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Unformatted text preview: Conduction- These potentials and the changes in ion concentration occur immediately around the membrane -> ECF and cytoplasm not charged - it’s neutral!- Membrane potential occurs only if diffusion occurring across membranes; i.e. potential created largely by continuous K+ diffusion = only tiny part of cell has this charge- Action potential created by brief in¡ux of Na+ ions; conduction along membrane relies totally on the electrical cable properties of neuron membrane -> electrical conduction: how well that membrane mimics a copper wire- The better the conductor, the further the action potential will spread Cable Properties- The conduction velocity of an a.p . along an axon depends on the membrane length constant, λ- Measures how quickly a potential diffce decays to zero as a fxn of distance- How far an a.p. is passively spread along a membrane before it is lost- How good the cable properties are of the membrane- ↑ λ ↑ the distance travelled- Mech of ↑ λ : ↑ axon diameter OR by ↑ membrane resistance- Length constant, λ : - Rm: resistance of membrane- Ro: resistance from outside by ECF, Ri: resistance from inside by cytoplasm- Rf so small, you can ignore it- Two hypothetical values of λ- Small λ : voltages decay rapidly from source of a.p., short distance travelled by a.p.- Longer λ : via either higher membrane resistance or axon diameter, voltages travel further along axon and can generate a new a.p. in adj. Neurons- Λ formally de¢ned as distance you can go before original voltage drops to below 30% original value- Translated this into voltages: a.p. = 100mV, depolarize to about 20mV; λ is a ballpark ¢gure Myelination- ↑ membrane resistance is the most ef¢cient means of ↑ conduction velocity ( λ )- Mech: Specialized glial cells (Schwann cells or oligodendrocytes) wrap around successive sections of an axon = myelin sheath (layers of membrane around axon, cell body sits on outside)- Schwann cell: PNS, wrap 1 axon- Oligodendrocytes: CNS, wrap several axons PSL302Y: Lecture 11, by Prof. MacKay Wed., Oct. 6, 2010- Small gap left btwn adj glial cells = node of Ranvier Saltatory Conduction- In myelinated axons, only the membrane exposed at the nodes is excitable- Where a.p.’s are produced by inFux of Na+ ions- Saltatory: jumps from node to node; not continuously generated at these nodes- A.p. jumps from one node to the next 5-10 nodes (travels very far and very quickly!)from one node to the next 5-10 nodes (travels very far and very quickly!...
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This note was uploaded on 03/27/2012 for the course PSL PSL300 taught by Professor Mackayfrench during the Fall '11 term at University of Toronto.
- Fall '11