ICL 3.1: Excitable Cell Membrane Flashcards
what is the function of neurons?
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
how do neurons transmit signals?
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
how are ion channels selectively permeable?
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
what are the 3 types of gated ion channels?
- mechanically-gated = stretch of the cell membrane required to open the ion channel
- ligand-gated channel = binding of a compound/NT is required to open the ion channel
- voltage-gated channel = change in the cell’s membrane potential required to open the ion channel
what are the 3 states of an ion channel?
- open
molecule structure of the ion channel has been changed to a different configuration which allows ions to flow
- closed
molecule structure of the ion channel has been changed to a different configuration which prevents ions to flow = closed but capable of opening
- inactive
the gates are open but channels are”plugged” which prevents ion movement –> so it’s open but incapable of opening
how do voltage-gated ion channels work?
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!
what is a membrane potential?
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
what is the resting potential?
the membrane potential of a cell at rest
for a neuron, it’s 70 mV
how does the Na/K pump work?
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
whats a potassium leak channel?
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
what is the equilibrium potential of a membrane?
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
what is the Nernst potential?
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!!
what is the Nernst equation?
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
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-?
+61
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
inside to out
ICF –> ECF
in such a case, the membrane potential will be closer to the equilibrium potential of K+
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?
+30.5
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?
+61.4
what is the goldman equation?
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
what does the resting membrane potential mainly depend on?
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
what creates a nerve impulse?
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
what is an action potential?
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
what are the 4 phases of an action potential?
- threshold
- depolarization
- repolarizatoin
- hyperpolarization
what is the depolarization phase of an action potential?
when the inside of a cell becomes less negative than the resting potential
usually this is Na+ moving into the cell
what is the repolarization phase of an action potential?
restoration of the negative resting potential following depolarization
Na+ channels close and the K+ channels remain open
what is the hyperpolarization phase of an action potential?
when the inside of a cell becomes more negative than the resting potential
what is the threshold of an action potential?
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).
in what order do the ion gates open during an action potential?
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
how do local anesthetics work?
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
summarize the events that happen during an action potential?
- resting state:
K+ conductance»_space;> Na+ conductance
- onset of action potential:
Na+ channels open and within a millisecond it closes
then the voltage-gated K+ channels open
- end of action potential
K+ channels close
during resting membrane potential, voltage gated sodium chanels are…
closed
the only thing open during resting membrane potential are K leak channels
rapid depolarization phase of an action potential is caused by….
an inward going Na+ current
0.2 ms after the peak of an action potential, Na channels are _____ and K+ channels are ______
inactivated, open
what is the absolute refractory period?
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
what is the relative refractory period?
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
how fast are Ca+2 voltage gated channels?
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
what is tetany?
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
how do Na+ channels perform positive feedback?
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)
how do you trigger a stronger response from an action potential?
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
how does the nervous system make sure uncontrolled action potential firing doesn’t happen?
- refractory period
- permeability of special K+ channels increases with high frequency firing (K channels will open up to balance the Na coming in)
- 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)
what is the all or nothing principle?
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
in what part of a neuron does an action potential start?
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
how are action potentials propagated along an axon?
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
why does an action potential only move forward in one direction?
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
what 3 factors does the speed of conduction of a signal depend on?
- temperature
the higher the temperature, the faster the speed
- axon diameter
the larger the diameter, the faster the speed
- axon myelination
what is saltatory conduction?
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
what happens during multiple sclerosis?
Multiple sclerosis is characterized by demyelination of the neurons in the CNS
what is hypokalemia?
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:
- hyperpolarization of the cells
- greater stimulus requirement for action potential
- delayed ventricular depolarization and arrhythmias
what is hyperkalemia?
high blood [K+] which means low [K} inside the cell
this results in:
- depolarization of cells
- inactivation of Na+ channels
- cells become refractory leading to major arrhythmias
what can electrolyte abnormalities cause?
major electrolytes abnormalities can lead to muscle spasms of skeletal muscles, dysrhythmias of cardiac muscles and seizure of CNS neurons
how does myelination increase the speed on an action potential?
it decreases the resistance to ion flow due to the large diameter
