The spinal cord is a long, tubular structure made up of nervous tissue, that extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column. It encloses the central canal of the spinal cord; the brain and spinal cord together make up the central nervous system. In humans, the spinal cord begins at the occipital bone where it passes through the foramen magnum, meets and enters the spinal canal at the beginning of the cervical vertebrae; the spinal cord extends down to between the second lumbar vertebrae where it ends. The enclosing bony vertebral column protects the shorter spinal cord, it is around 45 cm in men and around 43 cm long in women. The spinal cord has a varying width, ranging from 13 mm thick in the cervical and lumbar regions to 6.4 mm thick in the thoracic area. The spinal cord functions in the transmission of nerve signals from the motor cortex to the body, from the afferent fibers of the sensory neurons to the sensory cortex, it is a center for coordinating many reflexes and contains reflex arcs that can independently control reflexes.
It is the location of groups of spinal interneurons that make up the neural circuits known as central pattern generators. These circuits are responsible for controlling motor instructions for rhythmic movements such as walking; the spinal cord is the main pathway for information connecting the brain and peripheral nervous system. Much shorter than its protecting spinal column, the human spinal cord originates in the brainstem, passes through the foramen magnum, continues through to the conus medullaris near the second lumbar vertebra before terminating in a fibrous extension known as the filum terminale, it is about 45 cm long in men and around 43 cm in women, ovoid-shaped, is enlarged in the cervical and lumbar regions. The cervical enlargement, stretching from the C5 to T1 vertebrae, is where sensory input comes from and motor output goes to the arms and trunk; the lumbar enlargement, located between L1 and S3, handles sensory input and motor output coming from and going to the legs. The spinal cord is continuous with the caudal portion of the medulla, running from the base of the skull to the body of the first lumbar vertebra.
It does not run the full length of the vertebral column in adults. It is made of 31 segments from which branch one pair of sensory nerve roots and one pair of motor nerve roots; the nerve roots merge into bilaterally symmetrical pairs of spinal nerves. The peripheral nervous system is made up of these spinal roots and ganglia; the dorsal roots are afferent fascicles, receiving sensory information from the skin and visceral organs to be relayed to the brain. The roots terminate in dorsal root ganglia, which are composed of the cell bodies of the corresponding neurons. Ventral roots consist of efferent fibers that arise from motor neurons whose cell bodies are found in the ventral gray horns of the spinal cord; the spinal cord are protected by three layers of tissue or membranes called meninges, that surround the canal. The dura mater is the outermost layer, 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, it contains a network of blood vessels.
The arachnoid mater, the middle protective layer, is named for its spiderweb-like appearance. The space between the arachnoid and the underlying pia mater is called the subarachnoid space; the subarachnoid space contains cerebrospinal fluid, which can be sampled with a lumbar puncture, or "spinal tap" procedure. The delicate pia mater, the innermost protective layer, is 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 axons. Internal to this peripheral region is the grey matter, which contains the nerve cell bodies arranged in the three grey columns that give the region its butterfly-shape; this central region surrounds the central canal, an extension of the fourth ventricle and contains cerebrospinal fluid.
The spinal cord is elliptical in cross section, being compressed dorsolaterally. Two prominent grooves, or sulci, run along its length; the posterior median sulcus is the groove in the dorsal side, the anterior median fissure is the groove in the ventral side. The human spinal cord is divided into segments. Six to eight motor nerve rootlets branch out of right and left ventro lateral sulci in a orderly manner. Nerve rootlets combine to form nerve roots. Sensory nerve rootlets form off right and left dorsal lateral sulci and form sensory nerve roots; the ventral and dorsal roots combine to form one on each side of the spinal cord. Spinal nerves, with the exception of C1 and C2, form inside the intervertebral foramen; these rootlets form the demarcation between the peripheral nervous systems. The grey column, in the center of the cord, is shaped like a butterfly and consists of cell bodies of interneurons, motor neurons, neuroglia cells and unmyelinated axons; the anterior and posterior grey column present as projections of the grey matter and are known as the horns of the spinal cord.
