Postcentral Gyrus

The postcentral gyrus, or postcentral convolution, is a strip of cortical structure which lies just behind the central sulcus, running down adjacently and parallel to it. Bounded in back by the interparietal sulcus, the postcentral gyrus is located in the parietal lobe of the cerebral cortex. It is the seat of the processing center of somatosensory system, which is a sensory region of the brain.

Damage to the postcentral gyrus can cause agraphesthesia (disorientation in cutaneous space), astereognosia, loss of vibration, proprioception and fine touch (because the third-order neuron of the medial-lemniscal pathway cannot synapse in the cortex); and, if it affects the non-dominant hemisphere, it can also produce hemispatial neglect. It could also reduce nociception, thermoception and crude touch.

Postcentral gyrus

Decussation of the Pyramids

The decussation of the pyramids, or motor decussation, is the crossing over of cortical motor nerve fibers of the pyramidal tracts at the lower end of the medulla oblongata. About three-fifths of these pyramidal fibers leave the pyramids in successive bundles, and cross over to the opposite side in the anterior median fissure of the medulla oblongata, forming what is known as the pyramidal decussation. As a result, each of the anterior funiculus in the spinal cord is composed of motor fibers that originally come from the opposite side. The two pyramids consist of motor fibers which goes from the cerebral cortex to the medulla oblongata (via cerebral peduncles and pons) and to medulla spinalis, corticobulbar and corticospinal fibers. The word "decussation" means crossing, an X-shape crossing.

Decussation of the pyramids

Cross Section of Decussation of the Pyramids

Anterior Funiculus

The anterior funiculus is the bundle of white matter which lies on both sides of the spinal cord anterior median fissure and is bounded by the ventral roots of the spinal nerves. The anterior funiculus is composed of longitudinal motor fibers which come down from the cerebral cortex via the cerebral peduncles and the pyramid. Fibers from the anterior funiculus, along with axons from radial neurons in the anterior horn of the grey commisure, form the ventral roots of the spinal nerves.

Cross section of spinal cord showing the anterior funiculus

Ataxia

Ataxia is a neurological disorder which consists of gross lack of coordination of muscle movement; that is to say an abnormality of muscle control. Ataxia results in a jerky and unsteady motion of the trunk and limbs. It is a dysfunction of parts of the nervous system which coordinate movement. Cerebellar ataxia is a dysfunction of the cerebellum.

Not only can ataxia affect your movements, but also your speech, your eye movement and your ability to swallow. Ataxia results from damage to the cerebellum, stroke and cerebral paulsy.

Hemispatial Neglect

Hemispatial neglect is a neuropsychological condition which is characterized, after damage to one hemisphere of the brain, by an attentional deficit and awareness of one side of space. Although hemispatial neglect is very commonly contralateral to the damaged hemisphere, instances of ipsilesional neglect have been reported.

In other words, hemispatial neglect is a neurological disorder which is defined as a lack of attention for stimuli contra-lateral to the brain lesion. The assessment is traditionally done with basic pencil and paper tests and the rehabilitation programs are generally not well adapted. We propose a virtual reality system featuring an eye-tracking device for a better characterization of the neglect that will lead to new rehabilitation techniques. A stroke affecting the right parietal lobe of the brain can lead to neglect for the left side of the visual field, causing a patient with neglect to behave as if the left side of sensory space is nonexistent; although they can still turn left. In an extreme case, a patient with neglect might fail to eat the food on the left half of their plate, even though they complain of being hungry.

In hemispatial neglect there is a failure to represent information appearing in the hemispace contralateralto a brain lesion. In addition to the perceptual consequences of hemispatial neglect, several authors have reported that hemispatial neglect impairs visually guided movements. In some cases, neglect has been shown to impair visual perception without affecting visuomotor control in relation to the very same stimuli.

Cortical Development

Cortical development is the development of the cerebral cortex from the embryo state to maturity. It is very important to understand the mechanisms of cortical development, because it supplies us with the insight into the pathogenesis of many genetic and acquired developmental psychiatric disorders, which include autism, schizophrenia and learning disabilities.

