I. Neurons and Glial Cells comprise the Nervous System<\/p>\n
A. Overall function –Communication:<\/em>receive information, process it and transmit it via electrical and chemical signals. Also, function in stimulus-response mechanisms (reflexes)<\/em><\/p>\n B. Typical\u00a0Neuron<\/strong>–\u00a0dendrite for signal reception, cell body for signal integration, axon for signal conduction, & axon terminal for signal transmission<\/em><\/p>\n 1. Cell body<\/strong><\/p>\n a. Nuclei\u00a0<\/strong>Groups of cell bodies in CNS<\/em><\/p>\n b. Ganglia\u00a0<\/strong>Groups of cell bodies in PNS<\/em><\/p>\n 2. Cytoplasmic Processes<\/p>\n a. Dendrite\u00a0<\/strong>Receptive end<\/em><\/p>\n b. Axon\u00a0<\/strong>(nerve fibers)Conducting end<\/em><\/p>\n 3. Nerve\u00a0<\/strong>Bundle of axons in PNS<\/em><\/p>\n C. Neuroglia<\/strong>“Nerve glue”, supporting and protecting tissue<\/em><\/p>\n 1. Schwann Cells\u00a0<\/strong>PNS<\/em><\/p>\n a. Myelin<\/strong><\/p>\n 1. White matter<\/p>\n 2. Function –\u00a0Insulate and greatly speed up rate of nerve impulse transmission<\/em><\/p>\n b. Regeneration of axons [1-5 mm\/day]Possible due to Schwann cells providing structural scaffold. Slow process.<\/em><\/p>\n 2. Oligodendrocytes\u00a0<\/strong>Myelin production in CNS<\/em><\/p>\n 3. Astrocytes\u00a0<\/strong>Maintenance of blood brain barrier<\/em><\/p>\n II Electrical signals in neurons<\/p>\n A. Polarized; Depolarized; Hyperpolarized –resting membrane potential exists; inside of cell becomes less negative; inside of cell becomes more negative. All cells have a resting membrane potential but only nerve cells and muscle cells can change this RMP by depolarization and hyperpolarization.<\/em><\/p>\n B. Resting membrane potential can change –changes in the membrane potential are the result of changes in the permeability of the membrane to ions<\/em><\/p>\n 1. Graded potentials –for short distances<\/em><\/p>\n 2. Action potentials<\/p>\n C. Na+<\/sup> and K+<\/sup> membrane channels<\/p>\n 1. K+<\/sup> channels are nearly always open<\/p>\n 2. Gated channels –these are channels that open and close<\/em><\/p>\n a. Chemically regulated channels –ligand regulated and associated with local potentials<\/em><\/p>\n b. Mechanically regulated channels –also associated with local potentials<\/em><\/p>\n c. Voltage regulated channels –associated with action potentials and only present in muscle and nerve cells<\/em><\/p>\n D. Review of\u00a0Resting Membrane Potential\u00a0<\/strong>and the\u00a0Na+<\/sup>\/K+<\/sup> pump<\/strong><\/p>\n E. Graded Potentials<\/strong>–\u00a0change in resting membrane potential resulting in either depolarization or hyperpolarization<\/em><\/p>\n 1. Localized<\/p>\n 2. Magnitude varies\/graded<\/p>\n 3. AKA Receptor potentials, Synaptic potentials, Pacemaker potentials<\/p>\n 4. Stimulus results in change in membrane permeability<\/p>\n a. Light stimulating vertebrate photoreceptors result in hyperpolarization because membrane becomes LESS permeable to Na.<\/p>\n b. Light stimulating invertebrate photoreceptors results in hypopolarization or depolarization due to the membrane becoming MORE permeable to Na.<\/p>\n 5. Graded potentials decrease with distance –therefore graded potentials are important signaling over short distances<\/em><\/p>\n F. Action potentials<\/strong><\/p>\n 1. Very quick, very large and can propagate signals over long distances<\/p>\n 2. Only neurons and muscle cells<\/p>\n 3. Depolarization to threshold<\/strong>–\u00a0positive feedback loop<\/em><\/p>\n 4. Neurons & muscle cells have voltage-gated ion channels<\/p>\n 5. Action potential results from transient changes in the membrane permeability to Na+<\/sup> and K+<\/sup> due to opening and closing specific ion channels<\/p>\n 6. At rest, \u00a0K+<\/sup> leak channels are open but few Na+<\/sup> channels are open<\/p>\n 7. Depolarization<\/strong>: The stimulus causes Na+<\/sup> channels to open and since they are voltage regulated gates, this stimulates more to open and at about -55mV, threshold, this becomes a run away cycle with numerous Na+<\/sup> channels opening and Na+<\/sup> flooding into the cell so that it actually becomes positive inside the cell.<\/p>\n 8. Repolarization\u00a0<\/strong>occurs because the Na+<\/sup> channels are only open for a short time but the K+<\/sup> channels open and positive potassium ions leave the cell – in fact it hyperpolarizes the cell for a short time until all of the channels close.<\/p>\n 9. Na\/K pump\u00a0<\/strong>returns ion concentrations to the resting levels<\/p>\n 10. Action potentials are “All or None<\/strong>“<\/p>\n Question: If AP are all or none, how can you determine a weak stimulus from a strong stimulus?<\/em><\/p>\n 11. Refractory period<\/p>\n a. Absolute refractory period<\/strong>–\u00a0no stimulus can trigger another AP. This is due to Na+<\/sup> channel inactivation<\/em><\/p>\n b. Relative refractory period<\/strong>–\u00a0A stimulus that is very strong may trigger another AP. This is due to K+<\/sup> channels still being open<\/em><\/p>\n G. Propagation of Action Potentials =Nerve Impulse<\/strong><\/p>\n 1. Unmyelinated axons (gray)<\/p>\n a. Current flow<\/p>\n b. Unidirectional based on origin of AP & the fact that AP’s cannot go backwards due to refractory periods<\/p>\n c. Slow ~ 2mph<\/p>\n 2. Myelinated axons (white) –a fatty electrical insulator<\/em><\/p>\n a. Myelin\u00a0<\/strong>made by oligodendrocytes in CNS and Schwann cells in PNS<\/p>\n b. Not continuous, gaps are called\u00a0Nodes of Ravier<\/strong><\/p>\n c. Saltatory conduction<\/strong>–\u00a0leaping conduction from one node to the next and can be up to 250 mph<\/em><\/p>\n d. Newborns have incomplete myelination<\/p>\n e. Largest myelinated axons have fasted conduction because it is easier for ions to move through large axons<\/p>\n <\/p>\n <\/p>\n <\/p>\n <\/p>\n <\/p>\n","protected":false},"excerpt":{"rendered":" I. Neurons and Glial Cells comprise the Nervous System A. Overall function –Communication:receive information, process it and transmit it via electrical and chemical signals. Also, function in stimulus-response mechanisms (reflexes) B. Typical\u00a0Neuron–\u00a0dendrite for signal reception, cell body for signal integration, … Continue reading