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University of Toronto St. George
Yue Li

9 THE NERVOUS SYSTEM: CENTRAL NERVOUS SYSTEM General Anatomy of the Central Nervous System GLIAL CELLS 75-90% of the CNS is composed of glial cells (neuroglia) that provide support to neurons. 5 types: Schwann cells, oligodendrocytes, microglia, ependymal cells, and astrocytes. All glial cells release growth factors involved in the development of the nervous system. ASTROCYTES o Star-like appearance, most diverse. o Are necessary for the normal development of the nervous system as well as its continued support throughout life. o Direct development of special capillaries that restrict the movement of certain molecules between the blood and the CNS (blood-brain barrier). o They guide developing neurons to their correct destination and regulate the development and maintenance of synapses. Support the regeneration of damaged axons. o Critical for the maintenance of normal extracellular environment surrounding neurons, especially at synapses (i.e. maintaining extracellular K levels for neuron excitability). o Help remove biogenic amines and glutamate from the synaptic cleft (excess glutamate is toxic); protect neurons from oxidative stress and remove cellular debris. o They synthesize glutamine, which is then released into the interstitial fluid and picked up by other neurons to form glutamate (excitatory neurotransmitter). o Stores glycogen (used to make lactate for neurons as a source of energy). o They regulate synaptic communication between neurons, and have the ability to directly communicate with neurons and other glial cells (i.e. other astrocytes). o They can function with microglia in protecting neurons from toxic substances MICROGLIA o Protects the CNS from foreign matter (i.e. bacteria, dead or injured cells) by using phagocytosis and the release of cytokines. Also protect against oxidative stress. GLIAL CELLS IN NEURODEGENERATIVE DISEASES o Multiple sclerosis (autoimmune disease) results from the loss of myelin in the CNS. This slows down communication (i.e. blurred vision, muscle weakness, etc). o Alzheimers disease is caused by the loss of cholinergic neurons, and replacement of lost neurons with scar tissue (plaques). Astrocytes and microglia release inflammatory chemicals that enhance the degeneration of cholinergic neurons. o Parkinsons disease is a degenerative disease involving the loss of dopaminergic neurons. Similar to Alzheimers disease. PHYSICAL SUPPORT OF THE CENTRAL NERVOUS SYSTEM The outermost structures that protect the soft tissues of the CNS are the bony skull (cranium), which surrounds the brain, and the bony vertebral column, which surrounds the spinal cord. Between the bone and the nervous tissue are 3 membranes called the meninges and a layer of fluid called cerebrospinal fluid, which provide protection against impact. Meninges: 3 connective tissue membranes that separate the soft tissue of the CNS from the surrounding bone (dura mater, arachnoid mater, pia mater). Dura mater: outermost layer, closest to the bone; very tough fibrous tissue. Arachnoid mater: middle layer; web-like structure. Pia mater; innermost layer; immediately adjacent to the nervous tissue. Subarachnoid space: the space between the pia and the arachnoid filled with cerebrospinal fluid. Cerebrospinal fluid (CSF): clear, watery fluid that bathes the CNS. Surrounds the CNS and fills cavities within the brain and spinal cord. Acts as a shock absorber and interstitial fluid that bathes neurons and glial cells. Maintains neural ionic composition around neurons for normal excitability. Needs to be replenished by the blood supply to the CNS. The brain contains 4 cavities called ventricles which are continuous with each other: 2 C-shaped lateral ventricles are connected to a midline 3 ventricle by the interventricular foramen. The9 THE NERVOUS SYSTEM: CENTRAL NERVOUS SYSTEM rd th cerebral aqueduct connects the 3 ventricle to the 4 ventricle, which is continuous with the central canal (long thin cylindrical cavity that runs the length of the spinal cord). The lining of the ventricles and central canal is composed of glial cells called ependymal cells (type of epithelial cell). The lining of vascularized and forms a tissue called the choroid plexus (have pia mater, capillaries, and ependymal cells), and helps with the synthesis of CSF. Total volume of CSF is 125-150mL, but is recycled 3x a day therefore 400-500mL/day. CSF is circulated throughout the ventricular system and enters the subarachnoid space through openings of the fourth ventricle. CSF in the subarachnoid gets reabsorbed into venous blood through structures in the arachnoid mater called arachnoid villi at the top of the brain. BLOOD SUPPLY TO THE CNS CNS accounts for only 2% of the body weight but receives 15% of the blood that the heart pumps at resting conditions. CNS has a high metabolic rate, thus high demand for fuel and oxygen to meet its energy needs. The brain accounts for 20% of all the oxygen that the body consumes, and 50% of all the glucose consumed. The CNS is sensitive to interruptions in blood flow because cells in the CNS have very little glycogen and no access to fatty acids, therefore must obtain glucose directly from the blood. Nervous tissue cant obtain energy from anaerobic metabolism, so it requires an uninterrupted supply of oxygen and glucose to stay alive. CNS tissue can also use ketones to supply up to 2/3 of their energy needs. Ketones are a by- product of lipid catabolism when glucose supplies are limited. BLOOD-BRAIN BARRIERS The exchange of oxygen, glucose, and other materials between blood and cells in the CNS occurs across the walls of capillaries. Capillary walls are composed of a single layer of e
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