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
Chromosome 6 is one of the 23 pairs of chromosomes in humans. People have two copies of this chromosome. Chromosome 6 spans more than 170 million base pairs and represents between 5.5 and 6% of the total DNA in cells. It contains the Major Histocompatibility Complex, which contains over 100 genes related to the immune response, plays a vital role in organ transplantation; the human leukocyte antigen lies on chromosome 6, with the exception of the gene for β2-microglobulin, encodes cell-surface antigen-presenting proteins among other functions. In 2003, the entirety of chromosome 6 was manually annotated for proteins, resulting in the identification of 1,557 genes, 633 pseudogenes; the following are some of the newer gene count estimates. Because researchers use different approaches to genome annotation their predictions of the number of genes on each chromosome varies. Among various projects, the collaborative consensus coding sequence project takes an conservative strategy. So CCDS's gene number prediction represents a lower bound on the total number of human protein-coding genes.
The following is a partial list of genes on human chromosome 6. For complete list, see the link in the infobox on the right; the following are some of the genes located on p-arm of human chromosome 6: The following are some of the genes located on q-arm of human chromosome 6: The following diseases are some of those related to genes on chromosome 6: NotesSome text in this article was taken from http://ghr.nlm.nih.gov/chromosome=6 National Institutes of Health. "Chromosome 6". Genetics Home Reference. Retrieved 2017-05-06. "Chromosome 6". Human Genome Project Information Archive 1990–2003. Retrieved 2017-05-06. "Chromosome 6 Research Project". Parent-driven research for genotype-phenotype studies on chromosome 6 disorders. Retrieved 2017-06-17
Oligodendrocytes, or oligodendroglia, are a type of neuroglia whose main functions are to provide support and insulation to axons in the central nervous system of some vertebrates, equivalent to the function performed by Schwann cells in the peripheral nervous system. Oligodendrocytes do this by creating the myelin sheath, 80% lipid and 20% protein. A single oligodendrocyte can extend its processes to 50 axons, wrapping 1 μm of myelin sheath around each axon; each oligodendrocyte forms one segment of myelin for several adjacent axons. Oligodendrocytes are found only in the central nervous system, which comprises the brain and spinal cord; these cells were thought to have been produced in the ventral neural tube. They are the last cell type to be generated in the CNS, they were discovered by Pío del Río Hortega. Oligodendroglia, types of glial cells, arise during development from oligodendrocyte precursor cells, which can be identified by their expression of a number of antigens, including the ganglioside GD3, the NG2 chondroitin sulfate proteoglycan, the platelet-derived growth factor-alpha receptor subunit.
Most oligodendrocytes develop during embryogenesis and early postnatal life from restricted periventricular germinal regions. Oligodendrocyte formation in the adult brain is associated with glial-restricted progenitor cells, known as oligodendrocyte progenitor cells. SVZ cells migrate away from germinal zones to populate both developing white and gray matter, where they differentiate and mature into myelin-forming oligodendroglia. However, it is not clear. Between midgestation and term birth in human cerebral white matter, three successive stage of the human oligodendroglial cell lineage are found, viz the pre oligodendrocytes, the immature oligodendrocytes, the mature oligodendrocytes, it has been suggested that some undergo apoptosis and others fail to differentiate into mature oligodendroglia but persist as adult oligodendroglial progenitors. Remarkably, oligodendrocyte population originated in the subventricular zone can be expanded by administering epidermal growth factor; as part of the nervous system, oligodendrocytes are related to nerve cells, like all other glial cells, oligodendrocytes provide a supporting role for neurons as well as trophic support by the production of glial cell line-derived neurotrophic factor, brain-derived neurotrophic factor, or insulin-like growth factor-1.
In addition, the nervous system of mammals depends crucially on myelin sheaths, which reduce ion leakage and decrease the capacitance of the cell membrane. Myelin increases impulse speed, as saltatory propagation of action potentials occurs at the nodes of Ranvier in between Schwann cells and oligodendrocytes. Furthermore, impulse speed of myelinated axons increases linearly with the axon diameter, whereas the impulse speed of unmyelinated cells increases only with the square root of the diameter; the insulation must be proportional to the diameter of the fibre inside. The optimal ratio of axon diameter divided by the total fiber diameter is 0.6. In contrast, satellite oligodendrocytes are functionally distinct from other oligodendrocytes, they are not attached to neurons and, therefore, do not serve an insulating role. They remain opposed to regulate the extracellular fluid. Satellite oligodendrocytes are considered to be a part of the grey matter whereas myelinating oligodendrocytes are a part of the white matter.
