Central nervous system

The central nervous system (CNS) is the part of the nervous system consisting of the brain and spinal cord. The central nervous system is so named because it integrates the received information and coordinates and influences the activity of all parts of the bodies of bilaterally symmetric animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish—and it contains the majority of the nervous system. Many consider the retina[2] and the optic nerve (cranial nerve II),[3][4] as well as the olfactory nerves (cranial nerve I) and olfactory epithelium[5] as parts of the CNS, synapsing directly on brain tissue without intermediate ganglia. As such, the olfactory epithelium is the only central nervous tissue in direct contact with the environment, which opens up for therapeutic treatments. [5] The CNS is contained within the dorsal body cavity, with the brain housed in the cranial cavity and the spinal cord in the spinal canal. In vertebrates, the brain is protected by the skull, while the spinal cord is protected by the vertebrae.[6] The brain and spinal cord are both enclosed in the meninges.[6] In central nervous systems, the interneuronal space is filled with a large amount of supporting non-nervous cells called neuroglial cells.

Central Nervous System
1201 Overview of Nervous System
Schematic diagram showing the central nervous system in yellow, peripheral in orange
Details
Lymph224
Identifiers
LatinSystema nervosum centrale
pars centralis systematis nervosi[1]
Acronym(s)CNS
MeSHD002490
TAA14.1.00.001
FMA55675
Anatomical terminology

Structure

The central nervous system consists of the two major structures: the brain and spinal cord. The brain is encased in the skull, and protected by the cranium.[7] The spinal cord is continuous with the brain and lies caudally to the brain,[8] and is protected by the vertebrae.[7] The spinal cord reaches from the base of the skull, continues through[7] or starting below[9] the foramen magnum,[7] and terminates roughly level with the first or second lumbar vertebra,[8][9] occupying the upper sections of the vertebral canal.[4]

White and gray matter

1202 White and Gray Matter
Dissection of a brain with labels showing the clear division between white and gray matter.

Microscopically, there are differences between the neurons and tissue of the central nervous system and the peripheral nervous system. The central nervous system is divided in white and gray matter.[8] This can also be seen macroscopically on brain tissue. The white matter consists of axons and oligodendrocytes, while the gray matter consists of neurons and unmyelinated fibers. Both tissues include a number of glial cells (although the white matter contains more), which are often referred to as supporting cells of the central nervous system. Different forms of glial cells have different functions, some acting almost as scaffolding for neuroblasts to climb during neurogenesis such as bergmann glia, while others such as microglia are a specialized form of macrophage, involved in the immune system of the brain as well as the clearance of various metabolites from the brain tissue.[4] Astrocytes may be involved with both clearance of metabolites as well as transport of fuel and various beneficial substances to neurons from the capillaries of the brain. Upon CNS injury astrocytes will proliferate, causing gliosis, a form of neuronal scar tissue, lacking in functional neurons.[4]

The brain (cerebrum as well as midbrain and hindbrain) consists of a cortex, composed of neuron-bodies constituting gray matter, while internally there is more white matter that form tracts and commissures. Apart from cortical gray matter there is also subcortical gray matter making up a large number of different nuclei.[8]

Spinal cord

Sobo 1909 615
Diagram of the columns and of the course of the fibers in the spinal cord. Sensory synapses occur in the dorsal spinal cord (above in this image), and motor nerves leave through the ventral (as well as lateral) horns of the spinal cord as seen below in the image.
1508 Autonomic Control of Pupil Size
Different ways in which the central nervous system can be activated without engaging the cortex, and making us aware of the actions. The above example shows the process in which the pupil dilates during dim light, activating neurons in the spinal cord. The second example shows the constriction of the pupil as a result of the activation of the Eddinger-Westphal nucleus (a cerebral ganglion).

From and to the spinal cord are projections of the peripheral nervous system in the form of spinal nerves (sometimes segmental nerves[7]). The nerves connect the spinal cord to skin, joints, muscles etc. and allow for the transmission of efferent motor as well as afferent sensory signals and stimuli.[8] This allows for voluntary and involuntary motions of muscles, as well as the perception of senses. All in all 31 spinal nerves project from the brain stem,[8] some forming plexa as they branch out, such as the brachial plexa, sacral plexa etc.[7] Each spinal nerve will carry both sensory and motor signals, but the nerves synapse at different regions of the spinal cord, either from the periphery to sensory relay neurons that relay the information to the CNS or from the CNS to motor neurons, which relay the information out.[8]

The spinal cord relays information up to the brain through spinal tracts through the "final common pathway"[8] to the thalamus and ultimately to the cortex.