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A nerve is an enclosed, cable-like bundle of nerve fibres called axons, in the peripheral nervous system. A nerve provides a common pathway for the electrochemical nerve impulses called action potentials that are transmitted along each of the axons to peripheral organs or, in the case of sensory nerves, from the periphery back to the central nervous system; each axon within the nerve is an extension of an individual neuron, along with other supportive cells such as Schwann cells that coat the axons in myelin. Within a nerve, each axon is surrounded by a layer of connective tissue called the endoneurium; the axons are bundled together into groups called fascicles, each fascicle is wrapped in a layer of connective tissue called the perineurium. The entire nerve is wrapped in a layer of connective tissue called the epineurium. In the central nervous system, the analogous structures are known as tracts; each nerve is covered on the outside by a dense sheath of the epineurium. Beneath this is a layer of flat cells, the perineurium, which forms a complete sleeve around a bundle of axons.
Perineurial septae subdivide it into several bundles of fibres. Surrounding each such fibre is the endoneurium; this forms an unbroken tube from the surface of the spinal cord to the level where the axon synapses with its muscle fibres, or ends in sensory receptors. The endoneurium consists of an inner sleeve of material called the glycocalyx and an outer, meshwork of collagen fibres. Nerves are bundled and travel along with blood vessels, since the neurons of a nerve have high energy requirements. Within the endoneurium, the individual nerve fibres are surrounded by a low-protein liquid called endoneurial fluid; this acts in a similar way to the cerebrospinal fluid in the central nervous system and constitutes a blood-nerve barrier similar to the blood-brain barrier. Molecules are thereby prevented from crossing the blood into the endoneurial fluid. During the development of nerve edema from nerve irritation, the amount of endoneurial fluid may increase at the site of irritation; this increase in fluid can be visualized using magnetic resonance neurography, thus MR neurography can identify nerve irritation and/or injury.
Nerves are categorized into three groups based on the direction that signals are conducted: Afferent nerves conduct signals from sensory neurons to the central nervous system, for example from the mechanoreceptors in skin. Efferent nerves conduct signals from the central nervous system along motor neurons to their target muscles and glands. Mixed nerves contain both afferent and efferent axons, thus conduct both incoming sensory information and outgoing muscle commands in the same bundle. Nerves can be categorized into two groups based on where they connect to the central nervous system: Spinal nerves innervate much of the body, connect through the vertebral column to the spinal cord and thus to the central nervous system, they are given letter-number designations according to the vertebra through which they connect to the spinal column. Cranial nerves innervate parts of the head, connect directly to the brain, they are assigned Roman numerals from 1 to 12, although cranial nerve zero is sometimes included.
In addition, cranial nerves have descriptive names. Specific terms are used to describe their actions. A nerve that supplies information to the brain from an area of the body, or controls an action of the body is said to "innervate" that section of the body or organ. Other terms relate to whether the nerve affects the same side or opposite side of the body, to the part of the brain that supplies it. Nerve growth ends in adolescence, but can be re-stimulated with a molecular mechanism known as "Notch signaling". If the axons of a neuron are damaged, as long as the cell body of the neuron is not damaged, the axons would regenerate and remake the synaptic connections with neurons with the help of guidepost cells; this is referred to as neuroregeneration. The nerve begins the process by destroying the nerve distal to the site of injury allowing Schwann cells, basal lamina, the neurilemma near the injury to begin producing a regeneration tube. Nerve growth factors are produced causing many nerve sprouts to bud.
When one of the growth processes finds the regeneration tube, it begins to grow towards its original destination guided the entire time by the regeneration tube. Nerve regeneration is slow and can take up to several months to complete. While this process does repair some nerves, there will still be some functional deficit as the repairs are not perfect. A nerve conveys information in the form of electrochemical impulses carried by the individual neurons that make up the nerve; these impulses are fast, with some myelinated neurons conducting at speeds up to 120 m/s. The impulses travel from one neuron to another by crossing a synapse, the message is converted from electrical to chemical and back to electrical. Nerves can be categorized into two groups based on function: An afferent nerve fiber conducts sensory information from a sensory neuron to the central nervous system, where the information is processed. Bundles of fibres or axons, in the peripheral nervous system are called nerves, bundles of afferent fibers are known as sensory nerves.
