lecture 4- excitability 2 Flashcards
list 3 main functions of action potential
1- amplifies incoming stimuli (incoming stimuli can be weak, need to be amplified)
2- transmits information rapidly across long distances in the body (nerve impulses traveling along nervous system)
3- initiates & regulates effector responses (usually a muscle contraction)
define action potential
the discharge of potential energy on the membrane
remember conductance of membrane comes from…
and capacitance comes from…
conductance comes from active transport of sodium pump
capacitance comes from differential permeability (depletes inside of positive charge)
mechanism of action potential is also based on the ___ and ___ of sodium and potassium ion channels
structure and function
what determines whether sodium and potassium channels are open or closed?
voltage/charge on membrane
describe the mechanics behind the sodium channels activation/deactivation
the channels have a voltage sensor, when voltage changes- activate gate opens
- an incoming stimulus pushes voltage sensors upward and causes activation gate to swing open- allows sodium to diffuse through the channel down its concentration gradient
- as soon as that happens, change in shape of protein begins to automatically swing inactivation gate closed, so the ball&chain channel closes
- after some time, it will reset and be ready to respond to another stimulus
for the sodium channel, movement of electrical charge is called the ____
accumulation of what kind of charge and where?
__polarizes the membrane
sodium current
positive charge on inside of membrane- makes inside less negative
depolarizes
describe the mechanics behind the potassium channels activation/deactivation
slightly different structure than sodium, but same principle
- voltage sensor is more like a paddle- when voltage changes, paddle flips up and opens the channel, allowing K+ to diffuse out
- when the paddle flips up like this, it pushes against lipids, this hydrophobic interaction closes the gate again
for the potassium channel, movement of electrical charge is called the ___
accumulation of what kind of charge and where?
__polarizes the membrane
potassium current
outwardly directed, takes positive charge away from inside of membrane, makes it more negative
hyperpolarizes
sodium __polarizes the membrane
depolarizes
potassium __polarizes the membrane
hyperpolarizes
what organism has been used to study action potentials?
the giant axons of squid
when studying action potentials, can manipulate charge on membrane by turning on stimulator
- describe what happens when you give a -10 mV electrical shock
start off at -70 mV (resting) –> when you give a nerve a stimulus, giving it an electric chock
- giving a -10 shock causes membrane to go from -70 to -80 –> becomes more polarized (hyperpolarized) –> opens potassium current, K+ leaves, membrane becomes more negative (when u turn stimulator off, goes back to -70)
- this is a passive local response (passive b/c membrane potential changes in proportion to stimulus, local b/cit is transient- only lasts as long as stimulator is turned on)
when studying action potentials, can manipulate charge on membrane by turning on stimulator
- describe what happens when you give a -20 mV electric shock
goes from -70 to -90
- passive local response, larger stimulus opens more potassium channels, causing more K+ to leave membrane, leaving it more negative (hyperpolarized)
when studying action potentials, can manipulate charge on membrane by turning on stimulator
- describe what happens when you give a +10 mV electrical shock
goes from -70 to -60 –> membrane becomes more positive, depolarized –> activates sodium channels, increases inward sodium current
- passive local response
what is the threshold stimulus required on the membrane to cause an action potential
+15 mV (-70 –> -55) –> action potential
an action potential is generated at __ voltage, which holds this value
threshold voltage
+15 mV
threshold is the minimum ___ needed to elicit a ___
strength stimulus
physiological response
what happens with the sodium channels once the inside of membrane reaches threshold?
once inside of membrane reaches threshold, the + charge inside is enough to open the remaining sodium channels spontaneously (b/c channels are voltage-gated)
- stimulus itself has to reach certain voltage, -55 mV, but then becomes spontaneous
the action potential takes what shape and what does it represent?
AP takes the shape of a very steep wave of depolarization, this wave represents the release of all the stored energy of the membrane
what is the Hodgkin’s cycle
in an AP, the more the sodium channels open, the more inward Na+ current and the faster it diffuses through faster –> the more sodium channels then open again, more depolarized –> positive feedback
- each event feeds the other and the process becomes faster and faster as it continues, this is why wave of depolarization is so steep and fast
- the positive feedback system is called Hodgkin’s cycle
the positive feedback system that opens sodium channels faster and faster during an AP depolarization is called ___
Hodgkin’s cycle
what happens when the action potential reaches a voltage of +30?
this is the point of maximum overshoot, wave of depolarization stops –> change in voltage results in sodium channels closing b/c inactivation gates have all swung shut –> activates voltage-gated potassium channels
- K+ channels open, cause huge outward potassium current that takes the positive charge away from the inside of membrane, results in repolarization
describe the wave of repolarization and at what voltage it stops
wave of repolarization very steep and fast, goes all the way to -90 mV
-90 is called the after potential (at -90, K+ channels close and normal sodium pump and differential permeability restores resting potential)
what is the voltage of maximum overshoot where depolarization in AP stops
+30
what is the voltage of after potential where repolarization stops
-90
WHY does wave of repolarization stop at -90 mV?
