Lecture #2 - Action Potentials, Synaptic Transmission and EC Coupling

Question 1

Use of Ion Chanels

Answer 1

1. Signlaing Devices
2. Move ions across epithelia
3. Move ions in chemiostatsic circuts
4. Maintain Ion homeostasis

There are more ion channels in multi cellular organisms because they are important in communication across long distances

Question 2

Ion Chanels (Overall) - Why ion channels act as signaling devices

Answer 2

Ion channels open/close very fast (miliseconds)

Ion chanel opens –> chanel conducts millions of ions down electrochemical gradient –> ions moving causes a rapid and large change in membrane potential (change in charge difference) across membrane and change in ion concetration–> Change in membrane potential/ion concetration activates protein –> protein triggers downstream events
- Conductance = 10^6 ions per second (moves millions of ions/second)

NOTE - Voltage difference/charge difference/membrane potential = ALL same thing

Question 3

Downstream events that are caused by changes in membrane potential/ion flow (change in membrane potential can trigger)

Answer 3

1. Action Potential
2. Muscle contraction
3. Secretion

Ion chancels can signal for a cell to have an action potential or contract muscle or release vesicle

Question 4

Types of signaling events mediated by ion Channels

Answer 4

Ion Chanels can allow for:
1. Chemical signals to be converted to electrical signals (Example – Nuerotransmission at a synpase
2. Electrical signals to stimulate more electrical signals (Example – Action potentials moving a nerve)
- Propagation = electrical signals give rise to more electrical signals
3. Electrical signals to be converted to chemical signals (Example – Electrical signal at the end of nerve/ nerve going to muscle gives rise to chemical signals to causing excitation-contraction and excitation secretion coupling)
4. Spontaneous electrical signals (Example – Pacemaker activity in heart activating heart to beat using ion channels)

Question 5

Techniques used to measure ion channel activity

Answer 5

1. Intracelular microelectrode
2. Voltage Clamps
3. Patch Clamp

Question 6

Intracellular Microelectrode

Answer 6

Use - measures voltage across the membrane (measures membrane potential)

Micro-electrode uses a glass capillary pipette (pipette opening is very small ; pipette is filled with solution of conducting AgCl)

Process – insert the capillary pipette into a cell
- Property of the glass and the membrane being hydrophobic causes the glass pipette to form a tight seal (has high resistence) at the edges when it is inserted into the cell
- Because of the seal –> any movement of charge is through the electrode into the cell

Question 7

Measuring membrane potential using a Intracellular electrode

Answer 7

Experiment – Have bath that contains a excitable cell –> Intracellular electrode is inserted into the cell –> electrode measures the charge difference between the inside of the cell and the bath that the cell is in
- Need a reference electrode sitting n the bath to use the charge of the bath as a reference

Start the Volatge (V) is 0 –> electrode enters the cell and volatge decreases to -60mV - -90mv (cell is polarized) –> remove the electrode and the volatge goes back to 0
- Start V=0 –> because both the electrodes are in the bath
- Volatge of -60mV –> means the inside of the cell is more negative (Negative membrane potential)

Question 8

Polarized Vs. Depolarized Vs. Hyperpolarized

Answer 8

Polarized – inside if the cell is more negative compared to the outside (negative membrane potential) (Ex. -60mV inside cell)

Depolarized – Inside of the cell becomes less negative (Ex. Membrane potential goes from –60mV to 0mV)

HyperPolarized – Inside of cell/membrane potential becomes more negative

REMINDER - negative inside cell = negative membrane potential

Question 9

What does it mean for a cell to be excitable

Answer 9

ALL cells have membrane potential across the membrane BUT NOT all cells are excitable (nerve and muscle = excitable)

Excitable means the cell is able to become depolarized AND can fire an action potential (Non-excitable cell does not fire AP no matter how depolarized it gets

When cells are depilarized a little the pumps and transporters bring the cell back to resting potential BUT if depolarization crosses a certain threshold in excitable cells then cell fires an AP)

