Lec2

Lec2

• 1.
  Membrane Potential (Vm): -charge difference across the membrane - K+ Na+ K+ Na+ inside outside …how can passive diffusion of potassium and sodium lead to development of negative membrane potential?
• 2.
  Simplest Case If amembrane were permeable to only K+ then… inside outside K+ K+ K+ would diffuse down its concentration gradient until the electrical potential across the membrane countered diffusion.
• 3.
  Simplest Case K+ K+ Ifa membrane were permeable to only K+ then… The electrical potential that counters net diffusion of K+ is called the K+ equilibrium potential (EK). inside outside
• 4.
  The Potassium NernstPotential Example: If Ko = 5 mM and Ki = 140 mM EK = -61 log(140/4) EK = -61 log(35) EK = -94 mV EK = 61 log Ki Ko So, if the membrane were permeable only to K+, Vm would be -94 mV …also called the equilibrium potential
• 5.
  Simplest Case Na+ Na+ If amembrane were permeable to only Na+ then… The electrical potential that counters net diffusion of Na+ is called the Na+ equilibrium potential (ENa). inside outside Na+ would diffuse down its concentration gradient until potential across the membrane countered diffusion.
• 6.
  The Sodium NernstPotential Example: If Nao = 142 mM and Nai = 14 mM EK = -61 log(14/142) EK = -61 log(0.1) EK = +61 mV EK = 61 log Nai Nao So, if the membrane were permeable only to Na+, Vm would be +61 mV
• 7.
  Resting Membrane Potential 0mV EK -94 ENa +61 Vm -90 to -70 Why is Vm so close to EK? Ans. The membrane is far more permeable to K than Na..
• 8.
  The Goldman-Hodgkin-Katz Equation (alsocalled the Goldman Field Equation) Calculates Vm when more than one ion is involved. o Cl i Na i K i Cl o Na o K m Cl p Na p K p Cl p Na p K p V ] [ ' ] [ ' ] [ ' ] [ ' ] [ ' ] [ ' log . - + + - + + + + + + = 61 NOTE: P’ = permeability i Cl o Na o K o Cl i Na i K m Cl p Na p K p Cl p Na p K p V ] [ ' ] [ ' ] [ ' ] [ ' ] [ ' ] [ ' log . - + + - + + + + + + = -61 or
• 9.
  The Goldman-Hodgkin-Katz Equation Theresting membrane potential is closest to the equilibrium potential for the ion with the highest permeability!
• 10.
  ATP 3 Na+ 2 K+ ADP ActiveTransport K+Na+ Na+ K+ inside outside Remember: sodium is pumped out of the cell, potassium is pumped in...
• 11.
  Resting Membrane PotentialSummary
• 12.
   Cells:  containhigh a K+ concentration  have membranes that are essentially permeable to K+ at rest  Membrane electrical potential difference (membrane potential) is generated by diffusion of K+ ions and charge separation  measured in mV (=1/1000th of 1V)  typically resting membrane potentials in neurons are -70 to 90 mV Resting and action potentials + + + + + + + + + + + – – – – – – – – – – – Voltmeter – + 0 mV -80 mV +
• 13.
  Nerve Action Potential Nerve signals are transmitted by action potentials, which are rapid changes in the membrane potential that spread rapidly along the nerve fiber membrane  Action potential begins with a sudden change from the normal resting negative membrane potential to a positive potential and then ends with an almost equally rapid change back to the negative potential
• 14.
   Changes thatoccur at the membrane during the action potential  Transfer positive charges to the interior of the fiber at the onset and return positive charges to the exterior at its end Nerve Action Potential
• 15.
  Stages of actionpotential  Resting Stage.This is the resting membrane potential before the action potential begins. The membrane is said to be "polarized" during this stage because of the -90 millivolts negative membrane potential that is present
• 16.
  Depolarization Stage  Themembrane suddenly becomes very permeable to sodium ions, allowing tremendous numbers of positively charged sodium ions to diffuse to the interior of the axon.  The normal "polarized" state is neutralized by the inflowing positively charged sodium ions, with the potential rising rapidly in the positive direction  This is called depolarization
• 17.
  Repolarization Stage  Afterthe membrane becomes highly permeable to sodium ions, the sodium channels begin to close and the potassium channels open more than normal  Rapid diffusion of potassium ions to the exterior re-establishes the normal negative resting membrane potential  This is called repolarization of the membrane
• 18.
  The AP -membrane permeability • During the upstroke of an action potential:  Na permeability increases  due to opening of Na+ channels  memb. potential approaches ENa Na+ channels  K permeability increases  due to opening of K+ channels  mem. potential approaches EK • After hyperpolarization of membrane following an action potential: Membrane hyperpolarized resting potential K+ channels  There is increased K+ conductance  due to delayed closure of K+ channels • During the downstroke of an action potential:  Na permeability decreases  due to inactivation of Na+ channels 1 ms +61 0 (mV) -90 ENa EK
• 19.
  Properties of actionpotentials  Action potentials:  are all-or-none events  threshold voltage (sudden increase in the membrane potential) threshold -70 +60 mV 0 non- myelinated 0 800 400  have constant conduction velocity  Fibers with large diameter conduct faster than small fibers Fiber diameter (mm) 0 3 6 9 Myelinated 12 75 15 50 25 0  are initiated by depolarization  action potentials can be induced in nerve and muscle by extrinsic stimulation
• 20.
  Propagation: Rest Opening of Na+channels generates local current circuit that depolarizes adjacent membrane, opening more Na+ channels… Stimulated (local depolarization) Propagation (current spread)
• 21.
  Signal Transmission: Myelination • Schwanncells surround the nerve axon forming a myelin sheath • Sphingomyelin decreases membrane capacitance and ion flow 5,000-fold • Sheath is interrupted every 1-3 mm : node of Ranvier
• 22.
  Saltatory Conduction • AP’sonly occur at the nodes (Na channels concentrated here!) • increased velocity • energy conservation
• 23.
  Conduction velocity non-myelinated myelinated - non-myelinatedvs myelinated -