The adult human cerebral cortex has six neuronal layers, each consisting of specialized neurons with specific phenotypes and synaptic connections. In the majority of organs, cells are born near their eventual location, but in the embryonic cortical development neurons are generated some distance away. The final migration of these nerve cells and the establishment of proper interneuronal connections are critical for proper cortical functioning. Faults in this dynamic process provoke a number of developmental disorders of higher brain function.

During the embryonic development of the cortex, the first neurons destined to settle in the human cortex are produced during the first half of gestation, deep within the cerebrum and close to the cavity of the cerebral ventricles, in a area called the ventricular zone. Shortly after their last mitotic divisions, these neurons migrate outward toward the pial surface of the cortex where they make up a sheet of nerve cells called the cortical plate. Each successive generation of migrating neurons passes through the previously born cells before arriving at the final destination at the interface between the cortical plate and the marginal zone.

Neurons use a transient population of radial glial cells during migration as a scaffolding to aid their navigation. These glial cells form long fascicles that span the cerebral cortex and guide the migrating neurons through each cortical layer. According to the radial unit hypothesis of cortical development, the horizontal location of cortical neuron is determined by the position of its precursor cells in the proliferative ventricular zone, while its depth results from its birth order.

First, the cortex develops in an inside-out pattern in which the earliest born neurons are found in the deepest cortical layers while the later born neurons move to the more superficial layers. Second, the radial glial hypothesis provides an explanation for the columnar organization of the cortex. Each group of progenitor cells within the ventricular zone gives rise to a column of interrelated neurons above it. After the neurons have taken their proper laminar positions, they develop characteristic synaptic connections with nearby neurons as well as more distant neurons in associated regions of the cortex.

Phylogenetically, the sequence in which the cortex develops agrees with regionally relevant milestones in cognitive and functional development. Parts of the brain associated with more basic functions matured early: motor and sensory brain areas matured first, followed by areas involved in spatial orientation, speech and language development, and attention (upper and lower parietal lobes). Later to mature were areas involved in executive function, attention, and motor coordination (frontal lobes).

Middle Frontal Gyrus

The middle frontal gyrus, or middle frontal convolution, is a cortical strip which runs in an anteroposterior direction in the frontal lobe of the human brain. The middle frontal gyrus is located between the inferior frontal gyrus and the superior frontal gyrus. It constitutes one-third of the frontal lobe.

The middle frontal gyrus performs highly cognitive functions as it is involved with superior human consciousness. Its fibers link it with the primary motor cortex, the post central gyrus, and the cerebral peduncles.

Middle Frontal Gyrus

Superior Frontal Gyrus

The superior frontal gyrus is a strip of cerebral cortex which runs along longitudinally on the medial edge of the convex surface of the frontal lobe. The superior frontal gyrus constitutes one-third of the frontal lobe of the brain. It is bounded laterally by the superior frontal sulcus.

It is thought that the superior frontal gyrus is involved in self-awareness, in coordination with the action of the sensory system, contributing to higher cognitive functions and particularly to working memory.

Immune System

The immune system is a network of specialized cells and organs which protect the organism, of which they are part of, against disease-causing pathogens (bacteria, viruses, fungi, etc). The immune system works as a shield which protects the human body from foreign invaders that enter and use the host organism to multiply, altering its normal biological functions, and eventually causing its death.

Basically, the immune system consists of leukocytes (white blood cells), cytokines, spleen, thymus, bone marrow, intestinal flora, and skin. There are several types of white blood cells, such as basophils, monocytes, neutrophils, eosinophils, mast cells, and lymphocytes; they are found in the blood plasma and their function is to identify, kill, and phagocytose (engulf) foreign microorganisms such as bacteria, viruses, fungi, protozoans (parasites), and toxic proteins, inducing the reproduction of antibodies and lymphocytes T cells. Cytokines are types of proteins which trigger a chemical alarm, signalling the presence of pathogens for the white blood cells to act.

