Материал: Kaplan USMLE-1 (2013) - Anatomy
Section I • Histology and Cell Biology
Clinical Correlate
Drugs of Addiction and the
Blood-Brain Barrier
Heroin, ethanol, and nicotine are lipid soluble compounds that readily diffuse across the blood-brain barrier.
Clinical Correlate
Immune-Related Demyelinating Diseases, MS, and Guillain-Barre
Syndrome
In MS, myelin formed by oligodendrocytes undergoes an inflammatory reaction that impairs impulse transmission in axons in the CNS.
•Commonly, 2 or more CNS sensory or motor neural systems are affect ed in separate attacks.
•The only cranial nerve or spinal nerve affected by MS is the optic nerve because all of the myelin sheaths of its axons are formed by oligoden drocytes.
In Guillain-Barresyndrome, myelin formed by Schwann cells in the PNS under goes an acute inflammatory reaction after a respiratory or gastrointestinal illness. This reaction impairs or blocks impulse transmission ofaxons in the PNS, result ing in a polyneuropathy. Motor axons are always affected, producing weakness in the limbs. Weakness of cranial nerve (CN) innervated muscles (most commonly those innervated by CNs VI and VII) or respiratory muscles may be seen. Sen sory deficits are mild or absent.
Ependymalcellsline the ventricles in the adult brain. Some ependymal cells dif ferentiate into choroid epithelial cells, forming part of the choroid plexus, which produces cerebrospinal fluid (CSF). Ependymal cells are ciliated; ciliary action helps circulate CSF.
Tanycytes are specialized ependymal cells that have basal cytoplasmic processes in contact with blood vessels; these processes may transport substances between a blood vessel and a ventricle.
Blood-Brain Barrier
•The blood-brain barrier restricts access of micro-organisms, proteins, cells, and drugs to the nervous system.
•The blood-brain barrier consists of capillary endothelial cells, an under lying basal lamina, astrocytes, and pericytes.
•Cerebral capillary endothelial cells and their intercellular tight junctions are the most important elements of the blood-brain barrier.
•Astrocytes and pericytes are found at the blood-brain barrier outside the basal lamina. Astrocytes have processes with "end feet" that cover more than 95% of the basal lamina adjacent to the capillary endothelial cells.
Substances cross the blood-brain barrier into the CNS by diffusion, by se lective transport, and via ion channels. Oxygen and carbon dioxide are lipid soluble gases that readily diffuse across the blood-brain barrier. Glucose, amino acids, and vitamins K and D are selectively transported across the blood-brain barrier. Sodium and potassium ions move across the blood-brain barrier through ion channels.
62 MEDICAL
Chapter 5 • Nervous Tissue
Chapter Summary
•Neurons are composed of a cell body, dendrites, and an axon. They contain pigments such as melanin and lipofuscin.
•The cell body (soma or perikaryon) contains a nucleus, other cellular components, and rough endoplasmic reticulum (ER). Microtubules and neurofilaments form the cytoskeleton. They are important for axonal transport.
•Dendrites receive and transmit information to the cell body.
•Axons arise from the perikaryon or proximal dendrite. They contain microtubules and neurofilaments. Rapid axonal transport utilizes microtubules. Kinesins promote anterograde transport, whereas dynein promotes retrograde transport.
•Myelin is the covering of axons and is composed of phospholipids and cholesterol. Axons may be myelinated or nonmyelinated. Schwann cells myelinate a single peripheral nervous system (PNS) axon.
•Neurons may also associate with several axons (unmyelinated) without forming myelin. Oligodendrocytes form myelin in the central nervous system (CNS). One oligodendrocyte myelinates many axons.
•The node ofRanvier is a collar of naked axon between a proximal and distal bundle of myelin that has myelinated the axon. Its purpose is to allow rapid signal transport by rapidly moving from one node to the next. This process is called saltatory conduction.
MEDICAL 63
Immune Tissues |
6 |
LYMPH AND LYMPHATIC DRAINAGE
The lymphatic systemdrains lymph, excess fluid found in the extravascular space of most tissues. Lymphatic capillaries begin as blind tubes lined by endothelial cells with incomplete basal lamina and without tight junctions. The capillaries drain into largerlymphaticvessels, which have one-wayvalves, somewhat analo gous to those ofveins. Larger lymphatics have some surrounding smooth muscle.
Lymph Transport
Lymph is first transported to lymph nodes, then by lymphatic vessels from one lymph node to the next, and then back into the systemic venous circula tion, through the thoracic duct into the origin ofthe left brachiocephalic vein or through right lymphatic duct into the origin of the right brachiocephalic vein. Lymphatic vessels are not found in CNS, cartilage, bone, bone marrow, thymus, placenta, cornea, teeth and fingernails.
Most lymph is filtered through multiple lymph nodes on its way to the blood stream. The lymph nodes, mucosal associated lymphoid tissue (MALT), and spleen act as filters which can collect circulating lymphocytes, antigen sources, antigen-presenting cells, and macrophages from the lymph, providing further opportunities for lymphocyte antigen-interaction, lymphocyte proliferation, and maturation.
Normally, red blood cells are not seen in lymphatics, and lymph is clear and rela tivelycell-poor. Somelymph, such as that draining from the small intestine after a meal, canhave a high lipid content (chylomicrons) and appear opaque (chylous).
MEDICAL 65
