17.3 - Cardiac Muscle Tissue + Electrophysiology

What are the characteristics of Cardiac Muscle Cells?

• Branched

• Intercalated Disks

• Striated

• Single Nucleus

• Increased mitochondria = increased myoglobin for energy

• Makes ATP with fatty acids + glucose

What are the characteristics of Intercalated Disks?

• Connects pacemaker cells to contractile cell and connects contractile cells to each other.

• Contains desmosomes to hold cardiac muscle cells together

• Contains gap junctions to allow ions to pass from cell-cell.

  • They are electrical synapses; allows electrical activity to spread to al cardiac muscle cell.s

  • Allows heart to contract as a unit.

Contractile Cells:

• 99% of all cardiac cells

• Has an RMP

• Has a plateau phase

• Responsible for the contractions

Autorhythmic (Pacemaker) Cells

• 1% of all cardiac cells

• No RMP

• Has a prepotential phase

• Makes up conduction system

• Generates spontaneous action potentials that spread to contractile cells

Voltage Gated Na+ Channels

• Recall what it does

• Where are they found in this case?

Found in all cells except certain pacemaker cells

Voltage Gated Ca++ Channels

• Function

• Which cells contain them

• Voltage-gated Opening

• Time-gated Closing; closes after period of time regardless of the voltage.

• Present in all cardiac muscle cells.

K+ Ion Channels

• In all cardiac muscle cells

• Ligand or voltage gated

HCN Channel

• Unique to pacemaker cells

• Voltage-gated channel

• Activates upon hyperpolarization

• Causes Na+ to enter the cell while K+ still exists upon opening

Voltage

Difference in electrical potential between 2 points

Membrane Potential

Voltage/Charge difference that exists across membranes of all cells

Resting Membrane Potential

Membrane potential of a cell at rest

What is Pacemaker Potential?

• Action Potential in pacemaker cells

• AP results from reversal in membrane potential = from -60 mV to 10 mV

Characteristics of pacemaker cells electrically:

• Depolarization occurs slowly; lacks Na+ channels in the sarcolemma.

• Naturally hyperpolarize

• Contains HCN channels; causes cell to depolarize slowly to threshold upon opening of the channel

Steps to Pacemaker Potential:

1. Slow Initial Depolarization

   • Pacemaker potential is hyperpolarized = min. potential @ -60 mV

   • Hyperpolarization triggers voltage-gated HCN channels to open.

   • Na+ leak into cell faster than K+ leaks out = slow depolarization.

2. Full Depolarization

   • Membrane reaches threshold @ -40 mV.

   • Voltage-gated Ca++ open and enter the cell.

   • Membrane fully depolarizes.

3. Repolarization

   • Ca++ are time gated to close

   • Voltage-gated K+ channels open as Ca++ close = K+ exits the cell.

   • Membrane repolarizes

4. Minimum Potential Phase

   • K+ channels remain open until reaching min. potential.

   • Hyperpolarization triggers K+ to close.

• loops again

What is the Cardiac Conduction System?

Group of interconnected pacemaker cells that generate action potential and spread it to other parts of the cardiac conduction system

What are the 3 populations of pacemaker (autorhythmic) cells that make up the cardiac conduction system?

1. Sinoatrial Node (SA Node) = Primary pacemaker

2. Atrioventricular (AV Node) = Secondary pacemaker

3. Purkinje Fiber System = Slowest group of pacemaker

What is the Sinoatrial Node (SA Node)?

• Location

• Rate of depolarization

• Influence by

• Permeability

• Connected to what?

• The Primary Pacemaker

• Located in right atrium

• Cells of SA node have fastest rate of depolarization = 60-70 AP/min

• Influenced by PNS and SNS

• More permeable to Na+ = easier to increase MP and reach threshold = AP

• Connects do internodal pathways and the AV node; not directly connected to contractile cells

What is the Atrioventricular Node (AV Node)?

• Location

• Rate of depolarization

• Secondary Pacemaker

• Located between right atrium and ventricle.

• Rate of depolarization = 40-50 AP/min

• Takes over is generating AP if SA Node is nonfunctional

Purkinje Fiber System:

• Rate of depol.

• How are they different than the SA and AV Node

• Components

• Slowest pacemaker cells

• Rate of depolarization = 20 AP/min

• Atypical Pacemakers = AP rely on different ion channels and function differently.

