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L5 NPB 101 - Lecture 5 •  SmartSite: –  Lec 5...

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Unformatted text preview: Lecture 5 •  SmartSite: –  Lec 5 Notes •  Review –  Graded vs. AcDon PotenDal –  AcDon PotenDal •  Announcements: –  None •  Reading (Recommended): –  Chapter 4 (pp. 94 ­105) •  AcDon PotenDal (cont’d) •  ConducDon of AP •  Synapse 1 REV: AcDon PotenDal 1.  Rapid stereotyped change in Em, which occurs in response to a sDmulus. 2.  There are 6 primary characterisDcs of an AP: a)  Rapid depolarizaDon of Em (by large regeneraDve increase in PNa via PosiDve Feedback) b)  Rapid repolarizaDon 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 REV: Comparison of Graded vs. AcDon PotenDals Table 4 ­1, pg. 98 3 At Any Point of the AcDon PotenDal, The Em is a FuncDon of PNa and PK Fig. 4 ­11, pg. 99 4 Voltage ­Gated Channel ProperDes Affect the Neuronal Firing Rate •  Refractory Periods –  A new acDon potenDal usually cannot be iniDated in a region that just had an acDon potenDal –  Why? InacDvaDon of the populaDon of voltage ­gated Na+ channels Fig. 4 ­11, pg. 99 5 Voltage ­Gated Channel ProperDes Affect the Neuronal Firing Rate •  Absolute Refractory Period –  A new acDon potenDal ABSOLUTELY cannot be iniDated no maber how much sDmulaDon is given –  Why? •  Not enough voltage ­gated Na+ channels have recovered from inacDvaDon Fig. 4 ­11, pg. 99 6 Voltage ­Gated Channel ProperDes Affect the Neuronal Firing Rate •  RelaDve Refractory Period –  A new acDon potenDal MIGHT BE iniDated •  The sDmulus must be stronger than normal –  WHY? •  More voltage ­gated Na+ channels have recovered from inacDvaDon. –  BUT, •  a hyperpolarizing current sDll 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 sDmulus strength •  Briefly –  SDmuli of differing intensiDes elicit APs in sensory neurons –  The STRONGER sDmulus triggers a more APs per second –  The SIZE of the AP will NOT change, regardless the sDmulus strength (Remember AP is stereotypical). –  So AP frequency is one way to encode a signal and convey informaDon about characterisDcs of that signal to the nervous system. 8 ConducDon of AcDon PotenDal 9 AcDon PotenDal PropagaDon Fig. 4 ­9a, pg. 97 10 AcDon PotenDal PropagaDon Fig. 4 ­9b, pg. 97 11 ConducDon Velocity (CV) of AP •  Two types of propagaDon –  ConDguous conducDon •  ConducDon in unmyelinated fibers •  AcDon potenDal spreads along every porDon of the membrane –  Saltatory conducDon •  Rapid conducDon in myelinated fibers •  Impulse jumps over secDons of the fiber covered with insulaDng myelin 12 ConDguous ConducDon Fig. 4 ­10, pg. 98 13 ConducDon Velocity •  3 factors affect AP ConducDon 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 ConducDon Fig. 4 ­12a, 4 ­13, pg. 100 ­101 15 Saltatory ConducDon •  Propagates acDon potenDal faster than conDguous conducDon because acDon potenDal does not have to be regenerated at myelinated secDons •  Myelinated fibers conduct impulses about 50 Dmes 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 conducDon 17 Axon CommunicaDon  ­ The Synapse 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 SynapDc Convergence and Divergence Fig. 4 ­19, pg. 113 20 ...
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