Myelination continues into adulthood. The entire process is not complete until about 25–30 years of age. Myelination is an important component of intelligence. Neuroscientist Vincent J. Schmithorst proposes that there is a correlation with white matter and intelligence. People with greater white matter had higher IQs. A study done with rats by Janice M. Juraska showed that rats that were raised in an enriched environment had more myelination in their corpus callosum. Diseases that result in injury to the oligodendroglial cells include demyelinating diseases such as multiple sclerosis and various leukodystrophies. Trauma to the body, e.g. spinal cord injury, can cause demyelination. The immature oligodendrocytes, which increase in number during mid-gestation, are more vulnerable to hypoxic injury and are involved in periventricular leukomalacia; this congenital condition of damage to the newly forming brain can therefore lead to cerebral palsy. In cerebral palsy, spinal cord injury and multiple sclerosis, oligodendrocytes are thought to be damaged by excessive release of the neurotransmitter, glutamate.
Damage has been shown to be mediated by N-methyl-D-aspartate receptors. Oligodendrocyte dysfunction may be implicated in the pathophysiology of schizophrenia and bipolar disorder. Oligodendroglia are susceptible to infection by the JC virus, which causes progressive multifocal leukoencephalopathy, a condition that affects white matter in immunocompromised patients. Tumors of oligodendroglia are called oligodendrogliomas; the chemotherapy agent Fluorouracil causes damage to the oligodendrocytes in mice, leading to both acute central nervous system damage and progressively worsening delayed degeneration of the CNS. 2',3'-Cyclic-nucleotide 3
A base pair is a unit consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both DNA and RNA. Dictated by specific hydrogen bonding patterns, Watson–Crick base pairs allow the DNA helix to maintain a regular helical structure, subtly dependent on its nucleotide sequence; the complementary nature of this based-paired structure provides a redundant copy of the genetic information encoded within each strand of DNA. The regular structure and data redundancy provided by the DNA double helix make DNA well suited to the storage of genetic information, while base-pairing between DNA and incoming nucleotides provides the mechanism through which DNA polymerase replicates DNA and RNA polymerase transcribes DNA into RNA. Many DNA-binding proteins can recognize specific base-pairing patterns that identify particular regulatory regions of genes. Intramolecular base pairs can occur within single-stranded nucleic acids.
This is important in RNA molecules, where Watson–Crick base pairs permit the formation of short double-stranded helices, a wide variety of non-Watson–Crick interactions allow RNAs to fold into a vast range of specific three-dimensional structures. In addition, base-pairing between transfer RNA and messenger RNA forms the basis for the molecular recognition events that result in the nucleotide sequence of mRNA becoming translated into the amino acid sequence of proteins via the genetic code; the size of an individual gene or an organism's entire genome is measured in base pairs because DNA is double-stranded. Hence, the number of total base pairs is equal to the number of nucleotides in one of the strands; the haploid human genome is estimated to be about 3.2 billion bases long and to contain 20,000–25,000 distinct protein-coding genes. A kilobase is a unit of measurement in molecular biology equal to 1000 base pairs of DNA or RNA; the total amount of related DNA base pairs on Earth is estimated at 5.0×1037 and weighs 50 billion tonnes.
In comparison, the total mass of the biosphere has been estimated to be as much as 4 TtC. Hydrogen bonding is the chemical interaction. Appropriate geometrical correspondence of hydrogen bond donors and acceptors allows only the "right" pairs to form stably. DNA with high GC-content is more stable than DNA with low GC-content. But, contrary to popular belief, the hydrogen bonds do not stabilize the DNA significantly; the larger nucleobases and guanine, are members of a class of double-ringed chemical structures called purines. Purines are complementary only with pyrimidines: pyrimidine-pyrimidine pairings are energetically unfavorable because the molecules are too far apart for hydrogen bonding to be established. Purine-pyrimidine base-pairing of AT or GC or UA results in proper duplex structure; the only other purine-pyrimidine pairings would be AC and GT and UG. The GU pairing, with two hydrogen bonds, does occur often in RNA. Paired DNA and RNA molecules are comparatively stable at room temperature, but the two nucleotide strands will separate above a melting point, determined by the length of the molecules, the extent of mispairing, the GC content.