The immune system have two organs: the thymus and the spleen, which are the places where most leukocytes mature. Most leukocytes originate from hematopoietic stem cells in the bone marrow and then go in the blood stream to the thymus and spleen where they mature get stored. The intestinal flora is a natural barrier composed of millions of harmless and useful bacteria, thriving in the first section of the large intestine; these bacteria kill and engulf foreign harmful bacteria. The skin is the first line of defense, insulating the organism from the outside world.

Immune System

Pathogen

A pathogen is a disease-provoking microorganism, such as a bacterium or a virus, which enters a host and disrupts its normal functions, sometimes causing the death of the host body. A pathogen can penetrate a host through three main pathways: through the mouth (oral), mucous membrane (nose, eyes, etc), and through an open would.

The most common pathogens are bacteria, viruses, protozoans (parasites), fungi, and prion. Pathogens can be transmitted from person to person in a number of ways. The influenza virus is transmitted from person to person through the air, through sneezing and coughing. Escherichia coli is readily transmitted through water, food, and blood, but is not readily transmitted via air or the bite of an insect. Nevertheless, the body contains many natural order of defense against some of these pathogens; this natural defense is the immune system, which is composed of white blood cells.

Thymus

The thymus is a glandular organ which is involved in the establishment of the immune system from the 12th week of gestation until puberty. The thymus gland serves as the site of T cells (lymphocytes) maturation. Hematopoietic T lymphocyte progenitor cells produced in the bone marrow flow in the blood stream into the thymus where they are turned into small lymphocytes, called thymocytes, by thymic stem cells. Then, within this special thymic environment, these dividing lymphocytes go through a process of cellular differentiation in which immature thymocytes make distinct T cell receptors by a process of gene rearrangement.

The thymus is situated behind the sternum, in the upper antierior portion of the chest cavity, above the treachea. It is soft and pinkish-gray in color and is composed mainly of lymphatic tissue. At birth it is 5 cm long, 4 cm wide, and 0.6 cm thick. The thymus grows gradually in size and becomes more active until puberty; from then on, it slowly becomes atrophied and vestigial.


Thymus Gland

Myoglobin

Myoglobin is a globular protein composed of 153 amino acids. It is single-chained and has an iron-containing porphyrin in the center. Myoglobin is found in heart and skeletal muscles. As oxygen is bound to myoglobin, the muscle is able to maintain a high level of activity for a longer period of time it, using up the oxygen when they contract during physical activities.

Myoglobin has a molecular weight of 16,700 daltons. It is structurally related to hemoglobin and is the primary oxygen-carrying pigment of muscle tissues. Myoglobin combines with oxygen to form oxymyoglobin, acting as a small oxygen store of about 10 millilitres per kilogram of muscle. This source of oxygen is important during intermittent bursts of activity. Oxymyoglobin restored during recovery periods. Muscle rich in myoglobin is called red muscle. It has a preponderance of slow-twitch muscle fibers and is adapted to endurance activities.

Myoglobin arranges itself in pigments, which are responsible for making meat red. The red color which meat takes is partly determined by the charge of the iron atom in myoglobin and the oxygen attached to it. When meat is in its raw state, the iron atom is in the +2 oxidation state, and is bound to a dioxygen molecule (O2). The structure of myoglobin was figured out by John Kendrew and associates in 1958, using high-resolution X-ray crystallography.

Spleen

The spleen is a vascular organ situated in the left upper quadrant of the abdomen, below the diaphragm. The spleen function is to create lymphocytes for the destruction and removal of old red-blood cells. The spleen is also a blood reservoir in case of hemorrhagic shock. It also recycles iron.

The spleen produce antibodies in its white pulp, removing from the circulation antibody-coated bacteria and antibody-coated blood cells. It contains in reserve half the body's monocytes in its red pulp that upon moving to injured tissue such as the heart turn into dendritic cells and macrophages and aid wound healing. It is one of the centers of activity of the reticuloendothelial system, and can be considered analogous to a large lymph node. The absence of the spleen can lead to a predisposition to certain infections.

