Editing Central nervous system (section)

==Structure==
{{main article|Neuroanatomy}}
The CNS consists of two major structures: the [[brain]] and [[spinal cord]]. The brain is encased in the skull, and protected by the cranium.<ref name="Moore's" /> The spinal cord is continuous with the brain and lies [[anatomical terminology|caudally]] to the brain.<ref name="Kandel" /> It is protected by the [[vertebra]]e.<ref name="Moore's" /> The spinal cord reaches from the base of the skull, and continues through<ref name="Moore's" /> or starting below<ref name="Huijzen" /> the [[foramen magnum]],<ref name="Moore's" /> and terminates roughly level with the first or second [[lumbar vertebra]],<ref name=Kandel>{{cite book|vauthors= Kandel ER, Schwartz JH |title=Principles of neural science|year=2012|publisher=McGraw Hill|location=Appleton & Lange|isbn=978-0-07-139011-8|pages=338–343|edition=5.}}</ref><ref name="Huijzen">{{cite book|last=Huijzen|first=R. Nieuwenhuys, J. Voogd, C. van|title=The human central nervous system|year=2007|publisher=Springer|location=Berlin|isbn=978-3-540-34686-9|page=3|edition=4th}}</ref> occupying the upper sections of the [[vertebral canal]].<ref name="Fitzgerald">{{cite book|last=Estomih Mtui|first=M.J. Turlough FitzGerald, Gregory Gruener|title=Clinical neuroanatomy and neuroscience|publisher=Saunders|location=Edinburgh|isbn=978-0-7020-3738-2|page=38|edition=6th|year=2012}}</ref>

===White and gray matter===
{{main article|Gray matter|White matter}}
[[Image:1202 White and Gray Matter.jpg|thumb|right|Dissection of a human brain with labels showing the clear division between white and gray matter.]]
Microscopically, there are differences between the neurons and tissue of the CNS and the [[peripheral nervous system]] (PNS).<ref>{{cite book | vauthors = Miller AD, Zachary JF | chapter=Nervous System | title = Pathologic Basis of Veterinary Disease. | date=10 May 2020 | pages = 805–907.e1 | doi=10.1016/B978-0-323-35775-3.00014-X | pmc = 7158194 | isbn = 9780323357753 }}</ref> The CNS is composed of [[white matter|white]] and [[gray matter]].<ref name="Kandel" /> This can also be seen macroscopically on brain tissue. The white matter consists of [[axon]]s and [[oligodendrocyte]]s, while the gray matter consists of [[neuron]]s and unmyelinated fibers. Both tissues include a number of [[glia]]l cells (although the white matter contains more), which are often referred to as supporting cells of the CNS. Different forms of glial cells have different functions, some acting almost as scaffolding for [[neuroblast]]s 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 [[metabolite]]s from the [[brain tissue]].<ref name="Fitzgerald" /> [[Astrocyte]]s may be involved with both clearance of metabolites as well as transport of fuel and various beneficial substances to neurons from the [[capillary|capillaries]] of the brain. Upon CNS injury astrocytes will proliferate, causing [[gliosis]], a form of neuronal scar tissue, lacking in functional neurons.<ref name="Fitzgerald" />

The brain ([[cerebrum]] as well as [[midbrain]] and [[hindbrain]]) consists of a [[Cerebral cortex|cortex]], composed of neuron-bodies constituting gray matter, while internally there is more white matter that form [[Nerve tract|tract]]s and [[commissural fiber|commissures]]. Apart from cortical gray matter there is also [[subcortical gray matter]] making up a large number of different [[Nucleus (neuroanatomy)|nuclei]].<ref name="Kandel" />

