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+