The spleen is present in most vertebrates and lie in the abdominal cavity. It is an important part of the blood-forming system. It is also one of the largest lymphoid organs in the body and is involved in the defenses against diseases attributed to the reticuloendothelial system.

Interleukins

Interleukin is a type of cytokine which acts specifically as a mediator between leukocytes. Interleukins are proteins secreted mostly by T cells lymphocytes and macrophages, inducing growth and differentiation of lymphocytes and hematopoietic stem cells. Interleukins play an important role in the regulation of lymphocyte function. Several known types are crucial constituents of the body's immune system.

Interleukins function as chemical messengers between cells. They mediate and control the immunologic and inflamatory response. The term "interleukin" was coined by Dr. Paetkau, University of Victoria.

T Cell Receptor

The T cell receptor is a protein molecule located on the surface of T cells, whose function is to recognize antigens bound to major histocompatibility complex (MHC) molecules. The T cell receptors allow the T cell to recognize antigen presented in the context of major histocompatibility complex class I or class II molecules, which are expressed on infected cells or professional antigen-presenting cells.

A T cell receptor is a heterodimer which is composed of an alpha and beta chain in 95% of T cells, whereas 5% of T cells have TCRs consisting of gamma and delta chains. Engagement of the T cell receptor with antigen and major histocompatibility results in activation of its T lymphocyte through a series of biochemical events mediated by associated enzymes, co-receptors, specialized accessory molecules and activated or released transcription factors.

Plasmocytes

Plasmocytes, or plasma cells, are spherical cells which take part in the synthesis and release of antibodies. Plasmocytes and effector B cells produce large volumes of antibodies which are carried by the blood plasma and the lymphatic system. Although plasma cells originate in the bone marrow, they are produced in the bone marrow as B cells, before they finally differentiate into plasmocytes. This happens when B cells are activated by antigens. secrete high levels of antibodies,ranging from hundreds to thousands of antibodies per second per cell. Unlike their precursors, plasma cells can not switch antibody classes and act as antigen-presenting cells because they no longer display MHC-II.

Antibodies

Antibodies are Y-shaped gamma globulin proteins which are found on the surface of B cells lymphocytes (white blood cells). They are released into the blood stream and lymph in response to intruding antigens (bacteria, bacterial toxins, fungi, parasites, viruses, and cell proteins). Antibodies are used by the immune system to identify and neutralize these foreign microorganisms by binding specifically to them. B cells lymphocytes are capable of producing one type of antibody, each bearing sites on its membrane which binds to a specific antigen. When this binding takes place, it triggers the B cells lymphocyte to reproduce itself, forming a clone that manufactures vast amounts of its antibody. Antibodies are produced by a kind of white blood cell called a plasmocyte, which originates in the bone marrow as B cells.

The antibody molecule consists of four polypeptide chains; two identical light chains and two identical heavy chains-joined by disulfide bridges. The light chains have a variable portion that is different in each type of antibody and is the active portion of the molecule that binds with the specific antigen.

Antibodies

Blood

Blood is a bodily fluid which runs throughout a network of tube-like elastic vessels, distrubuting the necessary substances to the body cells, such as oxygen and nutrients, and transporting waste products away from those same cells. Blood is basically composed of three types of cells: 1) red blood cells, whose function is to carry nutrients, oxygen, and carbon dioxyde to and away from the body cells; 2) white blood cells, whose role is to protect the body from disease-inflicting foreign microorganisms such as bacteria, fungi, and viruses; 3) platelets, which are involved in hemostasis, causing blood to form clots at the site of a wound, stopping bleeding. These blood cells are suspended in plasma, which is composed of 90% water and 10% mineral ions, glucose, proteins, and hormones. The blood cells are produced in the bone marrow.

