Lecture 3 - Membrane Potentials Flashcards
Which species can pass through a cell membrane?
Eg. Water
Small molecules
Lipid soluble molecules
Uncharged molecules
Describe the features of diffusion
No energy input - passive
Spontaneous
Downhill flow
Which species cannot pass through a cell membrane
Charged particles
Large proteins
Eg. Ions, insulin
What is tonicity?
Concentration of Non-penetrating solutes
What is osmolarity?
Concentration of Total solutes
When comparing tonicity of cells, do we call the cell or the ECF hypo/hypertonic?
Always the extracellular fluid
What are the different units for osmolarity and tonicity?
Osmolality: Osm
Tonicity: no unit, relative measure
Describe Fick’s law
This law dictates the rate of diffusion across a membrane, depending on various variables:
- membrane thickness
- membrane permeability
- membrane surface area
- concentration gradient
Is the body in chemical equilibrium?
No - because certain solutes will have different concentrations on either side of the cell membrane.
eg. Na extracellularly and K intracellularly
Is the body in osmotic equilibrium?
Yes - equal concentration of water on either side of the membrane
–> Water moves across the membranes to ensure this
This means that overall solute concentrations are balanced
What is the typical osmolarity of the body?
300 mOsm
Is the body in electrical equilibrium?
Whole body level: yes - neutral
Cellular level: no - neurons are negative relative to the extracellular fluid
Describe the dogma of neuron communication between the PNS and CNS, afferent and efferent pathways
- Afferent neurons from the PNS enters the CNS and talks to an interneuron, entirely within the CNS
- Integration at the interneuron
- Interneuron talks to the efferent neuron, originating in the CNS
- Efferent neuron exits the CNS to do stuff in the periphery
Which ions are concentrated inside the cell?
Potassium
Which ions are concentrated outside the cell?
Sodium
Chlorine
Calcium
What is the electrical equilibrium of sodium and potassium individually, and what is the overall resting membrane potential?
Sodium: +60 mV
Potassium: -90 mV
Overall: -70 mV
What is the majority of RMP due to?
What else has an influence?
Majority is due to movement of ions down their gradients through leak channels
Also the Na-K ATPase pump
Why does potassium have a greater effect on the resting membrane potential (RMP)?
Because the membrane is more permeable to potassium, because there are more potassium leak channels
What is the Nernst potential?
This is the potential that is equal in magnitude to the potential for the ion to more across the membrane.
What does the magnitude of the Nernst potential rely on?
The ratio of the concentrations of the ion inside and outside the cell
What is the role of Na-K ATPase?
Maintains the chemical disequilibrium across the membrane
What are leak channels?
Another name for Open channels
ie not gated
They are always open for the passage of ions from ICF to ECF
What are the two types of protein that allow movement of a substance across a membrane?
- Channel
2. Carrier
Compare how channels are carriers move things across the membrane
Channel: direct connection between ICF and ECF through which things can move
Carrier: change in conformation allows movement of a molecule
What are the two types of channel protein?
- Open / leak channels
2. Gated channels
What are the different types of gated channels?
Chemically gated
Voltage gated
Mechanically gated
Compare concentration of Na+ inside and outside the cell
ICF: 15 mM
ECF: 150 mM
Compare concentration of K+ inside and outside the cell
ICF: 150 mM
ECF: 5 mM
Compare concentration of Cl- inside and outside the cell
ICF: 10 mM
ECF: 100 mM
Compare concentration of Ca2+ inside and outside the cell
ICF: 0.0001 mM
ECF: 1 mM
What is an electrochemical gradient?
The combination of the cell’s chemical and electrical gradient. These gradients are often in different directions:
K+:
• Chemical gradient out of cell
• Electrical gradient into cell
What is the equilibrium potential?
This is the point at which movement into and out of the cell due to chemical and electrical gradients is balanced
What happens if Na/K ATPase is inhibited?
- Na+ is no longer pumped back out
- K is no longer pumped back in
- RMP increases (cell more depolarised)
- Cell more likely to fire an action potential
What is an electrogenic pump?
A pump that moves ions unequally so that a potential across the cell membrane is set up
What happens when there is a change in the membrane’s ion permeability?
There will be movement of ions down their concentration gradients
• Change in membrane potential
What causes the change in a cell’s ion permeability?
An IPSP causes the opening of voltage gated ion channels
Voltage Gated Channels
What happens to the membrane potential if the cell becomes more permeable to Na?
Cell potential decreases; cell depolarises
What happens to the membrane potential if the cell becomes more permeable to K?
Cell potential increases; cell hyperpolarises
What are the stimuli for activation of Na and K voltage gated channels?
Na: depolarisation to threshold
K: same
Compare the speed of activation of Na and K voltage gated channels?
Na: fast
K: slow