1615 Locations Spinal Fiber Tracts

Schematic image showing the locations of a few tracts of the spinal cord.

1507 Short and Long Reflexes

Reflexes may also occur without engaging more than one neuron of the central nervous system as in the below example of a short reflex.

Cranial nerves

Apart from the spinal cord, there are also peripheral nerves of the PNS that synapse through intermediaries or ganglia directly on the CNS. These 12 nerves exist in the head and neck region and are called cranial nerves. Cranial nerves bring information to the CNS to and from the face, as well as to certain muscles (such as the trapezius muscle, which is innervated by accessory nerves[7] as well as certain cervical spinal nerves).[7]

Two pairs of cranial nerves; the olfactory nerves and the optic nerves[2] are often considered structures of the central nervous system. This is because they do not synapse first on peripheral ganglia, but directly on central nervous neurons. The olfactory epithelium is significant in that it consists of central nervous tissue expressed in direct contact to the environment, allowing for administration of certain pharmaceuticals and drugs. [5]

A peripheral nerve myelinated by Schwann cells (left) and a CNS neuron myelinated by an oligodendrocyte (right)

Periferal nerve myelination
Neuron with oligodendrocyte and myelin sheath

Brain

Rostrally to the spinal cord lies the brain.[8] The brain makes up the largest portion of the central nervous system, and is often the main structure referred to when speaking of the nervous system. The brain is the major functional unit of the central nervous system. While the spinal cord has certain processing ability such as that of spinal locomotion and can process reflexes, the brain is the major processing unit of the nervous system.

Brainstem

The brainstem consists of the medulla, the pons and the midbrain. The medulla can be referred to as an extension of the spinal cord, and its organization and functional properties are similar to those of the spinal cord.[8] The tracts passing from the spinal cord to the brain pass through here.[8]

Regulatory functions of the medulla nuclei include control of the blood pressure and breathing. Other nuclei are involved in balance, taste, hearing and control of muscles of the face and neck.[8]

The next structure rostral to the medulla is the pons, which lies on the ventral anterior side of the brainstem. Nuclei in the pons include pontine nuclei which work with the cerebellum and transmit information between the cerebellum and the cerebral cortex.[8] In the dorsal posterior pons lie nuclei that have to do with breathing, sleep and taste.[8]

The midbrain (or mesencephalon) is situated above and rostral to the pons, and includes nuclei linking distinct parts of the motor system, among others the cerebellum, the basal ganglia and both cerebral hemispheres. Additionally parts of the visual and auditory systems are located in the mid brain, including control of automatic eye movements.[8]

The brainstem at large provides entry and exit to the brain for a number of pathways for motor and autonomic control of the face and neck through cranial nerves,[8] and autonomic control of the organs is mediated by the tenth cranial (vagus) nerve.[4] A large portion of the brainstem is involved in such autonomic control of the body. Such functions may engage the heart, blood vessels, pupillae, among others.[8]

The brainstem also hold the reticular formation, a group of nuclei involved in both arousal and alertness.[8]

Cerebellum

The cerebellum lies behind the pons. The cerebellum is composed of several dividing fissures and lobes. Its function includes the control of posture, and the coordination of movements of parts of the body, including the eyes and head as well as the limbs. Further it is involved in motion that has been learned and perfected though practice, and will adapt to new learned movements.[8] Despite its previous classification as a motor structure, the cerebellum also displays connections to areas of the cerebral cortex involved in language as well as cognitive functions. These connections have been shown by the use of medical imaging techniques such as fMRI and PET.[8]

The body of the cerebellum holds more neurons than any other structure of the brain including that of the larger cerebrum (or cerebral hemispheres), but is also more extensively understood than other structures of the brain, and includes fewer types of different neurons.[8] It handles and processes sensory stimuli, motor information as well as balance information from the vestibular organ.[8]

Diencephalon

The two structures of the diencephalon worth noting are the thalamus and the hypothalamus. The thalamus acts as a linkage between incoming pathways from the peripheral nervous system as well as the optical nerve (though it does not receive input from the olfactory nerve) to the cerebral hemispheres. Previously it was considered only a "relay station", but it is engaged in the sorting of information that will reach cerebral hemispheres (neocortex).[8]

Apart from its function of sorting information from the periphery, the thalamus also connects the cerebellum and basal ganglia with the cerebrum. In common with the aforementioned reticular system the thalamus is involved in wakefullness and consciousness, such as though the SCN.[8]

