The commissural fibers or transverse fibers are axons that connect the two hemispheres of the brain. In contrast to commissural fibers, association fibers connect regions within the same hemisphere of the brain, projection fibers connect each region to other parts of the brain or to the spinal cord; the commissural fibers make up tracts that include the corpus callosum, the anterior commissure, the posterior commissure. The corpus callosum is the largest commissural tract in the human brain, it consists of about 200-300 million axons. The corpus callosum is essential to the communication between the two hemispheres. A recent study of individuals with agenesis of the corpus callosum suggests that the corpus callosum plays a vital role in problem solving strategies, verbal processing speed, executive performance; the absence of a developed corpus callosum is shown to have a significant relationship with impaired verbal processing speed and problem solving. Another study of individuals with multiple sclerosis provides evidence that structural and microstructural abnormalities of the corpus callosum are related to cognitive dysfunction.
Verbal and visual memory, information processing speed, executive tasks were shown to be impaired when compared to healthy individuals. Physical disabilities in multiple sclerosis patients seem to be related to abnormalities of the corpus callosum, but not to the same extent of other cognitive functions. Using diffusion tensor imaging, researchers have been able to produce a visualization of this network of fibers, which shows the corpus callosum has an anteroposterior topographical organization, uniform with the cerebral cortex; the anterior commissure is a tract that connects the two temporal lobes of the cerebral hemispheres across the midline, placed in front of the columns of the fornix. The great majority of fibers connecting the two hemispheres travel through the corpus callosum, over 10 times larger than the anterior commissure, other routes of communication pass through the hippocampal commissure or, via subcortical connections; the anterior commissure is a significant pathway that can be distinguished in the brains of all mammals.
Using diffusion tensor imaging, researchers were able to approximate the location of the anterior commissure where it crosses the midline of the brain. This tract can be observed to be in the shape of a bicycle as it branches through various areas of the brain. Through diffusion tensor imaging results, the anterior commissure was categorized into two fiber systems: 1) the olfactory fibers and 2) the non-olfactory fibers; the posterior commissure is a rounded nerve tract crossing the middle line on the dorsal aspect of the upper end of the cerebral aqueduct. It is important in the bilateral pupillary light reflex. Evidence suggests the posterior commissure is a tract that plays a role in language processing between the right and left hemispheres of the brain, it connects the pretectal nuclei. A case study described in The Irish Medical Journal discussed the role the posterior commissure plays in the connection between the right occipital cortex and the language centers in the left hemisphere; this study explains how visual information from the left side of the visual field is received by the right visual cortex and transferred to the word form system in the left hemisphere though the posterior commissure and the splenium.
Disruption of the posterior commissure can cause alexia without agraphia. It is evident from this case study of alexia without agraphia that the posterior commissure plays a vital role in transferring information from the right occipital cortex to the language centers of the left hemisphere; the lyra or hippocampal commissure. Aging Age-related decline in the commissural fiber tracts that make up the corpus callosum indicate the corpus callosum is involved in memory and executive function; the posterior fibers of the corpus callosum are associated with episodic memory. Perceptual processing decline is related to diminished integrity of occipital fibers of the corpus callosum. Evidence suggests that the genu of the corpus callosum does not contribute to any one cognitive domain in the elderly; as fiber tract connectivity in the corpus callosum declines due to aging, compensatory mechanisms are found in other areas of the corpus callosum and frontal lobe. These compensatory mechanisms, increasing connectivity in other parts of the brain, may explain why elderly individuals still display executive function as a decline of connectivity is seen in regions of the corpus callosum.