• 3 components:

  • AV Bundle (Bundle of His)

  • Left and Right Bundle Branches

  • Terminal Branches

What is the AV Bundle (Bundle of His) of the Purkinje Fiber System?

• Component of the Purkinje Fiber System that penetrates the heart's skeleton in the inferior interatrial and interventricular septum.

• Traveling across the cardiac skeleton causes the signal to slow down

What are the Left and Right Bundle Branches of the Purkinje Fiber System?

• Component that courses along left and right sides of the interventricular septum

• Carries signal from interventricular septum to the apex of the heart

What are the Terminal Branches of the Purkinje Fiber System?

Component of the Purkinje Fiber System that penetrates the ventricles and contacts the contractile cells

What are Internodal Pathways?

• Pathways that connect directly to contractile cells.

• Spreads the AP that originated from the SA node --> contractile cells of the atria

What are Purkinje Cells?

• Cells that connect directly to contractile cells.

• Moves AP from apex --> atria.

What are the steps of the Conduction Pathway through the Heart?

• Uses the Cardiac Conduction System

1. SA Node generates AP = 60-70 per minute

   • AP spreads to atrial cells.

   • Impulses conduct to the AV Node.

     
     

2. AP conduction slows down at AV Node = 40-50 per minute.

   • Allows atria to depolarize and contract before the ventricles; gives ventricles time to fill.

   • Prevents backflow into atria.

     
     

3. AP conducts to R + L bundle branches of the Purkinje Fiber System = 20 AP/min.

   • AP conducts to Purkinje fibers = spreads to contractile cells of the ventricles

Characteristics of Contractile Cell AP:

Pacemaker potential conducts down and triggers action potentials of contractile cells to occur

Phases of Contractile Cell Action Potential:

1. Rapid Depolarization Phase

   • Pacemaker AP conducts down to contractile cells, activating Na+ in sarcolemma.

   • Influx of Na+ cause rapid depol. of membrane from its RMP of -85 mV.

     
     

2. Initial Repolarization Phase

   • Small, initial repolarization immediately after depolarization.

   • Caused by abrupt inactivation of Na+ and small outflow of K+ through channels that were briefly opened.

     
     

3. Plateau Phase

   • Depol. sustained at about 0 mV.

   • Caused by opening of Ca++ channels and influx of Ca++.

   • As Ca++ enter, K+ leave = very little net change in MP.

   • Ca+ close slowly and timely, allowing the plateau phase to last longer.

     
     

4. Repolarization Phase

   • Na+ and Ca++ channels close

   • K+ channels open = K+ efflux

   • MP returns to RMP of -85 mV

Why is there a plateau phase in contractile cell AP?

• Lengthens the cardiac AP = slows heart down to provide the time required for the heart to fill with blood.

  • Prolonged AP makes the twitch last long = develops more force (strength)

  • Prolonged AP allows Ca++ to enter, which is needed for the sliding-filament mech. of sarcomere.

What is the Effective Refractory Period?

• The phase during an AP in which the cell cannot generate more AP.

• Plateau phase lengthens during this period in contractile cell AP.

• Prevents cells from entering a tetany state = sustained contraction.

• Allows heart to relax = ventricles fill with blood.

What is an Electrocardiogram (ECG/EKG)?

• Function

• Different Waves

• Graphic depiction of electrical activity in cardiac muscle cells over a period of time.

  • Does NOT display mechanical events.

• Waves:

  • P Wave

  • QRS Wave

  • T Wave

What is the P Wave of the ECG?

• Atrial Depolarization; except the SA node.

  • = Atria are contracting; but is not depicted in ECG.

What is the QRS Complex of the ECG?

• Ventricular Depolarization depicted in 3 separate waves.

  • Q = 1st downward deflection

  • R = Large upward deflection

  • S = 2nd downward deflection

  • QRS waves are represented with a large magnitude due to the ventricles being much larger than the atria.

  • = ventricles are contracting mechanically

    
    

• Atrial Repolarization

  • Produces small electrical signal, but is hidden by the QRS complex.

  • = atria are relaxing mechanically

What is the T Wave of the ECG?

• Ventricular Repolarization

  • Upward Deflection; sometimes inverted

  • = ventricles are relaxing mechanically

ECG Waves