Platelets

Platelets are irregularly-shaped blood cells without nucleus. A platelet measure between 2 and 3 µm (micrometers) in diameter. The average lifespan of a platelet is 12 days. They are produced by the fragmentation of precursor megakaryocytes. Platelets take part in hemostasis as they cause the blood to coagulate by forming clots to stop bleeding. When blood from a wound flows out, the platelets gather at the site of the damaged tissue to stop the bleeding. Calcium, vitamin K, and a protein called fibrinogen help the platelets form a clot.

When the number of platelets is too high, blood clots called thrombosis can form. This obstructs blood vessels and result in a stroke. An increase in the number of platelets is called thrombocytosis. A low number of platelets is called thrombocytopenia. Platelets release growth factors, which include platelet-derived growth factor (PDGF), a potent chemotactic agent, and a transforming growth factor-ß, that stimulates the deposition of extracellular matrix. Both of these growth factors have been shown to play a significant role in the repair and regeneration of connective tissues.

Hemostasis

Hemostasis is the process by which blood changes from a fluid to a solid state, stopping the hemorrhage through a damaged blood vessel. In other words, hemostasis is the arrest of bleeding by a natural process. But major arterial bleeding is unlikely to be stopped by this process; in this case it requires a surgical process. The process of hemostasis has three steps: 1) vasoconstriction; 2) temporary blockage of a break by a platelet plug; and 3) blood coagulation, which is the formation of a clot that seals the hole until the tissue is repaired.

Leukocytes

Leukocytes are the white blood cells of the blood tissue and the immune system, which protects the body against both infectious diseases and foreign materials by identifying, killing, and fagocytizing bacteria, fungi, and viruses. There are five different types of leukocytes: 1) neutrophils, which protect us against bacterial and fungal infection; 2) basophils, which are responsible for allergic and antigen response, releasing the chemical histamine causing inflammation; 3) eosinophils, which defend against parasitic infection; 4) monocytes, which divide and differentiate into macrophages to carry out the job of cleaning and phagocytizing bacteria, viruses and debris ; 5) Lymphocytes, which perform the most complex function of the immune system: recognize, identify, signaling, and destroy bacteria and viruses. All leukocytes are produced and derived from a multipotent cell in the bone marrow known as a hematopoietic stem cell. Leukocytes are found throughout the body, specially in the blood and lymphatic system.

Mast Cells

A mast cell is a connective and fatty tissue cell which produces basophilic and cytoplasmatic granules which contain histamine and heparin, which play an important role during allergic reactions and inflammation response. Mast cells are derived from an undifferentiated precursor of monocytic origin in the perivascular connective tissue. During inflammatory processes, the mast cells releases their granules, as well as various hormonal mediators into the interstitium. Mast cells can be stimulated to degranulate by direct injury, cross-linking of Immunoglobulin E (IgE) receptors, or by activated complement proteins. Evidence suggests that mast cells have a fundamental role in innate immunity as they elaborate a vast array of important cytokines and other inflammatory mediators, expressing multiple pattern recognition receptors thought to be involved in recognizing broad classes of pathogens.

Mast Cells

Cytokines

Cytokines are singaling soluble proteins which are secreted by the immune system cells and act as intercellular mediators in the generation of immune response, sending messages that are delivered to the same cell (autocrine), an adjacent cell (paracrine), or a distant cell (endocrine). Cytokines take part in reproduction, growth and development, normal homeostatic regulation, response to injury and repair, blood clotting, and immunity and tolerance. The cytokines bind to a specific receptor, provoking a change in function of the target cell.

Based on their function, cell of secretion, or target of action, cytokines can be divided into three groups: interleukins, chemokines, and lymphokines. Interleukin is a protein which is produced by one type of lymphocyte or macrophage and act on other leukocytes. Chemokine is a specific class of cytokines that mediates chemoattraction (chemotaxis) between cells. Lymphokines are produced by a type of immune cell known as a lymphocyte. Lymphokines direct the immune system response by signaling between its cells. Lymphokines play important roles, such as the attraction of other immune cells, like macrophages and other lymphocytes, to an infected site and their subsequent activation prepare them to attack the invaders.