===Spinal cord===
{{main article|Spinal cord}}
[[File:Sobo 1909 615.png|thumb|right|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.]]
[[Image:1508 Autonomic Control of Pupil Size.jpg|thumb|right|Different ways in which the CNS 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 nerve|spinal]] [[nerve]]s (sometimes segmental nerves<ref name="Moore's" />). The nerves connect the spinal cord to skin, joints, muscles etc. and allow for the transmission of [[Efferent nerve fiber|efferent]] motor as well as [[Afferent nerve fiber|afferent sensory signals]] and stimuli.<ref name="Kandel" /> 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,<ref name="Kandel" /> some forming plexa as they branch out, such as the [[brachial plexus|brachial plexa]], [[sacral plexus|sacral plexa]] etc.<ref name="Moore's" /> 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.<ref name="Kandel" />

The spinal cord relays information up to the brain through spinal tracts through the final common pathway<ref name="Kandel" /> to the [[thalamus]] and ultimately to the cortex.
<gallery>
File:1615 Locations Spinal Fiber Tracts.jpg|Schematic image showing the locations of a few tracts of the spinal cord.
File:1507 Short and Long Reflexes.jpg|Reflexes may also occur without engaging more than one neuron of the CNS as in the below example of a short reflex.
</gallery>

====Cranial nerves====
Apart from the spinal cord, there are also peripheral nerves of the PNS that synapse through intermediaries or [[Ganglion|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 nerve]]s<ref name="Moore's" /> as well as certain [[cervical spinal nerve]]s).<ref name="Moore's" />

Two pairs of cranial nerves; the [[olfactory nerve]]s and the [[optic nerve]]s<ref name="Purves 2000"/> are often considered structures of the CNS. This is because they do not synapse first on peripheral ganglia, but directly on CNS neurons. The olfactory epithelium is significant in that it consists of CNS tissue expressed in direct contact to the environment, allowing for administration of certain pharmaceuticals and drugs.
<ref name="Gizurarson S. 2012 566–582"/>

{{multiple image
| align       = left
| direction   = horizontal
| total_width = 640
| image1      = Periferal nerve myelination.jpg
| alt1        = Image showing the way Schwann cells myelinate periferal nerves.
| width1      = 822
| height1     = 232
| caption1    =
| image2      = Neuron with oligodendrocyte and myelin sheath.svg
| alt2        = A neuron of the CNS, myelinated by an oligodendrocyte
| width2      = 410
| height2     = 485
| caption2    =
| footer      = A peripheral nerve myelinated by [[Schwann cell]]s (left) and a CNS neuron myelinated by an [[oligodendrocyte]] (right)
}}{{clear left}}

===Brain===
{{main article|Brain}}
At the anterior end of the spinal cord lies the brain.<ref name="Kandel" /> The brain makes up the largest portion of the CNS. It is often the main structure referred to when speaking of the nervous system in general. The brain is the major functional unit of the CNS. While the spinal cord has certain processing ability such as that of [[spinal locomotion]] and can process [[reflex]]es, the brain is the major processing unit of the nervous system.<ref>{{cite web | vauthors = Thau L, Reddy V, Singh P | title = Anatomy, Central Nervous System | publisher = StatPearls | date=January 2020 | url = https://www.ncbi.nlm.nih.gov/books/NBK542179/ | access-date = 13 May 2020 | pmid = 31194336}}</ref><ref>{{cite web|url=http://www.cancer.ca/en/cancer-information/cancer-type/brain-spinal/brain-and-spinal-tumours/the-brain-and-spinal-cord/?region=on|title=The brain and spinal cord – Canadian Cancer Society|website=www.cancer.ca|language=en|access-date=2019-03-19}}</ref>

====Brainstem====
{{main article|Brainstem}}
The brainstem consists of the [[medulla oblongata|medulla]], the [[pons]] and the [[midbrain]]. The medulla can be referred to as an extension of the spinal cord, which both have similar organization and functional properties.<ref name="Kandel" /> The tracts passing from the spinal cord to the brain pass through here.<ref name="Kandel" />

Regulatory functions of the medulla nuclei include control of [[blood pressure]] and [[breathing]]. Other nuclei are involved in [[balance (ability)|balance]], [[taste]], [[hearing]], and control of muscles of the [[face]] and [[neck]].<ref name="Kandel" />