Older adults compared to younger adults show poorer performance in balance tests. A decline in white matter integrity of the corpus callosum in older individuals may explain declines in the ability to balance. Changes in the white matter integrity of the corpus callosum may be related to cognitive and motor function decline as well. Decreased white matter integrity effects proper transmission and processing of sensorimotor information. White matter degeneration of the genu of the corpus callosum is associated with gait, balance impairment, the quality of postural control; the corpus callosum allows for communication between the two hemispheres and is found only in placental mammals, while it is absent in monotremes and marsupials, as well as other vertebrates such as birds, reptiles and fish. The anterior commissure serves as the primary mode of interhemispheric communication in marsupials, which carries all the commissural fibers arising from the neocortex, whereas in pl
Astrocytes known collectively as astroglia, are characteristic star-shaped glial cells in the brain and spinal cord. The proportion of astrocytes in the brain is not well defined. Depending on the counting technique used, studies have found that the astrocyte proportion varies by region and ranges from 20% to 40% of all glia, they perform many functions, including biochemical support of endothelial cells that form the blood–brain barrier, provision of nutrients to the nervous tissue, maintenance of extracellular ion balance, a role in the repair and scarring process of the brain and spinal cord following traumatic injuries. Research since the mid-1990s has shown that astrocytes propagate intercellular Ca2+ waves over long distances in response to stimulation, similar to neurons, release transmitters in a Ca2+-dependent manner. Data suggest that astrocytes signal to neurons through Ca2+-dependent release of glutamate; such discoveries have made astrocytes an important area of research within the field of neuroscience.
Astrocytes are a sub-type of glial cells in the central nervous system. They are known as astrocytic glial cells. Star-shaped, their many processes envelop synapses made by neurons. Astrocytes are classically identified using histological analysis. Several forms of astrocytes exist in the central nervous system including fibrous and radial; the fibrous glia are located within white matter, have few organelles, exhibit long unbranched cellular processes. This type has "vascular feet" that physically connect the cells to the outside of capillary walls when they are in proximity to them; the protoplasmic glia are the most prevalent and are found in grey matter tissue, possess a larger quantity of organelles, exhibit short and branched tertiary processes. The radial glial cells are disposed in planes perpendicular to the axes of ventricles. One of their processes abuts the pia mater, while the other is buried in gray matter. Radial glia are present during development, playing a role in neuron migration.
Müller cells of the retina and Bergmann glia cells of the cerebellar cortex represent an exception, being present still during adulthood. When in proximity to the pia mater, all three forms of astrocytes send out processes to form the pia-glial membrane. Astrocytes are macroglial cells in the central nervous system. Astrocytes are derived from heterogeneous populations of progenitor cells in the neuroepithelium of the developing central nervous system. There is remarkable similarity between the well known genetic mechanisms that specify the lineage of diverse neuron subtypes and that of macroglial cells. Just as with neuronal cell specification, canonical signaling factors like Sonic hedgehog, Fibroblast growth factor, WNTs and bone morphogenetic proteins, provide positional information to developing macroglial cells through morphogen gradients along the dorsal–ventral, anterior–posterior and medial–lateral axes; the resultant patterning along the neuraxis leads to segmentation of the neuroepithelium into progenitor domains for distinct neuron types in the developing spinal cord.
On the basis of several studies it is now believed that this model applies to macroglial cell specification. Studies carried out by Hochstim and colleagues have demonstrated that three distinct populations of astrocytes arise from the p1, p2 and p3 domains; these subtypes of astrocytes can be identified on the basis of their expression of different transcription factors and cell surface markers. The three populations of astrocyte subtypes which have been identified are 1) dorsally located VA1 astrocytes, derived from p1 domain, express PAX6 and reelin 2) ventrally located VA3 astrocytes, derived from p3, express NKX6.1 and SLIT1 and 3) and intermediate white-matter located VA2 astrocyte, derived from the p2 domain, which express PAX6, NKX6.1, reelin and SLIT1. After astrocyte specification has occurred in the developing CNS, it is believed that astrocyte precursors migrate to their final positions within the nervous system before the process of terminal differentiation occurs. In medical science, the neuronal network was considered the only important function of astrocytes, they were looked upon as gap fillers.
