M&R - Resting Potential Flashcards
What do changes in the membrane potential difference form?
Basis of cell signalling
What are the membrane potentials of:
Animal cells
Cardiac and skeletal muscle
Nerve Cells
What is the average membrane potential?
Animal cells = -20 to -90mV
Cardiac and skeletal muscle = -80 to -90mV
Nerve Cells = 50 to -75mV
Average membrane potential = -70mV
What ion are cells most permeable to at rest?
Why?
K+
Due to facilitated diffusion through K+ channel proteins
In which direction does K+ move in a resting cell?
Why does this cause a negative potential intracellularly?
Outwards down concentration gradient
160/4 mM/l
Large protein anions are unable to follow k+ out of the cell, a negative potential develops on intracellular face of plasma membrane
What happens at equilibrium?
No net movement of ions
Diffusional and electrical forces are balanced
Why does the resting membrane potential never reach potassium equilibrium constant (Ek)?
Plasma membrane is not totally impermeable to other ions and the passage of these ions through selective ion channels contributes to the overall membrane potential.
Note - very few ions need to move across plasma membrane to establish a membrane potential
What proteins are involved in setting the membrane potential?
Voltage insensitive K+ channels - which remain open despite changes across membrane are predominantly responsibile for K+ movement that establishes resting potential
Na+ K+ ATPase - provides outward ionic gradient for K+ (3Na+ out
2K+ in)
Contributes little to resting membrane potential (approx -5mV)
What causes a change in the membrane potential?
The permeability of the plasma membrane to a particular ion
What is used to measure a membrane potential and in what units is it measured?
Microelectrode - fine glass pipette
mV millivolts
What equation is used to calculate the equilibrium potential?
Nernest Equation
What would happen to the membrane potential if K+ and Cl- channels open?
If Na+ Ca2+ channels open?
Inside becomes more negative = hyperpolarisation
Inside becomes less negative = depolarisation
What are the two main mechanism of channel gating?
Ligand gating - binding of ligand to a receptor site on the channel results in channel opening or closing
Voltage gating - channel open or closes in response to changes in membrane potential
What happens in hyperkalaemia?
Extracellular K+ is raised K+ efflux from cell is reduced Less negative membrane potential Membrane potential becomes closer to threshold for Na+ channel opening Membrane becomes more excitable acutely
What does the membrane potential provide?
Basis of signalling in the nervous system as well as many other types of cells
How does the cell membrane infer selective permeability?
What properties do ion channels have that make them selective?
Ion channels - proteins that enable ions to cross cell membrane. They have an aqueous pore through which ions flow by diffusion
Selectivity - for one/few ion species
Gating - pore can open/close by a conformational change in protein
Rapid ion flow - always down the concentration gradient
How is the resting potential set?
Ion selectivity of channels and the types of channel that are open makes the whole cell membrane selectively permeable to ions.
Describe depolarisation and hyperpolarisation
Depolarisation - decrease in the size of the membrane potential from its normal value
Cell interior less negative
E.g. -70mV to -50mV
Hyperpolarisation
Increase in size of membrane potential from its normal value
Cell interior becomes more negative e
E.g. Change from -70mV to -90mV
What effect will increasing the membrane permeability to a particular ion have on the equilibrium potential?
Move the membrane potential towards the equilibrium potential for that ion.
What are changes in membrane potential caused by?
Changes in the activity of ion channels
What does membrane potential depend on?
Depends on the relative numbers of of channel open for each ion
Name three types of gating and describe
Ligand gating - channel open or closes in response to binding of a chemical ligand e.g. Channels that respond to intracellular messages
Voltage gating - channel open or closes in response to a change in membrane potential e.g. Channels involved in action potentials
Mechanical gating - channel open or closes in response to membrane deformation e.g. Stretch receptors sensitive to pull on membrane
Describe fast synaptic transmission
Receptor protein is also an ion channel.
Ligand gated channel that opens directly when ligand binds
Binding and channel opening linked directly = fast synapse
What is excitatory synapse?
What are they permeable to?
What is the resulting change in membrane potential called?
Name two features of an excitatory synapse?
Name two transmitters of excitatory synapse?
Excitatory transmitters open ligand gated channels that cause membrane depolarisation
Permeable to Na+ and Ca2+
Excitatory post-synaptic potential
Longer time course than action potential
Graded with amount of transmitter
Acetylcholine, Glutamate
What is an inhibitory synapse?
What are they permeable to?
What is the resulting change in membrane potential called?
Name two features of an inhibitory synapse?
Name two transmitters of inhibitory synapse?
Inhibitory synapse open ligand gated channels that cause hyperpolarisation
Permeable to K+ and Cl-
Inhibitory post synaptic potential
Makes it harder to bring the cell to a point of fire as it has taken it more negative to resting potential
Glycine and GABA
What is slow synaptic transmission?
Name two slow synaptic transmissions
Receptor and channel are separate proteins
Direct G protein gating
Gating via intracellular messenger
Describe G protein gating
Describe gating via an intracellular message
GTP binding protein
Ligand binds to receptor
G protein released from receptor and goes into the plane of membrane to find its effector
Ligand binds to receptor
G protein released, but instead of binding to ion channel it binds to enzyme
Enzyme converts inert substrate into signalling molecule
May start signalling cascade
Second messenger may bind to channel directly or may phosphorylate channel
What are the two other factors that can influence membrane potential
Changes in ion concentration
Most important is extracellular K+ concentration, sometimes altered in clinical situations
Electrogenic pumps
Na+K+ATPase - not primarily responsible but will contribute. One +ve charge moved out for each cycle (3Na+ out, 2K+ in)
What is a membrane potential?
How is it expressed?
An electrical potential different across a plasma membrane
Voltage inside relative to that of the outside
Transmembrane movement of which ion predominates in establishment of the resting membrane potential?
And in which direction do the ions move?
K+
Outwards
Which protein is most important in allowing ionic movement to establish resting membrane potential?
Voltage insensitive K+ channels
Give two reasons why the equilibrium potential for K+ -95mV but the resting potential is closer to -70mV?
Other ions are able to move through the membrane
Leak of ions with positive equilibrium potentials (Na+, Ca2+) across the membrane drives the membrane potential higher than the equilibrium potential
What is the consequence for the membrane potential of an increase in membrane permeability to Na+ ions?
Depolarisation - more Na+ move into the cell with a positive charge. Decreasing membrane potential
Increased transmembrane flux of which two ions may result in membrane hyperpolarisation?
K+
Cl-
Increased transmembrane flux of which two ions may result in membrane depolarisation?
Na+
Ca2+
What would happen to the resting potential in hyperkalaemia?
Does it move closer/further away from firing an action potential?
Become move positive
Closer to the threshold for firing an action potential
Which three ions is the nicotinic acetylcholine receptor ion channel permeable to?
Na+
K+
Ca2+
Which ion predominantly flows through then open nicotinic acetylcholine receptor ion channel?
Na+
What ion is the glycine receptor permeable to?
What effect on membrane potential does activation of the glycine receptor result in?
What effect does this have on electrical excitability?
Cl-
Membrane hyperpolarisation
Decreases membrane excitability
Which two ions open channels for excitatory postsynaptic potentials?
Na+
Ca2+
Which two ions open channels for inhibitory postsynaptic potentials?
K+
Cl-
