By S. Tragak. Monmouth College, Monmouth Illinois.
Demyelination refers to the primary loss of myelin with relative preservation of the axon (eg buy zovirax 200 mg free shipping hiv infection age group. Spheroids contain mixtures of lysosomes zovirax 800mg online hiv infection rates california, mitochondria order zovirax 400mg hiv infection rates by demographic, neurofilaments discount zovirax 800 mg with mastercard antiviral proteins, and other cytoplasmic constituents. Slowing or cessation of axoplasmic transport at sites of damage presumably account for spheroids. This is seen in many types of mental retardation, including congenital hypothyroidism (cretinism). Atrophy is a reduction in the volume and surface area of dendritic branches, commonly seen in neurodegenerative diseases. Neuritic plaques are collections of degenerating axons and dendrites, mixed with microglia and astrocytes and associated with the extracellular deposition of amyloid (beta-amyloid, see lecture on Neurodegenerative diseases). Status spongiosis refers to a spongy state of the neuropil, the formation of fine to medium sized vacuoles representing swollen neuronal and astrocytic processes. This change is typical of transmissible spongiform encephalopathies, such as Creutzfeldt- Jacob disease. They contact blood vessels, pial surfaces, and enfold synapses in their functions to maintain the concentration of ions, neurotransmitters, and other metabolites within normal levels in the extracellular space. Astrocytes undergo hypertrophy (enlargement) and hyperplasia (proliferation) in response to a great many pathological processes, including hypoxic-ischemic damage and trauma. Astrocytes develop abundant pink cytoplasm, either due to imbibing plasma proteins and fluid in the short-term (when the blood-brain-barrier is broken) or filling up with intermediate filaments (in long-term scarring). The descriptive term of reactive, hypertrophic or gemistocytic is often used to describe this change. They are thought to be related to the hyperammonemia of hepatic failure (see notes on Metabolic diseases). Corpora amylacea are spherical accumulations of polyglucosan (branched-chain glucose polymers), which increase in numbers with age, particularly in a subventricular and subpial locations, and in glial scars. Neoplasia: Astrocytomas represent a common form of brain tumor (see notes on neoplasia) 5. Note that oligodendrocytes or progenitors of oligodendrocytes are able to remyelinate demyelinated axons, and thus help to repair demyelinated lesions. Myelin edema: In certain toxic and metabolic settings, fluid accumulates within myelin sheaths, leading to intramyelinic edema. Cell loss of oligodendrocytes occurs in a variety of disorders, including immune mediated (multiple sclerosis), viral (papova virus of progressive multifocal leukoencephalopathy), and toxic (e. Viral inclusions form in oligodendrocytes in progressive multifocal leukoencephalopathy. Hypertrophy and hyperplasia of endothelial cells is commonly seen in ischemia and in the vicinity of primary and metastatic neoplasms. Schwannomas are common, usually benign, neoplasms of peripheral nerves (see notes on neoplasia). It is important to review gross neuroanatomy and appreciate the anatomic relationships among the medial temporal lobe, tentorium cerebelli, the brain stem and upper cranial nerves, and the vertebro-basilar artery system (posterior circulation). The falx cerebri acts as an incomplete partition separating the hemispheres in the sagittal plane, stopping just above the corpus callosum. The tentorium cerebelli, a horizontal reflection, which lies on the superior surface of the cerebellum, separates supra- from infra-tentorial spaces. The tentorium is open in the ventral midline to allow the midbrain to pass through (tentorial notch). Thus, each free edge of the tentorium lies adjacent to either side of the midbrain. Small increases in volume of the brain may be tolerated, since there is some room for expansion (compression of ventricles and subarachnoid space). Large increases in volume cannot be tolerated, as they may be in visceral organs, without serious consequences. Should rapid expansion occur in one part of the brain, there will be compromise of adjacent tissue. Local expansion leads to local increase in pressure, and consequently to pressure gradients within the brain. Thus, structures at a distance from the main focus of a lesion can also be compromised. Some of the important types of shifts, their pathological consequences, and clinical manifestations will be outlined below. Thus, most substances do not pass readily from blood vessels into the brain parenchyma. This is defined as an increase in volume and weight of the brain due to fluid accumulation.