Question 10

Voltage during Action potential

Answer 10

Cell fires an action potential –> the membrane potential goes from negative to 0 (depolarizes)–> membrane potential cross beyond 0 (becomes positive ; ‘overhoot’) –> membrane potential will return to a negative value (membrane is polarized again) and membrane potential goes below the resting potential (hyperpolarized) –> drifts back to resting potential

Question 11

Hogdkin and Huxley

Answer 11

Insert a micro-electrode into the squid axon –> measured the measuring membrane potential) –> Saw a huge and quick waveform

NOW we know this wave form was the action potential

Question 12

Current

Answer 12

Current = rate of charge movement over time

Question 13

Characteristics of Action potential

Answer 13

1. Steroyticpical size and shape —> AP always looks the same (Same size/shape) for any axon/exitable cell even if the same cell fires many times
2. When an AP is fired the mmebrane enters a refractory period –> Once AP fires/membrane is in refractory period membrane can’t fire a second action potential (can’t get 2 additive APs to get larger AP)
   
   • Means action potential is always a defined thing
3. If a cell is depolarized there is an all or none response (fire AP or nothing)
4. Propagation –> AP move down the length of the axon
   
   • IF you depolarize cell at one point in the axon –> THEN measure the membrane potential farther down the axon the AP propegates (AP fires in one place then fires nearby)
     - AP waveform is unchanged during propgation (AP does not fizzle out as it goes down the axon)

Question 14

Issue with looking at membrane potnetial/mechanism during action potential

Answer 14

Issue - Hard to find what is happening because at every point on the Action potential wave form there are many things happening/changing (many ions moving in and out/many currents)

Solution – Clamp/fix the voltage at any given point (study the cell when it is at a specific voltage)
- Example - Fix/clamp the voltage at resting potentials and see what the currents across the membrane –> THEN change and clamp at new voltage and see the currents going in/out of cell at the new voltage

Chart (Time Vs. Membrane potential) ; Yellow line = Action potential wave form ; Purple – current of Na Chanels ; Red = curent of K chanels

Question 15

Voltage Clamp

Answer 15

Overall – Keep the membrane potential constant (clamped ; fix membrane at a voltage) and measure the the currents flowing across the membrane at a given membrane potential

Process – As a current goes across the membrane and the voltage changes a second electrode connected to the feedback amplifier (FBA) injects or removes charges to compensate for the change in voltage due to ion channels and keep a constant voltage in the cell (keep constant membrane potential)

Question 16

Voltage clamp of squid Axon Experiment - Results

Answer 16

Top chart – start at -65mV then clamp at -9mV to give a 56mV depolarization (enough to get AP)
- Square trace = indicates the membrane potential

Chart – measuring current (Im = mA/cm)
- Line A (Total Current ; Have Na gradient) –> + charge (Na+) goes into cell (Negative Current ; Na gradient is the driving force) ; AT end have a positive curent
- Line B - No Na gradient –> no driving force –> No Na Current ; STILL have positive current (has to be caused by something other than Na)

Question 17

Voltage clamp of squid Axon Experiment - Results Line A - Line B

Answer 17

Line C - Line A – Line B –> shows the curent caused by the Na Ions
- Negtaive current at the start of AP = Na going into the cell BUT then Na stops (because Line C Na current goes to 0)

NOTE - know the time of current in the AP based on top trace

Question 18

Na and K current

Answer 18

Na is causing the negative current at the start of line A (based on line C) and K causes the K current at the end of line A/B ( based on line B)
- Positive current in line A/B is K leaving cell (know K because Na current never goes to positive in Line C –> + value in lineA/B has to come from a different ion)

Line B - shows the K chanels slowly activate at the start –> THE K channels stay open for as long as the membrane is depolarized (stay open until the membrane potential goes back to –65mV in the AP trace at top)

Question 19

Positive and negative current

Answer 19

Positive going into the cell = negative current

Positive charge going out of cell = positive current

Question 20

Patch Clamp

Answer 20

Use - Isolate and describe properties of one channel (look at 1 channel)

