rhythmical excitation of heart ( chapter 10)

Question 1

how much sooner do atria contract than ventricles?

Answer 1

about 1/6 of a second

Question 2

SA node location

Answer 2

superior posterolateral wall of right atrium. Its an ellipsoid shape

Question 3

Resting membrane potential of SA node

Answer 3

-55–60 mv in comparison to -85–90 mv of ventricular muslce. This is cause the SA fibers naturally leak sodium and calcium ions into the cell which makes the cell more possitive.

Question 4

why is the resting potential -55 mv?

Answer 4

Anytime the potential is less negative than -55 mv the fast sodium channels become blocked. The gates of these channels close and remain so. Therefore only the slow sodium-calcium channels can open ( activate) and cause the action potential.

Question 5

Self exiting nodal fibers

Answer 5

due to high sodium concentration in extracellular fluid outside of nodal fibers, the sodium leaks into the nodal fibers and causes the resting potential to gradually become less negative between heart beats. Once it reaches ~ -40 mv it becomes activated and fires ( so the leakiness of sodium and calcium is what allows the nodal fibers to self fire)

Question 6

Why does the leakiness to sodium and calcium not cause the sinus nodal fibers to remain depolarized all the time?

Answer 6

sodium-calcium channels inactivate 100-150 milliseconds after opening
then the potassium channels open releasing alot of possitive charge.

Question 7

hyperpolarization

Answer 7

excess negative inside the fiber

Question 8

anterior interatrial band

Answer 8

passes through the anterior walls of the atria to the left atra

Question 9

Anterior, middle, posterior internodal pathways

Answer 9

bands that curve through the anterior, lateral, and posterior atrial walls and terminate at the AV node. They have similar fibers to purkinje fibers of the ventricles

Question 10

what is the total delay time of the AV node?

Answer 10

.13 second plus .03 from travel from the SA node so a total of .16 second.

Question 11

what causes the slow conduction through the AV node?

Answer 11

less gap junctions

Question 12

purkinje fibers

Answer 12

Start at AV node and travel through the ventricles. They transmit the action potential 1.5-4 meters per second which is 6 times the norm of a ventricular fiber.

Question 13

what causes the purkinje fibers fast conduction?

Answer 13

high number of gap junctions in intercalated discs.

Question 14

what causes the one way conduction pathway?

Answer 14

the fibrous barrier between the atria and ventricles

Question 15

why is the SA node the pacemaker?

Answer 15

its rhythmical discharge is faster than any other part of the heart

Question 16

ectopic pacemaker

Answer 16

when pacemaker is any other than SA node

Question 17

AV block

Answer 17

conduction fales to pass into ventricles. This will result in a ectopic pacemaker in the purkinje system that will beat at a different rate ( usually like 15-40 beats/ min)

Question 18

stokes adam syndrome

Answer 18

during an AV block when the purkinje system is trying to establish a new pacemaker the ventricles dont pump blood for 5 to 20 seconds. The person faints after 4-5 seconds and blood stops going to the brain. ( can be lethal)

Question 19

parasympathetic influece

Answer 19

releases acetocholine twhich decreases the rate of sinus node and decreases excitability of AV junctional fibers between atrial muscle and AV node

Question 20

Acetylcholine in parasympathetic stimulation

Answer 20

Causes potassium to leak out which leaves the fiber in a hyperpolarized state which makes the tissue less excitable. When the SA node is hyperpolarized it reaches levels like -65 to -75 compared to -55 to -6.0 mv. This greatly slows the rate of rythmicity of these fibers.

Question 21

Sympathetic influence

Answer 21

releases noreepinephrine which turns into beta-1 adrenergic receptors which influences heart rate.