ICL 3.1: Excitable Cell Membrane

Question 1

what is the function of neurons?

Answer 1

they are specialized “excitable” cells that allow rapid communication throughout body

they receive information, transduce it into signals (electrical impulses called action potentials), and transmit those signals to distant sites in the body

Question 2

how do neurons transmit signals?

Answer 2

there are concentrations of ions that are different across a plasma membrane which creates a small electrical potential = the neuron is polarized and there’s a difference in charge across the membrane!

then rapid ion movement into or out of the cell creates an electrical signal

there are ion channels present that allow for the flow of ions across the neuron membrane since neurons are cells and have lipid bilayers just like every other cell

Question 3

how are ion channels selectively permeable?

Answer 3

channels are selective in allowing passage of charged molecules

internal structure determines the molecular “selectivity filter” of the channel

there’s a minimum degree of selectivity based on charge but usually they’re more selective than that –> like just because a channel lets in Na doesn’t mean it’ll also let K in

Question 4

what are the 3 types of gated ion channels?

Answer 4

1. mechanically-gated = stretch of the cell membrane required to open the ion channel
2. ligand-gated channel = binding of a compound/NT is required to open the ion channel
3. voltage-gated channel = change in the cell’s membrane potential required to open the ion channel

Question 5

what are the 3 states of an ion channel?

Answer 5

1. open

molecule structure of the ion channel has been changed to a different configuration which allows ions to flow

2. closed

molecule structure of the ion channel has been changed to a different configuration which prevents ions to flow = closed but capable of opening

3. inactive

the gates are open but channels are”plugged” which prevents ion movement –> so it’s open but incapable of opening

Question 6

how do voltage-gated ion channels work?

Answer 6

the ion channel responds to changes in membrane potential with conformational changes that lead to gating

these ion channels are seen the most in peripheral nerves and are responsible for the activation of muscles!

Question 7

what is a membrane potential?

Answer 7

the voltage difference between the cytosol of a cell and the extracellular medium

in most cases this membrane potential is negative- the inside of most cells is slightly negative compared to the outside

Question 8

what is the resting potential?

Answer 8

the membrane potential of a cell at rest

for a neuron, it’s 70 mV

Question 9

how does the Na/K pump work?

Answer 9

it continually transports 3 Na+ ions to the outside and 2 K+ ions to the inside of the cell

the inside of the cell will be more negative than the outside

this creates large concentration gradients for Na+ and K+ across the resting nerve membrane

Question 10

whats a potassium leak channel?

Answer 10

channels in the cell membrane that allow K+ to leak, even in a resting cell

the K leak channel is ALWAYS in an open confirmation

the reason that the K leaks out is because of the gradient that has been established with the Na/K pump that puts lots of K inside the cell

so the cell becomes more negative as K+ ions are moved to the outside down their concentration gradient

Question 11

what is the equilibrium potential of a membrane?

Answer 11

the concentration gradient for K+ moves the ion out of the cell since the Na/K pump pumps it out

however, the electrical gradient tends to move K+ into the cell to try and make the inside less negative

when no further net movement of K+ occurs because the electrical gradient exactly counterbalances the concentration gradient

Question 12

what is the Nernst potential?

Answer 12

the diffusion potential across a membrane that exactly opposes the net diffusion of a particular ion through the membrane

so it’s the voltage which would balance out the unequal concentration across the membrane for that ion

for example, a positive voltage (+55) inside the neuron would keep the high concentration of positive Na+ ions outside the cell = it’s the potential inside the membrane

***every ion has its own nernst potential!!

Question 13

what is the Nernst equation?

Answer 13

E = (-61/z)log[X]in/[X]out

z = electrical change of the ion

it calculates the Nernst potential for any ion at 37 C

Question 14

if the plasma membrane of a neuronal cell is permeable to only Cl- and the concentration of Cl- is ten times more inside the cell than the outside, what would be the equilibrium potential of Cl-?

Answer 14

+61

Question 15

if the extracellular K+ is 4 mM and intracellular K+ is 40 mM, and the cell membrane is permeable to K+, what will be the direction of the K+ movement

Answer 15

inside to out

ICF –> ECF

in such a case, the membrane potential will be closer to the equilibrium potential of K+

Question 16

if the plasma membrane is permeable to only Ca+2 and the concentration of Ca+2 ions is 10 times more outside the cell than the inside, what would be the equilibrium potential of Ca+2?

Answer 16

+30.5

Question 17

if the plasma membrane of a neuron is only permeable to Na+ and the concentration of Na+ is 14 mEq/L inside and 142 mEq/L outside the cell, what is the Nernst potential?