The hypothalamus engages in functions of a number of primitive emotions or feelings such as hunger, thirst and maternal bonding. This is regulated partly through control of secretion of hormones from the pituitary gland. Additionally the hypothalamus plays a role in motivation and many other behaviors of the individual.[8]

Cerebrum

The cerebrum of cerebral hemispheres make up the largest visual portion of the human brain. Various structures combine to form the cerebral hemispheres, among others: the cortex, basal ganglia, amygdala and hippocampus. The hemispheres together control a large portion of the functions of the human brain such as emotion, memory, perception and motor functions. Apart from this the cerebral hemispheres stand for the cognitive capabilities of the brain.[8]

Connecting each of the hemispheres is the corpus callosum as well as several additional commissures.[8] One of the most important parts of the cerebral hemispheres is the cortex, made up of gray matter covering the surface of the brain. Functionally, the cerebral cortex is involved in planning and carrying out of everyday tasks.[8]

The hippocampus is involved in storage of memories, the amygdala plays a role in perception and communication of emotion, while the basal ganglia play a major role in the coordination of voluntary movement.[8]

Difference from the peripheral nervous system

1205 Somatic Autonomic Enteric StructuresN
A map over the different structures of the nervous systems in the body, showing the CNS, PNS, and ENS.

This differentiates the central nervous system from the peripheral nervous system, which consists of neurons, axons and Schwann cells. Oligodendrocytes and Schwann cells have similar functions in the central and peripheral nervous system respectively. Both act to add myelin sheaths to the axons, which acts as a form of insulation allowing for better and faster proliferation of electrical signals along the nerves. Axons in the central nervous system are often very short (barely a few millimeters) and do not need the same degree of isolation as peripheral nerves do. Some peripheral nerves can be over 1m in length, such as the nerves to the big toe. To ensure signals move at sufficient speed, myelination is needed.

The way in which the Schwann cells and oligodendrocytes myelinate nerves differ. A Schwann cell usually myelinates a single axon, completely surrounding it. Sometimes they may myelinate many axons, especially when in areas of short axons.[7] Oligodendrocytes usually myelinate several axons. They do this by sending out thin projections of their cell membrane which envelop and enclose the axon.

Development

Top; CNS as seen in a median section of a 5 week old embryo.
Bottom; CNS seen in a median section of a 3 month old embryo.

Sobo 1909 621
Sobo 1909 622

During early development of the vertebrate embryo, a longitudinal groove on the neural plate gradually deepens and the ridges on either side of the groove (the neural folds) become elevated, and ultimately meet, transforming the groove into a closed tube called the neural tube.[10] The formation of the neural tube is called neurulation. At this stage, the walls of the neural tube contain proliferating neural stem cells in a region called the ventricular zone. The neural stem cells, principally radial glial cells, multiply and generate neurons through the process of neurogenesis, forming the rudiment of the central nervous system.[11]

The neural tube gives rise to both brain and spinal cord. The anterior (or 'rostral') portion of the neural tube initially differentiates into three brain vesicles (pockets): the prosencephalon at the front, the mesencephalon, and, between the mesencephalon and the spinal cord, the rhombencephalon. (By six weeks in the human embryo) the prosencephalon then divides further into the telencephalon and diencephalon; and the rhombencephalon divides into the metencephalon and myelencephalon. The spinal cord is derived from the posterior or 'caudal' portion of the neural tube.

As a vertebrate grows, these vesicles differentiate further still. The telencephalon differentiates into, among other things, the striatum, the hippocampus and the neocortex, and its cavity becomes the first and second ventricles. Diencephalon elaborations include the subthalamus, hypothalamus, thalamus and epithalamus, and its cavity forms the third ventricle. The tectum, pretectum, cerebral peduncle and other structures develop out of the mesencephalon, and its cavity grows into the mesencephalic duct (cerebral aqueduct). The metencephalon becomes, among other things, the pons and the cerebellum, the myelencephalon forms the medulla oblongata, and their cavities develop into the fourth ventricle.[8]

EmbryonicBrain

Diagram depicting the main subdivisions of the embryonic vertebrate brain, later forming forebrain, midbrain and hindbrain.

Development of the neural tube

Development of the neural tube

Central
nervous
system
Brain Prosencephalon Telencephalon

Rhinencephalon, Amygdala, Hippocampus, Neocortex, Basal ganglia, Lateral ventricles

Diencephalon

Epithalamus, Thalamus, Hypothalamus, Subthalamus, Pituitary gland, Pineal gland, Third ventricle

Brain stem Mesencephalon

Tectum, Cerebral peduncle, Pretectum, Mesencephalic duct

Rhombencephalon Metencephalon

Pons, Cerebellum

Myelencephalon Medulla oblongata
Spinal cord

Evolution

Top: the lancelet, regarded an archetypal vertebrate, lacking a true brain. Middle: an early vertebrate. Bottom: spindle diagram of the evolution of vertebrates.

