Layers of Brain and Spinal Cord - Report Example

Neuroanatomy Introduction: Brain is considered as the judgment centre of the body. It is present in head region in a cavity called cranium and protected by a bonny structure called skull. Human nervous system consists of three parts: Central nervous system Peripheral nervous system Autonomic nervous system The central nervous system comprises of brain and spinal cord, which is also a prolongation of brain and covered by spinal column. The peripheral nervous system comprises of nerves. Brain and spinal cord are surrounded by some protective membranes which are collectively known as meninges. These membranes cover the entire nervous system. Meninges consist of three layers. Dura mater which is the outer most layer, Arachnoid which is the middle one and Pia mater which is the inner most. The infections of meninges are known as meningitis. Meninges are layers made up of connective tissues which perform a variety of functions which will be discussed later in this paper. Dura mater: Dura is a Latin word which means ‘hard mother’. Dura mater is the outer most of the meningial layers. As the name indicates it is rigid and nonflexible. It separates the brain from the cranial cavity and also prevents the brain from translocation. It is a double layered structure consisting of connective tissues. The outer layer of Dura matter is called the periosteal layer which is attached to the bones of the cranium. The inner layer is much soft and thinner and is called the meningeal layer. It holds on to the brain but does not attach itself to the bone. Along the spinal cord it is a single membrane but is similar to the cranial meninges. The falx cerebri separates the hemispheres of the cerebrum. The falx cerebelli separates the lobes of the cerebellum. The tentorium cerebelli separates the cerebrum from the cerebellum. Dura mater is a fused membrane except at some regions where it is quite separate and possesses an appropriate lining. These separations allow the Dura membrane to cover the dural sinuses. These sinuses are accountable for the collection of blood in veins which is further collected into a single vein in the neck region. There are four distinct regions where dura membrane segregates. The superior sagittal sinus runs across the top of the brain in an anterior-posterior direction. Other sinuses are the straight sinus, the inferior sinus, and the transverse sinus. Dura mater covers the spinal cord in tubular form. It is quite thick and wraps the whole spinal cord up to its end. There is a space between dura mater and skull known as the epidural space. In case of any hemorrhage in the brain or any closed injury, blood is collected in this epidural space. There is also a potential space between dura mater and the middle layer of meninges called arachnoid which is known as the subdural space. In case of any shock the blood accumulates in this layer and pushes down the lower layers of the meninges. If further bleeding occurs, the pressure applied by this subdural space may result in brain damage. In adults blood accumulates in epidural space while in children blood accumulates in subdural space. Arachnoid: The term arachnoid refers to the spider web like appearance of the blood vessels within the space. Arachnoid is the middle layer of meninges and runs through the whole nervous system. It penetrates into the sinuses formed by the dura mater and form projections. These projections are called arachnoid granulation. They pass through the cerebrospinal fluid and ventricles and return to the blood stream. It also ceases the fractures or cracks of brain. There is a space between arachnoid matter and Pia mater, known as subarachanoid space. This space is filled with cerebrospinal fluid. All the blood vessels that enter the brain pass through this space. Cerebrospinal fluid is a filtrate of blood. It is a clear liquid that runs through the different spaces of brain such as ventricles and central canals. This cerebrospinal fluid also acts as a shock absorber and prevents the brain from damage. It also brings nutrients to the brain and spinal cord and removes waste from the system. There are four spaces in the brain which are filled by cerebrospinal fluid. These layers act as a cushion and provide protection to the brain. The two lateral ventricles spread across a large area of the brain. The frontal horns of these structures are situated in the frontal lobes of the brain. They expand posteriorly into the parietal lobes and their inferior horns are situated in the temporal lobes. The third ventricle is found between the two thalamic bodies. The massa intermedia passes through these thalamic bodies and the hypothalamus is found at its floor and at its lateral walls. The fourth ventricle is situated between the cerebellum and the pons. All these ventricles are associated with each other. Two formina or munro connect the two lateral ventricles with the third one and this is known as the interventricular foramina. The third and forth ventricles are associated with each other by Aqueduct of Sylvius. The fourth ventricle is associated to the subarachnoid space through two lateral foramina of Luschka and by one medial foramen of Magendie. Pia mater: The pia mater is the third and the deepest layer of meninges. It is a mesh-like substance which covers the whole brain and spinal cord. It is extremely thin and attaches the convolutions of the brain and the indiscretion of the spinal cord. It plays an important role in providing nourishment to cells in its surrounding area. Pia mater is extraordinarily