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Lecture 3

Neuro Lecture 3.docx

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University of Toronto St. George
Michelle French

Neuro:Lecture 3 October-28-12 1:39 AM Impulse conduction • When AP generated, Na rushes into cell • Reversal of potential difference across membrane • Local MP goes from - on inside to + on inside • Source of depolarizing current for adjacent membrane • Na channels opened in adjacent membrane • Once started, AP will propagate from it's origin across the rest of the cell Excitable cells • Most cells aren't excitable cuz they don't have Na channels • They can still pass on currents (passively) • Most cells don’t want to carry signals over a distance so they don’t have axons • Axon like a wire • Only neurons and muscle cells generate propagating action potentials • Biological tissue has resistance so voltage will change when going through membrane o i.e. loss of signal as travel across membrane Cable properties • Length constant lamda o Measures how quickly a potential difference disappears • Speed of an AP depends on membrane length constant lambda • Lambda is increased by increasing diameter • Increased by increasing membrane resistance • See formula on slide 12 • Defined as the distance you have to travel to get to where the voltage drops to 37% of original Myelination • Increases membrane resistance (most efficient method of increasing speed of conduction) • Glial cells assist nervous system, needed for nutrition and membrane resistance • Specialized glial cells (schwann cells in PNS, oligodendrocytes in CNS) wrap around successive sections of an axon -> myelin sheath • 50-100 layer wrapped around the axon reduced the amount of leakage out of the cell • Schwann cell wraps around a single portion of the one axon (cytoplasm all squeezed out) • Oligodendrocyte has a number of processes that streak out like an octopus and wraps a bunch of axons individually • Small gaps left between adjacent portions of the sheath -> node of Ranvier • MS Saltatory conduction • In myelinated axons, one the membrane exposed at the nodes is excitable • AP only generated at these nodes • AP will jump from one to the next • No AP generated as depolarizing current can travel for many nodes • Jumping mode of conducti
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