Branchiostoma lanceolatum
Haikouichthys cropped
Spindle diagram

Planaria

Planarians, members of the phylum Platyhelminthes (flatworms), have the simplest, clearly defined delineation of a nervous system into a central nervous system (CNS) and a peripheral nervous system (PNS).[12][13] Their primitive brains, consisting of two fused anterior ganglia, and longitudinal nerve cords form the CNS; the laterally projecting nerves form the PNS. A molecular study found that more than 95% of the 116 genes involved in the nervous system of planarians, which includes genes related to the CNS, also exist in humans.[14] Like planarians, vertebrates have a distinct CNS and PNS, though more complex than those of planarians.

Arthropoda

In arthropods, the ventral nerve cord, the subesophageal ganglia and the supraesophageal ganglia are usually seen as making up the CNS.

Chordata

The CNS of chordates differs from that of other animals in being placed dorsally in the body, above the gut and notochord/spine.[15] The basic pattern of the CNS is highly conserved throughout the different species of vertebrates and during evolution. The major trend that can be observed is towards a progressive telencephalisation: the telencephalon of reptiles is only an appendix to the large olfactory bulb, while in mammals it makes up most of the volume of the CNS. In the human brain, the telencephalon covers most of the diencephalon and the mesencephalon. Indeed, the allometric study of brain size among different species shows a striking continuity from rats to whales, and allows us to complete the knowledge about the evolution of the CNS obtained through cranial endocasts.

Mammals – which appear in the fossil record after the first fishes, amphibians, and reptiles – are the only vertebrates to possess the evolutionarily recent, outermost part of the cerebral cortex known as the neocortex.[16] The neocortex of monotremes (the duck-billed platypus and several species of spiny anteaters) and of marsupials (such as kangaroos, koalas, opossums, wombats, and Tasmanian devils) lack the convolutions – gyri and sulci – found in the neocortex of most placental mammals (eutherians).[17] Within placental mammals, the size and complexity of the neocortex increased over time. The area of the neocortex of mice is only about 1/100 that of monkeys, and that of monkeys is only about 1/10 that of humans.[16] In addition, rats lack convolutions in their neocortex (possibly also because rats are small mammals), whereas cats have a moderate degree of convolutions, and humans have quite extensive convolutions.[16] Extreme convolution of the neocortex is found in dolphins, possibly related to their complex echolocation.

Clinical significance

Diseases

There are many central nervous system diseases and conditions, including infections of the central nervous system such as encephalitis and poliomyelitis, early-onset neurological disorders including ADHD and autism, late-onset neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and essential tremor, autoimmune and inflammatory diseases such as multiple sclerosis and acute disseminated encephalomyelitis, genetic disorders such as Krabbe's disease and Huntington's disease, as well as amyotrophic lateral sclerosis and adrenoleukodystrophy. Lastly, cancers of the central nervous system can cause severe illness and, when malignant, can have very high mortality rates. Symptoms depend on the size, growth rate, location and malignancy of tumors and can include alterations in motor control, hearing loss, headaches and changes in cognitive ability and autonomic functioning.

Specialty professional organizations recommend that neurological imaging of the brain be done only to answer a specific clinical question and not as routine screening.[18]

