Heart 2- Excitation of the heart

Question 1

Ability of cardiac muscle to depolarize and contract is ——-

Answer 1

intrinsic
-rhythmic beating continues without connection to the nervous system
-intrinsic heart rate: 100 bpm
-relies on gap junctions and the cardiac conduction system

Question 2

Autorhythmic cells

Answer 2

-noncontractile
-unstable resting potential
-initiate and distribute impulses throughout the heart
includes: SA node, AV node, AV bundle, right and left bundle branches, purkinje fibers

initiate APs
spread through gap junctions throughout the heart which triggers rhythmic contractions

Question 3

Pacemaker cells in heart

Answer 3

-SA node has unstable resting membrane potential
-Spontaneously changing MPs=pacemaker potentials

Question 4

Unstable resting potential= pacemaker potential

Answer 4

-K+ channels closed from previous AP
-Slow depolarization caused by open Na+ channels

Question 5

Pace maker: action potential

Answer 5

-threshold met at -40mV
-voltage gated Ca2+ channels open
-> influx of calcium -> rapid depolarization

Question 6

Pacemaker: repolarzation

Answer 6

when 0mV met
-inactivation of Ca2+ channels
-activation of K+ channels

pacemaker potential starts over
-Na+ begins leaking again
-Na+ channels were always open

Question 7

Cardiac conduction system

Answer 7

the following areas have auto rhythmic cells: SA node, AV node, AV bundle, right and left bundle branches, Purkinje fibers
-impulses pass trough the heart in order through those areas

Question 8

Cardiac conduction system overview:

Answer 8

1. SA node generates impulses
   2.the impulses pause at the AV node
   3.The AV bundle connects the atria to the ventricles
   4.The bundle branches conduct the impulse through the interventricular septum
   5.The subendocardial conducting network depolarizes the contractile cells of both ventricles

Question 9

Time of conduction:

Answer 9

time of appearance of depolarization after initiation at SA node at various parts of the heart

Question 10

Defective SA node

Answer 10

SA node sets heart rate, as no other regions has faster depolarization rate
-sets rate at ~75 bpm

Question 11

Ectopic focus

Answer 11

abnormal pacemaker sets pace
-cause: defective SA node
-If AV node takes over, it sets junctional rhythm ay 40-60 bpm

Question 12

Extrasystole

Answer 12

premature contraction of either atria or ventricles
-impulse triggered before SA node triggers impulse, causing delay in next impulse
-heart as longer time to fill, so next contraction is felt as thud as large volume of blood is being pushed out
-can be from excessing caffeine or nicotine

Question 13

Heart blocks

Answer 13

-to reach ventricles, impulse must pass through AV node
-If AV node is defective, may cause heart block

few impulses (partial block) or no impulses (total block) reach ventricles
ventricles beat at own intrinsic rate ~30bpm too slow to maintain adequate circulation
-treatment: artificial pacemaker
recouples atria and ventricles

Question 14

Extrinsic control

Answer 14

autonomic nervous systems acts on the heart to alter intrinsic rate

Question 15

Parasympathetic nervous system

Answer 15

cardioinhibitory center
-reaches heart via vagus nerve
-carries impulse to SA,AV nodes
releases acetylcholine, hyper polarizes cells
decreases HR, force of contraction
decreases HR below intrinsic rate

Question 16

Sympathetic nervous system

Answer 16

cardioacceleratory center
-reaches heart through right and left cardiac nerves
-carries impulses to SA, AV nodes, heart muscle, coronary arteries
releases norepinephrine, facilitates depolarization
increases HR, force on contraction
increases HR above intrinsic HR

Question 17

Hormonal extrinsic control of heart activity

Answer 17

-epinephrine from adrenal medulla increases HR
-Thyroxine from thyroid increases HR

Question 18

Ionic extrinsic control of heart activity

Answer 18

Hypocalcemia (low Ca2+)- decreases HR
Hypercalcemia (high Ca2+)-increases HR
Hypokalemia (low K+)- K+ diffuses out, leading to hyperpolarization, weak heart beats and arrhythmia
Hyperkalemia (high K+)-K+ keeps membrane voltage elevated, inhibiting depolarization, can cause cardiac arrest

Question 19

Baroreceptors and chemoreceptors

Answer 19

-in aortic arch and carotid sinus
detect changes in BP, BP increase, heart rate increases

-detect changes in CO2 and pH, CO2 increases, heart rate increases