Revision: Resting Membrane Potential and the Action Potential Flashcards
Define resting membrane potential.
The voltage difference across the cell membrane which exists in all cells.
Define cation.
Positively charged ions.
Define anion.
Negatively charged ions.
Define Diffusion.
The movement of molecules from a region of higher concentration to a region of lower concentration down the concentration gradient.
What is the resting membrane potential voltage?
The resting membrane potential voltage inside the cell usually sits around -70mV and can go down to -90mV
What are the four main things that contribute to the resting membrane?
- Na+/K+ ATPase pump (active transporter)
- Leaking K+ channels (passive transporter)
- Leaking Na+ channels (passive transporter)
- Permeability to the cell
How does the cell/neuron maintain the resting membrane potential?
The Na+/K+ ATPase pump helps to establish the concentration gradient. It actively pumps 3 x Na+ ions out of the cell in exchange for 2 x K+ ions. Ultimately resulting in more Na+ outside the cell, and more K+ inside the cell.
When K+ is inside the cell it is actually bound to anions such as proteins or phosphates.
At the same time the leaky K+ channels allows K+ free movement of K+ down the concentration gradient, so in this case K+ will move from inside the cell to outside the cell. The K+ channels are not permeable to anions, so it will leave the anions behind make the inside of the cell more negative.
At the same time leaky Na+ channels allow the free movement of Na+, in this case the Na+ will move from outside the cell to inside, however the Na+ channel is less permeable than the K+ pump so this process is alot slower.
These processes help to keep the RMP around -70mV to -90mV.
What is the graded potential? Describe the processes.
It is a temporary change in the membrane potential of a neuron that occurs between a presynaptic cell or neuron and postsynaptic cell or neuron. This process can be either excitatory (depolarising) or inhibitory (hyperpolarising).
Excitatory (depolarising process): the presynaptic neuron releases stimulatory ligands or neurotransmitters such as glutamate. This binds to ligand gated ions channels and opens these channels to allows the flow of cations such as Na+ or Ca+ into the cell. As the Na+ and Ca+ moves inside the cell it makes it more positive and shift the voltage from RMP = -70mV to meet threshold potential -55mV,
Inhibitory (hyperpolarising) process: the presynaptic neuron will release and inhibitory neurotransmitter such as GABA. This will bind to voltage gated ion channels which will open the gate and allow the flow of Cl- ion into the cells and for K+ ions to move out of the cells leaving behind anions. This makes the inside of the cell more negative -70mV»_space; -90mV
Describe how an action potential occurs.
Following the depolarising process of the graded potential the threshold voltage of -55mV is met in the trigger zone or the axon hillock this activates the activation gate on the voltage gated sodium channels to open, this allows Na+ ions to rush inside the cell making it more positive to reach +30mV. The positive ions then move down the axon and triggers the next voltage gated sodium channel to open. This process of depolarisation continues down the axon body towards the terminal bulb. At the terminal bulb there is a voltage gated calcium channel which is activated at +30mV which allows calcium to rush inside the cells. Ca++ then causes snare protein which are attached to the synapse vesicles and the cell membrane of the axon terminal and causes then to fuse and release the neurotransmitters into the synaptic space. These neurotransmitters than bind on the post-synaptic membrane to exert either an inhibitor or stimulatory effects.
As the AP moves down the axon body or cell, followed closely behind is the cell/axon repolarising with subsequent hyperpolarisation. .
Describe repolarisation.
At +30mv, this voltage:
- Inactivate the voltage gated sodium channels to stop the following of Na+ into the cell.
- Activates the voltage gated K+ channels which allows for the movement of K+ from inside the cell to outside the cell again leaving the anions behind to make the cell more negative.
This process will continue until the voltage becomes -90mv or hyperpolarized.
Once the voltage inside the cell reaches -90mV, at this voltage, it actually inhibits the voltage gated calcium channels. So calcium can no longer move into the cell, this inhibits the release of neurotransmitters.
Eventually with the help of the Na+/K+ ATPase pump, the leaking Na+ and K+ channel RMP will return back to -70mv.