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]].<ref name="Kandel" />
In the dorsal posterior pons lie nuclei that are involved in the functions of breathing, sleep, and taste.<ref name="Kandel" />

The midbrain, or mesencephalon, is situated above and rostral to the pons. It includes nuclei linking distinct parts of the motor system, including the cerebellum, the [[basal ganglia]] and both [[cerebral hemisphere]]s, among others. Additionally, parts of the visual and auditory systems are located in the midbrain, including control of automatic eye movements.<ref name="Kandel" />

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,<ref name="Kandel" /> Autonomic control of the organs is mediated by the [[Vagus nerve|tenth cranial nerve]].<ref name="Fitzgerald" /><!-- page 98-199 --> A large portion of the brainstem is involved in such autonomic control of the body. Such functions may engage the [[heart]], [[blood vessels]], and [[pupil]]s, among others.<ref name="Kandel" />

The brainstem also holds the [[reticular formation]], a group of nuclei involved in both [[arousal]] and [[alertness]].<ref name="Kandel" />

====Cerebellum====
{{main article|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 through practice, and it will adapt to new learned movements.<ref name="Kandel" />
Despite its previous classification as a motor structure, the cerebellum also displays connections to areas of the cerebral cortex involved in language and [[cognition]]. These connections have been shown by the use of [[medical imaging]] techniques, such as [[fMRI|functional MRI]] and [[Positron emission tomography]].<ref name="Kandel" />

The body of the cerebellum holds more neurons than any other structure of the brain, including that of the larger [[cerebrum]], but is also more extensively understood than other structures of the brain, as it includes fewer types of different neurons.<ref name="Kandel" /> It handles and processes sensory stimuli, motor information, as well as balance information from the [[vestibular organ]].<ref name="Kandel" />

====Diencephalon====
{{main article|Diencephalon|Thalamus|Hypothalamus}}
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]]).<ref name="Kandel" />

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 wakefulness and consciousness, such as though the [[Suprachiasmatic nucleus|SCN]].<ref name="Kandel" />

The hypothalamus engages in functions of a number of primitive emotions or feelings such as [[hunger]], [[thirst]] and [[maternal bond]]ing. This is regulated partly through control of secretion of [[hormone]]s from the [[pituitary gland]]. Additionally the hypothalamus plays a role in [[motivation]] and many other behaviors of the individual.<ref name="Kandel" />

====Cerebrum====
{{main article|Cerebrum|Cerebral cortex|Basal ganglia|Amygdala|Hippocampus}}
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.<ref name="Kandel" />

Connecting each of the hemispheres is the [[corpus callosum]] as well as several additional commissures.<ref name="Kandel" />
One of the most important parts of the cerebral hemispheres is the [[Cortex (anatomy)|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.<ref name="Kandel" />

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.<ref name="Kandel" />

===Difference from the peripheral nervous system===
[[File:1205 Somatic Autonomic Enteric StructuresN.jpg|thumb|350px|right|A map over the different structures of the nervous systems in the body, showing the CNS, [[peripheral nervous system|PNS]], [[autonomic nervous system]], and [[enteric nervous system]].]]

The PNS consists of neurons, axons, and [[Schwann cells]]. Oligodendrocytes and Schwann cells have similar functions in the CNS and PNS, 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 CNS are often very short, barely a few millimeters, and do not need the same degree of isolation as peripheral nerves. Some peripheral nerves can be over 1 meter 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.<ref name="Moore's">{{cite book|first1=Arthur F.|last1=Dalley|first2=Keith L|last2=Moore|first3=Anne M.R.|last3=Agur|title=Clinically oriented anatomy|year=2010|publisher=Lippincott Williams & Wilkins, Wolters Kluwer|location=Philadelphia [etc.]|isbn=978-1-60547-652-0|pages=48–55,464,700,822,824,1075|edition=6th ed., [International ed.].}}</ref> Oligodendrocytes usually myelinate several axons. They do this by sending out thin projections of their [[cell membrane]], which envelop and enclose the axon.