Process – apply the micro electrode into a patch of the cell and apply gentle suction –> Pull away so the patch of membrane tears off –> membrane will stick tightly to the electrode and make a seal around the microelectrode pipette because the membrane is hydrophobic
- End – isolate a small patch of the membrane where the is only 1 or a few chanels –> NOW can put patch in bath –> measure the current across one/few ion chanels in the patch of membrane

There are many configurations of patch clamp - Whole cell gives Outside-out or On-Cell gives In-side out

Question 21

Patch recoridng of an ion channel

Answer 21

Start – Ionic current = 0 –> current becomes negative (dips) –> current stays at 0 for a while –> channels open and the current becomes negative
- When the current becomes negative –> channel opens and closes Vs. current stays at 0 the chanels are closed

Patch recording shows the ion channel has stochastic properties

Question 22

Stochastic properties of proteins

Answer 22

Single ion chanel has stochastic properties –> when looking at a single chanel don’t know when it will open/close

SAME applies for ALL enzymes (are stochastic)
- If watch a single enzyme you can’t know at which point the bond would break or product is release because it is random)

Stochastic properties of ion chanels:
1. Gating is independent and is described by a proability function
2. An ion chanel has only 2 states (ion channels is either open or closed)
- Ion chanel opens and ions move through –> then the chanel closes and the current (charges moving) stop
- Open Vs. Closed states = because the protein is moving from one state to another
3. Conductance is finate

Question 23

Stochastic properties of ion chanels - Gating is independent and is described by a proability function

Answer 23

Opening and closing (gating) of a channel is a independent property and is based on a probability function
- Don’t know when the channel opens/closes just know the probabiility that a chanel open (proability stays the same)
- Chanel open/close once is independent of the same chanel open/closing again AND each of those events there is an equal probaility that the chanel open/closes
- Chanel can bind to a ligand/be phosphorylated to increase the probabiliy of opening

Question 24

Stochastic properties of ion chanels - Conductance is finate

Answer 24

Conductance (dip in chart) is a property of the chanel
- Each chanel has its own conduacted (never changes for the chanel)

Conductance is finite –> goes to the same value of negative current for each dip

Low conductance = smaller dip (less change in membrane potential)
Big conductance = bigger dip (Bigger opening = more ions go through = bigger change in current = bigger dip)

Question 25

Properties of Ion chanels

Answer 25

1. Ion Selectivity

Question 26

What allows for selectivility (How are ion chanels selective; why only let K and Not Cl))

Answer 26

1. Charges lining the Pore - Example – K chanel has acidic residences that attracts the Positive K+ ion in and helps the K+ go through and repel negative ions 2. Size of Pore - Example pore is built so only small ions can go through (big ion can't fit) 3. Charge Density (Hydration Shell) - Ion selectivity is ultimately determined by the charge density which affects the hydration shell Issue in reason 1 and 2 - Why let only K through and not Na (Na is positive AND is smaller than K) --> BECASUE of charge density and the water around it

Question 27

Charge density And hydration shell

Answer 27

Charge density = Charge/size - Smaller radius (Ex. Li+)= higher charge density (charge is speads across a smaller area) - Bigger radius (Rb+) = smaller charge density (charge is spread over a large area) - Charge density orders the water molecules around it Hydration shell : water around the charge density of the ions (size depends on charge density) - Higher charge density (smaller radius) = larger hydration sheel - Smaller charge density (bigger radius) = larger hydration shell

Question 28

How does hydration shell/charge density affect Ion selectivity

Answer 28

Ions need to be stripped of the water before it can through the channel --> inside of the channel will use energy to remove the hydration shell before ion enters pore Energy that the pore uses to remove the hydration shell needs to match the size of the hydration shell (size of shell is affected by charge density) Example – The cost of removing the hydration shell from Li does not match by inside of K chanel = Li won’t go through the K chanel Vs. - THE ability of the K chanel to remove hydration shells matches the hydration shell around K because of the walls of the channel and the coordination of negative charges

