Lecture 3 Flashcards
What neuron is associated with an afferent nerve?
Sensory neuron
What neuron is associated with an efferent nerve?
Motor neuron
What is membrane potential (Vm)?
difference in electrical charge across the membrane
What is membrane potential when neuron is at rest?
steady negative electrical potential (-65 mV)
What is electrical potential?
the force exerted on a charged particle (ion)
How is the resting membrane potential established and maintained?
Charge separation: neuronal membrane
Selective permeability: ion channel proteins
Concentration gradients: ion pumps
What’s the key ingredient and feature in the cytosol and extracellular fluid?
Water! It’s a polar solvent
What (besides water) is in the cytosol and extracellular fluid?
Ions
- cations: net positive charge
- anions: net negative charge
- hydrophilic!!
What’s the main features of the phospholipid bilayer?
-Hydrophobic lipids (tails)
- Hydrophilic phosphate groups (head)
How does the phospholipid bilayer act as a wall?
Ions hydrated in solution can’t pass through the hydrophobic core of the bilayer
What are proteins made from and where can they be found?
Made from amino acids
found in: enzymes, cytoskeletal elements, receptors, ion channels
What is the role of transmembrane proteins?
- control resting membrane potential and action potential
- control synaptic transmission
- control extracellular to intracellular signaling - control
What is the structure of proteins?
many amino acids linked via peptide bonds to form polypeptide chains
- r groups alternate sides
What are the 4 levels of protein structure?
1) Primary: polypeptide chain
2) Secondary: coiling into alpha helix and beta-sheets
3) Tertiary: 3D folding of the coils and beta-sheets
4) Quaternary: different polypeptides bonding together
What are channel proteins and their role?
Polar R groups exposed to the cytosol or extracellular fluid and nonpolar t groups in the membrane
- ion selectivity and gating
What are the 3 types of ion channels and what do they affect?
1) Voltage gated: contribute to action potential
2) Ligand Gated: contribute to alterations in post-synaptic potentials
3) Leak: alter resting membrane potential
What is the role of Ion pumps?
- critical for establishing cellular concentration gradients
- formed by membrane-spanning proteins
- use energy from ATP breakdown
What does the sodium-potassium pump work?
(321NoKiA)
3 Na+ out
2 K+ in
1 ATP used
- uses ~70% of the total amount of ATP in the brain
Define diffusion force.
force on an ion due to its concentration gradient
- doesn’t change significantly under physiological conditions
Define electrostatic force.
force on an ion produced by the membrane voltage
- amount and direction of the force is a function of the membrane voltage and the charge of the ion
Define equilibrium potential
voltage that exactly offsets the diffusional force of the ion
- calculated by the Nernst Equation
Define driving force.
sum of the diffusion and electrostatic forces
- membrane voltage minus ionic Eion)
- controls the rate of ion flux if the membrane is permeable to the ion
- ion flux alters membrane voltage
Define ion flux.
mechanism by which membrane potential is changed.
- controlled by driving force of an ion and permeability of the membrane to that ion
What is the law of permeability?
membrane potential is always driven toward the equilibrium potential of the ion to which the membrane is most permeable
Define diffusion.
- dissolved ions distribute evenly
- Ions flow down concentration gradient:
– Channels are permeable to specific ions
– Concentration gradient exists across the membrane
What’s Ohm’s Law?
I=V/R
V=IR
I: Electrical current
V: Electrical potential
R: Resistance
What does equilibrium potential indicate?
- No net movement of ions when separated by a phospholipid membrane
- equilibrium reached with K+ channels in the phospholipid bilayer
- Electrical potential difference that exactly balances ionic concentration gradient
How do changes in ionic concentrations affect Vm?
Large changes in Vm can occur even with super small changes in ionic concentrations
Are electrical charges balanced from the inside or outside?
Inside and outside membrane surfaces
How is the rate of movement of ions across membrane calculated?
Determined by driving force
- driving force = Vm-Eion (membrane potential - equilibrium potential)
How can equilibrium potential be determined in concentration difference is known?
Nernst Equation
Eion = 2.303 RT/zF log [ion]o/[ion]in
What’s the relationship between charge and equilibrium potential?
Reversely proportional: an ion with twice the charge will have twice the Eion
Complete the following for K+:
Concentration outside in mM
Concentration inside in mM
Ratio out:in
Eion at 37C
Concentration outside in mM: 5
Concentration inside in mM: 100
Ratio out:in: 1:20
Eion at 37C: -80 mV
Complete the following for Na+:
Concentration outside in mM
Concentration inside in mM
Ratio out:in
Eion at 37C
Concentration outside in mM: 150
Concentration inside in mM: 15
Ratio out:in: 10:1
Eion at 37C: 62 mV
Complete the following for Ca++:
Concentration outside in mM
Concentration inside in mM
Ratio out:in
Eion at 37C
Concentration outside in mM: 2
Concentration inside in mM: 0.0002
Ratio out:in: 10000:1
Eion at 37C: 123 mV
Complete the following for Cl-:
Concentration outside in mM
Concentration inside in mM
Ratio out:in
Eion at 37C
Concentration outside in mM: 150
Concentration inside in mM: 13
Ratio out:in: 11.5:1
Eion at 37C: -65 mV
Nernst equation for potassium?
61.54 x log [ion]out/[ion]in
Nernst equation for sodium?
61.54 x log [ion]out/[ion]in
Nernst equation for chloride?
-61.54 x log [ion]out/[ion]in
Nernst equation for calcium?
30.77 x log [ion]out/[ion]in
Which direction is membrane potential driven towards?
Towards the equilibrium potential of the ion to which the membrane is most permeable
What is the theoretical value? The actual potential? When are they equal?
Individual ion equilibrium potentials = theoretical
Membrane potential = actual potential
They are the same if the membrane is permeable only to that ion
What is the order of permeability?
K+»_space; Cl-»_space; Na+
When do you use Goldman Equation?
To calculate actual resting membrane potential if relative ion permeability is known
What is the default state of the K+ leak channel? Where does it exist? What does it result in?
Default state: open
Exists in all zones of the neuronal membrane
Results in a high resting permeability to K+ (determines resting membrane potential)
What’s the structure of the K+ channels?
4 subunits
What does increased extracellular K+ lead to?
depolarization of the membrane
What’s the goldman equation?
61.54 x log (P[K+]out + P[Na+]out + P[Cl-]in)/(P[K+]in + P[Na+]in + P[Cl-]out)
P = relative membrane permeability