outward current of potassium stops at -90 mV because it is the potassium equilibrium potential (charge on the membrane that is equal and opposite in strength to K+)
WHY does wave of depolarization stop at +30?
theoretically, should stop at +60 b/c that is sodium’s equilibrium potential…but it doesn’t –> this is b/c of the unique nature of the sodium channel itself
- change in voltage opens activation gate and lets sodium diffuse through and then inactivation gate swings closed & channel has to reset
- as other Na+ channels begin to open, the other trailing ones behind it begin to close and the closing of these early channels weakens the strength of the depolarization, this makes it stop short of the +60 mV
- wave of depolarization still tells us the basis of AP- need +15 from stimulus, then electrical discharge takes itself from -55 to +30 (put 15 mV into the system, get 85 mV out), magnitude of AP amplifies the charge by 6x
in an action potential, is the whole cell filling up with sodium/positive charge?
NO, remember all these events are only taking place at a fairly tight spot on surface of the membrane, rest of cell remains neutral
how much does an action potential change the sodium gradient?
AP changes Na+ gradient by less than 1/100,000
- a nerve cell can generate a thousand action potentials per second (AP happens in 1 ms)
does a nerve ever fatigue?
no, there is an inexhaustible supply of energy in electrical gradient (a thousand action potentials a second only uses .1% of energy)
what does it mean that the threshold is an all-or-none response
says that a stimulus will either cause an action potential or it will do nothing
- also says the membrane will release either all of its stored energy or it will release none
- so, all action potentials are the same size (since membrane releases all of its stored energy)
firing of the gun analogy, flush the toilet
what is the refractory period
period of reduced excitability after the start of an action potential (less responsive to stimulus)
what are the 2 phases of a refractory period
1- absolute refractory period
2- relative refractory period
describe absolute refractory period
the most important phase of the refractory period
- corresponds to duration of AP itself, excitability drops to zero, completely unresponsive
- during AP, membrane is completely unresponsive to any other stimulus (AP is a piece of coded info in an animal’s nervous system- absolute refractory period preserves the integrity of this info)
what is the relative refractory period
membrane is resetting, partially responsive to another stimulus
what can be manipulated in a lab during a relative refractory period
possible during relative refractory period that if you give a stimulus greater than threshold, can create an AP that is smaller than normal magnitude
- membrane just doesn’t have 100% of its stored energy restored, so if 50% restored, AP will be 50% of its normal size
- almost never see relative refractory period in living systems (manipulated in lab)
- in order to access 100% energy of membrane, need 100% of the sodium channels ready
refractory period is really due to what being refractory?
sodium channels- open quickly through voltage changes, deactivation opens through a time-basis, takes a few seconds then to reset its conformation
- so when talking about restoring energy, really talking about restoring 100% of the sodium channels back online so they can reopen again
- refractory period is a function of the refractory of the sodium channels itself
accommodation is the ability of membrane to…
respond differently to the way a stimulus is delivered
- a membrane can tell how a stimulus is being delivered, if you change the way its delivered, you change the way the membrane responds
when a nerve or muscle cell is depolarized slowly or is held at a depolarized level, what happens?
the usual threshold potential may pass without an action potential having been fired, called accommodation
concerning accommodation, what are the 2 delivery systems for a threshold stimulus?
1- square-wave stimulus: give all 15 mV at once
2- slowly increase the 15 mV, in this slowly increasing mode, when you hit threshold, nothing happens (this is accommodation)
why does accommodation happen?
because of the nature of sodium channels
- for square-wave stimulus, 15 mV opens all sodium channels at once, enough to cause AP
- for slowly delivered stimulus, the Na+ channels that were opened initially, begin to shut down b/c of deactivation gate…when finally get to threshold, do not have enough sodium channels to cause AP
give an example of accommodation in the body
hyperkalemia- having too much potassium in your blood, K+ offsets the depolarization and allows initial Na+ channels time to close down before it reaches threshold
what is adaptation?
describe the situation in a stimulator
the reduction of action potential frequency/discharge in response to a continuous low-level stimulus
- set simulator to threshold, leave it on and give continuous low-level stimulus –> at first, membrane gives high frequency of AP’s, as soon as it regenerates, gives another AP –> after a while, the frequency of AP begins to decrease and stop entirely
why does adaptation happen?
with an AP, sodium channels open –> deactivate –> reset (changing shape constantly)
- after a while, becomes less flexible/responsive b/c each time sodium channel changes shape, it loses some potential energy and the energy has to be restored before it can fully respond again
adaptation is like blocking out ___ and allows ___
blacking out background noise (unimportant stimuli) and allows important stimuli to come in with more clarity
