Cardiac Muscle and Intrinsic Condition Study Guide

Question 1

anatomy of cardiac muscle

Answer 1

• Striated – like skeletal muscle
• Contracts via sliding filament mechanism – sarcomeres

Question 2

shape/size of cardiac myocytes vs skeletal muscle cells

Answer 2

• Cardiac myocytes are shorter and fatter than skeletal muscle cells, also more branched and interconnected
• Each cardiac myocyte has 1 or 2 centrally located nuclei (skeletal has 2+)
• Large mitochondria account for 25-35% of cardiac myocyte volume

Question 3

types of junctions that exist between cardiac muscles

Answer 3

• Desmosomes: prevent separation during contraction
• Gap junctions: allow ions to pass from cell to cell - transmitting current across entire heart

Question 4

intercalated discs

Answer 4

junctions that the plasma membranes of adjacent cardiac myocytes interlock at

Question 5

functional syncytium.

Answer 5

Myocardium acts as a single coordinated unit (not singular myocytes)

Question 6

role of calcium in cardiac muscle contraction

Answer 6

• Calcium delivery in the sarcomere – fewer, wider t tubules (1 per sarcomere) regulate calcium distribution
• Like skeletal muscle, cardiac muscle is contractile tissue – contractions are preceded by depolarizations in the form of action potentials
• action potential across the t tubules triggers release of calcium from the SR
• Calcium binds to troponin, moves tropomyosin, allows cross bridge cycling to begin
• Release of calcium: depolarization opens special, slow flow ca2+ channels in the cell membrane – combination of extracellular ca2+ and ca2+ from the SR allows contraction

Question 7

autorhythmicity

Answer 7

The ability to spontaneously depolarize

Question 8

types of cardiac cells with autorhythmicity

Answer 8

Pacemakers cells (self excitable, non contractile cardiac cells) in SA and AV nodes (1% of myocytes)

Question 9

functional syncytium

Answer 9

Cardiac muscle cells are tied together to form a functional syncytium, either all cardiac myocytes contract together, or the heart doesn’t contract. (Skeletal muscles contract via motor unit recruitment)

Question 10

summation + tetany in skeletal muscle

Answer 10

Summation is when a succession of stimuli occur, creating a stronger contraction. Tetany occurs when all the contractions combine to create one single long contraction
- In skeletal muscle, the refractory period is shorter than contraction, allowing for summation (and tetanus)

Question 11

How is tetany avoided in cardiac muscle?

Answer 11

the refractory period is much longer than contraction, preventing tetany

Question 12

depolarization

Answer 12

ca2+ channels open (around -40mV), ca2+ influxes leading to an action potential

Question 13

Repolarization

Answer 13

k+ channels open, K+ effluxes, cell’s interior becomes more negative

Question 14

sequence of excitation through the heart.

Answer 14

• Sinoatrial node (generated impulse 75 times/minute + sets pace for heart “sinus rhythm”)
• Atrioventricular node (bottle neck)
• Atrioventricular bundle
• Bundle branches
• Purkinje fibers (subendocardial conducting network)

Question 15

length of time it takes for an impulse to pass through the entire heart

Answer 15

0.22 seconds (220 ms)

Question 16

cause for delay at the AV node

Answer 16

0.1s allowing the atria to complete their contraction

Question 17

importance of delay at the AV node

Answer 17

ensures that all of the blood has moved from the atria into the ventricles before the ventricles contract

Question 18

ONLY electrical connection between the atria and ventricles

Answer 18

atrioventricular bundle

Question 19

What is the role of the Purkinje fibers in ensuring efficient “ejection” of blood from the ventricles?

Answer 19

The fibers provide the impulse for the bulk of ventricular depolarization, and ventricular contraction almost immediately follows depolarization. The wave of contraction moves from the apex toward the atria, and blood is ejected superiorly to the great vessels

Question 20

intrinsic rate set by the SA Node

Answer 20

75 times/minute

Question 21

intrinsic rate set by the AV node

Answer 21

50 times/minute

Question 22

intrinsic rate set by the AV bundle/purkinje fibers

Answer 22

30 times/minute

Question 23

Ectopic pacemaker

Answer 23

an abnormal pacemaker that takes over for the SA node

Question 24

Arrythmia

Answer 24

an irregular heart rhythm resulting from a defect in the intrinsic conduction system

Question 25

Fibrillation

Answer 25

rapid or irregular contractions of the heart
- Control of the heart by the SA Node is disrupted
- Fibrillating ventricles are not useful pumps

Question 26

Defibrillation

Answer 26

electrically shocking the heart to depolarize the entire myocardium - ideally, the SA node begins to function normally, and sinus rhythm is restored

Question 27

Implantable Cardioverter Defibrillators (ICDs)

Answer 27

devices that continually monitor heart rhythms; they will slow tachycardia and emit an electrical shock in the event of fibrillation

Question 27

PVC - Premature Ventricular Contraction or Extrasystole

Answer 27

Other ectopic pacemakers can appear even when the SA Node is functioning normally - small regions of the heart become hyperexcitable and the heart prematurely contracts (usually harmless)

Question 28

Why can a PVC feel like a thud in the chest?

Answer 28

After a PVC, the heart has a slightly longer time to fill

Question 29

Total heart block

Answer 29

no impulses get through, the ventricles beat at their own intrinsic rate – too slow for adequate tissue perfusion

Question 30

Partial heart block

Answer 30

only some impulses get through

Question 31

role of an artificial pacemaker

Answer 31

medical devices to recouple the atria and ventricles – pacemakers can be programmed to change with changing energy demand and interrogated to when symptoms appear (artificially sends electrical signals)

Question 32

What does the SNS do to heart rate?

Answer 32

accelerate heart rate and increase contractility

Question 33

What does the PNS do to heart rate?

Answer 33

decelerate the heart rate

Question 34

How do these autonomic systems change the heart’s rate

Answer 34

Fibers of the autonomic nervous system can modify heart rate and add variability

Question 35

How sympathetic impulses are transmitted from the brain to the heart

Answer 35

cardioacceleratory center in the medulla oblongata sends impulses through sympathetic trunk – stimulates sa node, av node, myocardium, and coronary arteries to increase heart rate and contractility

Question 36

How parasympathetic impulses are transmitted from the brain to the heart

Answer 36

cardioinhibitory center sends impulses through the vagus nerve to decrease heart rate