More the function of astrocytes has been reconsidered, they are now thought to play a number of active roles in the brain, including the secretion or absorption of neural transmitters and maintenance of the blood–brain barrier. Following on this idea the concept of a tripartite synapse has been proposed, referring to the tight relationship occurring at synapses among a presynaptic element, a postsynaptic element and a glial element. Structural: They are involved in the physical structuring of the brain. Astrocytes get their name because they are "star-shaped", they are the most abundant glial cells in the brain that are associated with neuronal synapses. They regulate the transmission of electrical impulses within the brain. Glycogen fuel reserve buffer: Astrocytes contain glycogen and are capable of gluconeogenesis; the astrocytes next to neurons in hippocampus store and release glucose. Thus, astrocytes can fuel neurons with glucose during periods of high rate of glucose consumption and glucose shortage.
A recent research on rats suggests there may be a connection between this activity and physical exercise. Metabolic support: They provide neurons with nutrients such as lactate. Glucose sensing: associated w
An autonomic ganglion is a cluster of nerve cell bodies in the autonomic nervous system. The two types are sympathetic ganglion and parasympathetic ganglion
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
Tanycytes are special ependymal cells found in the third ventricle of the brain, on the floor of the fourth ventricle and have processes extending deep into the hypothalamus. It is possible that their function is to transfer chemical signals from the cerebrospinal fluid to the central nervous system; the term tanycyte comes from the Greek word tanus. Tanycytes astrocytes, their form and location have led some authors to regard them as radial glia cells that remain in the hypothalamus throughout life. This has led some to believe. So, tanycytes display certain characteristics that distinguish them from radial glia cells. Tanycytes in rats begin to develop in the last two days of gestation and continue on until they reach their full differentiation in the first month of life. Radial glia cells on the other hand, are a key component of the embryonic brain. Tanycytes contain many proteins not found in radial glia cells. Thus, evidence now suggests that tanycytes are genealogical descendants of radial glia cells that do not develop into astrocytes, but rather into their own subpopulation.
Tanycytes in adult mammals are found in the circumventricular organs. They are most numerous in the third ventricle of the brain, but can be seen in the spinal cord radiating from the ependyma of the central canal to the spinal cord surface. Tanycytes represent 0.6% of the population of the lateral ventricular wall, as described by Doetsch et al. Tanycytes have been shown in vivo to serve as a diet-responsive neurogenic niche. Recent work suggests that tanycyte cells bridge the gap between the central nervous system via cerebrospinal fluid to the hypophyseal portal blood; this may link the CSF to neuroendocrine events. Researches in 2005 and 2010 found that tanycytes participate in the release of gonadotropin-releasing hormone. GnRH is released by neurons located in the rostral hypothalamus; these nerve fibers are concentrated in the region that matches the distribution of β1 tanycytes. It is thought that there are two different mechanisms by which tanycytes participate in the release of GnRH. One includes the cyclic remodeling of the spatial relationship between GnRH terminals, the tanycytes, the perivascular space.
The second is the cell to cell signaling mechanism mediated by specific tanycyte compounds. Recent evidence supports both mechanisms, the possibility that both are part of a single mechanism; the term tanycyte was coined by Horstmann in 1954 when he described a distinct structural feature of the cell, a single, long basal process that projects to a distinct region of the hypothalamus. During the 1970s and 1980s, tanycytes were the subject of many research publications, ranging from their morphology to function, but the lack of advanced methodological tools stalled research and led to disagreements between researchers about the full role of tanycytes. Recent advances in immunocytochemistry have allowed for new research in this area. Bibliography http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Nervous/Nervous.htm NIF Search - Tanycyte via the Neuroscience Information Framework
Anatomical terminology is a form of scientific terminology used by anatomists and health professionals such as doctors. Anatomical terminology uses many unique terms and prefixes deriving from Ancient Greek and Latin; these terms can be confusing to those unfamiliar with them, but can be more precise, reducing ambiguity and errors. Since these anatomical terms are not used in everyday conversation, their meanings are less to change, less to be misinterpreted. To illustrate how inexact day-to-day language can be: a scar "above the wrist" could be located on the forearm two or three inches away from the hand or at the base of the hand. By using precise anatomical terminology such ambiguity is eliminated. An international standard for anatomical terminology, Terminologia Anatomica has been created. Anatomical terminology has quite regular morphology, the same prefixes and suffixes are used to add meanings to different roots; the root of a term refers to an organ or tissue. For example, the Latin names of structures such as musculus biceps brachii can be split up and refer to, musculus for muscle, biceps for "two-headed", brachii as in the brachial region of the arm.