Question 29

Conductance

Answer 29

Conductance - Ease with which ions goes through the ion channel (C=1/R) - In charts conductance is the slope Ion channels are conducting devices in a circut --> as a conducting devide IF driving force increases (Ex. Voltage across the membrane) ions being pulled increases

Question 30

Chanel Conductance with NO concetration gradient

Answer 30

Charts (X-Axis = Voltage (E) ; (Y Axis = Current) - Shows Ohms Law --> g=I/E Start Have Equal K on both sides of membrane --> one side becomes Positive –-> ions goes from + to - side (repelled by the positive charge) --> the more positive the side becomes (more voltage) the more positive the current - See in Line 1 – At voltage of 0 the Current is 0 --> when the volatge increases the curret (Positive slope Increase Voltage so increase the ions being pulled (Current) - IF you flip volatge (have reverse curren)t --> volatge decreases so the curret Decreases (ions go in opposite directions) Line 2 and 3 = if added more chanels the slope increases --> rate of increase in current increases

Question 31

Chanel Conductance with WITH concetration gradient

Answer 31

Model is like a battery in a circuit (Pump is the battery (makes the gradient) - Modified Omhs law --> g=I/(E-Eion) Line 1- Voltage is 0 --> ion channels open --> have a current because have a concertation gradient driving the movement of ions (when have concetration gradient have current even though v=0) IF have no current BUT the insides becomes more negative --> Ion moves against the concentration gradient (get negative current) - As ions move (get a membrane potential --> Current becomes 0 --> volatge is negative (point E1) --> current becomes negative Line 1 shows K current and Line 2 is shows Na current

Question 32

What does ion channel conductance depend on

Answer 32

Ion chanel conductance depends on the membrane potential = depends on the volatge - Ion chanels open and shut in response to voltage Ion chanels are changed by volatge (open/shut in repsonse to voltage) AND they chnage volatge (let ions through to change voltage)

Question 33

Charts that show conductance is voltage dependent

Answer 33

Top chart (X-Axis = Volatge; Y-Axis = conductance) - Start – conductance is low (g = low ; chanels are shut) --> have a jump to higher conductance Bottom Chart --> model NOW is a a rectified device Right Chart --> line has a break in slope and therefore a break in conductance (conductance is not constant) - Start – slope is low (conductance is low) --> THEN increase voltage --> slope changes (increases) --> once slope increases have constant conductance - What happens at the jump break in conductance (jump in slope) = channels open (NOW conduct ions) - Charts shows the chanel depends on volatge (because when incerase volatge there is a incerase in conductance)

Question 34

Answer 34

Answer #1– All of the above E1 (0 on Y Axos) --> Curent in 0 - Know the current is zero at Nenst potentola and that Nest potentola is elctrochrmical equoliburm - Curent is 0 at E1 - E1 is nernst potential (net current is 0) - E1 is Reversal potentoal because on either side of the voltage the curernt is reverse (Curent is positive to the right and negative to the left of the point ) Answer #2 – Constant is constant (because the slope of the line is constant

Question 35

Conductance of Ion Chanels

Answer 35

Ion chanels have high conductance (move ions fast) Example – Ach (Acytley chone receptor chanel) --> Current of 6.6pA leads to conductance o 40 million ions/second - Carriers = 10^3 ions ; Pumps = 100 ions/second - Reach Equilibrium potentials quickly BUT the kinetics of membrane potential changes reflects the channel gating NOT conductance Amount of ions a transporter moves is related to how complicated the underlying process is (More straightforward to move the ions then the conductance is higher

Question 36

Gating

Answer 36

Gating = opening/closing Ion channels have 2 types of gates (BOTH need to be open for ion to pass): 1. Activation gate (opens when actiavted) 2. Inactivation gate Activation chanel can stay open becaise the activation signal is still there BUT the inactivation gate can shut = ions can't go through - Chanels can spontaneously close because the inactivation gate can shut - Once the inactivation gate closed --> need to have a confirmation chnage before the channel goes back to closed resting --> once chanel goes to closed resting stage (active closed ; inactive open) it is ready to be activated again

