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Chapter 12

Chapter 12.doc

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Department
Anatomy and Physiology
Course
ANP1106
Professor
Jacqueline Carnegie
Semester
Winter

Description
Chapter 12: The Central Nervous System Protection of the Brain -the brain is protected by bone (the skull), membranes (the meninges), a water cushion (cerebrospinal fluid) -the brain is protected from harmful substances in the blood by the blood-brain barrier Meninges -made up of 3 connective tissue membranes that lie just external to the CNS organs -used to cover and protect the CNS, protect blood vessels and enclose sinuses, contain cerebrospinal fluid (CSF), and forms partitions in the skull -from external to internal of the meninx: 1) Dura Mater -the strongest meninx -is a 2-layered sheet of fibrous connective tissue -the more superficial periosteal layer is attached to the inner surface of the skull (the periosteum) -no dural periosteal layer surrounding the spinal cord -the deeper meningeal layer forms the true external covering of the brain and continues caudally in the verterbral canaal as the spinal dura mater -the 2 dura maters are fused together except in certain areas where they separate to enclose dural venous sinuses that collect venous blood from the brain and direct it into the internal jugular veins of the neck -dural septa is made up of meningeal dura mater that extends inward to form flat partitions that subdivide the cranial cavity -they help to limit excessive movement of the brain within the cranium -made up of: a) Falx Cerebri -sickle-shaped fold that dips into the longitudinal fissure between the cerebral hemispheres -attaches to the crista galli of the ethmoid bone anteriorly b) Falx Cerebelli -continue inferiorly from the posterior falx cerebri -runs along the vermis of the cerebellum c) Tentorium Cerebelli -like a tent over the cerebellum -a nearly horizontal dural fold that extends into the transverse fissure between the cerebral hemispheres (which it helps to support) and the cerebellum 2) Arachnoid Mater -the middle meninx -forms a loose brain covering -never dips into the sulci of the cerebral surface -separated from the dura mater by the subdural space which contains a film of fluid -the subarachnoid space lies beneath the arachnoid membrane -contains weblike extensions that span the space and secure the arachnoid mater to the underlying pia mater -filled with CSF and contains the largest blood vessels serving the brain -these blood vessels are poorly protected because the arachnoid is find and elastic -arachnoid villi are knoblike projections of the arachnoid mater that protrude superiorly through the dura mater and into the superior sagittal sinus -CSF is absorbed into the venous blood of the sinus by these valve-like villi 3) Pia Mater -composed of delicate connective tissue -rich with tiny blood vessels -the only meninx that clings tightly to the brain and follows every convolution of the brain -small arteries entering the brain carry parts of pia mater inward with them for small distances Meningitis -inflammation of the meninges -bacterial or viral meningitis may spread to the CNS -diagnosed by obtaining a sample of CSF via lumbar tap and examining for microbes Encephalitis- brain inflammation Cerebrospinal Fluid -found in and around the brain and spinal cord -forms a liquid cushion that gives buoyancy to the CNS structures -reduces brain weight by 97% and prevents the brain from crushing under its own weight -helps nourish the brain and carries chemical signals throughout the brain -formed from blood plasma and has a similar composition -contains less protein than plasma -contains more Na+, Cl-, and H+ than blood; less Ca2+ and K+ -the choroid plexuses that hang from the roof of each ventricle forms CSF -made up of a cluster of broad, thin-walled capillaries enclosed by pia mater and then by a layer of ependymal cells lining the ventricles -capillairies are fairly permeable and tissue fluid filters continuously from the bloodstream -the ependymal cells are joined by tight junctions and have ion pumps that allow them to modify this filtrate by actively transporting only certain ions across their membranes into the CSF pool -regulation of CSF is important because it mixes with extracellular fluid and this influences its composition -ion pumping also sets up ionic gradients that cause water to diffuse into the ventricles -in adults, total CSF volume is about 150 mL which is replaced every 8 hours -500 mL of CSF is formed daily -choroid plexues help cleanse the CSF by removing waste products and unnecessary solutes -CSF moves freely through the ventricles -some circulates into the central canal of the spinal cord -most CSF enters the subarachnoid space via the lateral and median apertures in the walls of the 4th ventricle -cilia of ependymal cells lining the ventricles help to keep the CSF in constant motion -in the subarachnoid space, CSF bathes the outer surfaces of the brain and spinal cord and then returns the blood in the dural sinuses via the arachnoid villi hydrocephalus- accumulation of CSF and the exertion of pressure on the brain -caused by obstructed circulation (i.e.: tumor) -in babies, it causes the head to enlarge because their skull bones have not fused yet -in adults, it may damage the brain because the skull is rigid and hard and accumulating fluid compresses blood vessels of the brain and crushes soft nervous tissue -treated by inserting a shunt into the ventricles to drain excess fluid into the abdominal cavity Blood-Brain Barrier -a protective mechanism that helps to maintain the stable environment for the brain -if the brain was exposed to chemical variations, the neurons would fire uncontrollably because hormones and amino acids serve as neurotransmitters and certain ions (i.e.: K+) modify the threshold for firing -bloodbourne substances in the brain's capillaries must pass through 3 layers before they reach the neurons: 1) the endothelium of the capillary wall 2) thick basal lamina surrounding the external face of each capillary 3) the bulbous feet of the astrocytes clinging to capillaries -supply the required signals to the endothelial cells, causing them to make tight junctions -tight junctions join the endothelial cells together, forming the blood-brain barrier and making these the least permeable capillaries in the body -blood-brain barrier is selective -nutrients like glucose, essential amino acids, and some electrolytes move passively by facilitated diffusion through the endothelial cell membranes -bloodborne metabolic wastes, proteins, some toxins, and most drugs cannot enter -small non-essential amino acids and potassium ions are prevented from crossing; are also actively pumped from the brain across the capillary endothelium -barrier is ineffective against fats, fatty acids, oxygen, carbon dioxide, and other fat- soluble molecules that diffuse easily through plasma membranes -bloodborne alcohol, nicotine, and anesthetics can affect the brain -structure of blood-brain barrier is not completely uniform -capillaries of the choroid plexuses are very porous -the ependymal cells surrounding them have tight junctions -blood-brain barrier is absent in some areas surrounding the 3rd and 4th ventricles and their capillary endothelium is permeable -e.g.: the vomiting centre of the brain stem -monitors blood for poisonous substances -e.g.: hypothalamus -regulates water balance, body temperature, and other metabolic activities -the lack of a blood-brain barrier is essential to allow the hypothalamus to sample chemical compositions of the blood -blood-brain barrier is incomple
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