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Module 12.3 Protection of the Brain

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MODULE 12.3 PROTECTION OF THE BRAIN © 2016 Pearson Education, Inc. Brain Protection Three features within protective shell of skull provide additional shelter for delicate brain tissue: • Cranial meninges – three layers of membranes that surround brain • Cerebrospinal fluid (CSF) – fluid that bathes brain and fills cavities • Blood-brain barrier – prevents many substances in the blood from gaining access to the cells of the brain © 2016 Pearson Education, Inc. Brain Protection • Cranial meninges – three of them, are protective membranes of mostly dense irregular collagenous tissue • Structural arrangement from superficial to deep: dura mater, arachnoid mater, and pia mater(Figure 12.18) © 2016 Pearson Education, Inc. Brain Protection • Cranial meninges (continued): • Epidural space – between inner surface of cranial bones and outer surface of dura mater; only a potential space as dura is normally tightly bound to bone only allowing for passage of blood vessels • Dura mater (dura) – outermost meninx; thickest and toughest of three meningeal layers; tough, leathery membrane with abundant collagen fibers and very few elastic fibers © 2016 Pearson Education, Inc. Brain Protection • Cranial meninges (continued): • Dura mater (dura) (continued): • Has two layers(Figure 12.18a): • Periosteal dura – outer layer; attached to inner surface of bones of cranial cavity; functions as periosteum with extensive blood supply in epidural space • Meningeal dura – inner layer; avascular and lies superficial to arachnoid mater • Mostly fused, creating a single inelastic membrane except in regions where cavities called dural sinuses are found © 2016 Pearson Education, Inc. Brain Protection • Cranial meninges (continued): • Dura mater (dura) (continued): • Dural sinuses – venous channels; drain CSF and deoxygenated blood from brain’s many veins; may also be found where meningeal dura folds over itself and courses between structures in brain © 2016 Pearson Education, Inc. Brain Protection • Cranial meninges (continued): • Dura mater (dura) (continued): • Dural folds include falx cerebri, tentorium cerebelli, and falx cerebelli (Figure 12.14b) • Falx cerebri – within longitudinal fissure; partition between left and right cerebral hemispheres; superior sagittal sinus (large dural sinus) found superior to falx cerebri • Tentorium cerebelli – partition between cerebellum and occipital lobe of cerebrum • Falx cerebelli – partition between left and right hemispheres of cerebellum © 2016 Pearson Education, Inc. Brain Protection • Cranial meninges (continued): • Subdural space – found deep to dura, thin serous fluid-filled space; houses veins that drain blood from brain • Arachnoid mater – middle meninx, deep to subdural space; named for its resemblance to a spider web, composed of dense irregular collagenous tissue, thinner and somewhat more elastic than dura (Figure 12.18c) © 2016 Pearson Education, Inc. Brain Protection • Cranial meninges (continued): • Arachnoid mater (continued): • Arachnoid trabeculae – inward extensions, composed of collagen fiber bundles and fibroblasts; anchor arachnoid to deep pia mater • Arachnoid granulations (villi) – small bundles of arachnoid; project superficially through meningeal dura into the dural sinuses; play important role in the return of CSF to bloodstream © 2016 Pearson Education, Inc. Brain Protection • Cranial meninges (continued): • Subarachnoid space – found deep to arachnoid mater and superficial to pia mater; contains CSF and the major blood vessels of brain • Pia mater – deepest meningeal layer; only meninx in physical contact with brain tissue © 2016 Pearson Education, Inc. Brain Protection • Cranial meninges (continued): • Pia mater (continued): • Follows contour of brain, dives into sulci and fissures • Permeable to substances in brain extracellular fluid and CSF; allows substances to move between these two fluids; helps to balance concentration of different solutes in them © 2016 Pearson Education, Inc. Brain Protection Figure 12.18a, b Structure of the cranial meninges and dural sinuses. © 2016 Pearson Education, Inc. Brain Protection Figure 12.18c Structure of the cranial meninges and dural sinuses. © 2016 Pearson Education, Inc. The Ventricles and Cerebrospinal Fluid • Four ventricles within brain are linked cavities that are continuous with central canal of spinal cord (Figures 12.19, 12.20) • Lined with ependymal cells • Filled with cerebrospinal fluid © 2016 Pearson Education, Inc. The Ventricles and Cerebrospinal Fluid • Right and left lateral ventricles (first and second ventricles); within their respective cerebral hemisphere (Figure 12.19): • Resemble ram’s horns when observed in anterior view; horseshoe-shaped appearance in lateral view • Three regions: anterior horn, inferior horn, and posterior horn © 2016 Pearson Education, Inc. The Ventricles and Cerebrospinal Fluid • Third ventricle – narrow cavity found between two lobes of diencephalon; continuous with lateral ventricles by interventricular foramen • Fourth ventricle – between pons and cerebellum; connected to third ventricle by cerebral aqueduct (small passageway through midbrain) • Continuous with central canal of spinal cord • Contains several posterior openings that allow CSF in ventricles to flow into subarachnoid space (Figure 12.19a) © 2016 Pearson Education, Inc. The Ventricles and Cerebrospinal Fluid Figure 12.19 Ventricles of the brain. © 2016 Pearson Education, Inc. The Ventricles and Cerebrospinal Fluid • Cerebrospinal fluid (CSF) – clear, colorless liquid similar in composition to blood plasma; protects brain in following ways: • Cushions brain and maintains a constant temperature within cranial cavity • Removes wastes and increases buoyancy of brain; keeps brain from collapsing under its own weight © 2016 Pearson Education, Inc. The Ventricles and Cerebrospinal Fluid • Most CSF is formed within each of the four ventricles by structures called choroid plexuses, located where blood capillaries come into direct contact with ependymal cells (which also produce some CSF) lining the ventricles. • The capillaries within the choroid plexuses are fenestrated, cells have small gaps between them and within their membranes that allow fluid and electrolytes to escape the blood and enter the ECF. © 2016 Pearson Education, Inc. The Ventricles and Cerebrospinal Fluid • Choroid plexuses (continued): • About 150 ml (about 2/3 cup) of CSF circulates through brain and spinal cord • 750–800 ml of CSF is produced daily so old CSF must be removed as choroid plexuses make new CSF • Process of CSF production and removal occurs constantly; roughly 50% of total CSF is completely replaced every 5–6 hours © 2016 Pearson Education, Inc. The Ventricles and Cerebrospinal Fluid • Pathway for formation, circulation, and reabsorption of CSF (Figure 12.20): • Fluid and electrolytes leak out of capillaries of choroid plexuses into ECF of ventricles • Taken up into ependymal cells; then secreted into ventricles as CSF • Circulated through and around brain and spinal cord in subarachnoid space; CSF circulates through ventricles with the help of the ciliated ependymal cells, some of this CSF then flows into the subarachnoid space around the spinal cord. • Some CSF is reabsorbed into blood in dural sinuses via arachnoid granulations © 2016 Pearson Education, Inc. The Ventricles and Cerebrospinal Fluid Figure 12.20 Formation and flow of cerebrospinal fluid (CSF). © 2016 Pearson Education, Inc. The Blood-Brain Barrier The brain is protects not only from outside threats, but also from inside. Most chemicals and disease-causing organisms (like bacteria and viruses) are denied access to the cells of the brain by the blood- brain barrier – keeps CSF and brain ECF separate from the blood(Figure 12.21) • Consists mainly of simple squamous epithelial cells (endothelial cells) of blood capillaries, their basal laminae, and astrocytes © 2016 Pearson Education, Inc. The Blood-Brain Barrier • Unique features of endothelial cells found in barrier: • More tight junctions than cells of most capillaries • Limit endocytosis and exocytosis- many molecules in the blood cannot enter ECF of brain, the opposite is true as well, many substances in the ECF of brain cannot enter the blood. © 2016 Pearson Education, Inc. The Blood-Brain Barrier • Substances that easily pass through plasma membranes are able to pass through blood-brain barrier; include water, oxygen, carbon dioxide, and nonpolar, lipid-based molecules • Protein channels or carriers allow for passage of other molecules, include glucose, amino acids, and ions • Most large, polar molecules are effectively prevented from crossing blood-brain barrier in any significant amount; while barrier is protective, it can hinder access of medications into brain © 2016 Pearson Education, Inc. The Blood-Brain Barrier Figure 12.21 The blood-brain barrier. © 2016 Pearson Education, Inc. Concept Boost: Where Exactly Is Blood- Brain Barrier? • No single structure is labeled “blood-brain barrier” on any figure because blood-brain barrier isn’t around brain; it’s within brain • Not located in one distinct place but found throughout entire brain • To understand this, we must first understand body’s tiniest blood vessels; capillaries are vessels that deliver oxygen and nutrients to body’s cells and remove any wastes produced by cells © 2016 Pearson Education, Inc. Concept Boost: Where Exactly Is Blood- Brain Barrier? • Capillaries found in most organs and tissues are fairly leaky; allow a wide variety of substances to move from blood to extracellular
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