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Resting membrane potential. 1 mV= 0.001 V membrane separates intra- and extracellular compartments inside negative (-80 to -60 mV) due to the asymmetrical distribution of ions across the cell membrane AND the differential permeability of the membrane to these ions.
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Resting membrane potential • 1 mV= 0.001 V • membrane separates intra- and extracellular compartments • inside negative (-80 to -60 mV) • due to the asymmetrical distribution of ions • across the cell membrane • AND the differential permeability of the membrane to these ions
Channels allow ions to diffuse across membranes Voltage-gated: Na+ channels, K+ channels, Ca2+ channels Ligand-gated: neurotransmitters (acetylcholine, glutamate)
Potassium Equilibrium Potential Figure 5-34a
Resting membrane potential is due mostly to high potassium permeability Figure 5-34c
The Nernst equation describes an ion’s equilibrium potential • where: • R is the gas constant (8.314 X 107 dyne-cm/mole degree), • T is the absolute temperature in o Kelvin, • z is the charge on the ion • F is the Faraday (the amount of electricity required to chemically alter one gram equivalent weight of reacting material = 96,500 coulombs).
A simpler version of the Nernst equation At 37ºC: When ions can move across a membrane, they will bring the membrane potential to their equilibrium potential.
Calculating the membrane potential for a cell that is only permeable to K+ [K+]out = 5 mM [K+]in = 150 mM Ek = 61 x (-1.5) = -92 mV
Sodium Equilibrium Potential ENa = 61 x 1 = +61 mV
The Na+-K+-ATPase (“sodium pump”) works to keep intracellular K+ high and Na+ low
Predicting the membrane potential (Vm) • The membrane potential can be described by the relationship between ion permeabilities and their concentrations • The Goldman equation: • Vm = PNa[Na+]out+ PK[K+]out+ PCl[Cl-]in 61 log PNa[Na+]in+ PK[K+]in+ PCl[Cl-]out • At the resting potential • a. K+ is very close to equilibrium. • b. Na+ is very far from its equilibrium. • c. PK >> PNa
Real neurons and “Dynamic Polarization” Purkinje cell Cerebellum Pyramidal cell Layer V neocortex Input Dendrites Dendrites Axon collaterals Collateral branch Axon Output Axon Santiago Ramon y Cajal, 1900
Electrical Signals: Ion Movement • Resting membrane potential determined by • K+ concentration gradient • Cell’s resting permeability to K+, Na+, and Cl– • Gated channels control ion permeability • Mechanically gated • Ligand gated • Voltage gated
Current flow through ion channels leads to changes in membrane potential Ohm’s Law: V = I * R V = voltage, I = current (Amps), R = resistance (Ohms) I = V/R or I = V * G G = conductance (Siemens) For current to flow, there must be a driving force (Vm - Eion) > or < 0, thus I = (Vm - Eion) * G If current flows across a resistance--the cell membrane acts like one--there is a change in voltage (membrane potential).
Graded Potentials Graded potentials can be: EXCITATORY or INHIBITORY (action potential (action potential is more likely) is less likely) The size of a graded potential is proportional to the size of the stimulus. Graded potentials decay as they move over distance.
Action Potential 1 ms • All-or-none • Not due to “membrane breakdown” +40 “Overshoot” 0 mV Shock -80
Electrical Signals: Action Potentials Figure 8-9 (1 of 9)
Electrical Signals: Action Potentials Figure 8-9 (2 of 9)
Electrical Signals: Action Potentials Figure 8-9 (3 of 9)
Electrical Signals: Action Potentials Figure 8-9 (4 of 9)
Electrical Signals: Action Potentials Figure 8-9 (5 of 9)
Electrical Signals: Action Potentials Figure 8-9 (6 of 9)
Electrical Signals: Action Potentials Figure 8-9 (7 of 9)
Electrical Signals: Action Potentials Figure 8-9 (8 of 9)
Electrical Signals: Action Potentials Why is AP peak < ENa? Figure 8-9 (9 of 9)
Electrical Signals: Voltage-Gated Na+ Channels Na+ channels have two gates: activation and inactivation gates Figure 8-10a
Electrical Signals: Voltage-Gated Na+ Channels Figure 8-10c
Electrical Signals: Voltage-Gated Na+ Channels Figure 8-10d
How does an AP travel down an axon? Figure 8-14
Speed of AP conduction is governed by: • Diameter of the axon • Resistance of the axon membrane to ion leakage
Axon size matters 1 mm
Myelination increases conduction velocity Top speed=225 mph Top speed=170 mph Kawasaki Z750S
Electrical Signals: Graded Potentials Subthreshold and suprathreshold graded potentials