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University of Toronto St. George
Michelle French

CNS November-05-12 3:47 PM Consists of the brain and the spinal chord Neurons outside the CNS make up the peripheral nervous system • The PNS includes neurons entirely outside the CNS, and also parts of neurons that project into or out of the CNS • The PNS comprises the somatic nervous system, for controlling voluntary action via the skeletal muscle • Part of the autonomic nervous system is the enteric nervous system, which controls digestion and movements of the intestines o Receives lots of input from the spinal chord, but can also operate independently of the CNS Most of the body's neurons are in the CNS • Dominate for information processing • Brain has ~86 billion neurons and spinal chord has about 1 Billion • Estimates for the enteric nervous system range from 100-600 million and the total for the PNS is probably not much more • Both CNS and PNS also have cells called glia, which support and protect neurons, and are likely about as numerous • Most of the neurons in the brain are in the cerebellum • only 10% in cerebrum The brain is protected by bone, meninges and cerebrospinal fluid • Between the skull |(cranium) and the brain lie 3 tissue layers called meninges: the dura mater, the arachnoid mater and the pia mater • The subarachnoid space contains cerebrospinal fluid (CSF) The same layers protect the spinal chord CSF is produced in fluid filled spaces called ventricles • From top to bottom: lateral -> third -> fourth -> central canal of spinal chord CSF is secreted into the ventricles and flows throughout the subarachnoid space • Secreted by choroid plexus in ventricle walls • Then reabsorbed by arachnoid villi • Cushions the brain and maintains a chemical environment with lower pH and [K+] than plasma, no blood cells and little protein CSF is secreted by the choroid plexus Goes from capillary to ependymal cells into ventricle • The choroid plexus consists of specialized regions in the walls of the ventricles • It's ependymal cells transport ions, vitamins, nutrients and water from the blood into the ventricles CSF is reabsorbed into the blood by arachnoid villi • The arachnoid villi are projections of the arachnoid membrane through the dura, into the venous sinus • By this circulation, from choroid plexus to arachnoid villi, the entire volume of CSF is recycled ~3 times a day Glial cells called astrocytes help create myelin sheaths and the blood brain barrier • Extensions of the astrocyte wrap around axons to form myelin sheaths, which speed up nerve conduction • Other extension called foot processes contact capillaries to form the blood brain barrier The blood brain barrier helps keep harmful substances out of the interstitial fluid • Astrocytes secrete paracrines that promote tight junction between the cells of the capillary endothelium • Patients with the motor disorder Parkinsons disease have too little of the neurotransmitter dopamine. Dopamine can't cross the blood brain barrier, so dopamine pills don't help. But the smaller precursor L-dopa can cross The CNS is fueled mainly with oxygen and glucose • The brain receives 15% of the blood pumped by the heart. Oxygen from the membrane crosses the blood brain barrier • Membrane transporters move glucose from the plasma into the interstitial fluid of the CNS • The brain is responsible for half the body's glucose consumption • Efficient energy consumption because heat is not released • Whole body uses 100W, brain uses 20% = 20% 20W The CNS includes gray and white matter • Gray matter consists of unmyelinated nerve cell bodies, dendrites, and axon terminals. The cell bodies are arranged either in layer (in parts of the brain) or in clusters called nuclei • White matter consists of myelinated axons running in bundles called tracts • In the peripheral nervous system, clusters of neurons are called ganglia, and bundles of axons are called nerves Scientists can trace neural pathways in the CNS by various methods • One method is to inject and enzyme, horseradish peroxidase, near axon terminals. The HRP is brought into the neurons by endocytosis and carried, by fast axonal transport, to the cell body and dendrites • Throughout the cell, the HRP takes part in reaction yielding coloured or fluorescent products that let scientists see the whole neuron • Another method is to modify mice genetically so their neurons produce novel proteins which can then be tagged by fluorescent anti-bodies, making neurons light up. Spinal chord It is arranged in 31 segments, each with a pair of spinal nerves • The dorsal root of each spinal nerve carries incoming sensory signals • The dorsal root ganglion contains cell bodies of sensory neurons • The ventral root carries signals from the central nervous system to muscles and glands (ventral = near belly) Gray matter in the spinal chord consists of sensory and motor nuclei • V
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