The first word describes what is being spoken about, the second describes it, the third points to location. When describing the position of anatomical structures, structures may be described according to the anatomical landmark they are near; these landmarks may include structures, such as the umbilicus or sternum, or anatomical lines, such as the midclavicular line from the centre of the clavicle. The cephalon or cephalic region refers to the head; this area is further differentiated into the cranium, frons, auris, nasus and mentum. The neck area is called cervical region. Examples of structures named according to this include the frontalis muscle, submental lymph nodes, buccal membrane and orbicularis oculi muscle. Sometimes, unique terminology is used to reduce confusion in different parts of the body. For example, different terms are used when it comes to the skull in compliance with its embryonic origin and its tilted position compared to in other animals. Here, Rostral refers to proximity to the front of the nose, is used when describing the skull.
Different terminology is used in the arms, in part to reduce ambiguity as to what the "front", "back", "inner" and "outer" surfaces are. For this reason, the terms below are used: Radial referring to the radius bone, seen laterally in the standard anatomical position. Ulnar referring to the ulna bone, medially positioned when in the standard anatomical position. Other terms are used to describe the movement and actions of the hands and feet, other structures such as the eye. International morphological terminology is used by the colleges of medicine and dentistry and other areas of the health sciences, it facilitates communication and exchanges between scientists from different countries of the world and it is used daily in the fields of research and medical care. The international morphological terminology refers to morphological sciences as a biological sciences' branch. In this field, the form and structure are examined as well as the changes or developments in the organism, it is functional.
It covers the gross anatomy and the microscopic of living beings. It involves the anatomy of the adult, it includes comparative anatomy between different species. The vocabulary is extensive and complex, requires a systematic presentation. Within the international field, a group of experts reviews and discusses the morphological terms of the structures of the human body, forming today's Terminology Committee from the International Federation of Associations of Anatomists, it deals with the anatomical and embryologic terminology. In the Latin American field, there are meetings called Iberian Latin American Symposium Terminology, where a group of experts of the Pan American Association of Anatomy that speak Spanish and Portuguese and studies the international morphological terminology; the current international standard for human anatomical terminology is based on the Terminologia Anatomica. It was developed by the Federative Committee on Anatomical Terminology and the International Federation of Associations of Anatomists and was released in 1998.
It supersedes Nomina Anatomica. Terminologia Anatomica contains terminology for about 7500 human gross anatomical structures. For microanatomy, known as histology, a similar standard exists in Terminologia Histologica, for embryology, the study of development, a standard exists in Terminologia Embryologica; these standards specify accepted names that can be used to refer to histological and embryological structures in journal articles and other areas. As of September 2016, two sections of the Terminologia Anatomica, including central nervous system and peripheral nervous system, were merged to form the Terminologia Neuroanatomica; the Terminologia Anatomica has been perceived with a considerable criticism regarding its content including coverage and spelling mistakes and errors. Anatomical terminology is chosen to highlight the relative location of body structures. For instance, an anatomist might describe one band of tissue as "inferior to" another or a physician might describe a tumor as "superficial to" a deeper body structure.
Anatomical terms used to describe location
Node of Ranvier
Nodes of Ranvier known as myelin-sheath gaps, occur along a myelinated axon where the axolemma is exposed to the extracellular space. Nodes of Ranvier are uninsulated and enriched in ion channels, allowing them to participate in the exchange of ions required to regenerate the action potential. Nerve conduction in myelinated axons is referred to as saltatory conduction due to the manner in which the action potential seems to "jump" from one node to the next along the axon; this results in faster conduction of the action potential. Many vertebrate axons are surrounded by a myelin sheath, allowing rapid and efficient saltatory propagation of action potentials; the contacts between neurons and glial cells display a high level of spatial and temporal organization in myelinated fibers. The myelinating glial cells; the internodal glial membranes are fused to form compact myelin, whereas the cytoplasm-filled paranodal loops of myelinating cells are spirally wrapped around the axon at both sides of the nodes.
