Physio Ch. 13 Part 1 Electrical Activity in the Heart & ECG

Cardiovascular System

• Made of the heart and blood vessels

• System that moves blood around the body

- Brings oxygen and nutrients to tissue

- Removes carbon dioxide and waste from tissues

- Regulation of blood volume and pressure

- Bodily communication via hormone transport

Atrioventricular (AV) valves

• Valves that separate atria and ventricles

- Right AV valve / tricuspid valve

- Left AV valve / bicuspid valve / mitral valve

Semilunar valves

• Valve between ventricles and vessels they empty into

- Right ventricle: Pulmonary valve

- Left ventricle: Aortic valve

Right AV valve/tricuspid valve

• Located between the right atria and ventricle

• Opens passively

- When atrial pressure is greater than ventricular

• Cause first heart sound when they close

- “Lub”

Left AV valve/bicuspid valve/mitral valve

• Located between the left atria and ventricle

• Opens passively

- When atrial pressure is greater than ventricular

• Cause first heart sound when they close

- “Lub”

Pulmonary valve

• Located between the right vertical and pulmonary trunk

• Opens passively

- When ventricular pressure is greater than arterial pressure

• Cause second heart sound when they close

- “Dub”

Aortic valve

• Semilunar valve

• Located between the left vertical and aorta

• Opens passively

- When ventricular pressure is greater than arterial pressure

• Cause second heart sound when they close

- “Dub”

Pacemaker/nodal cells

• Located throughout the heart but concentrate in Sinoatrial (SA) node and Atrioventricular (AV) node

• Able to generate their own action potential

Sinoatrial (SA) node

• Bundle of pacemaker cells that act as the pacemaker of the heart

• Sets heart rate at 70-80 AP per minute

• Makes a pacemaker potentials

Atrioventricular (AV) node

• Bundle of pacemaker cells that act as the pacemaker of the heart

• Sets heart rate at 40-60 AP per minute

• Acts as the backup system for the pacemakers

- “Ectopic pacemaker”

Conduction fibers

• Long cells that spread out across the heart to conduct electrical impulses

• Large diameter for fast conduction

Myocardial contractile cells

• The muscle cells of the heart that contract

Electrical synapses

• Gap junctions between heart muscle cells

- Allow for the fast conduction of electrical impulses

Electrical Events of a Heartbeat

• Step 1: SA node initiates an AP

• Step 2: AV node transmits AP

• Step 3: Impulse travels through atrioventricular bundle/bundle of His in interventricular septum

• Step 4: Impulse travels down the left and right bundle branches to each ventricle

• Step 5: Impulse spreads through the Purkinje fibers (“subendocardial conducting network”) throughout the ventricular myocardium

AV bundle/Bundle of His

• Produces 20-40 AP per minute

Purkinje fibers

• Produces 30-40 AP per minute

• Acts as the backup system for the pacemakers

- “Ectopic pacemaker”

Pacemaker potential

• A slow depolarization that is sent out to muscular cells of the heart after contraction

• Continues until threshold is reached

Ion Channel Changes in Pacemaker Cell

• Use K, Na, Ca to drive action potential production

• Multi step process that changes permeability

- Step 1: Initial spontaneous depolarization, K permeability decreases and Na permeability increases

- Step 2: Later spontaneous depolarization, Ca permeability increases and Na permeability decreases

- Step 3: Rapid depolarization, Ca permeability increases

- Step 4: Repolarization, Ca permeability decreases and K permeability increases

Funny channels

• Channels that are are necessary for pacemaker cells to produce and action potential

- Responsible for initial period of spontaneous depolarization

• Allows Na to move in and K to move out

- Na moves in at much greater value and depolarized the cell

• Open as soon as cell hyper-polarizes to -70mV and closes when the charge nears -55mV

T-type calcium channels

• Channels that are are necessary for pacemaker cells to produce and action potential

- Responsible for later period of spontaneous depolarization

• Allows Ca to move in

• Open as soon as cell hyper-polarizes to -55mV

L-type calcium channels

• Channels that are are necessary for pacemaker cells to produce and action potential