References

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  2. ^ a b Purves, Dale (2000). Neuroscience, Second Edition. Sunderland, MA: Sinauer Associates. ISBN 9780878937424. Archived from the original on 11 March 2014.
  3. ^ "Medical Subject Headings (MeSH): Optic Nerve". National Library of Medicine. Archived from the original on 2 October 2013. Retrieved 28 September 2013.
  4. ^ a b c d e Estomih Mtui, M.J. Turlough FitzGerald, Gregory Gruener. Clinical neuroanatomy and neuroscience (6th ed.). Edinburgh: Saunders. p. 38. ISBN 978-0-7020-3738-2.
  5. ^ a b c Gizurarson S (2012). "Anatomical and histologica\ ]=\ factors affecting intranasal drug and vaccine delivery". Current Drug Delivery. 9 (6): 566–582. doi:10.2174/156720112803529828. PMC 3480721. PMID 22788696.
  6. ^ a b 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.
  7. ^ a b c d e f g h i Arthur F. Dalley, Keith L. Moore, Anne M.R. Agur (2010). Clinically oriented anatomy (6th ed., [International ed.]. ed.). Philadelphia [etc.]: Lippincott Williams & Wilkins, Wolters Kluwer. pp. 48–55, 464, 700, 822, 824, 1075. ISBN 978-1-60547-652-0.
  8. ^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab ac ad Kandel ER, Schwartz JH (2012). Principles of neural science (5. ed.). Appleton & Lange: McGraw Hill. pp. 338–343. ISBN 978-0-07-139011-8.
  9. ^ a b Huijzen, R. Nieuwenhuys, J. Voogd, C. van (2007). The human central nervous system (4th ed.). Berlin: Springer. p. 3. ISBN 978-3-540-34686-9.
  10. ^ Gilbert, Scott F.; College, Swarthmore; Helsinki, the University of (2014). Developmental biology (Tenth ed.). Sunderland, Mass.: Sinauer. ISBN 978-0878939787.
  11. ^ Rakic, P (October 2009). "Evolution of the neocortex: a perspective from developmental biology". Nature Reviews. Neuroscience. 10 (10): 724–35. doi:10.1038/nrn2719. PMC 2913577. PMID 19763105.
  12. ^ Hickman, Jr., Cleveland P.; Larry S. Roberts; Susan L. Keen; Allan Larson; Helen L'Anson; David J. Eisenhour (2008). Integrated Princinples of Zoology: Fourteenth Edition. New York, NY, USA: McGraw-Hill Higher Education. p. 733. ISBN 978-0-07-297004-3.
  13. ^ Campbell, Neil A.; Jane B. Reece; Lisa A. Urry; Michael L. Cain; Steven A. Wasserman; Peter V. Minorsky; Robert B. Jackson (2008). Biology: Eighth Edition. San Francisco, CA, USA: Pearson / Benjamin Cummings. p. 1065. ISBN 978-0-8053-6844-4.
  14. ^ Mineta K, Nakazawa M, Cebria F, Ikeo K, Agata K, Gojobori T (2003). "Origin and evolutionary process of the CNS elucidated by comparative genomics analysis of planarian ESTs". PNAS. 100 (13): 7666–7671. doi:10.1073/pnas.1332513100. PMC 164645. PMID 12802012. Archived from the original on 24 September 2015.
  15. ^ Romer, A.S. (1949): The Vertebrate Body. W.B. Saunders, Philadelphia. (2nd ed. 1955; 3rd ed. 1962; 4th ed. 1970)
  16. ^ a b c Bear, Mark F.; Barry W. Connors; Michael A. Paradiso (2007). Neuroscience: Exploring the Brain: Third Edition. Philadelphia, PA, USA: Lippincott Williams & Wilkins. pp. 196–199. ISBN 978-0-7817-6003-4.
  17. ^ Kent, George C.; Robert K. Carr (2001). Comparative Anatomy of the Vertebrates: Ninth Edition. New York, NY, USA: McGraw-Hill Higher Education. p. 409. ISBN 0-07-303869-5.
  18. ^ American College of Radiology; American Society of Neuroradiology (2010). "ACR-ASNR practice guideline for the performance of computed tomography (CT) of the brain". Agency for Healthcare Research and Quality. Reston, VA, USA: American College of Radiology. Archived from the original on 15 September 2012. Retrieved 9 September 2012.

External links

Acute radiation syndrome

Acute radiation syndrome (ARS) is a collection of health effects that are present within 24 hours of exposure to high doses of ionizing radiation. It is also called radiation poisoning, radiation sickness and radiation toxicity.

The onset and type of symptoms depend on the amount of radiation exposure, both in any one dose, and cumulative exposure. Relatively smaller doses result in gastrointestinal effects, such as nausea and vomiting, and symptoms related to falling blood counts, and predisposition to infection and bleeding. Relatively larger doses can result in neurological effects, including but not limited to seizures, tremors, lethargy, and rapid death. Treatment of acute radiation syndrome is generally supportive with blood transfusions and antibiotics, with some extreme cases requiring more aggressive treatments, such as bone marrow transfusions.The radiation causes cellular degradation due to damage to DNA and other key molecular structures within the cells in various tissues. This destruction, particularly because it affects the ability of cells to divide normally, in turn causes the symptoms. The symptoms can begin within one hour and may last for several months. The terms refer to acute medical problems rather than ones that develop after a prolonged period.Similar symptoms may appear months to years after exposure as chronic radiation syndrome when the dose rate is too low to cause the acute form. Radiation exposure can also increase the probability of developing some other diseases, mainly different types of cancers. These later-developing diseases are sometimes also described as radiation sickness, but they are never included in the term acute radiation syndrome.

