Chapter 3: The Neuronal Membrane at Rest

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9 Apr 2012
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Chapter 3- the Neuronal Membrane at Rest
Introduction
Simple reflex: must collect distribute and integrate information
Axon carry ions not free electrons, therefore cytosol less conductive than a copper wire, also less
insulated
Action potential: nerve impulse with fixed size and duration, frequency and distribution codes
information; brief reversal in potential
Excitable membrane: cells capable of generating and conducting action potentials; muscle and nerve
cells
Resting potential difference: difference in electrical charge across the membrane; negative
The Cast of Chemicals
Cytosol and the Extracellular Fluid
Water
o Polar: effective solvent of other polar molecules
Ions
o Have net electrical charge
o Charged portion of water has stronger attraction for ions that they have for each other
o Sphere of hydration: cloud of water around each ion the insulate the ions from each other
o Monovalent vs divalent; cation vs anion
o Imp. Na+ K+ Cl- Ca2+
The Phospholipid Membrane
Hydrophilic vs hydrophobic
Lipid: water insoluble, barrier to water soluble ions and water
The phospholipid bilayer
o Phospholipid: polar phosphate head and non polar hydrocarbon tail
o Phospholipid bilayer: two layers of phospholipids that isolates cytosol from extracellular
environment
Protein
Enzymes, cytoskeleton, receptors
Protein structure
o alphaC: bonded to carboxyl, amino, hydrogen and R group (can be hydrophobic or hydrophilic)
o Peptide bonds: join amino group to carboxyl group
o Polypeptide: proteins of a single chain of amino acid
o Primary structure:
o Secondary structure: hydrogen bonds, alpha helix beta pleated sheet
o Tertiary structure: R group interactions; globular
o Quaternary structure: bonding of different chains together, each chain is subunit
Channel Proteins
o Protein chemically heterogeneous so hydrophobic in bilayer and hydrophilic on either side
o Ion channels: membrane spanning proteins
4-6 proteins with pore inbetween
Varying subunits give varying properties
o Ion selectivity: based on diameter of pore and nature of R groups lining it
o Gating:
Ion Pumps
o Membrane spanning proteins use ATP to move ions across membrane
o Ex. pump Ca2+ and Na+
The Movement of Ions
Diffusion
Diffusion: random movement that leads to net movement of ions from high concentration to low
concentration
o Faster with increasing temperature
o Ions diffuse when there are channels permeable to the ion and when there is a concentration
gradient across the membrane
Concentration gradient: difference in concentration, ions move down gradient
Electricity
Opposite charges attract, like charges repel
o Therefore Na moves toward anode and Cl toward cathode
Electrical current (I amps): the movement of electrical charge, positive in the direction of positive
charge movement
Electrical potential /voltage (V-volts): difference in charge between anode and cathode; force on a
charged particle
Electrical conductance (g-seimens): the relative ability of an electrical charge to migrate from one
point to another
o Depends on the number of particles available to carry electrical charge and the ease with
which these particles can travel through space
Electrical resistance (R-ohms): the relative inability of an electrical charge to migrate; R=1/g
Ohms Law: I=gV
Electrical ion movement: requires selective permeability and electrical potential difference across
membrane
The Ionic Basis of the Resting Membrane Potential
Membrane potential (Vm): the voltage across the neuronal membrane at any moment
Microelectrode: a thin glass tube with an extremely fine tip (d=0.5 microm) that penetrates with
minimal damage and is filled with an electrically conductive salt solution connected to voltmeter
which is also connected to wire placed outside the cell
Resting potential: -65 mV
Equilibrium Potentials
If no permeability of membrane and equal amounts of salt on both sides (but different
concentrations) then Vm=0
Ionic equilibrium potential/equilibrium potential (Eion): the electrical potential difference that
exactly balances an ionic concentration gradient; diffusional and electrical forces are equal and
opposite and there is no net movement of ions
o Need ionic concentration gradient and selective ionic permeability
o If the membrane was only permeable to that ion
Important Points
o Large changes in membrane potential caused by minuscule changed in ionic concentrations
o The net difference in electrical charge occurs at the inside and outside surfaces of the
membrane
Ions on either side attract to each other at the membrane so they localize at membrane
Capacitance: membrane stores electrical charge
o Ions are driven across the membrane at a rate proportional to the difference between the
membrane potential and the equilibrium potential
Ionic driving force: Vm-Eion
o If the concentration difference across the membrane is known for an ion and equilibrium
potential can be calculated for that ion
Nernst equation
o To calculate equilibrium potential
o Eion=2.303 RT/zF log ([ion]o/[ion]i)
o Dont need selectivity or permeability of membrane
o Ek=-80mV
o Ena=+62 mV
The Distribution of Ions Across the Membrane
Sodium potassium pump: enzyme that uses ATP to pump 3 Na out of cell and 2 K into cell
o Uses 70% energy in brain
Calcium pump: actively transports Ca2+ out of the cytosol across the membrane
Relative Ion Permeabilities of the Membrane at Rest
Goldmann equation: takes in to consideration the relative permeability of the membrane to
different ions
o Ex. If K and Na resting membrane potential is -65mV
The Wide World of Potassium Channels
o Lily and Yuh Nung Jan: determined amino acid sequence of one potassium channel using
Shaker fruit flies (shake to ether)
o Many different kinds of potassium channels
o Most have four subunits
o Pore loop: contributes to selectivity filter that only lets potassium in
o Chris Miller and Roderick MacKinnon: studied scorpion toxin that blocks K channels to solve
3D structure of channel
o Weaver mouse: mutation of single amino acid in pore loop of a potassium channel in
cerebellum causes difficulty in maintaining posture and moving properly
Sodium and potassium ions can now pass through
The importance of regulating the external potassium concentration
o Depolarization: a change in membrane potential from normal resting value (-65) to less
negative
o Blood brain barrier: specialization of the walls of brain capillaries that limits the movement of
potassium into the extracellular fluid of the brain
o Astrocytes: potassium spatial buffering: take in excess potassium with their membrane
potassium pumps and dissipates it over a large area
o Muscle cells do no have blood brain barrier or astrocytes and are affected by potassium levels
Death by Lethal Injection
o KCl injection gets rid of membrane potential and without depolarization cardiac muscle cells
cant make heart beat