L17 Cardiac Automaticity

Question 1

Define cardiac automaticity

Answer 1

Ability of cardiac tissue to generate an action potential

Question 2

 what is the resting membrane potential in cardiac cells

Answer 2

-85 mV

Question 3

Concentration of intracellular potassium and extracellular

Answer 3

Intracellular=140 mmol/L

Extracellular= 4 mmol/L

Question 4

What is the EK and what does it mean

Answer 4

Equilibrium potential for potassium
It’s the membrane potential at which the net flow of potassium ions out of a cell down the diffusive gradient is exactly balanced by their net movement into a cell down the electrical gradient

Question 5

What are the properties of cardiac ion channels and explain them

Answer 5

▪️Selectivity which means they’re only permeable to a single type of ion based on their physical configuration
▪️Voltage sensitive gating:a specific membrane potential range is required for particular channel to be in open configuration at all membrane potential outside the range the channel will be closed and impermeable to ions
▪️Time dependence: some ion channels are configured to close a fraction of a second after opening they can all be open again until the membrane potential is back to resting levels thereby preventing further excessive influx

Question 6

What are the phases of the action potential in cardiac tissue

Answer 6

Five phases
0 to 4 beginning and ending with phase 4
1- initial rapid depolarization
2-rapid partial early repolarization
3-prolonged period of slow repolarization which is plateau phase
4-rapid repolarization phase
5-complete repolarization

Question 7

Describe phase 4

Answer 7

The resting phase and the resting potential is about -85 mV

Question 8

Describe phase 0

Answer 8

▪️Depolarization
▪️As a result of excitation wave generated by pacemaker tissue the sodium gates open and the permeability to sodium increases(fast sodium channels)
▪️The large sodium influx rapidly depolarizes the membrane to 0 mV and slightly above 0 mV for a transient period of time called the overshoot
▪️As a result the membrane potential rise between 20 to positive 30 mV
▪️At this point the sodium gates are inactivated by the electric charge distribution across the cell membrane and the sodium permeability falls

Question 9

Describe phase 1

Answer 9

▪️Early repolarization
the membrane potential is now slightly positive and the permeability to potassium ions begins to increase and some potassium leaves the cell at an increased rate down both a favorable concentration and electrical gradient

Question 10

Why doesn’t membrane potential immediately fall during phase 1

Answer 10

Membrane potential does not immediately fall because there is a simultaneous opening of L type voltage gated calcium ion channels an inward flow of calcium ions from outside the cell

Question 11

Describe phase 2

Answer 11

▪️The plateau phase
▪️L-type Ca2+ are still open and there is a small constant inward current of Ca2+
▪️K+ leaks out down its conc gradient through delayed rectifier K+ channels
▪️Electrically balanced and TMP maintained just below zero

Question 12

Describe phase 3

Answer 12

▪️Repolarization
▪️Ca2+ channels are gradually inactivated Persistent outflow of K+ now exceeding Ca2+ inflow brining membrane potential back to -85mV

Question 13

What causes the plateau in the action potential

Answer 13

▪️Slow calcium channels are voltage activated and calcium sodium trying to slow to open and remain open for several tenths of a second
▪️Prolonged opening of the slow calcium sodium channels allows a large quantity of the calcium inflow to the interior of the cardiac muscle fiber causing plateau
Decreased permeability of the cardiac muscle membrane for potassium decreases potassium outflux during the action potential plateau
Voltage gated potassium channels are slower to open this delays the return of the membrane potential to -80 to -90
The presence of a plateau in the action potential causes ventricular contraction to last as much as 15 times as long in cardiac muscle as in skeleton muscle

Question 14

What is the criteria of pacemaker tissues

Answer 14

▪️Automaticity unlike other cardio myocytes pacemaker cells do not require external stimulation to initiate the action potential they are capable of self initiated the polarization in a rhythmic fashion
▪️ pacemaker cells have an unstable membrane potential
▪️ no rapid depolarization phase
▪️ reduction in outflux of potassium
▪️ membrane potential is never lower than -60
▪️ fast sodium iron channels are inactivitated so there is no rapid depolarization phase

Question 15

What is intrinsic rhythm of the SAN

Answer 15

110 to 120 bpm

Tonic parasympathetic vagal tone reduces heartbeat to 70 bpm

Question 16

Overall of pace maker potential

Answer 16

▪️After firing an action potential the membrane potential decreases from -60 to -45 at this level in action potential is fired and the cycle is repeated
▪️Spontaneous gradual depolarization occurs during diastole of the heart
And it just called the pace maker potential or pre-potential
Only has 3 phases

Question 17

Describe phase 4 of the pacemaker prepotential

Answer 17

Diastolic depolarization
Spontaneous influx of sodium slowly depolarizer membrane above -60 mV(funny sodium channels)
Calcium entry through the voltage gated T type calcium channels
Inactivation of potassium channels

Question 18

Describe phase zero of the pacemaker action potential

Answer 18

 mainly due to calcium influx through L-type calcium channels depolarization phase is relatively slow to develop and of smaller magnitude
Maximum depolarization goes up positive or -10 mV compared to the +30mv for the contractile myocardial cells

Question 19

Describe the repolarization phase of the pacemaker action potential

Answer 19

Immediately follows depolarization calcium channel inactivitation and opening of voltage gated potassium channels occurs just after a peak of action potential

Question 20

What does the Vagus and sympathetic innervation do

Answer 20

Vagus: SAN cells are hyperpolarized and the rate of rise to phase 4 resting potential is reduced
Sympathetic: Simulate SAN and increased phase 4 depolarization and the threshold potential is reached more quickly with increase in heart rate

Question 21

What does increase in potassium lead to

Answer 21

Hyperkalemia which leads to the depolarization of cardiac myocytes with increased resting membrane potential to zero therefore returned negative membrane potential to open all sodium in channels will not occur causing cardiac arrest