vascular and along with arachnoid it performs special functions. It provides covering to all the four ventricles. It combines with the ependyma, the membranous lining of the ventricles to form special structures called the choroid plexes. These choroid plexes are responsible for the production cerebrospinal fluid. It also provides support in attaching the dura mater to the spinal cord through lateral extensions along the spinal cord. These extensions are known as ligamentum denticulatum. Meningitis: Meningitis is a clinical manifestation which is characterized by inflammation of the meninges. Some common symptoms that appear in meningitis are headache, nuchal rigidity, photophobia) and an increased white blood cells count in the cerebrospinal fluid (CSF; pleocytosis). On the basis of symptoms meningitis can be classified into two types of infections. Acute and chronic, in acute meningitis symptoms appear in hours to several minutes while in chronic infections it may takes hours to weeks. Meningitis may be caused by certain infectious and noninfectious agents. Few of the noninfectious causes of meningitis are medications (eg, nonsteroidal anti-inflammatory drugs, antibiotics) and carcinomatosis. The main causes of Infectious meningitis are the biological agents. On the basis of these biological agents meningitis can be classified. Bacteria are considered as the main cause of acute meningitis i.e. some fungi and parasites can also cause meningitis such as cryptococcal meningitis, Histoplasma meningitis, and amebic meningoencephalitis. There are three main pathways by which an infectious agent (i.e., bacteria, virus, fungus, parasite) can entre the central nervous system and cause meningitis. First of all the infectious agent colonize within the host body. This colonization may occur in skin, nasopharynx, respiratory tract, gastrointestinal tract, or genitourinary tract. Usually the route of entry of infectious agent is by respiratory track i.e. Neisseria meningitides and Staphylococcus pneumoniae (pneumococcus) entre in body by nasopharyngeal route. After colonization the pathogen invade into the sub mucosal epithelium. From here it enters into the blood stream and through it finally reaches to central nervous system. This route of entry is followed in meningococcal, cryptococcal, syphilitic, and pneumococcal meningitis. The second route of entry is a retrograde neuronal pathway (i.e. olfactory and peripheral nerves). This route of entry is followed by Naegleria fowleri, Gnathostoma spinigerum. Third one is direct contiguous spread (i.e., sinusitis, otitis media, congenital malformations, trauma, and direct inoculation during intracranial manipulation). Since there are limited host defenses (eg, immunoglobulins, neutrophils, complement components), the multiplication of infectious agent become uncontrolled which lead to the inflammation of meninges. Cytokines and other pro inflammatory molecules (eg, tumor necrosis factor-alpha [TNF-alpha], interleukin [IL]–1), chemokines (IL-8), and other proinflammatory molecules play a vital role in damaging. The concentration of cerebrospinal fluid of TNF-alpha, IL-1, IL-6, and IL-8 is also found to be increased in bacterial meningitis. The toxic product released by the bacteria are exposed to cells (eg, endothelium, leukocytes, microglia, astrocytes, meningeal macrophages) promotes the synthesis of cytokines and proinflammatory mediators. An endogenous pyrogen IL-1 is also synthesized by activated mononuclear phagocytes which induce fever. Many secondary mediators, such as IL-6, IL-8, nitric oxide, prostaglandins (PGE2), and platelet activation factor (PAF) enhance the inflammatory response. TNF-alpha induces chemokine IL-8 mediates neutrophil chemoattractant. IL-1 Nitric oxide is a free radical molecule induces cytotoxicity. cyclooxygenase, play role in the induction of increased blood-brain barrier (BBB) permeability. PAF is considered to mediate the formation of thrombi and the activation of clotting factors within the vasculature. In response of this inflammation bleeding starts in subarachnoid space which results in edema and high protein levels. Neutrophils entre the blood stream in response of cytokines and chemotactic molecules and then penetrates in damaged BBB which increases the viscosity of cerebrospinal fluid which dimities venous outflow and lead to interstitial edema. This edema results in decreased cerebral blood flow and intracranial hypertension. Further complications meningitis include is the increased intracranial pressure (ICP). These complications may promote proinflammatory molecules. Interstitial edema (secondary to obstruction of CSF flow, as in hydrocephalus), cytotoxic edema (swelling of cellular elements of the brain through the release of toxic factors from the bacteria and neutrophils), and vasogenic edema (increased BBB permeability) also occur in such condition. Bibliography Top of Form Martin, John H. Neuroanatomy: Text and Atlas. New York, N.Y.: McGraw-Hill, 2003. Bottom of Form Top of Form Snell, Richard S., and Richard S. Snell. Clinical Neuroanatomy. Philadelphia: Lippincott Williams & Wilkins, 2006. Bottom of Form Top of Form Blumenfeld. Neuroanatomy. Sinauer Associates Inc, 2009. Bottom of Form Top of Form Bosman, M. C. Neuroanatomy. Pretoria, RSA: University of Pretoria, 2008. Bottom of Form Top of Form Nolte, John, and John W. Sundsten. The Human Brain: An Introduction to Its Functional Anatomy. St. Louis, Mo: Mosby, 2002. Bottom of Form Top of Form Schünke, Michael, Lawrence M. Ross, Edward D. 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