An efferent nerve fiber conducts signals from a motor neuron in the central nervous system to muscles. Bundles of these fibres are known as efferent nerves; the nervous system is the part of an animal that coordinates its actions by transmitting signals to and from different parts of its body. In vertebrates it consists of two main par
Myelin is a lipid-rich substance formed in the central nervous system by glial cells called oligodendrocytes, in the peripheral nervous system by Schwann cells. Myelin insulates nerve cell axons to increase the speed at which information travels from one nerve cell body to another or, for example, from a nerve cell body to a muscle; the myelinated axon can be likened to an electrical wire with insulating material around it. However, unlike the plastic covering on an electrical wire, myelin does not form a single long sheath over the entire length of the axon. Rather, each myelin sheath insulates the axon over a single section and, in general, each axon comprises multiple long myelinated sections separated from each other by short gaps called Nodes of Ranvier; each myelin sheath is formed by the concentric wrapping of an oligodendrocyte or Schwann cell process around the axon. More myelin speeds the transmission of electrical impulses called action potentials along myelinated axons by insulating the axon and reducing axonal membrane capacitance.
This results in saltatory conduction whereby the action potential "jumps" from one node of Ranvier, over a long myelinated stretch of the axon called the internode, before "recharging" at the next node of Ranvier, so on, until it reaches the axon terminal. Nodes of Ranvier are the short unmyelinated regions of the axon between adjacent long myelinated internodes. Once it reaches the axon terminal, this electrical signal provokes the release of a chemical message or neurotransmitter that binds to receptors on the adjacent post-synaptic cell at specialised regions called synapses; this "insulating" role for myelin is essential for normal motor function, sensory function and cognition, as demonstrated by the consequences of disorders that affect it, such as the genetically determined leukodystrophies. Due to its high prevalence, multiple sclerosis, which affects the central nervous system, is the best known disorder of myelin; the process of generating myelin is called myelination or myelinogenesis.
In the CNS, cells called oligodendrocyte precursor cells differentiate into mature oligodendrocytes, which form myelin. In humans, myelination begins early in the 3rd trimester, although only little myelin is present in either the CNS or the PNS at the time of birth. During infancy, myelination progresses with increasing numbers of axons acquiring myelin sheaths; this corresponds with the development of cognitive and motor skills, including language comprehension, speech acquisition and walking. Myelination continues through adolescence and early adulthood and although complete at this time, myelin sheaths can be added in grey matter regions such as the cerebral cortex, throughout life. Myelin is considered a defining characteristic of the jawed vertebrates, but axons are ensheathed by glial cells in invertebrates, although these glial-wraps are quite different from vertebrate compact myelin, formed, as indicated above, by concentric wrapping of the myelinating cell process multiple times around the axon.
Myelin was first described in 1854 by Rudolf Virchow, although it was over a century following the development of electron microscopy, that its glial cell origin and its ultrastructure became apparent. In vertebrates, not all axons are myelinated. For example, in the PNS, a large proportion of axons are unmyelinated. Instead, they are ensheathed by non-myelinating Schwann cells known as Remak SCs and arranged in Remak bundles. In the CNS, non-myelinated, intermingle with myelinated ones and are entwined, at least by the processes of another type of glial cell called the astrocyte. CNS myelin differs in composition and configuration from PNS myelin, but both perform the same "insulating" function. Being rich in lipid, myelin appears white. Both CNS white matter tracts and PNS nerves each comprise thousands to millions of axons aligned in parallel. Blood vessels provide the route for oxygen and energy substrates such as glucose to reach these fibre tracts, which contain other cell types including astrocytes and microglia in the CNS and macrophages in the PNS.