- Responsible for rapid depolarization phase of action potential production

• Allows large amounts of Ca to move in

• Open as soon as cell reaches threshold and close as soon as cell reaches peak of AP

Potassium channels effect on AP production

• Channels that are are necessary for pacemaker cells to produce and action potential

- Responsible for repolarization phase of action potential production

• Allows large amounts of K to move out of the cell

• Open as soon the cell reaches peak of AP and closes when cell has repolarized to -70mV

Refractory in Cardiac Contractile Cells

• Long refractory period prevents summation and tetanus

Cardiac Contractile Cell’s response to Action Potential

• Multi step process

- Step 0: Depolarization phase

- Step 1: Brief drop in potential

- Step 2: Plateau phase

- Step 3: Repolarization of membrane potential

- Step 4: Resting membrane potential

Depolarization phase

• Phase 0 of Cardiac Contractile Cell Action Potential

• Na channels open

- Peak of 40mV caused

Brief drop in potential

• Phase 1 of Cardiac Contractile Cell Action Potential

• Na channels close and depolarization is set in motion

- L calcium channels open and membrane depolarizes

Plateau phase

• Phase 2 of Cardiac Contractile Cell Action Potential

• Membrane stays in a depolarized state

- K channels stay closed

- Ca channels stay open

Repolarization of membrane potential

• Phase 3 of Cardiac Contractile Cell Action Potential

• Potassium channels “delayed rectifier channels” open

• Inward rectifier channels open

• Calcium channels close

Resting membrane potential

• Phase 4 of Cardiac Contractile Cell Action Potential

• All ions are at resting values

• -90mV

Excitation-Contraction Coupling in Cardiac Contractile Cells

• AP spreads to cardiomyocyte through gap junctions which causes depolarizes to threshold

- AP triggers opening of voltage-gated calcium channels on SR + PM

• Calcium removal from cytosol

- Ca2+-ATPase in SR membrane

- Ca2+-ATPase in PM

- Na+-Ca2+ exchanger in PM

Electrocardiogram (ECG/EKG)

• Monitors electrical activity of heart

- Record of the overall spread of electrical current through the heart as a function of time during the cardiac cycle

P wave

• Part of an electrocardiogram reading

• Upward deflection caused by atrial depolarization

QRS complex

• Part of an electrocardiogram reading

• Sharp upward and downward deflections, ventricular depolarization

• Corresponds to phase 0 of ventricular contractile cell AP

• Atrial repolarization occurs at this time but is usually not detected by ECG

T wave

• Part of an electrocardiogram reading

• Upward deflection, ventricular repolarization

• Corresponds to phase 3

Isoelectic line

• Part of an EKG

• Horizontal line between waves, no electrical activity occurring

P-Q/P-R interval

• Between onset of P wave and onset of QRS complex

• Estimate of time of conduction through AV node

• Atrial systole

Q-T interval

• From onset of QRS complex to end of T wave

• Estimate of time ventricles are contracting

T-Q segment

• From end of T wave to beginning of QRS complex

• Estimate of time ventricles are relaxing

R-R interval

• Between peaks of two successive QRS complexes

• Time between heartbeats

Bradycardia

• Slow heart rate

- Below 50 bpm

Tachycardia

• Fast heart rate

- Above 100 bpm

Ventricular tachycardia

• Increased rate of contraction in ventricles

• Leads to ventricular fibrillation and death

Flutter

• Extremely fast (200 to 300 bpm) but coordinated contractions

Fibrillation

• Uncoordinated pumping between two similar heart valves

- Either both atria’s or both ventricles

Atrial fibrillation

• Atrial muscle fibers depolarize independently

- Not deadly as long as ventricular contraction remains functional

Ventricular fibrillation

• Ventricular muscle fibers depolarize

independently → blood cannot be efficiently pumped to tissues

• Defibrillation: apply large external current to depolarize all muscle cells at the

same time & return synchronous electrical activity to heart

Chordate tendineae

• Tendons that extend from the ventrals to the AV valves

• Pull downward on the valve cusps to preventing the AV valves from being pushed into the atria (prolapsing)