Brain tumor

A brain tumor occurs when abnormal cells form within the brain. There are two main types of tumors: malignant or cancerous tumors and benign tumors. Cancerous tumors can be divided into primary tumors, which start within the brain, and secondary tumors, which have spread from elsewhere, known as brain metastasis tumors. All types of brain tumors may produce symptoms that vary depending on the part of the brain involved. These symptoms may include headaches, seizures, problems with vision, vomiting and mental changes. The headache is classically worse in the morning and goes away with vomiting. Other symptoms may include difficulty walking, speaking or with sensations. As the disease progresses, unconsciousness may occur.The cause of most brain tumors is unknown. Uncommon risk factors include inherited neurofibromatosis, exposure to vinyl chloride, Epstein–Barr virus and ionizing radiation. The evidence for mobile phone exposure is not clear. The most common types of primary tumors in adults are meningiomas (usually benign) and astrocytomas such as glioblastomas. In children, the most common type is a malignant medulloblastoma. Diagnosis is usually by medical examination along with computed tomography or magnetic resonance imaging. The result is then often confirmed by a biopsy. Based on the findings, the tumors are divided into different grades of severity.Treatment may include some combination of surgery, radiation therapy and chemotherapy. Anticonvulsant medication may be needed if seizures occur. Dexamethasone and furosemide may be used to decrease swelling around the tumor. Some tumors grow gradually, requiring only monitoring and possibly needing no further intervention. Treatments that use a person's immune system are being studied. Outcome varies considerably depending on the type of tumor and how far it has spread at diagnosis. Glioblastomas usually have poor outcomes, while meningiomas usually have good outcomes. The average five-year survival rate for all brain cancers in the United States is 33%.Secondary, or metastatic, brain tumors are more common than primary brain tumors, with about half of metastases coming from lung cancer. Primary brain tumors occur in around 250,000 people a year globally, making up less than 2% of cancers. In children younger than 15, brain tumors are second only to acute lymphoblastic leukemia as the most common form of cancer. In Australia, the average lifetime economic cost of a case of brain cancer is $1.9 million, the greatest of any type of cancer.

Cavernous hemangioma

Cavernous hemangioma, also called cavernous angioma, cavernoma, or cerebral cavernous malformation (CCM) (when referring to presence in the brain) is a type of blood vessel malformation or hemangioma, where a collection of dilated blood vessels form a lesion. Because of this malformation, blood flow through the cavities, or caverns, is slow. Additionally, the cells that form the vessels do not form the necessary junctions with surrounding cells. Also, the structural support from the smooth muscle is hindered, causing leakage into the surrounding tissue. It is the leakage of blood, known as a hemorrhage from these vessels that causes a variety of symptoms known to be associated with this disease.

Central nervous system depression

Central nervous system depression is a physiological state that can result in a decreased rate of breathing, decreased heart rate, and loss of consciousness possibly leading to coma or death. It is the result of inhibited or suppressed brain activity.

Central nervous system disease

Central nervous system diseases, also known as central nervous system disorders, are a group of neurological disorders that affect the structure or function of the brain or spinal cord, which collectively form the central nervous system (CNS).

Central nervous system viral disease

The Central Nervous System controls most of the functions of the body and mind. It comprises the brain, spinal cord and the nerve fibers that branch off to all parts of the body. The Central Nervous System viral diseases are caused by viruses that attack the CNS. Existing and emerging viral CNS infections are major sources of human morbidity and mortality. Virus infections usually begin in the peripheral tissues, and can invade the mammalian system by spreading into the peripheral nervous system and more rarely the CNS. CNS is protected by effective immune responses and multi-layer barriers, but some viruses enter with high-efficiency through the bloodstream and some by directly infecting the nerves that innervate the tissues. Most viruses that enter can be opportunistic and accidental pathogens, but some like herpes viruses and rabies virus have evolved in time to enter the nervous system efficiently, by exploiting the neuronal cell biology. While acute viral diseases come on quickly, chronic viral conditions have long incubation periods inside the body. Their symptoms develop slowly and follow a progressive, fatal course.

Demyelinating disease

A demyelinating disease is any disease of the nervous system in which the myelin sheath of neurons is damaged. This damage impairs the conduction of signals in the affected nerves. In turn, the reduction in conduction ability causes deficiency in sensation, movement, cognition, or other functions depending on which nerves are involved.