In terms of total mass, myelin comprises 40% water. Protein content includes myelin basic protein, abundant in the CNS where it plays a critical, non-redundant role in formation of compact myelin. In the PNS, myelin protein zero has a similar role to that of PLP in the CNS in that it is involved in holding together the multiple concentric layers of glial cell membrane that constitute the myelin sheath; the primary lipid of myelin is a glycolipid called galactocerebroside. The intertwining hydrocarbon chains of sphingomyelin strengthen the
Inflammation is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants, is a protective response involving immune cells, blood vessels, molecular mediators. The function of inflammation is to eliminate the initial cause of cell injury, clear out necrotic cells and tissues damaged from the original insult and the inflammatory process, initiate tissue repair; the five classical signs of inflammation are heat, redness and loss of function. Inflammation is a generic response, therefore it is considered as a mechanism of innate immunity, as compared to adaptive immunity, specific for each pathogen. Too little inflammation could lead to progressive tissue destruction by the harmful stimulus and compromise the survival of the organism. In contrast, chronic inflammation may lead to a host of diseases, such as hay fever, atherosclerosis, rheumatoid arthritis, cancer. Inflammation is therefore closely regulated by the body. Inflammation can be classified as either chronic.
Acute inflammation is the initial response of the body to harmful stimuli and is achieved by the increased movement of plasma and leukocytes from the blood into the injured tissues. A series of biochemical events propagates and matures the inflammatory response, involving the local vascular system, the immune system, various cells within the injured tissue. Prolonged inflammation, known as chronic inflammation, leads to a progressive shift in the type of cells present at the site of inflammation, such as mononuclear cells, is characterized by simultaneous destruction and healing of the tissue from the inflammatory process. Inflammation is not a synonym for infection. Infection describes the interaction between the action of microbial invasion and the reaction of the body's inflammatory response—the two components are considered together when discussing an infection, the word is used to imply a microbial invasive cause for the observed inflammatory reaction. Inflammation on the other hand describes purely the body's immunovascular response, whatever the cause may be.
But because of how the two are correlated, words ending in the suffix -itis are sometimes informally described as referring to infection. For example, the word urethritis means only "urethral inflammation", but clinical health care providers discuss urethritis as a urethral infection because urethral microbial invasion is the most common cause of urethritis, it is useful to differentiate inflammation and infection because there are typical situations in pathology and medical diagnosis where inflammation is not driven by microbial invasion – for example, trauma and autoimmune diseases including type III hypersensitivity. Conversely, there is pathology where microbial invasion does not cause the classic inflammatory response – for example, parasitosis or eosinophilia. Acute inflammation is a short-term process appearing within a few minutes or hours and begins to cease upon the removal of the injurious stimulus, it involves a coordinated and systemic mobilization response locally of various immune and neurological mediators of acute inflammation.
In a normal healthy response, it becomes activated, clears the pathogen and begins a repair process and ceases. It is characterized by five cardinal signs:An acronym that may be used to remember the key symptoms is "PRISH", for pain, immobility and heat; the traditional names for signs of inflammation come from Latin: Dolor Calor Rubor Tumor Functio laesa The first four were described by Celsus, while loss of function was added by Galen. However, the addition of this fifth sign has been ascribed to Thomas Sydenham and Virchow. Redness and heat are due to increased blood flow at body core temperature to the inflamed site. Loss of function has multiple causes. Acute inflammation of the lung does not cause pain unless the inflammation involves the parietal pleura, which does have pain-sensitive nerve endings; the process of acute inflammation is initiated by resident immune cells present in the involved tissue resident macrophages, dendritic cells, Kupffer cells and mast cells. These cells possess surface receptors known as pattern recognition receptors, which recognize two subclasses of molecules: pathogen-associated molecular patterns and damage-associated molecular patterns.
