Nerve conduction Flashcards
(46 cards)
extracellular concentrations of all the ions
sodium 140mM
potassium 4mM
Calcium 2.4 mM
Chloride 100mM
Bicarbonate 25mM
Hydrogen 40nM
Intracellular concentrations
sodium 5-25 mM
potassium 140mM
Calcium 0.1 μM
Chloride - less than extracellular
Bicarbonate 10-20mM
Hydrogen 50-100nM
Why does intracellular chloride not have a value?
Varies significantly between tissue
Membrane potential definition
An electrical potential difference between the inside of the cell and its surroundings
Explain the concept of gross electrochemical neutrality
The distribution of a single ion across the membrane is imbalanced, however the charge disparity is countered by an unequal distribution of an ion of an opposite charge
Explain the movement of potassium at resting potential
- 4mM outside the cell, 140 nM inside so there is a net diffusion of potassium ions through the potassium leak channels in the cell membrane
- However, the loss of the positive anions creates a charge separation, meaning that the intracellular matrix is more negative.
- This negativity pulls back potassium ions down their electrical gradient
- eventually reaches an electrochemical equilibrium, at -90mv
Electrical potential definition
The potential at which an equilibrium occurs where ion efflux is balanced by influx. E.g Ek = -90mV
What occurs to sodium ions at rest?
Sodium ion channels are shut, preventing the electrochemical diffusion into the cell
Nernst equation
RT/zF x Ln X+O/X+I
Explain what each symbol means
R= Gas constant T= temperature in kelvin z= valency F= Faraday's constant X+O= ion concentration outside X+I = ion concentration inside
What is valency? + an example
The charge so Ca2+ valency is 2+ whereas Cl- is -1
What temperature does it take place at?
37.5 degrees- body temp
What does the Nernst Equation calculate?
The membrane potential
How did Goldman vary Nernst’s equation?
took into account that the electric field would vary if there was a loss of ions across membranes
Requirements of the Donnan equation
Presence of an impermeant ion on one Side of the membrane.
osmotic balance
sodium is non diffusible
How was the impermeability of sodium disproved?
Radioactive iostope sodium-24 was injected, and thus could see travel of sodium in the frog’s sartorial muscle fibre. Shown that there was a steady gain of sodium
What counteracts this gain of sodium?
Sodium potassium ATPase pump
Explain the sodium potassium pump functions
pumps 3 sodium ions out and 2 potassium ions in, thus acts an electrogenic antiporter.
Apart from sodium and potassium ions, what else effects the resting potential?
Chloride ions
How does the diffusion of chloride ions impact the resting potential?
Chemical gradient higher on the outside, so chloride wants to move in, making the membrane more positive. It’s electrical potential takes place at about -40mV, thus makes the resting potential less negative.
What is the resting potential?
-70mV
How is an action potential generated?
- Stimulus occurs and exceeds a particular threshold. Potassium channels shut
- Voltage gated sodium channels open, allowing sodium channels to rapidly flood into the axon, depolarising it
- eventually becomes +42 mV, and sodium channels become inactivated.
- Delayed rectifier voltage gated potassium channels open, allowing potassium ions to flow out of the axon, repolarising it.
- Becomes too negative- hyper polarised.
What is the refractory period?
A period of time in which an action potential cannot be generated?
Two types of refractory periods + explained
Absolute refractory- all the voltage gated sodium channels are inactivated, so no action potential can form
relative refractory period- occurs during hyper polarisation. Action potential could form however would require a very strong stimulus to reach threshold.
