Neurophysiology Flashcards
What is a resting membrane potential?
- Resting membrane potential is an electrochemical gradient (ie intracellular ion concentration is different from extracellular ion concentration). Since these are different - it has the POTENTIAL to change
- Extracellular vs intracellular fluid ion concentration at rest
- Ions important for neurophysiology:
Cations = Na+, K+, Ca2+,
Anion = Cl
There are many negative proteins in the cell
The difference in electric charge b/n ICF and ECF gives a potential.
The many proteins (-) and Chloride ions (Cl-) inside the cell make the overall resting potential of the inside of the cell negative
The difference in electric charge b/n ICF and ECF means that some ions will easily move across the plasma membrane if their channels are open
How is Na+ and K+ regulated?
Ions cross the plasma membrane via proteins called ion channels At REST, K+ channels are open, K+ flows in AND out
* K+ions move into the cell due to electrical gradient and move out of the cell due to the concentration gradient..so why is there more K+ inside than out??
* There is a Sodium/Potassium (Na+/K+) pump in the cell membrane. This pump brings in 2 K+ and removes 3 Na+ ions. More K+ ions are then inside and more Na+ ions are outside of the cell.
* The pump requires energy (Adenosine Triphosphate - ATP) to work and gets the ion concentrations in the neuron ready for action potentials (ready to fire). ATP is required as sodium is being transported out of the cell and K+ is being transported into the cell both travelling against their concentration gradient
There are also Na+ Channels: these are closed in a resting neuron
On the axon Na+ channels are voltage-gated
They will only open with certain membrane potentials (eg -50 to +30 mV)
There are also voltage-gated K+ Channels: these are also closed in a resting neuron
Voltage-gated K+ channels only open when the cell becomes positive
How do we measure an action potential?
The intracellular and extracellular electrical potentials can be measured by recording electrodes and an oscilloscope
Action potentials produce a rapid reversal in potential
Where do action potentials begin?
Action Potentials begin at the Axon Hillock
Communication with dendrites from other neurons brings positive or negative ions into the cell
Enough positive charge at the hillock will fire the neuron
What do changes in charge in the axon cause?
Changes of charge in the axon alters how ion channels work
Less negative charge in axon opens sodium (Na+) channels
There is a threshold of excitation (potential) required to open these channels (-50mV)
Massive influx of positive charge (Na+) into the cell
Polarity of the membrane switches for milliseconds
Changes in charge occur in a wave along the axon
After the change in charge has occurred, the membrane returns to it’s original resting membrane potential - let’s take a closer look
What happens in a neuron at rest that receives positive charges?
At rest: K+channels open and at equilibrium, voltage-gated K+channels closed
Na+channels are closed,
Na+/K+pump is setting up potential
The axon hillock receives positive charge which reaches cell threshold (-50 mV)
The Na+ channels closest to the change in potential change their molecular conformation: they OPEN, massive influx of Na+ ions -
Depolarisation Some K+ ions leave as now more negative outside! Na+/K+ pump is closed
When the potential becomes positive the Na+ channels close: K+ GOES OUT
The less negative (depolarised) part of the cell continues to open sodium channels along the length of the axon: the first part of the axon is now in it’s refractory period: this part of the neuron is still too positive to fire:
Voltage-gated K+ ions channels open - big efflux of K+ ions
The middle section of the axon is now in it’s refractory period, the first part is recovering
The third part of the axon has now become depolarised. The first is hyperpolarised
The actual concentration of ions doesn’t change that much, a small movement of ions causes a large change in POLARITY: more positive inside and more negative outside
What are the 2 refractory periods?
Absolute refractory period: 1 ms after action potential
The cell is positive and cant fire
Relative refractory period: 2-4 ms after action potential
The neuron can not fire again until the resting potential is restored
The potential is below the resting potential, it could fire,it would just take more positive charges
How is the membrane potential restored?
Na+/K+pump and open K+channels restore the RMP to -70 mV
The membrane of the neuron is ready to fire again approx4 msafter the action potential has occurred in it’s section of the membrane
How do we make up for the slow speed of channels opening and closing?
Waiting for all of the channels to open and close takes time
There is also the potential for current to ‘leak’ with many channels in the membrane
Therefore the neuron is insulated with myelin
Myelination speeds up axon conduction
Myelin wraps the axon - with small gaps in between - the Nodes of Ranvier
The Nodes of Ranvier are where channels and pumps are concentrated along the axon
Myelination insulates the axon so charge speeds along between nodes - energy efficient
Myelin & Saltatory Conduction: Propagation of the Action Potential
The unmyelinated nodes are 1000 times smaller than the myelinated sections
Due to refractory periods the action potential can only proceed in one direction
