Unit 1 - CV System The Heart PART D Flashcards
Unlike skeletal muscle, cardiac muscle…
does NOT require a stimulus from the nervous system in order to contract
The myocardium of the heart has a built-in network of non-contractile cells (the conduction system), that are capable of…
This electrical activity…
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.
Cardiac Muscle Cells split
Cardiac Muscle Cells
- -> Modified/Specialized = Intrinsic Conduction System
- -> Autorhythmic cells - generate APs OR –> Conducting cells
Cardiac Muscle Cells
–> “Normal” = Myocardium –> Contractile cells
Conduction System
Composed of non-contractile (no sarcomeres) cardiac cells that generate and conduct action potentials
What are the 2 parts of the Conduction System?
a. Autorhythmic pacemaker cells
b. Conducting cells
Where is the Sinoatrial (SA) Node located?
- In right atrium
What is the rate that the Sinoatrial (SA) Node generates APs?
- generates action potentials (APs) at a rate of 100 APs/min (modified by parasympathetic (PSNS) innervation to be 75 APs/min AT REST)
The Sinoatrial (SA) Node generates APs faster than other areas of the heart, therefore…
acts as the natural PACEMAKER OF THE HEART.
Where is the Atrioventricular (AV) node located?
- in right atrium
The Atrioventricular (AV) node generates APs…
at a rate of 50 APs/min
The Atrioventricular (AV) node is…
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
The rate of APs of both SA and AV nodes are influenced by…
the nervous and endocrine systems
Allows changes in heart for activities like exercise, sleep, etc
Autorhythmic cells…
spontaneously fire APs
- depolarizations of the autorhythmic cells then spread rapidly to adjacent contractile cells through gap junctions
AV node & Purkinje fibers DO NOT usually set the heart beat b/c…
their rhythm is SLOWER than that of the SA node (unusual RPs)
Describe the ejection of blood
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
Conducting cells
large diameter conducting cells (can be autorhythmic)
List the 4 kinds of Conducting cells
i. Interatrial pathway
ii. Internodal pathway
iii. Atrioventricular (AV) bundle
iv. Purkinje Fibers (also known as ”subendocardial conducting network”)
Interatrial pathway
Carries signals from SA node to left atrium
Internodal pathway
Carries signals from SA node to AV node
Atrioventricular (AV) bundle
- 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.
Purkinje Fibers (also known as ”subendocardial conducting network”)
Network of terminal branches that transmit impulses (action
potentials) to contractile cells
Describe the pathway of the conducting system of the heart
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
Explain the 5 steps of the conducting system of the heart
- SA node depolarizes
- Electrical activity goes RAPIDLY to AV node via internodal pathways
- Depolarization spreads more SLOWLY across atria. Conduction SLOWS through AV node.
- Depolarization moves RAPIDLY through ventricular conducting system to the apex of the heart
- Depolarization wave spreads upward from the apex
What happens if the SA node is damaged?
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.
Action Potentials In Pacemaker Cells (SA node and AV node)
These (Myocardial Autorhythmic) cells produce unstable membrane potentials called PACEMAKER POTENTIALS that are capable of spontaneously producing action potentials.
Pacemaker potentials
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)
List the 3 steps of Action Potentials In Pacemaker Cells (SA node and AV node)
a. Pacemaker potential
b. Depolarization phase
c. Repolarization phase
Step 1. Pacemaker potential
- If (funny) channels open (when the cell MP is -60mV), and Na+ enters the cell causing a slow depolarization toward threshold.
Step 2. Depolarization phase
- 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).
Step 3. Repolarization phase
- 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).
If (funny) channels
permeable to both K+ & Na+
- allow current (I) to flow
- have unusual properties
What are the Action Potentials In Myocardial Contractile Cells Phases?
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
Phase 4: Resting Membrane Potential
Is -90mv in contractile cardiac cells
Phase 0: Depolarization phase
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.
Phase 1: Initial Repolarization Phase
Na+ channels close, fast K+ channels open, K+ leaves cell causing repolarization.
Phase 2: Plateau
Voltage gated Ca++ channels open, fast K+ channels close. Ca++ entry and decreased permeability to K+ prolongs depolarization.
Phase 3: Repolarization phase
Voltage gated slow K+ channels open, K+ exits cell and membrane potential returns to resting levels.
Brief description of the APs in Myocardial Contractile Cells phases 0-4
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
The RAPID _______ phase of the AP is the result of Na+ ______, & the STEEP ______ phase is due to K+ ______ the cell
DEPOLARIZATION
ENTRY
REPOLARIZATION
LEAVING
Why do myocardial cells have a LONGER AP?
due to Ca2+ entry
Cardiac contractile cell action potentials are much longer in duration (~250 msec) than in skeletal muscle cells (1-2 msec) due what?
due to the PLATEAU PHASE
What 2 imp. effects does the long APs have?
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
What is prevented by the long absolute refractory period?
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
The long absolute refractory period allows…
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).
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)?
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.
