electrophysiology of the heart

Question 1

syncytium

Answer 1

• well organized muscle fibers in myofibrils

Question 2

working cells vs specialized cells

Answer 2

• working cells- contract to pump blood and are well organized into myofibrils called syncytium- connections between cells cause electrical activity to spread via gap junctions and intercalated disks; no intrinsic pacemaker activity
• specialized cells- function to set and coordinate electrical activity of heart; pacemaker cells- intrinsic rhythm generator

Question 3

Sarcolemma

Answer 3

-hearts name for phospholipid bilayer that controls flow of solutes (via membrane receptors, ion channels) and regulates activity via gap junctions and transverse tubules

Question 4

what is important to note about equilibrium potential

Answer 4

-it does not equal resting potential- equilibrium potential is different for each ion -put all together and we can find the resting membrane potential

Question 5

what’s the membrane potential-concentration gradient for

ca, K, cl, protein, Na

Answer 5

protein and K have higher conc inside the cell and want to move out
Ca, Na, and Cl are higher outside the cell and wanna move in

Question 6

what’s the electrical gradient for

ca, K, cl, protein, Na

Answer 6

• the contents of the cell on the inside is negative so all of the negative ions want to move in- Na, K, Ca
• Cl and protein want to move toward extracellular space

Question 7

driving force

Answer 7

• potential that is available to DRIVE ions across the membrane where they wanna go (don’t just flow freely)
• difference between membrane voltage and equilibrium potential

Question 8

ohms law

Answer 8

I(current) = (Vm-Veq)/R which is the difference between where the membrane potential is minus where it ions will stop flowing due to equilibrium over the resistance to flow thru the membrane

Question 9

Describe how the resting membrane potential for sodium is maintained

Answer 9

• electrical and concentration gradients BOTH favor Na into the cell but cell is not permeable to Na at rest.
• Na-K ATPase- maintains low Na intracellularly and hi outside

Question 10

K membrane potential

Answer 10

concentration grad wants to move K out of cell
electrical grad wants to move K into the cell

• cell membrane is permeable to K at rest
• resting membrane potential is nearly at equilibrium potential for K which reduces potential driving force for K to move

Question 11

membrane potential for Chloride

Answer 11

-opposing gradients:
-conc gradient favors CL into cell
-electrochemical grad favors out
membrane somewhat permeable to Cl

Question 12

Calcium’s membrane potential

Answer 12

-concentration/electrochem grad- into cell
membrane permeability to Ca2+ is low and concentrations are maintained by Ca2+ efflux out of the cell and sequestration via the sarcoplasmic reticulum in the cell

Question 13

Describe a cardiac action potential in terms of the ion channels

Answer 13

• Na+ channels open letting Na in and allow for rapid depolarizing on NON-NODAL muscle cells
• Ca++ channels then open and let ions in due to this change in membrane potential and then
• K channels open and let K out repolarizing the heart
• “funny” channels open during hyperpolarization/repolarization that are activated by Cyclic nucleoide-gated channel “pacemaker channels” allow POSITIVE charge to enter cell = allows for signal to rest of heart

Question 14

Explain how the heart and skeletal mucle differ in terms of electrolyte influx/efflux via walking thru what happens during an action potential

Answer 14

• Depolarization occurs and Na channels open for a short time and then slap shut (same as skeletal)
• Ca2++ channels open SLOWWWW but open longer than Na and there is a PLATEAU PHASE- note, Ca2++ INFLUX is required for muscle contraction; while skeletal relies on sr/er for Ca2++
• without influx of Ca2+, heart will not contract
• Decreased K permeability during AP for heart when skeletal remains the same- this allows for plateau
• once Ca++ and Na+ channels close, K permeability increases to return RMP

Question 15

describe the phases of myocyte contraction

Answer 15

• phase 0- depolarization- fast due to open of Na Channels (vertical line)
• phase 1- early repolarization- due to Na channels closing fast and K channels being a bit open but repolarization is incomplete (peak of vertical line)
• Phase 2- Plateau- SLOW Ca channels allow for plateau as they are counterbalanced with K+ coming out of the cell => ALLOWS FOR BLOOD TO BE PUMPED OUT OF THE HEART (horizontal line)
• Phase 3- Rapid repolarization where Ca channels close and K channels fully open (fall of line)
• Phase 4- Resting membrane potential- only K channels are open and rmp maintained until new stimulus

Question 16

refractory periods

Answer 16

• relative refractory period- More difficult to excite but not impossible- some ions reset and some are not (.05 sec)
• Absolute refractory period- NO WAY DUDE! Ventricle (0.25-.3 sec) and atria (0.15)

Question 17

describe the changes in Na channel when activated by stimulus

Answer 17

• Na channel at rest with pore close and activation gate is open
• stimulus occurs, threshold reached, and pore opens allowing Na thru into the cell
• Time dependent- shortly after- Na gate closes however pore remains open allowing for Na to enter but not enter the cell
• shortly after, pore closes as well due to a conformation change stimulated by change in membrane potential
• as cell returns to lower membrane potential (-90) the activation gate will open bringing us back to start position (with pore still closed)

Question 18

when does the relative refractory period occur? effective refractory period?

Answer 18

• effective is when INACTIVATION GATE IS CLOSED and pore is open
• relative is when inactivation gate is OPEN but pore is closed

Question 19

how do myocytes refrain from being tetanized?

Answer 19

• it can only be stimulated when it’s at rest due to the prolonged effective refractory period (when gate is closed)
• skeletal muscle has a short effective refractory period so it and respond to each stimulus up to the limits of developed tension

Question 20

what can cause ventricular fib

Answer 20

-when not all cells and channels go back to ready position for the next stimulus- interrupts normal flow fo the AP

Question 21

on the membrane potential chart where is the effective and relative refractory periods for myocytes

Answer 21

absolute is from start of phase 0 to middle of falling phase of phase of 3 (b/c can’t restimulate heart when Na channels already open)
relative is middle of phase 3 to where 4 meets the resting potential line

Question 22

Cellular permeability and cardiac action potential for Nodal cells (phases and what occurs)

Answer 22

• NO phase 1 and 2
• Phase 0- once threshold potential met, Ca++ channels open = NO FAST NA CHANNELS -SLOW RESPONSE (gradual up slope)
• Phase 3- after depolarization, K channels open (cause downslope)
• Phase 4- comes down to a unstable line that is not flat like myocytes because there is a leak of positive ions (Na) coming into the cell via “funny current” and they change membrane potential until threshold is reached

Question 23

how are nodal cells effected by the autonomic nervous system?

Answer 23

• Norepi causes increase in permeability of Ca into the cell causing increase membrane potential allowing the cell to reach threshold faster to contract (increases contractility)
• Ach- increases K permeability-decreasing membrane potential and bringing it to resting potential (decreases contraction)