Question 37

Types of Gating

Answer 37

1. Voltage Gating (field variations) --> Chanel will open/close as the membrane potential changes - Electric field across the bilayer is high --> electric feild can be felt by the ion channel itself --> channels feels the charge and has a confirmation change (chanel open/shut) - For a protein to feel charge it itself needs to be charged 2. Ligand Gating (receptor chanels) --> When the ligand binds the chanel opens - Example ligands --> Nuerotranistters OR G proteins OR ATP or Ca or IP3

Question 38

Ion chanels in Action Potential

Answer 38

ALL voltage gated chanels 1. Leak K channels - Open at rest + Low conductance (small pore) + Closed upon depolaizration and opens when the cell is repolzarized 2. Na Chanels - Closed at rest + Open quickly upon depolorization (when inside becomes positive) + Inactive within 1-2msec of opening (Na chanel is transiently open) 3. Delayed rectifier K channels - Closed at rest + Open upon depolarization + 10 fold slower to open than Na Chanels

Question 39

Time course of the action potential

Answer 39

1. At rest (membrane potential is -60mV ; inside negative) – Leak K chanels are open - K leaving the cell starts the AP - In chart - K leave chanels are open before and after the Peak of the AP 2. Have a stimulus ( positive charge from somewhere else) that causes the membrane to depolarize --> leak channels close and the Na channels open 3. Threshod is Reached 4. Threshold is surpassed --> the Na current overhwhelmes the leak K current --> Membrane potential goes Ena (very positive) 5. Na channels inactivate and delayed K channels open --> K leaves cell --> membrane repolarizes to Ek (negative) - When all of the Na Chanels closes the delayed K chanels open --> go to Ek (hyperpolarized) 6. Leak chanels open and Delayed K channels close --> membrane goes to resting potential

Question 40

Why is the inside of the cell negative at rest

Answer 40

Resting memebrane portential (-70mV) depends on leak K channels Negative membrane potential at rest because the leak K channels are open --> K is going out if the cell --> inside becaomes negtaive (leaves more negative inside) Resting potential is negative BU the resting potential is NOT at Ek (resting potential is more depolarized (more positive) than Ek) - WHY - because there factors can afect membrane potnetial (pumps and carriers) and because K chanels are not very selective and might let other ions through which affects the membrane potential (the leak K chanels mainly move K but is NOT ONLY moving K)

Question 41

Na Chanels opening in AP

Answer 41

Na chanels open --> Na+ goes into cell --> inside the cell becomes more positive --> cell becomes more depolarized --> more and more Na chanels open - The cell was already becoming depoarized BUT once the Na chanels open the cell depolarized even more - Na chanels open after initial depolarzaion and not the K Chanels because the Na chanels open faster than the K chanels chart - yellow line going slowly = membrane deplarizing

Question 42

Threshold in AP

Answer 42

Threshold = when the Na current (Na ions coming in) balances the leak K current (K going out) - Threshold - The level of depolarization that is needed to initiate the all or none action potental

Question 43

Surpassing Threshold in AP

Answer 43

Threshold is surpassed --> the Na current overhwhelmes the leak K current (ALL Na chanels open = more Na coming in compared to K going out) --> membrane rapidly depolarized (Membrane potential goes Ena) - Membrane rapidly depolarizating = burst of a ALL Na channels opening (Positive feedback of More Na going in --> cell is more depolarized --> mere Na open --> more Na goes in) In chart – Sharp peak in purple and in AP trace (yellow) = all Na channels open (Point when the cell is most depolarized) - Peak of the Na trace (purple) corresponds to the peak of the yellow (AP trace) - Have rapid change in membrane potential (have peak of depolarization) because all of the Na chanels open and close fast

Question 44

Does the membrane potential actually reach Ena

Answer 44

When all of the Na chanels open and Leak K close the membrane potential does NOT go to ENa WHY not Ena? 1. Na chanels spontenouly inactivate (close in 1-2 ms = not open long enough to reach ENa) 2. When the Na channels closes the Delayed K chanels open

