Membrane Potentials

What is the resting membrane potential?

• voltage difference across cell membrane, when the cell is at rest

• - 70 mV

What determines the resting membrane potential?

1.) Sodium + Potassium Pumps

• Pump 3 Na+ out, 2 K+ in

• Makes the inside of the cell slightly more negative compared to the outside

2.) The membrane is impermeable to negatively charged molecules

• Anions and negatively charged proteins remain in the cell

• Helping contribute to the negative nature inside

3.) Leaky Channels

• Because of the Na/K pumps, K+ will move out of these leaky channels at a relatively high amount

• There is a moderate amount of permeability for Cl- ions which leave the cell as well (Contributing to increased positive charge inside cell)

• Some channels allow sodium to to enter the cell, moving down it’s concentration gradient, however these channels have very LOW permeability compared to K+ channels, this contributes to some positive charge inside the cell

Together explain why the resting potential is around - 70mv.

What is a graded potential?

• A shift above or below a membrane’s resting potential resulting from a stimulus

• Unlike action potentials, which are one size, graded potentials vary in size and can be bigger or smaller depending on the strength of the signal

• So these are the initial, flexible responses, that could ultimately lead to an action potential if there is enough change in charge

What is the significance of the axon Hillock?

• Trigger zone

• Once you hit particular voltage, you can trigger an action potential by opening voltage gated sodium channels, concentrated at this region

Describe the process of the action potential.

1.) Stimulus reaches threshold (~ -55 mV)

2.) Activates voltage gated sodium channels, allowing an influx of sodium ions, making the inside of the cell VERY positive

3.) Once the inside of the cell reaches a certain amount of positivity, the sodium channel closes

4.) The action potential or depolarizing wave (Inside of cell becoming positive) moves down the axon terminal

5.) The increase in positive charge inside the cell activates the potassium pumps, which allows potassium to exit the cell

6.) As this happens, the voltage becomes more negative, this is re-polarization

7.) For a moment, the cell becomes hyperpolarized to around -90 mV, this is because the potassium gates close very slowly, making the cell slightly more negative inside

8.) The potassium gate slowly closes and thanks to the three factors affecting resting membrane potential we will return to -70mv

• Due to leaky sodium and chloride channels

What is the absolute refractory period? Can an action potential occur in this state?

• When the voltage-gated Na+ channels open, the neuron absolutely CANNOT respond to another stimulus

• Includes time from opening Na+ channels until resetting of channels

• Ensures that each AP is an all or nothing event

What is the relative refractory period? Can another action potential occur in this state?

• Follows absolute refractory period

• Most Na+ channels return to resting state, some K+ channels still open and re-polarization is occuring

• You can have another action potential but the threshold is very elevated since the inside is more negative than in the normal resting state

• Would need a very strong stimulus

Are action potentials different depending on stimulus intensity?

No, all action potentials are alike and independent of stimulus intensity

• But the CNS can tell the difference between a weak and strong stimulus:

• Strong stimuli cause action potentials to occur more frequently, so CNS determines stimulus intensity by the frequency of impulses (Or # per second)

• So higher frequency is a stronger stimulus

All cells have a ______ _____ across their cell _____, due to an _______ distribution of ___.

potential difference, membranes, unequal, charge

In general, what ions will you find in the highest concentrations inside and outside the cell membrane during the resting state?

• Inside = K+

• Outside = Na+

What are the four types of cell membrane ion channels?

How is each regulated

Are they all regulated?

Regulated: Open or Closed Depending on Environmental Condition

1.) Ligand Gated

• Need certain chemicals to bind

2.) Mechanically Gated

• Need physical stimulus

3.) Voltage Gate

• Open or close depending on potential difference

Un-Regulated

4.) Always Open - Leaky

• Allow ions to leak through membrane

What would happen if the cell membrane was only permeable to K+?

• Would move out of the cell, down their concentration gradient until the electrical force opposing ion movement equals the force created by the concentration gradient

• This would mean the MAXIMUM negative membrane is - 90 mV

What would happen if the cells were only permeable to Na+?

• Na+ would move into the cell

• This would mean the MAXIMUM positive membrane potential would be + 60 mV

What equation gives the equilibrium ion potential?

Nernst equation

What determines the resting potential of the cell?

• In a few words.

• The relative permeability of the membrane to ALL IONS

Changes in the permeability of the membrane to any ion results in a change in the..?

membrane potential of that cell

Graded potentials are suited to ____ information transfer. Action potentials are suited to sharing information over _____ distances.

short, long

What is decremental transfer?

Graded potentials decrease in strength as they travel along a neuron, meaning as the voltage spreads toward the cell body or axon it weakens, it will not trigger an action potential if it doesn’t reach a threshold

• Which is why it is for short signaling

In simple terms what is the threshold essentially?

movement of Na+ into the cell exceeds the movement of K+ out of the cell

What is stimulus size in each of the following:

• Action Potentials

• Graded Potentials

• Stimulus size related to action potential frequency

• Stimulus size directly related to the size of the response

Is there a maximum number of frequencies we can have?

Yes, it is limited by the absolute refractory period because this determines how quickly a neuron can recover, ready to fire again

How do australian funnel-web spiders paralyze their prey?

• Venom contains atroctoxins

• Cause opening of the sodium channels, thus excessive neural activity and paralysis

How do tetraodontidae puffer fish protect themselves by paralyzing predators?

• Skin and certain organ contains tetrodotoxins

• 2nd Most poisonous vertebrate to man

• Blocks opening of sodium channels, thus blocking all neural activity, leading to paralysis

How do local anaesthetics act?

• By blocking voltage gated Na+ channels

• Or can access inside of cells via different channels