Session 3 - What is a membrane potential? Flashcards
What is a membrane potential?
Membrane potential: the potential difference across a cell’s plasma membrane
How do you measure membrane potential?
- Micropipette/ microelectrode, connected to voltmeter, penetrates the cell membrane
When membrane potential is measured, what is it expressed as?
- Membrane potentials expressed as voltage of inside of a cell relative to the outside
What is animal cell range of membrane potential?
Animal cell range: -20 to -90 mV
What is skeletal and cardiac muscle range of membrane potential?
Skeletal & cardiac muscle: -80 to -90 mV
What is nerve cells membrane potential?
Nerve cells: -50 to 75 mV
How is a membrane potential set up?
Membrane potentials are set up because the membrane is selectively permeable to different ions due to transmembranous channel proteins.
Give three characteristics of ion channels
Monspecificity
Gating
A high rate of ion flow
What is monospecificity of ion channels?
Channels that let through one (or a few) ion species
What is gating of an ion channel?
Channel can bbe open or closed by a conformational change in the protein molecule
What is special about the high rate of ion flow through an ion channel?
It is always down the electrochemical gradient for the ion
Depending on which channels are open…
The resting membrane can be selectively permeable to certain ion species
What is the intracellular and extracellular ion conc of Na+
Na+
Intracellular ~10mM
Extra cellular -145mM
What is the intracellular and extracellular ion conc of K+
Intracellular - 160 mM
Extracellular - 4.5 mM
What is the intracellular and extracelular ion conc of Cl-?
Intracellular - 3mM
Extracellular - 114mM
What is A- in cell ion terminology?
Represents anions other than Cl-, including phosphate, amino acids and charged groups on proteins
What is the intracellular concentration of A-? What is the extracellular?
167 mM intracellular
40mM extracellular
How is a membrane potential generated?
- K+ channels open \ membrane selectively permeable to K+
- K+ diffuse out of cell down the concentration gradient
- Anions can’t follow \ cell is negatively charged on the inside with respect to the outside
- This membrane potential will oppose outward movement of K+ \ system comes into equilibrium
What is the equilibrium potential?
Equilibrium potential: the membrane potential at which the electrical and diffusion forces balance one another out so that there is no further net movement of an ion
What is the unit of measurement used in the Nernst equation
mV
What is theoretical membrane potential if the membrane was 100% permeable to K+?
-94.6 mEquilibrium potential: the membrane potential at which the electrical and diffusion forces balance one another out so that there is no further net movement of an ion
What does the Nernst equation calculate?
The potassium equilibrium potential
CHECK EQUATIONS IN WORKBOOK
DONE?
Why is the true membrane potential around 75mV rather than 94.6 mV?
selectively permeable to K+. In reality the cell is also permeable to other cations such as Na+ and Ca2+ which move into the cell and make the membrane less negative
Define depolarisation
Depolarization: decrease in membrane potential so that inside of the cell becomes less negative
Define hyperpolarisation
Hyperpolarisation: increase in membrane potential so that the inside of the cell becomes more negative
How can membrane permeability for a particular ion be increased?
By opening its channels
How does depolarisation occur?
Opening of Na+ and Ca2+ channels
How does hyperpolarisation occur?
Opening of K+ or Cl- channels
K+ is postive, why does opening its channels hyperpolarise the cell?
Because higher intracellular conce, lower extracellular - Moves out of cell
What does increasing selective permeability of an ion cause?
The membrane potential is pushed towards the equilibrium potential for that ion
What two things could have occured if there is a depolarisation of cell membrane potential (must outline two DIFFERENT mechanisms)
Permeability of membrane to Na+ increases
Ions enter cell
Inside more +’ve>depolarisation
A-
Ions leave cell
Inside more +’ve > depolarisation
What happens if selective permeability of membrane to Cl- increases?
Inside more -‘ve, hyperpolarisation (reduced membrane excitability
What happens if selective permeability of membrane to K+ decreases?
Inside more +’ve > depolarisation
increased membrane excitability
Give five roles of membrane excitation
- Actions potentials in nerve and muscle cells
- Triggering and control of muscle contraction
- Control of secretion of hormones and neurotransmitters
- Transduction of sensory information into electrical activity by receptors
- Postsynaptic actions of fast synaptic transmitters
How can we control channel activity?
How can we control channel activity? Ligand, mechanical (membrane deformation) or voltage gating
What is ligand gating
The channel is opened (or closed) by binding of a chemical ligand, which may be an extracellular transmitter or an intracellular messenger
How does voltage gating work?
The channels opens or closes in response to changes in the membrane potential
What is fast synaptic transmission?
- Receptor protein is also an ion channel
- Transmitter binding causes the channel to open
- Change in membrane potential:
What is an excitatory synapse and how is the action potential caused?
· Excitatory transmitters open ligand gated channels that cause depolarization
· Resulting change is called an Excitatory post-synaptic potential (EPSP) – relatively long to ensure that voltage sensitive channels have sufficient time to open
· Transmitters include: acetylcholine, glutamate
How does a fast inhibitory synapse work?
Name two neurotransmitters which stimulate it
· Hyperpolarization
· Inhibitory post-synaptic potential (IPSP)
· Transmitters include: glycine, g-aminobutyric acid (GABA)
What are two mechanisms of slow synaptic transmission?
- Direct G-protein gating
2. Gating via intracellular messenger
How does direct G-protein gating work?
- Localized and quite rapid
* NT binding causes G-protein dissociation > binds to ion channel > ion channel opens
How does gating via intracellular messenger work?
- NT binding causes G-protein dissociation > binds to enzyme effector > which initiates a signaling cascade > intracellular messenger binds to ion channel > ion channel opens
What does digoxin do
It sabilizes the heart