Question 45

Why do the delayed K channels close

Answer 45

When the cell gets repolarized (because K+ leave the cell) --> the delayed K changes shut because the cell is no longer depolarized (leak K chanels open ONLY when the cell is depolarized) When the cell is repolarized (negative membrane potential) the leak K chanels open --> membrane goes back to resting potential - Goes back to resting potential because the Na/K ATPase pump and Leak channels are open

Question 46

Overall AP Process

Answer 46

OVERALL - Leak open --> stomulus cuases membrabe to be depolarized --> leak close ; Na chanels opne --> beyond thershold all NA chanels open and mebrane potentila increases --> Na close and Delayed K open --> membrabe potential goes back down and the cell is repolarzied

Question 47

Why is there a sterotypical size and shape of AP (why is it the same every time)

Answer 47

Because every cell has its specific expression of channels = it will always have the same number of chanels --> always gets the same membrane potential values The number of channels can change during differentiating and development = the shape of the spike can be different - Different cell type or differention state can have different shape

Question 48

Why is there a refractory period

Answer 48

Two types of refractry periods: 1. Absolute --> no second Action potential can be seen (no additive AP) 2. Relative --> can be another action potential BUT there would need to be a HUGE stimulus to get the all of Na channels to open simultaneously Why is there a refractory periods --> because the inactivation property of Na (after channels open/close they are channels are slow to open again ; can't get AP untol Na chanels are ready to open) - IN the relative refractory period some of the Na chanels are inactive BUT if there is a large enough stimulus --> open ALL of the Na chanels --> get another AP - In Abosulte refractory period all of the Na chanels are inactivted --> can’t get another AP

Question 49

Why All of nothing response in AP

Answer 49

Because of the threshold Once past the threshold all Na chanell open and cell has an Action potential IF the cell does not reach the threshold ALL Na chanels don't open then there is no Action potential

Question 50

Why is there propagation

Answer 50

Propegation can spread depolarization down a nerve fiber In the region where the AP is firing in the neuron the membrane portnetial is reverse (+ inside ; normally have - inside) - Some of the + charge will drift in the cytoplasm --> positive charge comes close to the next patch of membrane --> the next patch of membrane will be depolarized and will have an action potential (allows for propegation) and the first patch resets In vertabrates AP does not go at every point in the membrane in vertabrtes INSTEAD AP hops from one point with no mylination to the next (faster)

Question 51

Answer 51

I THINK C

Question 52

Answer 52

Answer – B --> because the Na chanels are inactivates so they can’t open again

Question 53

Why does the AP only move in 1 directions

Answer 53

AP only moves in the direction of naive membrane that hasn’t fired before WHY – because the past part of the membrane that just had the action potential has Na channels that are inactived (can’t fire again) - NOW only the next part of the membrane that hasn’t fired and therefore does not have Na chanels that are reseting can fire - Inactivation of the Na channels causes te action potential to only go in one direction

Question 54

Normal cardiac Action potential

Answer 54

Normal cardiac action potential is broader --> Ca channels open for longer --> Ca goes into the cell for longer --> the depolarizaion is longer than normal - Delayed K chanels still brings the AP down

Question 55

Genetic Disorders from voltage gated ion chanels

Answer 55

EGC trace has a Q wave and T Wave (between waves is QT interval) Long QT syndrome - prolongation of AP leads to cardiac arrhythmias Mutations can change the shape of an action potential (broader and get Tordes De Poimtes): 1. Mutations in HERG K+ channel (delayed rectifier channel) channels is slower to open --> depolarizations is longer and repolarization is delayed - Action potential is braoder at the top --> QT interval is longer --> causes Arrythmia --> get Torsae De pointes --> heart goes crazy 2. Dominant mutations in voltage gated Na+ channel: channels fail to inactivate --> more Na comes in --> depolarization is longer (broader) --> QT is longer --> get Arrythmia --> Get torsade de pontes