This organization demands a tight developmental control and the formation of a variety of specialized zones of contact between different areas of the myelinating cell membrane. Each node of Ranvier is flanked by paranodal regions where helicoidally wrapped glial loops are attached to the axonal membrane by a septate-like junction; the segment between nodes of Ranvier is termed as the internode, its outermost part, in contact with paranodes is referred to as the juxtaparanodal region. The nodes are encapsulated by microvilli stemming from the outer aspect of the Schwann cell membrane in the PNS, or by perinodal extensions from astrocytes in the CNS; the internodes are the myelin segments and the gaps between are referred to as nodes. The size and the spacing of the internodes vary with the fiber diameter in a curvilinear relationship, optimized for maximal conduction velocity; the size of the nodes span from 1–2 µm whereas the internodes can be up to 1.5 millimetres long, depending on the axon diameter and fiber type.
The structure of the node and the flanking paranodal regions are distinct from the internodes under the compact myelin sheath, but are similar in CNS and PNS. The axon is exposed to the extra-cellular environment at the node and is constricted in its diameter; the decreased axon size reflects a higher packing density of neurofilaments in this region, which are less phosphorylated and are transported more slowly. Vesicles and other organelles are increased at the nodes, which suggest that there is a bottleneck of axonal transport in both directions as well as local axonal-glial signaling; when a longitudinal section is made through a myelinating Schwann cell at the node, three distinctive segments are represented: the stereotypic internode, the paranodal region, the node itself. In the internodal region, the Schwann cell has an outer collar of cytoplasm, a compact myelin sheath, inner collar of cytoplasm, the axolemma. At the paranodal regions, the paranodal cytoplasm loops contact thickenings of the axolemma to form septate –like junctions.
In the node alone, the axolemma is contacted by several Schwann microvilli and contains a dense cytoskeletal undercoating. Although freeze fracture studies have revealed that the nodal axolemma in both the CNS and PNS is enriched in intra-membranous particles compared to the internode, there are some structural differences reflecting their cellular constituents. In the PNS, specialized microvilli project from the outer collar of Schwann cells and come close to nodal axolemma of large fibers; the projections of the Schwann cells are perpendicular to the node and are radiating from the central axons. However, in the CNS, one or more of the astrocytic processes come in close vicinity of the nodes. Researchers declare that these processes stem from multi-functional astrocytes, as opposed to from a population of astrocytes dedicated to contacting the node. On the other hand, in the PNS, the basal lamina that surrounds the Schwann cells is continuous across the node; the nodes of Ranvier contain Na+/K+ ATPases, Na+/Ca2+ exchangers and high density of voltage-gated Na+ channels that generate action potentials.
A sodium channel consists of a pore-forming α subunit and two accessory β subunits, which anchor the channel to extra-cellular and intra-cellular components. The nodes of Ranvier in the central and peripheral nervous systems consist of αNaV1.6 and β1 subunits. The extra-cellular region of β subunits can associate with itself and other proteins, such as tenascin R and the cell-adhesion molecules neurofascin and contactin. Contactin is present at nodes in the CNS and interaction with this molecule enhances the surface expression of Na+ channels. Ankyrin has been found to be bounded to βIV spectrin, a spectrin isoform enriched at nodes of Ranvier and axon initial segments; the PNS nodes are surrounded by Schwann cell microvilli, which contain ERMs and EBP50 that may provide a connection to actin microfilaments. Several extracellular matrix proteins are enriched at nodes of Ranvier, including tenascin-R, Bral-1, proteoglycan NG2, as well as phosphacan and versican V2. At CNS nodes, the axonal proteins include contactin.
The molecular organization of the nodes corresponds to their specialized function in impulse propagation. The level of sodium channels in the node versus the internode suggests that the number IMPs co