~bn09 membrane potential.ppt
TRANSCRIPT
Neural Signaling:The Membrane
PotentialLecture 9
Membrane Structure
Barrier Compartmentalization
Semipermeable selectively leaky
Fluid Mosaic Model Phospholipids Proteins ~
Phospholipid Bilayer
Hydrophilic heads
(phosphate)
Hydrophobic tails (lipid)
Membrane Proteins
Channels Pumps
active transport Receptor protein sites
bind messenger molecules Transducer proteins:
2d messenger systems Structural proteins
form junctions with other neurons ~
Membrane Proteins: Ionophores
Ions Channels Nongated
always open Gated
chemically-gated electrically-gated mechanically-gated ~
Chemically-Gated Channels
ligand-gated Ionotropic
receptor protein = channel direct control ---> fast
Metabotropic second messenger system indirect ---> slow ~
Membrane Proteins
OUTSIDE
INSIDE
Metabolic pumps
Membrane proteins Pump ions
require energy Na+ - K+ Ca++ (calcium)
Also various molecules nutrients neurotransmitters ~
Biolelectric Potential
Communication within neuron electrical signal
electric current = movement of electrons
Bioelectric: movement of ions ~
Ion Distribution
Particles / molecules electrically charged
Anions negatively charged
Cations positively charged ~
Anions (-) Large intracellular proteins Chloride ions Cl-
Cations (+) Sodium Na+ Potassium K+ ~
Ion Distribution
Resting Membrane Potential
Membrane
outside
inside
Na+
Na+
Cl-
Cl-K+
K+
A-
+ + + + + + + + + + +
-----------
+ + + + + + + + + + +
-----------
more negative particles in than out Bioelectric Potential
like a battery Potential for ion movement
• current ~
Membrane is polarized
INSIDE
POS
NEG
Bioelectric Potential
OUTSIDE
Forces That Move Ions
Concentration (C) particles in fluid move from area of
high to area of low concentration diffusion, random movement
Electrostatic (E) ions = charged particles like charges repel opposite charges attract ~
Equilibrium Potential
Also called reversal potential Distribution of single ion across
membrane e.g., EK+, ENa+, ECl-
Potential for movement of ion if channel opens units millivolts (mV) Potential outside = 0, by convention ~
Equilibrium Potential
R = gas constant F = Faraday constant T = temperature (K) Z = valence (charge) of ion ~
i
o
K K
K
ZF
RTE
][
][log
Equilibrium Potential
i
o
K K
K
Z
mVE
][
][log
58
K+: z = +1
Cl-: z = -1
Mg++: z = +2
Equilibrium Potential
Constants never change Assume 25 oC Use log10 ~
mVmVEK
75400
20log58
Equilibrium Potential
mVmVENa
5550
440log58
10
i
o
Na Na
Na
ZF
RTE
][
][log
Membrane Potential
Net bioelectric potential for all ions units = millivolts (mV)
Balance of both gradients concentration & electrostatic
Vm = -65 mV given by Goldman equation ~
icliNaiK
ocloNaoK
m ClPNaPKP
ClPNaPKP
F
RTV
][][][
][][][log
Membrane Potential: Goldman Equation
P = permeability at rest: PK: PNa: PCl = 1.0 : 0.04 : 0.45
Net potential movement for all ions known Vm:Can predict direction of movement of any ion ~
C
Organic anions - Membrane impermeableOpposing electrical force not required
A-
Vm = -65 mV
Chloride ion
C
E
Cl-
Vm = -65 mV
Concentration gradient equal to electrostatic gradient.
Leaks out neuron ECl- = - 65 mV ~
K+ C
EVm = -65 mV
Potassium ion
Concentration gradient greater than electrostatic gradient.
Leaks out neuron EK = - 75 mV ~
Sodium ion
Na+
C EVm = -65 mV
Concentration gradient and electrostatic gradient into neuron.
ENa+ = +55 mV ~
Metabolic Pumps
Active Transport mechanisms Require energy
Move materials against gradient Na+ - K+ Calcium - Ca++ Nutrients, etc.~
Na+ - K+ Pump
Maintains gradients at rest Pumps 3 Na+ out of cell 2 K+ into cell Energy = ATP ~
Inside Outside
Na+
Na+
Na+
K+
K+K+K+
Na+
Na+
Na+
ATP
Inside Outside
Na+ Na
+Na+
K+
K+
K+
K+