Invertebrates, the brain and spinal cord are both enclosed in themeninges.[2] The meninges providea barrier to chemicals dissolved in the blood, protecting the brain from mostneurotoxins commonly found in food. Within the meninges the brain and spinal cord are bathed incerebral spinal fluid which replaces thebody fluid found outside the cells of allbilateral animals.
In vertebrates, the CNS is contained within thedorsal body cavity, while the brain is housed in thecranial cavity within theskull. The spinal cord is housed in thespinal canal within thevertebrae.[2] Within the CNS, the interneuronal space is filled with a large amount of supporting non-nervous cells called neuroglia orglia from the Greek for "glue".[3]
The CNS consists of two major structures: thebrain andspinal cord. The brain is encased in the skull, and protected by the cranium.[8] The spinal cord is continuous with the brain and liescaudally to the brain.[9] It is protected by thevertebrae.[8] The spinal cord reaches from the base of the skull, and continues through[8] or starting below[10] theforamen magnum,[8] and terminates roughly level with the first or secondlumbar vertebra,[9][10] occupying the upper sections of thevertebral canal.[6]
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 theperipheral nervous system (PNS).[11] The CNS is composed ofwhite andgray matter.[9] This can also be seen macroscopically on brain tissue. The white matter consists ofaxons andoligodendrocytes, while the gray matter consists ofneurons and unmyelinated fibers. Both tissues include a number ofglial 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 forneuroblasts to climb duringneurogenesis such asbergmann glia, while others such asmicroglia are a specialized form ofmacrophage, involved in theimmune system of the brain as well as the clearance of variousmetabolites from thebrain tissue.[6]Astrocytes may be involved with both clearance of metabolites as well as transport of fuel and various beneficial substances to neurons from thecapillaries of the brain. Upon CNS injury astrocytes will proliferate, causinggliosis, a form of neuronal scar tissue, lacking in functional neurons.[6]
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 ofspinalnerves (sometimes segmental nerves[8]). The nerves connect the spinal cord to skin, joints, muscles etc. and allow for the transmission ofefferent motor as well asafferent 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 thebrachial 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 thethalamus and ultimately to the cortex.
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.
Apart from the spinal cord, there are also peripheral nerves of the PNS that synapse through intermediaries organglia directly on the CNS. These 12 nerves exist in the head and neck region and are calledcranial nerves. Cranial nerves bring information to the CNS to and from the face, as well as to certain muscles (such as thetrapezius muscle, which is innervated byaccessory nerves[8] as well as certaincervical spinal nerves).[8]
Two pairs of cranial nerves; theolfactory nerves and theoptic 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]
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 ofspinal locomotion and can processreflexes, the brain is the major processing unit of the nervous system.[12][13]
The brainstem consists of themedulla, thepons and themidbrain. 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]
The next structure rostral to the medulla is the pons, which lies on the ventral anterior side of the brainstem. Nuclei in the pons includepontine nuclei which work with thecerebellum and transmit information between the cerebellum and thecerebral 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, thebasal ganglia and bothcerebral 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,[9] Autonomic control of the organs is mediated by thetenth cranial nerve.[6] A large portion of the brainstem is involved in such autonomic control of the body. Such functions may engage theheart,blood vessels, andpupils, among others.[9]
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 andcognition. These connections have been shown by the use ofmedical imaging techniques, such asfunctional MRI andPositron emission tomography.[9]
The body of the cerebellum holds more neurons than any other structure of the brain, including that of the largercerebrum, but is also more extensively understood than other structures of the brain, as it includes fewer types of different neurons.[9] It handles and processes sensory stimuli, motor information, as well as balance information from thevestibular organ.[9]
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 wakefulness and consciousness, such as though theSCN.[9]
The hypothalamus engages in functions of a number of primitive emotions or feelings such ashunger,thirst andmaternal bonding. This is regulated partly through control of secretion ofhormones from thepituitary gland. Additionally the hypothalamus plays a role inmotivation and many other behaviors of the individual.[9]
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 thecorpus callosum as well as several additional commissures.[9]One of the most important parts of the cerebral hemispheres is thecortex, made up of gray matter covering the surface of the brain. Functionally, thecerebral 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]
The PNS consists of neurons, axons, andSchwann cells. Oligodendrocytes and Schwann cells have similar functions in the CNS and PNS, respectively. Both act to addmyelin 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 theircell membrane, which envelop and enclose the axon.