Some demyelinating diseases are caused by genetics, some by infectious agents, some by autoimmune reactions, and some by unknown factors. Organophosphates, a class of chemicals which are the active ingredients in commercial insecticides such as sheep dip, weed killers, and flea treatment preparations for pets, etc., also demyelinate nerves. Neuroleptics can also cause demyelination.Demyelinating diseases are traditionally classified in two kinds: demyelinating myelinoclastic diseases and demyelinating leukodystrophic diseases. In the first group, a normal and healthy myelin is destroyed by a toxic, chemical, or autoimmune substance. In the second group, myelin is abnormal and degenerates. The second group was denominated dysmyelinating diseases by Poser.In the most known example, multiple sclerosis, good evidence shows that the body's own immune system is at least partially responsible. Acquired immune system cells called T-cells are known to be present at the site of lesions. Other immune-system cells called macrophages (and possibly mast cells) also contribute to the damage.Vitamin B12 deficiency can cause demyelination.

Depressant

A depressant, or central depressant, is a drug that lowers neurotransmission levels, which is to depress or reduce arousal or stimulation, in various areas of the brain. Depressants are also occasionally referred to as "downers" as they lower the level of arousal when taken. Stimulants or "uppers" increase mental and/or physical function, hence the opposite drug class of depressants is stimulants, not antidepressants.

Depressants are widely used throughout the world as prescription medicines and as illicit substances. Alcohol is a very prominent depressant. Alcohol can be and is more likely to be a large problem among teenagers and young adults. When depressants are used, effects often include ataxia, anxiolysis, pain relief, sedation or somnolence, and cognitive/memory impairment, as well as in some instances euphoria, dissociation, muscle relaxation, lowered blood pressure or heart rate, respiratory depression, and anticonvulsant effects, and even complete anesthesia or death at high doses. Cannabis may sometimes be considered a depressant. THC may slow brain function to a small degree, while reducing reaction to stimuli. Cannabis may also treat insomnia, anxiety and muscle spasms similar to other depressive drugs. Other depressants can include drugs like xanax and a number of opiates.

Depressants exert their effects through a number of different pharmacological mechanisms, the most prominent of which include facilitation of GABA, and inhibition of glutamatergic or monoaminergic activity. Other examples are chemicals that modify the electrical signaling inside the body. The most prominent of these being bromides and channel blockers.

Encephalomyelitis

Encephalomyelitis is inflammation of the brain and spinal cord. Various types of encephalomyelitis include:

Acute disseminated encephalomyelitis or postinfectious encephalomyelitis, a demyelinating disease of the brain and spinal cord, possibly triggered by viral infection.

Encephalomyelitis disseminata, a synonym for multiple sclerosis.

AntiMOG associated encephalomyelitis, one of the underlying conditions for the phenotype neuromyelitis optica and in general all the spectrum of MOG autoantibody-associated demyelinating diseases.

Equine encephalomyelitis, also called equine encephalitis, a potentially fatal mosquito-borne viral disease that infects horses and humans.

Myalgic encephalomyelitis, a disease involving presumed inflammation of the central nervous system with symptoms of muscle pain and fatigue; the term has sometimes been used interchangeably with chronic fatigue syndrome, though there is still controversy over the distinction.

Experimental autoimmune encephalomyelitis (EAE), an animal model of brain inflammation.

Progressive encephalomyelitis with rigidity and myoclonus (PERM) – A kind of stiff person syndrome.

AIDS related encephalomyelitis, caused by opportunistic Human T-lymphotropic virus type III (HTLV-III) infection.

Extrapyramidal system

In anatomy, the extrapyramidal system is a part of the motor system network causing involuntary actions. The system is called extrapyramidal to distinguish it from the tracts of the motor cortex that reach their targets by traveling through the pyramids of the medulla. The pyramidal tracts (corticospinal tract and corticobulbar tracts) may directly innervate motor neurons of the spinal cord or brainstem (anterior (ventral) horn cells or certain cranial nerve nuclei), whereas the extrapyramidal system centers on the modulation and regulation (indirect control) of anterior (ventral) horn cells.

Extrapyramidal tracts are chiefly found in the reticular formation of the pons and medulla, and target lower motor neurons in the spinal cord that are involved in reflexes, locomotion, complex movements, and postural control. These tracts are in turn modulated by various parts of the central nervous system, including the nigrostriatal pathway, the basal ganglia, the cerebellum, the vestibular nuclei, and different sensory areas of the cerebral cortex. All of these regulatory components can be considered part of the extrapyramidal system, in that they modulate motor activity without directly innervating motor neurons.