Higher GC content results in higher melting temperatures. On the converse, regions of a genome that need to separate — for example, the promoter regions for often-transcribed genes — are comparatively GC-poor. GC content and melting temperature must be taken into account when designing primers for PCR reactions; the following DNA sequences illustrate pair double-stranded patterns. By convention, the top strand is written from the 5' end to the 3' end. A base-paired DNA sequence: ATCGATTGAGCTCTAGCG TAGCTAACTCGAGATCGCThe corresponding RNA sequence, in which uracil is substituted for thymine in the RNA strand: AUCGAUUGAGCUCUAGCG UAGCUAACUCGAGAUCGC Chemical analogs of nucleotides can take the place of proper nucleotides and establish non-canonical base-pairing, leading to errors in DNA replication and DNA transcription; this is due to their isosteric chemistry. One common mutagenic base analog is 5-bromouracil, which resembles thymine but can base-pair to guanine in its enol form. Other chemicals, known as DNA intercalators, fit into the gap between adjacent bases on a single strand and induce frameshift mutations by "masquerading" as a base, causing the DNA replication machinery to skip or insert additional nucleotides at the intercalated site.
Most intercalators are known or suspected carcinogens. Examples include ethidium acridine. An unnatural base pair is a designed subunit of DNA, created in a laboratory and does not occur in nature. DNA sequences have been described which use newly created nucleobases to form a third base pair, in addition to the two ba
Protein Data Bank
The Protein Data Bank is a database for the three-dimensional structural data of large biological molecules, such as proteins and nucleic acids. The data obtained by X-ray crystallography, NMR spectroscopy, or cryo-electron microscopy, submitted by biologists and biochemists from around the world, are accessible on the Internet via the websites of its member organisations; the PDB is overseen by an organization called the Worldwide Protein Data Bank, wwPDB. The PDB is a key in areas such as structural genomics. Most major scientific journals, some funding agencies, now require scientists to submit their structure data to the PDB. Many other databases use protein structures deposited in the PDB. For example, SCOP and CATH classify protein structures, while PDBsum provides a graphic overview of PDB entries using information from other sources, such as Gene ontology. Two forces converged to initiate the PDB: 1) a small but growing collection of sets of protein structure data determined by X-ray diffraction.
In 1969, with the sponsorship of Walter Hamilton at the Brookhaven National Laboratory, Edgar Meyer began to write software to store atomic coordinate files in a common format to make them available for geometric and graphical evaluation. By 1971, one of Meyer's programs, SEARCH, enabled researchers to remotely access information from the database to study protein structures offline. SEARCH was instrumental in enabling networking, thus marking the functional beginning of the PDB; the Protein Data Bank was announced in October 1971 in Nature New Biology as a joint venture between Cambridge Crystallographic Data Centre, UK and Brookhaven National Laboratory, USA. Upon Hamilton's death in 1973, Tom Koeztle took over direction of the PDB for the subsequent 20 years. In January 1994, Joel Sussman of Israel's Weizmann Institute of Science was appointed head of the PDB. In October 1998, the PDB was transferred to the Research Collaboratory for Structural Bioinformatics; the new director was Helen M. Berman of Rutgers University.
In 2003, with the formation of the wwPDB, the PDB became an international organization. The founding members are PDBe, RCSB, PDBj; the BMRB joined in 2006. Each of the four members of wwPDB can act as deposition, data processing and distribution centers for PDB data; the data processing refers to the fact that annotate each submitted entry. The data are automatically checked for plausibility; the PDB database is updated weekly. The PDB holdings list is updated weekly; as of 17 October 2018, the breakdown of current holdings is as follows: 120,052 structures in the PDB have a structure factor file. 9,734 structures have an NMR restraint file. 3,486 structures in the PDB have a chemical shifts file. 2,531 structures in the PDB have a 3DEM map file deposited in EM Data BankThese data show that most structures are determined by X-ray diffraction, but about 10% of structures are now determined by protein NMR. When using X-ray diffraction, approximations of the coordinates of the atoms of the protein are obtained, whereas estimations of the distances between pairs of atoms of the protein are found through NMR experiments.
Therefore, the final conformation of the protein is obtained, in the latter case, by solving a distance geometry problem. A few proteins are determined by cryo-electron microscopy; the significance of the structure factor files, mentioned above, is that, for PDB structures determined by X-ray diffraction that have a structure file, the electron density map may be viewed. The data of such structures is stored on the "electron density server". In the past, the number of structures in the PDB has grown at an exponential rate, passing the 100 registered structures milestone in 1982, the 1,000 in 1993, the 10,000 in 1999, the 100,000 in 2014. However, since 2007, the rate of accumulation of new protein structures appears to have plateaued; the file format used by the PDB was called the PDB file format. This original format was restricted by the width of computer punch cards to 80 characters per line. Around 1996, the "macromolecular Crystallographic Information file" format, mmCIF, an extension of the CIF format started to be phased in.