During early development of the vertebrate embryo, a longitudinalgroove on theneural plate gradually deepens and the ridges on either side of the groove (theneural folds) become elevated, and ultimately meet, transforming the groove into a closed tube called theneural tube.[14] The formation of the neural tube is calledneurulation. At this stage, the walls of the neural tube contain proliferatingneural stem cells in a region called theventricular zone. The neural stem cells, principallyradial glial cells, multiply and generateneurons through the process ofneurogenesis, forming the rudiment of the CNS.[15]
Theneural tube gives rise to bothbrain andspinal cord. The anterior (or 'rostral') portion of the neural tube initially differentiates into three brainvesicles (pockets): theprosencephalon at the front, themesencephalon, and, between the mesencephalon and the spinal cord, therhombencephalon. (By six weeks in the human embryo) the prosencephalon then divides further into thetelencephalon anddiencephalon; and the rhombencephalon divides into themetencephalon andmyelencephalon. The spinal cord is derived from the posterior or 'caudal' portion of the neural tube.
Top: thelancelet, regarded an archetypal vertebrate, lacking a true brain. Middle: an earlyvertebrate. Bottom: spindle diagram of the evolution of vertebrates.
Planarians, members of the phylumPlatyhelminthes (flatworms), have the simplest, clearly defined delineation of a nervous system into a CNS and aPNS.[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]
The CNS ofchordates differs from that of other animals in being placeddorsally in the body, above the gut andnotochord/spine.[20] The basic pattern of the CNS is highly conserved throughout the different species ofvertebrates and during evolution. The major trend that can be observed is towards a progressive telencephalisation: thetelencephalon of reptiles is only an appendix to the largeolfactory bulb, while in mammals it makes up most of the volume of the CNS. In the human brain, the telencephalon covers most of thediencephalon and the entiremesencephalon. Indeed, theallometric 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 throughcranial 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 thecerebral cortex (main part of the telencephalon excluding olfactory bulb) known as theneocortex.[21] This part of the brain is, in mammals, involved in higher thinking and further processing of all senses in thesensory cortices (processing for smell was previously only done by its bulb while those for non-smell senses were only done by thetectum).[22] The neocortex ofmonotremes (the duck-billedplatypus and several species ofspiny anteaters) and ofmarsupials (such askangaroos,koalas,opossums,wombats, andTasmanian devils) lack the convolutions –gyri andsulci – found in the neocortex of mostplacental 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 indolphins, possibly related to their complexecholocation.
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]
^abMaton, 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, US: Prentice Hall. pp. 132–144.ISBN0-13-981176-1.
^abHuijzen, R. Nieuwenhuys, J. Voogd, C. van (2007).The human central nervous system (4th ed.). Berlin: Springer. p. 3.ISBN978-3-540-34686-9.{{cite book}}: CS1 maint: multiple names: authors list (link)
^Gilbert, Scott F.; College, Swarthmore; Helsinki, the University of (2014).Developmental biology (Tenth ed.). Sunderland, Mass.: Sinauer.ISBN978-0878939787.
^Hickman, Cleveland P. Jr.; Larry S. Roberts; Susan L. Keen; Allan Larson; Helen L'Anson; David J. Eisenhour (2008).Integrated Princinples of Zoology: Fourteenth Edition. New York, NY, US: McGraw-Hill Higher Education. p. 733.ISBN978-0-07-297004-3.
^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, US: Pearson / Benjamin Cummings. p. 1065.ISBN978-0-8053-6844-4.
^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
^Kent, George C.; Robert K. Carr (2001).Comparative Anatomy of the Vertebrates: Ninth Edition. New York, NY, US: McGraw-Hill Higher Education. p. 409.ISBN0-07-303869-5.
