Membrane Potentials and Action Potentials Flashcards
what is voltage?
generated by ions to produce a charge gradient = potential difference
what is current?
movement of ions due to potential difference
what is resistance?
barrier that prevents the movement of ions
which three things can cause ion channels to open/close?
- transmembrane voltage
- mechanical force
- binding of ligands
what is an electrochemical equilibrium?
when the concentration gradient exactly balances the electrical gradient ie. electric force= concentration force
what does the Nernst equation enable you to calculate?
electrochemical equilibrium potential
eg. potential at which the concentration of potassium moving out of the cell is exactly balanced by the electrical gradient that occurs as you move potassium out of the cell
what is the concentration of sodium extracellularly and intracellularly, respectively?
150mM and 10mM
what is the concentration of potassium extracellularly and intracellularly, respectively
5mM and 150mM
why is the resting potential -70mV and not -90mV?
some sodium can still enter the cell
what does the Goldman-Hodgkin_Katz (GHK) equation describe?
resting membrane potential
depolarisation meaning
membrane potential moves towards 0mV
repolarisation
membrane potential becomes more negative towards resting membrane potential
overshoot
membrane potential becomes more positive from 0
hyperpolarisation
when potential becomes more negative than resting potential
what happens to the charge as a small depolarisation propagates down an axon? What is this called?
charge leakage resulting in the depolarisation fading away - graded potentials
Where does a graded potential occur? What do they do in terms of action potentials? What are they in response to?
- synapses and sensory receptors
- initiate or prevent
- stimulus
where do action potentials occur?
exciteable cells - muscle, cardiac tissue, neurons (nerve impulse) and some endrocrine tissue
what does membrane potential depend on
movement of potassium through CHANNELS not pumps
what are the 5 phases of an action potential?
- resting potential
- depolarising stimulus
- upstroke
- repolarisation
- after - hyperpolarisation
how is resting potential maintained?
K+ flow out of cell, small amount of Na+ into cell. Permeability of K+ is greater than Na+. Eq potential of K+ = 90mV
what happens during a stimulus?
depolarisation, if it reaches a threshold value then an action potential is produced
at the threshold potential, what happens to the permeability of the membrane to sodium?
increases permeability
What happens during upstroke?
VGSC open quickly, sodium enters cell, VGPC open slowly, potassium leaves cell. Membrane potential moves towards Na+ equilibrium potential.
what happens during repolarisation?
Sodium channels inactivated (plug stops flow), Na+ entry stops. More K+ channels open and remain open as K+ moves down its electrochemical gradient. Membrane potential moves towards K+ equilibrium potential
what happens during the absolute refractory period?
nerve cannot be restimulated as sodium channels are inactivated, sodium cannot exit cell.
what happens during hyperpolarisation?
K+ channels still open and K+ continues to leave cell down electrochemical gradient. As membrane potential reaches closer to K+ equilibrium, potential some K+ channels close. Membrane potential returns to resting.
what is the name of the refractory period during hyperpolarisation?
relative refractory period
what happens during the relative refractory period?
inactivation gate of Na+ channels opens, so a stronger than normal stimulus is needed to stimulate an action potential
what happens to the decay in action potential depending on the diammeter / myelation of the neuron and why?
slower decay with a larger diammeter or small myelinated neuron because there is lower resistance
at what speed does an action potential flow in a small diammeter, non myelinated axon?
1m/s
at which speed does an action potential spread in a large diammeter, myelinated axon?
120m/s
what can decrease conduction velocity?
- reduced axon diammeter eg. regrowth after injury
- reduced myelination eg. multiple sclerosis and diptheria, cold, anoxia, compression and drugs (some anaesthetics)
what are the three main factors that influence the movement of ions accross a membrane?
- concentration of ion on both sides of the membrane
- charge on ion
- voltage accross membrane
Why is the Na+ equilibrium potential -ve eg. -70mV and the K+ equilibrium potential +ve eg. +40mV, when both are positive ions?
- More K+ inside, so tend to flow out
- More Na+ outside, so tend to flow in
- A potential of -70mV is needed to attract K+ and stop net outward flow
- A potential of +40mV is needed to repel Na+ from entering the cell
which ion is important for the upstroke and which is important for the falling portion of the action potential? In which directions do these ions move?
Na+ - in
K+ - out
respectively
which factors influence the speed and propagation of an action potential along an axon?
- myelination - linear relationship between conduction and myelin thickness
- axon diammeter - lower resistance, ions move faster
- Conduction velocity equivalent to square root of axon diammeter
Which equation is used to calculate real membrane potential?
Goldman-Hodgkin-Katz equation