Some of these substances promote coagulation generic zovirax 400mg amex hiv infection using condom, called 133 procoagulants (clotting factors) buy zovirax 800 mg lowest price hiv infection rates brazil, and the others that inhibit coagulation called anticoagulants zovirax 800 mg with amex hiv infection urethra. Whether blood will coagulate depends on the balance between these two groups of substances purchase zovirax 800mg hiv infection symptoms after 2 years. In the blood stream, the anticoagulants normally predominate, so that the blood does not coagulate while it is circulating in the blood vessels. But when a vessel is ruptured, procoagulants in that area of tissue damage become “activated” and override the anticoagulants. Clotting factors Blood clotting is mediated by the sequential activation of a series of coagulation factors, proteins synthesized in the liver that circulate in the plasma in an inactive state. This is composed of mainly phospholipids from the membranes of the tissue plus a lipoprotein complex. Figure 42: Stages of blood coagulation 136 (c) Effect of activated factor X (Xa) to form prothrombin activator The factor Xa combines immediately with tissue phospholipids that are part of tissue factor or combine with factor V to form the complex called prothrombin activator. That is, activated factor X combines with factor V and platelet/tissue phospholipids to form the complex called prothrombin activator. Prothrombin is enzymatically split into two 137 fragments: one inert and the other possessing the properties of thrombin. Initially the conversion of prothrombin proceeds too slowly to produce significant amounts of thrombin needed for coagulation. Thrombin itself, however, increases its own rate of formation by converting proaccelerin (factor V) into accelerin which then accelerates the formation of thrombin. The clot is composed of a meshwork of fibrin fibers running in all directions and entrapping blood cells, platelets and plasma proteins. Excessive bleeding Excessive bleeding can result from deficiency of any of the many blood-clotting factors. With few exceptions, almost all the blood clotting factors are formed by the liver. Therefore, diseases of the liver such as hepatitis, and cirrhosis can sometimes depress the clotting system. In absence of vitamin-K, subsequent insufficiency of these coagulation factors in the blood can lead to serious bleeding tendencies. Symptoms Hemophilia is characterized by spontaneous or traumatic subcutaneous hemorrhage, blood in urine, and bleeding in the mouth, lips, tongue, and within the joints. A platelet count of 100,000/cu mm or less is generally considered to constitute thrombocytopenia, although the bleeding tendency does not become evident until the count falls below 40,000/cu mm. The drop in platelets may occur because of either of the following reasons: (2) Thromboembolic conditions The pathologic converse to hemostasis is called thrombosis. Thrombosis can be thought of as the formation of blood clot (thrombus) in uninjured vessels, or thrombotic occlusion of a vessel after relatively minor injury. Like hemostatic mechanism, thrombosis also depends on three general components: the vascular wall, platelets, and the coagulation cascade. It is particularly important in thrombus formation in the heart and arterial circulation, for example, within the cardiac chambers when there has been endocardial injury (e. It is important to note that endothelium does not need to be denuded or physically disrupted to contribute to the development of thrombosis; any perturbation in the dynamic balance of prothrombotic and antithrombotic effects can influence local clotting events. Thus, significant endothelial dysfunction may occur from the hemodynamic stresses of hypertension, or bacterial endotoxins. Normal blood flow is laminar such that the platelets elements flow centrally in the vessel lumen separated from the endothelium by a slower-moving clear zone of plasma. Stasis and turbulence therefore: (a) Disrupt laminar flow and bring platelets into contact with the endothelium (b) Prevent dilution of activated clotting factors by fresh-flowing blood (c) Retard the inflow of clotting inhibitors and permit the build-up of thrombi (d) Promote endothelial cell activation. It is loosely defined as any alteration of the coagulation pathways that predisposes thrombosis, and it can be divided into primary (genetic) and secondary (acquired) disorders. The characteristic alteration is a mutant factor Va that cannot be inactivated by protein C; as a result, an important antithrombotic counter-regulatory pathway is lost. Use of oral contraceptives and the hyperestrogenic state of pregnancy are some common examples of this category. The systemic circulation, which supplies all the tissues, is a high-resistance system with a large pressure difference between the arteries and veins. The arteries are highly elastic and muscular; they distribute blood to the smaller arterioles and ultimately to the network of capillaries, where exchange of fluid, small molecules and nutrients occurs across the thin walls. Human heart is a four chambered pump, well adapted to separation of oxygen rich and oxygen poor blood handled by left and the right side of the heart respectively. Thin wall atria receive blood, which reaches into thick-walled ventricles that pump blood into systemic and pulmonary circuits through great vessels. The pulmonary circulation is a low-pressure, low-resistance system handling the same amount of blood at the same time as systemic circulation to keep the same amount of blood in the right and left side of the heart. The left ventricle is more muscular and heavier than the right ventricle, which pumps against the low resistance of the pulmonary circulation; the left pumps against the high resistance of the systemic circulation.