PAMPs are compounds that are associated with various pathogens, but which are distinguishable from host molecules. DAMPs are compounds that are associated with host-related cell damage. At the onset of an infection, burn, or other injuries, these cells undergo activation and release inflammatory mediators responsible for the clinical signs of inflammation. Vasodilation and its resulting increased blood flow causes increased heat. Increased permeability of the blood vessels results in an exudation of plasma proteins and fluid into the tissue, which manifests itself as swelling; some of the released mediators such as bradykinin increase the sensitivity to pain. The mediator molecules alter the blood vessels to
Anatomical terms of location
Standard anatomical terms of location deal unambiguously with the anatomy of animals, including humans. All vertebrates have the same basic body plan – they are bilaterally symmetrical in early embryonic stages and bilaterally symmetrical in adulthood; that is, they have mirror-image left and right halves if divided down the middle. For these reasons, the basic directional terms can be considered to be those used in vertebrates. By extension, the same terms are used for many other organisms as well. While these terms are standardized within specific fields of biology, there are unavoidable, sometimes dramatic, differences between some disciplines. For example, differences in terminology remain a problem that, to some extent, still separates the terminology of human anatomy from that used in the study of various other zoological categories. Standardized anatomical and zoological terms of location have been developed based on Latin and Greek words, to enable all biological and medical scientists to delineate and communicate information about animal bodies and their component organs though the meaning of some of the terms is context-sensitive.
The vertebrates and Craniata share a substantial heritage and common structure, so many of the same terms are used for location. To avoid ambiguities this terminology is based on the anatomy of each animal in a standard way. For humans, one type of vertebrate, anatomical terms may differ from other forms of vertebrates. For one reason, this is because humans have a different neuraxis and, unlike animals that rest on four limbs, humans are considered when describing anatomy as being in the standard anatomical position, thus what is on "top" of a human is the head, whereas the "top" of a dog may be its back, the "top" of a flounder could refer to either its left or its right side. For invertebrates, standard application of locational terminology becomes difficult or debatable at best when the differences in morphology are so radical that common concepts are not homologous and do not refer to common concepts. For example, many species are not bilaterally symmetrical. In these species, terminology depends on their type of symmetry.
Because animals can change orientation with respect to their environment, because appendages like limbs and tentacles can change position with respect to the main body, positional descriptive terms need to refer to the animal as in its standard anatomical position. All descriptions are with respect to the organism in its standard anatomical position when the organism in question has appendages in another position; this helps avoid confusion in terminology. In humans, this refers to the body in a standing position with arms at the side and palms facing forward. While the universal vertebrate terminology used in veterinary medicine would work in human medicine, the human terms are thought to be too well established to be worth changing. Many anatomical terms can be combined, either to indicate a position in two axes or to indicate the direction of a movement relative to the body. For example, "anterolateral" indicates a position, both anterior and lateral to the body axis. In radiology, an X-ray image may be said to be "anteroposterior", indicating that the beam of X-rays pass from their source to patient's anterior body wall through the body to exit through posterior body wall.
There is no definite limit to the contexts in which terms may be modified to qualify each other in such combinations. The modifier term is truncated and an "o" or an "i" is added in prefixing it to the qualified term. For example, a view of an animal from an aspect at once dorsal and lateral might be called a "dorsolateral" view. Again, in describing the morphology of an organ or habitus of an animal such as many of the Platyhelminthes, one might speak of it as "dorsiventrally" flattened as opposed to bilaterally flattened animals such as ocean sunfish. Where desirable three or more terms may be agglutinated or concatenated, as in "anteriodorsolateral"; such terms sometimes used to be hyphenated. There is however little basis for any strict rule to interfere with choice of convenience in such usage. Three basic reference planes are used to describe location; the sagittal plane is a plane parallel to the sagittal suture. All other sagittal planes are parallel to it, it is known as a "longitudinal plane".
The plane is perpendicular to the ground. The median plane or midsagittal plane is in the midline of the body, divides the body into left and right portions; this passes through the head, spinal cord, and, in many animals, the tail. The term "median plane" can refer to the midsagittal plane of other structures, such as a digit; the frontal plane or coronal plane divides the body into ventral portions. For post-embryonic humans a coronal plane is vertical and a transverse plane is horizontal, but for embryos and quadrupeds a coronal plane is horizontal and a transverse plane is vertical. A longitudinal plane is any plane perpendicular to the transverse plane; the coronal plane and the sagittal plane are examples of longitudinal planes. A transverse plane known as a cross-section, divides the body into cranial and caudal portions. In human anatomy: A transverse plane is an X-Z plane, parallel to the ground, which s