L5_NPB_101 - Lecture 5 •  SmartSite: –  Lec 5...

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Unformatted text preview: Lecture 5 •  SmartSite: –  Lec 5 Notes •  Review –  AcCon PotenCal •  Announcements: –  None •  Reading (Recommended): –  Chapter 4 (pp. 94 ­105) •  AcCon PotenCal (cont’d) •  ConducCon of AP •  Synapse 1 REV: AcCon PotenCal 1.  Rapid stereotyped change in Em, which occurs in response to a sCmulus. 2.  There are 6 primary characterisCcs of an AP: a)  Rapid depolarizaCon of Em (by large regeneraCve increase in PNa via PosiCve Feedback) b)  Rapid repolarizaCon of Em (by decrease in PNa and slow increase in PK) c)  Every AP followed by 2 refractory periods d)  Every AP is All ­or ­None e)  AP conducted over the axon f)  AP are conducted rapidly 2 At Any Point of the AcCon PotenCal, The Em is a FuncCon of PNa+ and PK+ Fig. 4 ­11, pg. 99 3 Voltage ­Gated Channel ProperCes Affect the Neuronal Firing Rate •  Refractory Periods –  A new acCon potenCal usually cannot be iniCated in a region that just had an acCon potenCal –  Why? InacCvaCon of the populaCon of voltage ­gated Na+ channels Fig. 4 ­11, pg. 99 4 Voltage ­Gated Channel ProperCes Affect the Neuronal Firing Rate •  Absolute Refractory Period –  A new acCon potenCal ABSOLUTELY cannot be iniCated no maaer how much sCmulaCon is given –  Why? •  Not enough voltage ­gated Na+ channels have recovered from inacCvaCon Fig. 4 ­11, pg. 99 5 Voltage ­Gated Channel ProperCes Affect the Neuronal Firing Rate •  RelaCve Refractory Period –  A new acCon potenCal MIGHT BE iniCated –  The sCmulus must be stronger than normal Fig. 4 ­11, pg. 99 6 Voltage ­Gated Channel ProperCes Affect the Neuronal Firing Rate •  RelaCve Refractory Period –  WHY? •  More voltage ­gated Na+ channels have recovered from inacCvaCon. –  BUT, •  a hyperpolarizing current sCll flows through the voltage ­ gated K+ channels. Fig. 4 ­11, pg. 99 7 Refractory Periods Limit AP Frequency •  This is important for telling us things such as sCmulus strength (MORE LATER) •  Briefly –  SCmuli of differing intensiCes elicit APs in sensory neurons –  The STRONGER sCmulus triggers a more APs per second –  The SIZE of the AP will NOT change, regardless the sCmulus strength (Remember AP is stereotypical). –  So AP frequency is one way to encode a signal and convey informaCon about characterisCcs of that signal to the nervous system. 8 ConducCon of AcCon PotenCal 9 AcCon PotenCal PropagaCon Fig. 4 ­9a, pg. 97 10 AcCon PotenCal PropagaCon Fig. 4 ­9b, pg. 97 11 ConducCon Velocity (CV) of AP •  Two types of propagaCon –  ConCguous conducCon •  ConducCon in unmyelinated fibers •  AcCon potenCal spreads along every porCon of the membrane –  Saltatory conducCon •  Rapid conducCon in myelinated fibers •  Impulse jumps over secCons of the fiber covered with insulaCng myelin 12 ConCguous ConducCon Fig. 4 ­10, pg. 98 13 ConducCon Velocity •  3 factors affect AP ConducCon Velocity (CV) •  Axon resistance: o  (Larger) diameter ⇒ ➡(Less) resistance ⇒ (Faster) CV (Faster) CV •  Membrane (Current) leaks: o  ➡ (Fewer) leaks ⇒ •  Membrane capacitance (ability to store charge): o  ➡ (Less) capacitance ⇒ (Faster) CV 14 Saltatory ConducCon Fig. 4 ­12a, 4 ­13, pg. 100 ­101 15 Saltatory ConducCon •  Propagates acCon potenCal faster than conCguous conducCon because acCon potenCal does not have to be regenerated at myelinated secCon •  Myelinated fibers conduct impulses about 50 Cmes faster than unmyelinated fibers of comparable size •  Myelin –  Primarily composed of lipids –  Formed by Glial Cells •  Oligodendrocytes in Central Nervous System (CNS) •  Schwann cells in Peripheral Nervous System (PNS) 16 Current Flow in Unmyelinated and Myelinated Axons “Many Small steps” Saltatory conducCon 17 The Synapse – Axon CommunicaCon 18 Morphology of a Chemical Synapse Input Zone Dendrites and Cell body Nucleus Trigger Zone Axon hillock Conducting Zone Axon (may be from 1mm to more than 1m long) Arrows indicate the direction in which nerve signals are conveyed. Output Zone Axon terminals See Fig. 4 ­8, pg. 95; Fig. 4 ­14, pg. 104 19 SynapCc Convergence and Divergence Fig. 4 ­19, pg. 113 20 ...
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