Lesson 12 Neural Tissue 31 10 2011 20 23 00 Neuroglia Half the volume of the nervous system Many types of neuroglia in CNS and PNS Four Types of Neuroglia in the CNS o Ependymal Cells produce Cerebrospinal Fluid CSF Cells with highly branched processes contact neuroglia directly From epithelium called ependyma Line central canal of spinal cord and ventricles of brain Secrete cerebrospinal fluid CSF Have cilia or microvilli that circulate CSF Moniter CSF Contain stem cells for repair o Astrocytes support cells Large cell bodies with many processes Maintain blood brain barrier isolates CNS Solutes in blood fluid do not have free access to interstitial fluid of CNS Astrocytes regulate flow of material from capillary Create 3D framework for CNS Repair damaged neural tissue fairly difficult they don t undergo mitosis Guide neuron development and interconnections Control interstitial environment fluid o Oligodendrocytes Smaller cell bodies with fewer processes Involved in myelination of axons Produces Myelin that insulates axons functions to Increase speed of action potentials Make nerves appear white Wrapping of axon is not complete there are gaps nodes Nodes also called nodes of Ranvier Internodes myelinated segments of axon Myelination causes nervous tissue to appear white White matter regions of CNS with many myelinated nerves axons Gray matter unmyelinated areas of CNS neuronal cells bodies Notes Shwann Cells are found in the PNS not the CNS Can only wrap around one axon Smallest and least numerous neuroglia with my fine branched processes Migrate through neural tissue Clean up cellular debris waste products and pathogens o Microglia Neuroglia of the Peripheral Nervous System Satellite cells o Also call amphicytes Shwann Cells o Surround ganglia masses of neuron cell bodies in the PNS o Regulate environment around neuron o Also called neurilemma cells o Form myelin sheath neurilemma around peripheral axons o One Schwann cell sheaths one segment of axon many Shwann cells are required to sheath an entire axon o One Shwann cell wraps around one axon in a process called myelination Summary of Neurons and Neuroglia Neurons perform all communication information processing and control functions of the nervous system Neuroglia preserve physical and biochemical structure of neural tissue Neuroglia are essential to survival and function of neurons Neural Responses to Injuries in the PNS Wallerian degeneration o Axon distal to injury site degenerates o Macrophages migrate into area and remove debris o Shwann cells form path for new growth Proliferate and form solid cellular cord following path of original axon Wrap new axon in myelin Nerve Regeneration in CNS limited by o Many more axons likely to be involved o Astrocytes that Release chemicals that block growth Produce scar tissue physically interfere with regrowth of axon Peripheral Nerve Regeneration after Injury o Fragmentation of axon and myelin occurs in distal stump o Shwann cells form cord grow into cut and unite stumps Macrophages engulf degenerating axon and myelin o Axon sends buds into network of Schwann cells and then starts growing along cord of Schwann cells o Axon continues to grow into distal stump and is enclosed by Schwann cells Lessons 13 14 Membrane Potential Synaptic Transmission Transmembrane Potential written here as TMP Ion Movements and Electrical Signals o All plasma cell membranes produce electrical signals by ion movements o TMP is particularly important to neurons Five main membrane processes in neural activities o Resting potential TMP of resting cells o Graded potential temporary localized change in resting potential caused by stimulus to the cells o Action potential an electrical impulse produced by graded potential propagates along surface of axon to synapse o Synaptic activity releases neurotransmitters at presynaptic membrane produces graded potentials in postsynaptic membrane o Information processing response integration of stimuli of postsynaptic cell The Transmembrane Potential written here as TMP Three important concepts o The extracellular fluid ECF and intracellular fluid cytosol differ greatly in ionic composition Concentration gradient of ions NA K Cells have selectively permeable membranes Membrane permeability varies by ion Due to the presence of passive leak channels Passive Forces acting across the plasma membrane o Chemical gradients concentration gradients chemical gradient of ions Na K o Electrical gradients separates charges of positive and negative ions result in potential difference Electrical Currents and resistance o Electrical current movement of charges to eliminate potential difference opposite charges attract o Resistance the amount of current a membrane restricts The electrochemical gradient o For a particular ion Na K the electrochemical gradient is The sum of chemical and electrical forces acting on the ion across the plasma membrane This is a form of potential energy Active Forces across the membrane o Sodium potassium ATPase exchange pump Powered by ATP it carries 3 Na out and 2K in Balances passive forces of diffusion through passive channels Maintains resting potential 70mV The Resting Potential potassium ions o Because the plasma membrane is highly permeably to o The electrochemical gradient for sodium ions is very large but the membrane s permeability to these ions is very low o The sodium potassium exchange pump ejects 3 Na ions for every 2 K ions that it brings into the cell It serves to stabilize the resting potential when the ratio of Na entry to K loss through passive channels is 3 2 o At the normal resting potential these passive and active mechanisms are in balance The resting potential varies widely with the type of cell A typical neuron has a resting potential of approximately 70mV Changes in the TMP o TMP rises or falls In response to temporary changes in membrane permeability from opening or closing specific membrane channels Sodium and Potassium Channels o Membrane permeability to Na and K determines TMP o These channels are either passive or active Passive channels are always open permeability changes with conditions flow is from greater to lesser Active channels open and close in response to stimuli At resting potential most of these gated channels Three States of Gated Channels are closed o Closed but capable of opening o Open activated o Closed but not capable of opening inactivated Three classes of Gated Channels o Chemically gated channels Open in
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