Another important function performed by cytokines is wound healing. Cytokines make sure that the restorative sequences are executed in the appropriate order by signaling blood cells and vascular endothelium to coagulate and fill in a wound opening, recruiting and signaling macrophages and neutrophils to engulf microbes, and guiding protective skin epidermal cells to grow over the wounded area.

Macrophages

A macrophage is type of white blood cell whose main function is to destroy and engulf bacteria and foreign microorganism within tissue. That is why it is said that a macrophage is a phagocytic cell, which means it phagocytizes (devours) bacteria, bacteria-infected cells, and debris. Macrophages are also involved in the immune response, as they are activated by cytokines released by T cells lymphocytes; once activated macrophages begin not only to kill and phagocytize foreign bodies, but also to divide and multiply as bigger and stronger macrophages.

Macrophages measure 21 micrometers in diameter. They are produced by the division of monocytes. Another importan function performed by the macrophages is the removal of necrotic cellular debris in the lungs. The removal dead cell material is important in chronic inflammation, as the early stages of inflammation are dominated by neutrophil granulocytes, which are ingested by macrophages when they die.

Lymphocytes

A lymphocyte is a type of leukocyte (white blood cell) which is responsible for immune responses. Lymphocytes make up 25% of the white blood cells and they regulate and take part in acquired immunity. According to their physical appearence under the microscope, there are categories of lymphocytes: the large granular lymphocytes and the small lymphocytes. According to their functions, there are three types of lymphocytes: 1) natural killer cells, which are large granular lymphocytes; 2) T cells, which are small lymphocytes; 3) B cells, also small.

Natural killer cells are toxin-containing lymphocytes which kill virus-infected cells and tumor cells. T cells are lymphocytes which are involved in cell-mediated immunity; they have the ability to recognize specific peptide antigens through the receptors on their cell surface. B cells recognize specific “non-self” antigens, during a process known as antigen presentation; once they have identified the foreign microorganism, the cells generate specific responses to eliminate specific pathogens or pathogen infected cells; when B cells are activated by antigens, or pathogens, they bind them by their protein-receptor and begin to produce large quantities of antibodies which then neutralize foreign objects like bacteria and viruses.

Lymphocytes are produced in the bone marrow, but some them mature in the thymus and the spleen.

B Cells

A B cell is a class of lymphocyte (white blood cell) that secretes antibodies against antigens. B cells perform the job of immune surveillance. They begin to produce antibodies when they are fully activated by an antigen (foreign bacterial or viral substance). Each B cell has a special protein wrapping around its surface. This protein is a membrane-bound immunoglobulin that functions as a receptor, which is referred to as the B cell receptor, or BCR.

The B cell receptor will bind to a foreign antigen and activate the B cell, which, in turn, engulfs the bound antigen molecule by receptor-mediated endocytosis. The antigen is destroyed and its fragments are displayed on the cell surface. Then, T helper cells bind the B cell and secrete lymphokines which stimulate the B cell to begin a cell cycle and, through repeated mitosis, develop into a clone of cells with identical B cell receptor (BCR).

Immature B cells are produced in the bone marrow. Then these immature B cells migrate to the spleen, where they are called transitional B cells, and some of these cells differentiate into mature B lymphocytes.

B Cells

Antigen

An antigen is any foreign substance which generates antibodies, provoking an immune response in the organism it has entered. Antigens include bacteria, viruses, foreign blood cells, and the toxins they secrete. When a human body suffers from autoimmune disorders, its own immune system reacts to its own antigens (self-antigens), which are the white blood cells and its toxins with which they would attack bacteria and viruses under normal and healthy circumstances. The term "antigen" is formed from the words "antibody generator". Specially prepared microbial antigens which are used to induce protective antibodies are called vaccines.

An antigen is bound by an antibody at its antigen-binding site, then it is destroyed by the host's phagocytic white blood cells. It is important to bear in mind that antibodies tend to discriminate between the specific molecular structures presented on the surface of the antigen . Antigens are also proteins or polysaccharides from the surface or the inside of a viral or bacterial cell.