MmCIF is now the master format for the PDB archive. An XML version of this format, called PDBML, was described in 2005; the structure files can be downloaded in any of these three formats. In fact, individual files are downloaded into graphics packages using web addresses: For PDB format files, use, e.g. http://www.pdb.org/pdb/files/4hhb.pdb.gz or http://pdbe.org/download/4hhb For PDBML files, use, e.g. http://www.pdb.org/pdb/files/4hhb.xml.gz or http://pdbe.org/pdbml/4hhbThe "4hhb" is the PDB identifier. Each structure published in PDB receives a four-character alphanumeric identifier, its PDB ID; the structure files may be viewed using one of several free and open source computer programs, including Jmol, Pymol, VMD, Rasmol. Other non-free, shareware programs
In genetics, complementary DNA is DNA synthesized from a single-stranded RNA template in a reaction catalyzed by the enzyme reverse transcriptase. CDNA is used to clone eukaryotic genes in prokaryotes; when scientists want to express a specific protein in a cell that does not express that protein, they will transfer the cDNA that codes for the protein to the recipient cell. CDNA is produced by retroviruses and integrated into the host's genome, where it creates a provirus; the term cDNA is used in a bioinformatics context, to refer to an mRNA transcript's sequence, expressed as DNA bases rather than RNA bases. CDNA is derived from mRNA, so it contains only exons but no introns. Although there are several methods for doing so, cDNA is most synthesized from mature mRNA using the enzyme reverse transcriptase; this enzyme, which occurs in retroviruses, operates on a single strand of mRNA, generating its complementary DNA based on the pairing of RNA base pairs to their DNA complements. To obtain eukaryotic cDNA whose introns have been removed: A eukaryotic cell transcribes the DNA into RNA.
The same cell processes the pre-mRNA strands by removing introns, adding a poly-A tail and 5’ Methyl-Guanine cap This mixture of mature mRNA strands is extracted from the cell. The poly-A tail of the post-transcriptional mRNA can be taken advantage of with oligo beads in an affinity chromatography assay. A poly-T oligonucleotide primer is hybridized onto the poly-A tail of the mature mRNA template, or random hexamer primers can be added which contain every possible 6 base single strand of DNA and can therefore hybridize anywhere on the RNA Reverse transcriptase is added, along with deoxynucleotide triphosphates; this synthesizes one complementary strand of DNA hybridized to the original mRNA strand. To synthesize an additional DNA strand, traditionally one would digest the RNA of the hybrid strand, using an enzyme like RNase H, or through alkali digestion method. After digestion of the RNA, a single stranded DNA is left and because single stranded nucleic acids are hydrophobic, it tends to loop around itself.
It is that the ssDNA forms a hairpin loop at the 3' end. From the hairpin loop, a DNA polymerase can use it as a primer to transcribe a complementary sequence for the ss cDNA. Now, you should be left with a double stranded cDNA with identical sequence as the mRNA of interest. Complementary DNA is used in gene cloning or as gene probes or in the creation of a cDNA library; when scientists transfer a gene from one cell into another cell in order to express the new genetic material as a protein in the recipient cell, the cDNA will be added to the recipient, because the DNA for an entire gene may include DNA that does not code for the protein or that interrupts the coding sequence of the protein. Partial sequences of cDNAs are obtained as expressed sequence tags. With amplification of DNA sequences via polymerase chain reaction now commonplace, one will conduct reverse transcription as an initial step, followed by PCR to obtain an exact sequence of cDNA for intra-cellular expression; this is achieved by designing sequence-specific DNA primers that hybridize to the 5' and 3' ends of a cDNA region coding for a protein.
Once amplified, the sequence can be cut at each end with nucleases and inserted into one of many small circular DNA sequences known as expression vectors. Such vectors allow for self-replication, inside the cells, integration in the host DNA, they also contain a strong promoter to drive transcription of the target cDNA into mRNA, translated into protein. On June 13, 2013, the United States Supreme Court ruled in the case of Association for Molecular Pathology v. Myriad Genetics that while occurring human genes cannot be patented, cDNA is patent eligible because it does not occur naturally; some viruses use cDNA to turn their viral RNA into mRNA. The mRNA is used to make viral proteins to take over the host cell. An example of this first step from viral DNA to cDNA can be seen in the HIV cycle of infection. Here, the host cell membrane becomes attached to the virus’ lipid envelope which allows the viral capsid with two copies of viral genome RNA to enter the host; the cDNA copy is made though reverse transcription of the viral RNA, a process facilitated by the chaperone CypA and a viral capsid associated reverse transcriptase.
CDNA library cDNA microarray RNA-Seq H-Invitational Database Functional Annotation of the Mouse database Complementary DNA tool
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