Central nervous system

Source: Wikipedia, the free encyclopedia.
Central 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
TA98A14.1.00.001
TA25364
FMA55675
Anatomical terminology

The central nervous system (CNS) is the part of the

lancelets
.

The rest of this article exclusively discusses the vertebrate central nervous system, which is radically distinct from all other animals.

Overview

In

bilateral animals
.

In vertebrates, the CNS is contained within the

vertebrae.[2] Within the CNS, the interneuronal space is filled with a large amount of supporting non-nervous cells called neuroglia or glia from the Greek for "glue".[3]

In vertebrates, the CNS also includes the

therapeutic agents which cannot otherwise cross the meninges barrier.[7]

Structure

Main article: Neuroanatomy

The CNS consists of two major structures: the

vertebral canal.[6]

White and gray matter

Main articles:
Gray matter and White matter
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

brain tissue.[6] 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.[6]

The brain (

subcortical gray matter making up a large number of different nuclei.[9]

Spinal cord

Main article: Spinal cord
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.
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 nerves (sometimes segmental nerves[8]). 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.[9] 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,[9] some forming plexa as they branch out, such as the brachial plexa, sacral plexa etc.[8] 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.[9]

The spinal cord relays information up to the brain through spinal tracts through the final common pathway[9] to the thalamus and ultimately to the cortex.

  • Schematic image showing the locations of a few tracts of the spinal cord.
    Schematic image showing the locations of a few tracts of the spinal cord.
  • Reflexes may also occur without engaging more than one neuron of the CNS as in the below example of a short reflex.
    Reflexes may also occur without engaging more than one neuron of the CNS 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

cervical spinal nerves).[8]

Two pairs of cranial nerves; the olfactory nerves and the optic nerves[4] 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. [7]

Image showing the way Schwann cells myelinate periferal nerves.
A neuron of the CNS, myelinated by an oligodendrocyte
A peripheral nerve myelinated by Schwann cells (left) and a CNS neuron myelinated by an oligodendrocyte (right)

Brain

Main article: Brain

At the anterior end of the spinal cord lies the brain.[9] 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 reflexes, the brain is the major processing unit of the nervous system.[12][13]

Brainstem

Main article: 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, which both have similar organization and functional properties.[9] The tracts passing from the spinal cord to the brain pass through here.[9]

Regulatory functions of the medulla nuclei include control of blood pressure and breathing. Other nuclei are involved in balance, taste, hearing, and control of muscles of the face and neck.[9]

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.[9] In the dorsal posterior pons lie nuclei that are involved in the functions of breathing, sleep, and taste.[9]

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 hemispheres, among others. Additionally, parts of the visual and auditory systems are located in the midbrain, including control of automatic eye movements.[9]

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,

blood vessels, and pupils, among others.[9]

The brainstem also holds the reticular formation, a group of nuclei involved in both arousal and alertness.[9]

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.[9] Despite its previous classification as a motor structure, the cerebellum also displays connections to areas of the cerebral cortex involved in language and

functional MRI and Positron emission tomography.[9]

The body of the cerebellum holds more neurons than any other structure of the brain, including that of the larger

vestibular organ.[9]

Diencephalon

Main articles: Diencephalon, Thalamus, and 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).[9]

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.[9]

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.[9]

Cerebrum

Main articles: Cerebrum, Cerebral cortex, Basal ganglia, Amygdala, and 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.[9]

Connecting each of the hemispheres is the corpus callosum as well as several additional commissures.[9] 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.[9]

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.[9]

Difference from the peripheral nervous system

A map over the different structures of the nervous systems in the body, showing the CNS, PNS, autonomic nervous system, and enteric nervous system.

This differentiates the CNS from the PNS, which 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.[8] Oligodendrocytes usually myelinate several axons. They do this by sending out thin projections of their cell membrane, which envelop and enclose the axon.

Development

CNS seen in a median section of a 5-week-old embryo.
CNS seen in a median section of a 3-month-old embryo.
Top image: CNS as seen in a median section of a 5-week-old embryo. Bottom image: CNS seen in a median section of a 3-month-old embryo.
Main article:
Neural development

During early development of the vertebrate embryo, a longitudinal

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 CNS.[15]

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

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.[9]

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

Lancelets or amphioxus are regarded as similar to the archetypal vertebrate form, and possess to true brain.
A neuron of the CNS, myelinated by an oligodendrocyte
Traditional spindle diagram of the evolution of the vertebrates at class level.
Top: the lancelet, regarded an archetypal vertebrate, lacking a true brain. Middle: an early vertebrate. Bottom: spindle diagram of the evolution of vertebrates.
See also: Cephalization and Archicortex

Planaria

Platyhelminthes (flatworms), have the simplest, clearly defined delineation of a nervous system into a CNS and a PNS.[16][17]
Their primitive brains, consisting of two fused anterior ganglia, and longitudinal nerve cords form the CNS. Like vertebrates, have a distinct CNS and PNS. The nerves projecting laterally from the CNS form their 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.[18]

Arthropoda

In

motor neurons due to their small size.[19]

See also: Lateral horn of insect brain

Chordata

The CNS of

cranial endocasts
.

Mammals

eutherians).[23]
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.[21] 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.[21] Extreme convolution of the neocortex is found in dolphins, possibly related to their complex echolocation.

Clinical significance

Diseases

Main article: Central nervous system disease

There are many CNS diseases and conditions, including

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.[24]

References

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  4. ^
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  5. ^ "Medical Subject Headings (MeSH): Optic Nerve". National Library of Medicine. Archived from the original on 2 October 2013. Retrieved 28 September 2013.
  6. ^
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    ISBN 978-3-540-34686-9.{{cite book}}: CS1 maint: multiple names: authors list (link
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  12. PMID 31194336. Retrieved 13 May 2020. {{cite journal}}: Cite journal requires |journal= (help
    )
  13. ^ "The brain and spinal cord – Canadian Cancer Society". www.cancer.ca. Retrieved 19 March 2019.
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  19. PMID 24965579
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  20. ^ Romer, A.S. (1949): The Vertebrate Body. W.B. Saunders, Philadelphia. (2nd ed. 1955; 3rd ed. 1962; 4th ed. 1970)
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  22. ^ Feinberg, T. E., & Mallatt, J. (2013). The evolutionary and genetic origins of consciousness in the Cambrian Period over 500 million years ago. Frontiers in psychology, 4, 667. https://doi.org/10.3389/fpsyg.2013.00667
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  24. ^ 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, US: American College of Radiology. Archived from the original on 15 September 2012. Retrieved 9 September 2012.

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