Answer 17

+61.4

Question 18

what is the goldman equation?

Answer 18

it calculates the diffusion potential when the membrane is permeable to several different ions

this is how we get the resting potential of a neuron is -70 mV because we look at the Nernst potential of Na, K and Cl

Na+, K+, and Cl- are most important in the development of membrane potentials in nerve fibers –> the contribution of each ion is proportional to its membrane permeability

Question 19

what does the resting membrane potential mainly depend on?

Answer 19

K+

K+ diffusion contributes far more to the membrane potential Na+ because K+ is highly permeable to the membrane compared to Na+

that’s why the resting membrane potential is much closer to the equilibrium potential of K+ (-90 mV) than it is to Na+ (+65 mV)

so this just reinforces that the more permeable the plasma membrane is to a given ion, the more that ion will contribute to the membrane potential

Question 20

what creates a nerve impulse?

Answer 20

changes in the membrane potential of a neuron give rise to nerve impulses

membrane potentials are changed by responses to stimuli such as temperature, light, or pressure because energy from the stimulus causes the ion channels to open

these sensory signals are converted to electrical signals via depolarization of sensory neuron membranes

Question 21

what is an action potential?

Answer 21

rapid changes in the membrane potential that spread rapidly along the nerve fiber membrane

it’s the brief depolarization seen in the inside of thenerve cell during the transmission ofnerveimpulses

an action potential begins with a sudden change of the resting (negative) membrane potential to a positive potential and ends with an almost equally rapid change back to the negative potential

Question 22

what are the 4 phases of an action potential?

Answer 22

1. threshold
2. depolarization
3. repolarizatoin
4. hyperpolarization

Question 23

what is the depolarization phase of an action potential?

Answer 23

when the inside of a cell becomes less negative than the resting potential

usually this is Na+ moving into the cell

Question 24

what is the repolarization phase of an action potential?

Answer 24

restoration of the negative resting potential following depolarization

Na+ channels close and the K+ channels remain open

Question 25

what is the hyperpolarization phase of an action potential?

Answer 25

when the inside of a cell becomes more negative than the resting potential

Question 26

what is the threshold of an action potential?

Answer 26

in a neuron, it is the level that a depolarization must reach for the generation of an action potential

so not every signal is transformed into an action potential

anactionpotential will not occur until the initial rise in membrane potential reaches the threshold potential (~ - 65 mv).

Question 27

in what order do the ion gates open during an action potential?

Answer 27

voltage-gated sodium channels open first but the same stimuli closes the inactivation gate only after fractions of a second later

the membrane potential starts to return towards the resting membrane state and the Na pump will only reopen when the membrane returns close to the original resting membrane potential

this protective mechanism is what prevents stuff like seizures!

voltage-gated K+ channels open later after the Na+ channels – they open around the same time that the Na+ channels start to close

when the K+ gates open, K+ diffuses outwards which depolarizes the cell back to -70 mV

Question 28

how do local anesthetics work?

Answer 28

they block the Na+ channels so that action potentials can’t fire

they only effect the local area because it can only diffuse across cell membranes in their un-ionized form

as soon as the anesthetic enters the cell it becomes ionized and i can no longer diffuse to other cells in the area

Question 29

summarize the events that happen during an action potential?

Answer 29

1. resting state:

K+ conductance&raquo_space;> Na+ conductance

2. onset of action potential:

Na+ channels open and within a millisecond it closes

then the voltage-gated K+ channels open

3. end of action potential

K+ channels close

Question 30

during resting membrane potential, voltage gated sodium chanels are…

Answer 30

closed

the only thing open during resting membrane potential are K leak channels

Question 31

rapid depolarization phase of an action potential is caused by….

Answer 31

an inward going Na+ current

Question 32

0.2 ms after the peak of an action potential, Na channels are _____ and K+ channels are ______

Answer 32

inactivated, open

Question 33

what is the absolute refractory period?

Answer 33

at the peak of the action potential, all Na+channels become inactivated

once inactivated they cannot be immediately opened until resting membrane potential is restored (refractory state)

this period during which additional depolarizing stimuli do not lead to new action potentials is referred to as theabsolute refractory period

so no matter how much stimulus you give, another action potential CANNOT be triggered during the absolute refractory period

Question 34

what is the relative refractory period?

Answer 34

after the absolute refractory period, Na+ channels begin to recover from inactivation

the cell is now hyper polarized so a stronger than normal stimulus is needed in order to elicit an action potential

during the relative refractory period, K+ conductance remains really high –> the continued K+flow out of the cell opposes the depolarization caused by opening of recovered Na+channels

Question 35

how fast are Ca+2 voltage gated channels?

