Unit 1 - CV System The Heart PART D

Question 1

Unlike skeletal muscle, cardiac muscle…

Answer 1

does NOT require a stimulus from the nervous system in order to contract

Question 2

The myocardium of the heart has a built-in network of non-contractile cells (the conduction system), that are capable of…

This electrical activity…

Answer 2

spontaneously creating and conducting the action potentials that will stimulate contraction of contractile cardiac muscle cells

spreads as an orderly wave throughout the myocardium and ensures that the atria and ventricles contract at the appropriate times to form the heartbeat.

Question 3

Cardiac Muscle Cells split

Answer 3

Cardiac Muscle Cells

• -> Modified/Specialized = Intrinsic Conduction System
• -> Autorhythmic cells - generate APs OR –> Conducting cells

Cardiac Muscle Cells
–> “Normal” = Myocardium –> Contractile cells

Question 4

Conduction System

Answer 4

Composed of non-contractile (no sarcomeres) cardiac cells that generate and conduct action potentials

Question 5

What are the 2 parts of the Conduction System?

Answer 5

a. Autorhythmic pacemaker cells

b. Conducting cells

Question 6

Where is the Sinoatrial (SA) Node located?

Answer 6

• In right atrium

Question 7

What is the rate that the Sinoatrial (SA) Node generates APs?

Answer 7

• generates action potentials (APs) at a rate of 100 APs/min (modified by parasympathetic (PSNS) innervation to be 75 APs/min AT REST)

Question 8

The Sinoatrial (SA) Node generates APs faster than other areas of the heart, therefore…

Answer 8

acts as the natural PACEMAKER OF THE HEART.

Question 9

Where is the Atrioventricular (AV) node located?

Answer 9

• in right atrium

Question 10

The Atrioventricular (AV) node generates APs…

Answer 10

at a rate of 50 APs/min

Question 11

The Atrioventricular (AV) node is…

Answer 11

Composed of small diameter cells with few gap junctions that slow down the AP conduction speed. Creates ~100 msec DELAY in AP conduction that ensures the atria contract and are fully empty before ventricular contraction begins

Question 12

The rate of APs of both SA and AV nodes are influenced by…

Answer 12

the nervous and endocrine systems

Allows changes in heart for activities like exercise, sleep, etc

Question 13

Autorhythmic cells…

Answer 13

spontaneously fire APs

- depolarizations of the autorhythmic cells then spread rapidly to adjacent contractile cells through gap junctions

Question 14

AV node & Purkinje fibers DO NOT usually set the heart beat b/c…

Answer 14

their rhythm is SLOWER than that of the SA node (unusual RPs)

Question 15

Describe the ejection of blood

Answer 15

as the muscles contract, they pull the apex & base of the heart closer together, squeezing blood out the openings at the top of the ventricles

Question 16

Conducting cells

Answer 16

large diameter conducting cells (can be autorhythmic)

Question 17

List the 4 kinds of Conducting cells

Answer 17

i. Interatrial pathway
ii. Internodal pathway
iii. Atrioventricular (AV) bundle
iv. Purkinje Fibers (also known as ”subendocardial conducting network”)

Question 18

Interatrial pathway

Answer 18

Carries signals from SA node to left atrium

Question 19

Internodal pathway

Answer 19

Carries signals from SA node to AV node

Question 20

Atrioventricular (AV) bundle

Answer 20

• only pathway through which APs are carried from atria to ventricles
• Carries signal quickly through ventricular septum where bundle splits into two branches (BUNDLE BRANCHES) that carry the signal to the apex of heart.

Question 21

Purkinje Fibers (also known as ”subendocardial conducting network”)

Answer 21

Network of terminal branches that transmit impulses (action

potentials) to contractile cells

Question 22

Describe the pathway of the conducting system of the heart

Answer 22

Sinoatrial (SA) –> Interatrial pathway –> contractile cells of atrial myocardium

Sinoatrial (SA) –> Internodal pathway –> AV node –> AV bundle –> AV bundle branches –> Purkinje Fibers –> contractile cells of ventricular myocardium upwards starting at apex

Question 23

Explain the 5 steps of the conducting system of the heart

Answer 23

1. SA node depolarizes
2. Electrical activity goes RAPIDLY to AV node via internodal pathways
3. Depolarization spreads more SLOWLY across atria. Conduction SLOWS through AV node.
4. Depolarization moves RAPIDLY through ventricular conducting system to the apex of the heart
5. Depolarization wave spreads upward from the apex

Question 24

What happens if the SA node is damaged?

Answer 24

If the SA node is damaged, the atria may not contract and action potentials in the heart will be produced at the rate of the AV node (50 APs/min). This may not be high enough to sustain life functions. In this case, a manmade pacemaker (consisting of a battery and electrode) can be surgically implanted under the skin to artificially stimulate the AV node cells at a rate that is within normal range.

Question 25

Action Potentials In Pacemaker Cells (SA node and AV node)

Answer 25

These (Myocardial Autorhythmic) cells produce unstable membrane potentials called PACEMAKER POTENTIALS that are capable of spontaneously producing action potentials.

Question 26

Pacemaker potentials

Answer 26

Unlike neurons and skeletal muscle cells, these cells have no resting membrane potential and a threshold of -40mV.