The extrapyramidal tracts include parts of the following:

rubrospinal tract

pontine reticulospinal tract

medullary reticulospinal tract

lateral vestibulospinal tract

tectospinal tract

Grey matter

Grey matter (or gray matter) is a major component of the central nervous system, consisting of neuronal cell bodies, neuropil (dendrites and myelinated as well as unmyelinated axons), glial cells (astrocytes and oligodendrocytes), synapses, and capillaries. Grey matter is distinguished from white matter in that it contains numerous cell bodies and relatively few myelinated axons, while white matter contains relatively few cell bodies and is composed chiefly of long-range myelinated axon tracts. The colour difference arises mainly from the whiteness of myelin. In living tissue, grey matter actually has a very light grey colour with yellowish or pinkish hues, which come from capillary blood vessels and neuronal cell bodies.

Intracranial hemorrhage

Intracranial hemorrhage (ICH), also known as intracranial bleed, is bleeding within the skull. Subtypes are intracerebral bleeds (intraventricular bleeds and intraparenchymal bleeds), subarachnoid bleeds, epidural bleeds, and subdural bleeds.Intracerebral bleeding affects 2.5 per 10,000 people each year.

Leukoencephalopathy with vanishing white matter

Leukoencephalopathy with vanishing white matter (VWM disease) is an autosomal recessive neurological disease. The cause of the disease are mutations in any of the 5 genes encoding subunits of the translation initiation factor EIF-2B: EIF2B1, EIF2B2, EIF2B3, EIF2B4, or EIF2B5. The disease belongs to a family of conditions called the Leukodystrophies.

Medial lemniscus

The medial lemniscus, also known as Reil's band or Reil's ribbon, is a large ascending bundle of heavily myelinated axons that decussate in the brainstem, specifically in the medulla oblongata. The medial lemniscus is formed by the crossings of the internal arcuate fibers. The internal arcuate fibers are composed of axons of nucleus gracilis and nucleus cuneatus. The axons of the nucleus gracilis and nucleus cuneatus in the medial lemniscus have cell bodies that lie contralaterally.

The medial lemniscus is part of the dorsal column–medial lemniscus pathway, which ascends from the skin to the thalamus, which is important for somatosensation from the skin and joints, therefore, lesion of the medial lemnisci causes an impairment of vibratory and touch-pressure sense.

Nerve

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, and each fascicle is wrapped in a layer of connective tissue called the perineurium. Finally, 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.

Oxygen toxicity

Oxygen toxicity is a condition resulting from the harmful effects of breathing molecular oxygen (O2) at increased partial pressures. Severe cases can result in cell damage and death, with effects most often seen in the central nervous system, lungs, and eyes. Historically, the central nervous system condition was called the Paul Bert effect, and the pulmonary condition the Lorrain Smith effect, after the researchers who pioneered the discoveries and descriptions in the late 19th century. Oxygen toxicity is a concern for underwater divers, those on high concentrations of supplemental oxygen (particularly premature babies), and those undergoing hyperbaric oxygen therapy.

The result of breathing increased partial pressures of oxygen is hyperoxia, an excess of oxygen in body tissues. The body is affected in different ways depending on the type of exposure. Central nervous system toxicity is caused by short exposure to high partial pressures of oxygen at greater than atmospheric pressure. Pulmonary and ocular toxicity result from longer exposure to increased oxygen levels at normal pressure. Symptoms may include disorientation, breathing problems, and vision changes such as myopia. Prolonged exposure to above-normal oxygen partial pressures, or shorter exposures to very high partial pressures, can cause oxidative damage to cell membranes, collapse of the alveoli in the lungs, retinal detachment, and seizures. Oxygen toxicity is managed by reducing the exposure to increased oxygen levels. Studies show that, in the long term, a robust recovery from most types of oxygen toxicity is possible.

Protocols for avoidance of the effects of hyperoxia exist in fields where oxygen is breathed at higher-than-normal partial pressures, including underwater diving using compressed breathing gases, hyperbaric medicine, neonatal care and human spaceflight. These protocols have resulted in the increasing rarity of seizures due to oxygen toxicity, with pulmonary and ocular damage being mainly confined to the problems of managing premature infants.

In recent years, oxygen has become available for recreational use in oxygen bars. The US Food and Drug Administration has warned those suffering from problems such as heart or lung disease not to use oxygen bars. Scuba divers use breathing gases containing up to 100% oxygen, and should have specific training in using such gases.

Vasculitis

Vasculitis is a group of disorders that destroy blood vessels by inflammation. Both arteries and veins are affected. Lymphangitis is sometimes considered a type of vasculitis. Vasculitis is primarily caused by leukocyte migration and resultant damage.

Although both occur in vasculitis, inflammation of veins (phlebitis) or arteries (arteritis) on their own are separate entities.

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