Question 56

What happens when a AP gets to the end of an Axon

Answer 56

Action potential moves along the axon and gets to the end of the axon where the axon meets an effector (Ex. muscle) --> Nerve needs to transmit information from the neuron to the muscle so the muscle will contract OVERALL at the neuromuscular junction - signal goes from the neuron across the cleft to the muscles - Signal goes from electrical to chemical to electrical

Question 57

Nueromuscular junction Process

Answer 57

1. AP arrives at the Pre-synaptic terminal --> depolarizes the pre-synaptic terminal (becomes more positive) 2. Depolarization of the presynaptic terminal causes the Ca voltage gated channels to open --> Ca goes into the synaptic neuron 3. Ca going into the pre-synpatic neuron causes the synaptic vesicles to fuse with the membrane - The synaptic vesicles are docks on the membrane in presynaptic neuron 4. When the synaptic vesicles fuse with the membrane --> Neurotransmitters (packed in the synpatic vesicles) go into the synpatic cleft --> Nuertotramistters will diffuse quickly across the cleft and reach the post synaptic membrane 5. Nuerotramitter (Ach) binds to the ligand gated chanel on the post synpatic membrane --> chanel open --> Na goes into the cell and K leaves the cell --> get a net effect of membrane depolarization (motor end plate depolarizes) --> action potential is fired at the post synaptic membrane 6. Action propgate at the post synpatic membrane will propegate along the surface of the muscle 7. Removal of ACh terminates activation of the receptor

Question 58

What happens when Nuerotranimitters binds ti the ligand gated chanel on the pos-synpatic membrame

Answer 58

5. Nuerotramitter (Ach) binds to the ligand gated chanel on the post synpatic membrane --> chanel open --> Na goes into the cell and K leaves the cell --> get a net effect of membrane depolarization (motor end plate depolarizes) --> action potential is fired at the post synaptic membrane 6. Action propgate at the post synpatic membrane will propegate along the surface of the muscle 7. Removal of ACh terminates activation of the receptor - Acytylchone will be hydrolyzed and broken down --> choline goes back into the presynaptic cell where it will be packaged again into vesicles

Question 59

Morphology of the muscle

Answer 59

Surface plasma membrane has T-Tubulues (invaginations of the plasma membrane deep into muscle fiber)--> invaginations allows the AP running along the surface to go into the thickness of the muscle (no invaginations the muscle would only contract near the surface) Muscle cell cytoplasm has actin and myosin motors Wave of depolarization goes down into the T-Tubules --> depolarizes causes the Ca in the SR to open --> the Ca in the SR will leave --> Ca binds to troponins --> allows actin and myosin to interact --> muscle contracts - Ca – second messenger

Question 60

Intercation between T Tubulues and Muscle SR

Answer 60

T-Tubules and the SR form a junction --> at the junction SR will expand --> forms the Tripartite junction - Junction = SR on both sides of the T-Tubulue NOTE - ER of the muscle = sarcoplasmic reticulum

Question 61

T Tubulus and SR image

Answer 61

In imgae – Pill shape in middle of junction = T-Tubulus ; Things on both sides of T-Tubule = the SR membrane ; electron dense structures between the SR and the T Tubulue are the Ca ion chanels

Question 62

How does Skelatal muscle contract rapidly

Answer 62

ER (SR) chanel and the plasma membare are in close contact (because of teh T Tubule invaginations) --> volatge is passed through the ER protein (Ca ion chanel) --> ER protein opens (chanel opens) --> allows Ca to leave SR quickly - Ca leaves very quickly once have the AP because the the signal from the membrane is in contact with the SR (because of the invaginations of the T-Tubulus)

Question 63

Why is smooth muscle slower to contract

Answer 63

Smooth muscle – Have Ca Chanel in the plasma membrane --> Action potential causes the Ca channel to open and Ca goes into the cell --> Ca binds to the Ca receptors on the ER --> ER Ca channels open and Ca leaves the ER Skeletal muscles need to be faster = have direct contact between the chanles on the plasma membarne and the SR due to the invagbations from the T-Tubulue