Answer 35

they’re slow but they stay open longer!

so they’re slow but they result in more sustained depolarization

Ca+2 pumps transport Ca+2 ions outside the cell and then Ca+2 channels pump them back in

Question 36

what is tetany?

Answer 36

the concentration of Ca2+ in the ECF affects sodium channel activation

when there is a deficit of Ca2+ ions, the Na+ channels become activated (opened) by a small increase of the membrane potential

so when there’s decreased calcium it leads to muscle tetany since the Na+ channels are easily polarized –> normally, calcium makes sure that Na+ channels aren’t overly active

[Ca2+] needs to fall only 50% below normal before spontaneous discharge occurs in some peripheral nerves = muscle “tetany”

muscle tetany can be lethal like if there’s tetanic contractionof the respiratory muscles

Question 37

how do Na+ channels perform positive feedback?

Answer 37

when one Na+ channel opens, others in the are also open

once an event changes membrane potential from −90 mv toward zero, it causes many voltage-gated Na+ channels to open –> this causes rapid inflow of Na+ ions, further increasing the membrane potential

this opens more voltage-gated Na+ channels in the surrounding and more streaming of Na+ ions into the interior of the nerve fiber

this positive-feedback cycle continues until all the voltage-gated Na+ channels are activated (opened)

Question 38

how do you trigger a stronger response from an action potential?

Answer 38

an action potential doesn’t increase in magnitude, it increases in frequency!

aka back to back firing of action potentials is what gives you a larger response to a stimulus

the stronger the stimulus, the higher the frequency at which action potentials are generated –> the frequency of action potentials is directly related to the intensity of the stimulus

Question 39

how does the nervous system make sure uncontrolled action potential firing doesn’t happen?

Answer 39

1. refractory period
2. permeability of special K+ channels increases with high frequency firing (K channels will open up to balance the Na coming in)
3. astroglia removes excess K+ from ECF since ECF [K+] increases (removing excess K outside the cell ensures that more K can continue to be brough outside the cell and balance Na+ going into the cell)

Question 40

what is the all or nothing principle?

Answer 40

once anactionpotential has been elicited at any point on the membrane of a normal fiber, the depolarization process travels over the entire membrane OR it does not travel at all

Question 41

in what part of a neuron does an action potential start?

Answer 41

the axon hillock which is at the initial segment of the axon

current spreads from the dendrites and soma towards the axon hillock and Na+ builds up until there’s enough to reach the threshold and have an action potential

action potentials can be generated in the dendrites but the threshold is much higher

Question 42

how are action potentials propagated along an axon?

Answer 42

positive electrical charges are carried by the inward-diffusing Na+ ions to the adjacent resting membrane areas

the Na+ channels in the new areas immediately openand theactionpotential spreads –> the new depolarized areas causes more adjacent Na+ channels to open

this progressively leads to more and more depolarization so the depolarization process travels along the entire length of the fiber - nerveimpulse

Question 43

why does an action potential only move forward in one direction?

Answer 43

the reason the action potential doesn’t go backwards is because there’s a refractory period that prevents that! so even if the Na+ drifts backwards, that part of the axon can’t be restimulated

Question 44

what 3 factors does the speed of conduction of a signal depend on?

Answer 44

1. temperature

the higher the temperature, the faster the speed

2. axon diameter

the larger the diameter, the faster the speed

3. axon myelination

Question 45

what is saltatory conduction?

Answer 45

the action potential in myelinated axons jump large distances from node to node (1mm), a process called saltatory propagation

this increases the speed of propagation dramatically

Question 46

what happens during multiple sclerosis?

Answer 46

Multiple sclerosis is characterized by demyelination of the neurons in the CNS

Question 47

what is hypokalemia?

Answer 47

low [K+] in the blood –> low K in the ECF means there’s more inside the cell so then you need a really large stimulus to induce an action potential

this results in:

1. hyperpolarization of the cells
2. greater stimulus requirement for action potential
3. delayed ventricular depolarization and arrhythmias

Question 48

what is hyperkalemia?

Answer 48

high blood [K+] which means low [K} inside the cell

this results in:

1. depolarization of cells
2. inactivation of Na+ channels
3. cells become refractory leading to major arrhythmias

Question 49

what can electrolyte abnormalities cause?

Answer 49

major electrolytes abnormalities can lead to muscle spasms of skeletal muscles, dysrhythmias of cardiac muscles and seizure of CNS neurons

Question 50

how does myelination increase the speed on an action potential?

Answer 50

it decreases the resistance to ion flow due to the large diameter