below the vertebra with the same name.) (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 nerves (exit spinal column through IVF below corresponding vertebra T1-T12)
• 5 lumbar segments forming 5 pairs of lumbar nerves (exit spinal column through IVF, below corresponding vertebra L1-L5)
• 5 (or 1) sacral segments forming 5 pairs of sacral nerves This article was originally based on an entry from a public domain edition of Gray's Anatomy. As such, some of the information contained herein may be outdated (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 A vertebra is an individual bone in the flexible column that defines vertebrate animals, e.g., humans. The vertebral column encases and protects the spinal cord, which runs from the base of the cranium down the dorsal side of the animal until reaching the pelvis. From there, vertebra continue into the tail 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 The conus medullaris is the terminal end of the spinal cord. It occurs near lumbar vertebral levels 1 and 2 (L2). 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 upper end of the conus medullaris is usually not well. 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 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. The space in which the cerebrospinal fluid is present is actually an extension of the subarachnoid space.

There are two regions where the spinal cord enlarges:

• Cervical enlargement - corresponds roughly to the brachial plexus The brachial plexus is an arrangement of nerve fibers, running from the spine, formed by the ventral rami of the lower four cervical and first thoracic nerve roots . It proceeds through the neck, the axilla (armpit region), and into the arm nerves, which innervate the upper limb In human anatomy, the upper limb refers to the region distal to the deltoid. 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 plexus nerves, which innervate the lower limb According to Terminologia Anatomica, it includes the pelvic girdle, buttocks, hip, and thigh, as well as the components distal to the knee. 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 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. The neural groove gradually deepens as the neural folds become elevated, and ultimately the folds meet and coalesce in the middle line and convert the groove into a closed tube, the neural tube or neural during development. As the neural tube begins to develop, the notochord begins to secrete a factor known as Sonic hedgehog Sonic hedgehog homolog is one of three proteins in the mammalian signaling pathway family called hedgehog, the others being desert hedgehog (DHH) and Indian hedgehog (IHH). SHH is the best studied ligand of the hedgehog signaling pathway. It plays a key role in regulating vertebrate organogenesis, such as in the growth of digits on limbs and 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 In vertebrates, the term motor neuron classically applies to neurons located in the central nervous system (or CNS) that project their axons outside the CNS and directly or indirectly control muscles. The motor neuron is often associated with efferent neuron, primary neuron, or alpha motor neurons. Meanwhile, the overlying ectoderm Generally speaking, the ectoderm differentiates to form the nervous system, tooth enamel and the epidermis secretes bone morphogenetic protein Bone Morphogenetic Proteins are a group of growth factors and cytokines were originally discovered by their ability to induce the formation of bone and cartilage, but are now considered to constitute a group of pivotal morphogenetic signals, orchestrating tissue architecture throughout the body . The important functioning of BMP signals in (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 netrins. 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.

Somatosensory organization

Somatosensory organization is divided into the dorsal column-medial lemniscus tract (the touch/proprioception/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.

Motor organization

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 Brodmann areas 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 lower limb is something different from upper limb & upper trunk.there is a 4 neuron pathaway for lower limbs proprioception.this pathway initiaLLY follow with dorsal spino cerebellar path way. the pathway 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.

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 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 decussate 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).^[2] 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.^[3]

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 bone. 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, or full body paralysis (called Quadriplegia) 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.)

Spinal cord genomic map

The Allen Institute for Brain Science, 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.^[4] 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.^[5][6]

Additional images

See also

• Cauda equina
• Conus medullaris
• Meninges
• Spinal nerves
• Lumbar puncture
• Neutral spine
• Brown-Sequard Syndrome

References

1. ^ Maton, Anthea; Jean Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna Quon Warner, David LaHart, Jill D. Wright (1993). Human Biology and Health. Englewood Cliffs, New Jersey, USA: Prentice Hall. pp. 132–144. ISBN 0-13-981176-1.
2. ^ Saladin. Anatomy and Physiology, 5th Ed.
3. ^ Saladin. Anatomy and Physiology, 5th Ed.
4. ^ "Gene Search :: Spinal Cord". mousespinal.brain-map.org. http://mousespinal.brain-map.org/. Retrieved 2010-02-23.
5. ^ msnbc.msn.com, Gene map charts spinal cord mysteries
6. ^ sciencenews.org/view, MapQuest for the mouse spinal cord

External links

• Spinal Cord Histology - A multitude of great images from the University of Cincinnati
• 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.

Categories: Central nervous system | Spinal cord | Vertebral column

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