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Chapter 9

Chapter 9, Kee

10 Pages

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Michelle French

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Chapter Nine: The Nervous System: Central Nervous System chains of neuron-neuron connections (neural pathways) allow the CNS to carry out all its complex functions CNS responsible for everything we perceive, do, feel, and think + gives us unique personality and sense of self-identity + performs many critical functions and typically escape our notice - ie: coordinates activities of all our organ systems necessary for homeostasis CNS + contains about 100 biollion (10^11) neurons + contains 100 trillion (10^14) synapses + all contained within the brain and the spinal cord General Anatomy of the Nervous System CNS consists of the brain and spinal cord + made up of soft tissue vulnerable to damage by physical trauma + protected by glial cells, bone, connective tissue, and cerebrospinal fluid Glial Cells 75-90% of CNS also called neuroglia provide support to neurons role: neural communication five types + Schwann cells + oligodendocytes + microglia + ependymal cells + astrocytes all release growth factors involved in the development of the nervous system glial cells and neurons communicate each other Astrocytes: star-like appearance most diverse of glial cells many functions in CNS necessary for normal development of nervous system & continued support throughout life + direct development of special capillaries that restrict the movement of certain molecules between blood and CNS blood-brain barrier + Schwann cells and oligodendrocytes also contribute to development of neurons but to lesser extent guide developing neurons to their correct destination and regulate the development and maintenance of synapses + also support the regeneration of damaged axons critical to the maintenance of the normal extracellular environment surrounding neurons, especially at synapses + especially important in maintaining normal extracellular potassium levels a state that's critical to neuron excitability also remove certain neurotransmitters (ie: glutamate, biogenic amines) from synaptic cleft + excess levels of glutamate are toxic and contribute to the spread of neurological damage during a stroke synthesize and store molecules for use by neurons + synthesize glutamine released into the interstitial fluid and picked up by other neurons to form glutamate, an excitatory neurotransmitter store some glycogen, broken down to lacate transported to neurons where it serves as an imprtant energy source in active areas of the brain neurons communication function with microglia in protecting neurons from toxic substances protect neurons from oxidative stress and help remove cellular debris Microglia protect CNS from foreign matters (ie: bacteria, remnants of dead/injured cells) + via phagocytosis & release of cytokines in a manner similar to certain blood cells protect neurons against oxidative stress Glial Cells in Neurodegenerative Diseases multiple sclerosis results from the loss of myelin in the CNS + autoimmune disease immune system attacks a part of the body oligodendrocytes + loss of myelin (and some axons) slows down/stops communication along certain neural pathways + symptoms: blurred vision, muscle weakness, and difficulty maintaining balance Alzheimer's disease + caused by loss of cholinergic neurons in certain brain areas and replacement of lost neurons with scar tissue plaques + during degeneration of cholinergic neurons, astrocytes and microglia become overly active release inflammatory chemicals that enhance further degeneration of cholinergic neurons vicious cycle takes place + early signs: loss of memory & confusion + later signs: motor dysfunction, loss of communication skills, and decrease in cognitive functions Parkinson's disease + degenerative disease loss of dopaminergic neurons glial cells protect neurons in their immediate vicinity Physical support of CNS outermost structures that protect the soft tissues of CNS are bony skull (cranium) that surrounds the brain, and the bony vertebral column, which surrounds the spinal cord between bone and nervous tissue + series of soft membranes meninges and a layer of fluid called cerebrospinal fluid provide protection against harsh impacts meninges + three connective tissue membranes that separate the soft tissue of CNS from surrounding bone + three meningeal membranes: dura mater, arachnoid mater, & pia mater dura mater + outermost layer, closest to the bone. + hard, durable + very tough, fibrous tissue arachnoid mater + middle layer + web-like structure + no space exists between dura and arachnoid pia mater + innermost layer + immediately adjacent to the nervous tissue + tender + space between pia and arachnoid mater subarachnoid space filled with cerebrospinal fluid Cerebrospinal fluid(CSF) + clear, watery fluid that bathes CNS + similar in composition to plasma + completely surrounds CNS and fills a number of cavities located within the brain and spinal cord brain contains four cavities called ventricles + continuous with each other + two C-shaped lateral ventricles connected to a midline third ventricleby interventricular foramen + cerebral aqueduct connects third ventricle to fourth ventricle, continuous with central canal a long thin cylindrical cavity that runs the length of the spinal cord + lining of ventricles and central canal composed of glial cells ependymal cells, a type of epithelial cell lining of ventricles is vascularized and forms a tissue choroid plexus consists of pia mater, capillaries, and ependymal cells and functions in the synthesis of CSF + total volume of CSF (1250150mL) because it's recycled three times per day choroid plexus must produce 400-500mL/day+ as CSF is fluid, it circulates the ventricular system and enters the subarachnoid space through openings of the fourth ventricle + CSF in subarachnoid space eventually gets reabsorbed into venous blood through special structures in the arachnoid mater arachnoid villi, located at the top of the brain CSF has several functions in the brain + acts as a shock absorber that prevents the soft nervous tissue from colliding with the hard bone because CNS essentially floats in CSF + the interstitial fluid that bathes neurons and glial cells, providing these cells essential nutrients and removing waste products + contributes to the maintenance of normal ionic composition around neurons, which is essential for normal excitability of neurons + to carry out its metabolic functions, it must be replenished by the blood supply to CNS Blood Supply to the CNS CNS accounts for only 2% of body weight, receives about 15% of the blood that the heart pmps to all the body's organs and tissues under resting conditions + large blood supply necessary CNS tissue has a high rate of metabolic activity compared to most other body tissues = has a high demand for fuel and oxygen to meet its energy needs under resting conditions, + body accounts for 20% of all oxygen that the body consumes + 50% of all the glucose consumed + to ensure delivery of these needed materials, adequate blood flow to the CNS must be maintained at all times CNS so dependent on this blood supply that disruption of blood flow for even a few minutes can result in irreversible damage to CNS tissue stroke blood flow is interrupted because of a blocked/ruptured blood vessel in the brain CNS sensitive to interruptions in blood flow because cells in CNS have very little gylogen must obtain glucose directly from blood + most cells in CNS don't have access to fatty acids for energy which increases their demand for glucose unlike many other tissues which can obtain energy from anaerobic metabolism during periods of reduced oxygen availability, nervous tissue cannot do this, so it requires an uninterrupted supply of
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