Question 64

Excitaion contraction coupling

Answer 64

The ER Ca chanel has an extracellular domain --> extracellular domain interacts with the plasma membrane Ca chanel - Using theses chanels the muscle causes the action potential to go to the mueslce cytoplasm --> Action potential causes the Ca to leave the ER --> Actin and myosin contract When Actin and myosin stop being in contact --> Ca pumps will put all of the Ca back in the ER --> actin and myosin won’t interact --> muscle relaxes --> can restart the process

Question 65

Ca chanel on ER Vs. Plasma membarne

Answer 65

Ca Chanel at the plasma membrane (voltage gated) = Dihydropydrimidine recpetor Ca chanel on ER (ligand gated)= Rhayodin receptor chanel

Question 66

Parts of volatge gated ion chanels

Answer 66

1. Part of the channel that respond to voltage 2. Parts of the channel that are responsible to passing the ions

Question 67

P Loop

Answer 67

Example – K channel (transmembrane channel) has a P loop (permeability loop) that is dedicated to permeability (allows K through) - Loop is a tetramer --> 4 loops come together to allow K to pass Experiment - Mutation in P loop --> changes if inhibitors can access the loop - Example – TEA access on both sides of the membrane is changed when mutant P loop - Showed P loop is important for permeability (because inhibitors couldn't get in (the P loop was not permeable) when the P loop was mutant)

Question 68

S4 Helix

Answer 68

S4 helix = voltage sensor Helix S4 = lies horizontally along membrane When mutate S4 = channel can’t sense voltage = channel won’t open or shut in response to voltage

Question 69

How does S4 helix sense Volatge

Answer 69

S4 helix has positively charged residues (Arg or Lys) at repeating every three residues - Charged residues sense the charge difference across the membrane HOW - IF have positive on one side of the membrane THEN the helix is oriented a certain way ; IF the charge is revered on that side of membrane (becomes negative on that side of the membrane) THEN the helix moves

Question 70

How does S4 helix voltage sensing open chanel

Answer 70

When the helix changes orientation (moves) when charge across membrane changes S4 moves within the plane of the lipid bilayer --> moves the rest of the channel protein = the channel opens/shuts

Question 71

Gating currents

Answer 71

Movement of charges in channel proteins can be detected as “gating currents” Evidence of the S4 helix moving - IF activate the chanel BUT there are no ions for the chanel to actually move you still see a small spike - Spike = the gating current --> movement of charges in the plan of the bilayer (charges are NOT crossing the membrane ; they are are moving in the bilayer itself) - Only see gating current when there is no ions are conducting (no ions moving)

Question 72

How does the Inactivation gate close?

Answer 72

Example - When the Na channel opens it is committed to closing/inactivating How does inactivation gate close --> proposed a ball and chain model - Charged ball that is hanging off of the chanel on cytoplasmic side --> when the channel opens the charges in the pore are rearranged --> the rearrangement of charges attracts the ball --> ball swings and shuts the channel NOTE - Chanel is inactived using amino acids in the N terminus of the chanel

Question 73

Experimental proof of ball and chain model - Showing K channels close

Answer 73

Experiment - Have K channels that inactivate after opening - Know that the K channels inactivate after opening at all voltages because of the traces (all line up) ; At one voltage get one trance and at a different voltage get new trace BUT even with bigger voltage the channels all inactivate and there is no current - Bigger voltage = bigger stimulus = chanels open faster

Question 74

Experimental proof of ball and chain model

Answer 74

In experiment – deleted specific amino acids in the N terminus --> NOW the channel doesn't inactive (stays open) --> NOW the current is always high (channel doesn't close) - SHOWS that there is a protein compoent that is causing the inactivation of the chanel THEN – took the peptide that they cutoff and put the peptide back in the bath - IF they add enough of the peptide they can recover the inactivation of the channel --> Suggests that the N terminus (part cut off) is the part that moves and shuts the pore of the chanel (inactivates the channel)

Question 75

Summary

Answer 75