• called this b/c it never “rests” @ a constant value
• starts @ -60mV & slowly goes up toward threshold (unstable)

Question 27

List the 3 steps of Action Potentials In Pacemaker Cells (SA node and AV node)

Answer 27

a. Pacemaker potential
b. Depolarization phase
c. Repolarization phase

Question 28

Step 1. Pacemaker potential

Answer 28

• If (funny) channels open (when the cell MP is -60mV), and Na+ enters the cell causing a slow depolarization toward threshold.

Question 29

Step 2. Depolarization phase

Answer 29

• At threshold, If (funny) channels close (no more Na+ entry), and voltage gated Ca++ channels open. Calcium enters cell quickly (cell becomes more and more positive).

Question 30

Step 3. Repolarization phase

Answer 30

• At peak, voltage gated Ca++ channels close, slow K+ channels open. K+ exits the cells, decreasing membrane potential.
• At -60mV, voltage gated K+ channels close, If channels reopen, and the next pacemaker potential begins (results in a continuous cycle of action potentials).

Question 31

If (funny) channels

Answer 31

permeable to both K+ & Na+

• allow current (I) to flow
• have unusual properties

Question 32

What are the Action Potentials In Myocardial Contractile Cells Phases?

Answer 32

a. Phase 4: Resting Membrane Potential
b. Phase 0: Depolarization phase
c. Phase 1: Initial Repolarization Phase
d. Phase 2: Plateau
e. Phase 3: Repolarization phase

Question 33

Phase 4: Resting Membrane Potential

Answer 33

Is -90mv in contractile cardiac cells

Question 34

Phase 0: Depolarization phase

Answer 34

Depolarization of autorhythmic cells spreads through gap junctions to contractile cells. Voltage gated Na+ channels open; Na+ enters cell until membrane is depolarized to +20 mV.

Question 35

Phase 1: Initial Repolarization Phase

Answer 35

Na+ channels close, fast K+ channels open, K+ leaves cell causing repolarization.

Question 36

Phase 2: Plateau

Answer 36

Voltage gated Ca++ channels open, fast K+ channels close. Ca++ entry and decreased permeability to K+ prolongs depolarization.

Question 37

Phase 3: Repolarization phase

Answer 37

Voltage gated slow K+ channels open, K+ exits cell and membrane potential returns to resting levels.

Question 38

Brief description of the APs in Myocardial Contractile Cells phases 0-4

Answer 38

Phase 0: Na+ channels OPEN

Phase 1: Na+ channels CLOSE

Phase 2: Ca2+ channels OPEN; FAST K+ channels OPEN

Phase 3: Ca2+ channels CLOSE; SLOW K+ channels OPEN

Phase 4: Resting Potential

Question 39

The RAPID _______ phase of the AP is the result of Na+ ______, & the STEEP ______ phase is due to K+ ______ the cell

Answer 39

DEPOLARIZATION

ENTRY

REPOLARIZATION

LEAVING

Question 40

Why do myocardial cells have a LONGER AP?

Answer 40

due to Ca2+ entry

Question 41

Cardiac contractile cell action potentials are much longer in duration (~250 msec) than in skeletal muscle cells (1-2 msec) due what?

Answer 41

due to the PLATEAU PHASE

Question 42

What 2 imp. effects does the long APs have?

Answer 42

a. The ABSOLUTE REFRACTORY PERIOD is almost as long as the contractile response (contraction + relaxation)
b. The long ABSOLUTE REFRACTORY period ensures that a second contraction cannot be initiated before the first has completed

Question 43

What is prevented by the long absolute refractory period?

Answer 43

SUMMATION - b/c by the time a 2nd AP can take place, the myocardial cell has almost completely relaxed

TETANUS - b/c a series of APs occurring in rapid succession, is not happening so the sustained contraction/tetanus does not happen

Question 44

The long absolute refractory period allows…

Answer 44

Allows for alternation of contraction and relaxation of the myocardium with enough time in between beats for the chambers to fill with blood (i.e. heartbeat).

Question 45

Excitation Contraction Coupling in Myocardial Cells

How does the action potential in a cardiac contractile cell stimulate contraction of the cell (and therefore the heart)?

Answer 45

a. Contractile cell depolarizes and action potential on sarcolemma causes….
b. Opening of voltage gated Ca++ channels (L-TYPE CALCIUM CHANNELS) on the membrane (T-tubule). Calcium enters the cell.
c. Ca++ binds to RYANODINE RECEPTOR (RyR) Ca++ release channel on the membrane of the sarcoplasmic reticulum (SR).
d. RyR channel opens and Ca++ is released into the cytosol. This process is described as Ca++ INDUCED Ca++ RELEASE., which leads to a Ca++ signal.
e. Ca++ ions bind to troponin, initiating the contraction cycle and crossbridge formation (myosin binds to actin, powerstroke, etc).
f. Relaxation of the cell involves: Ca++ ATPase pumping Ca++ into the SR. Na+/ Ca++ Exchanger (NCX), an antiport that moves Ca++ out of cell in exchange for Na+. As Ca++ levels in the cytosol decrease, Ca++ unbinds from troponin, stopping the contraction cycle.