Neurons, Synapses & Cardiac Action Potential

• Conduction pathway of the heart focuses on electrical activity and refractoriness of cardiac cells
• Discusses ECG interpretation, lead placement, and rhythm analysis

Refractoriness

• The period of recovery cells need after being discharged so they can to respond to stimulus
• Refractory period is longer than contraction

Absolute Refractory Period

• Effective Refractory Period
• Cell can't respond to further stimulation
• Myocardial mechanical cells can't contract
• Electrical conduction system can't conduct electrical impulse
• Tetanic contractions can't be provoked in cardiac muscle
• Phases 0-3 of Cardiac Action Potential
• From the onset of the QRS complex to the peak of the T wave

Relative Refractory Period

• Vulnerable period
• Cardiac cells have repolarized to their threshold potential
• Cells can be stimulated to respond (depolarize) to stronger than normal stimulus
• Appears during downslope of the T wave

Supernormal Period

• After relative refractory period
• Weaker than normal stimulus depolarizes cardiac cells
• Dysrhythmias develop
• End of the T wave

Conduction System: Sinoatrial (SA) Node

• SA node firing leads to electrical impulse to produce normal heart rate
• Located in the upper posterior right atrium
• Less than 1 mm from epicardial surface
• Richly supplied by para/sympathetic nerve fibers
• HR decreases from vagus nerve stimulation and during rest/sleep
• HR increases from sympathetic activity, exercise, and stress
• Receives blood from SA node artery
• SA node artery originates from R coronary artery in 60% of people
• Fibers from SA node connect directly with atrial fibers
• SA node leads to R atrium, interatrial septum, and L atrium
• In adults, it's usually 10-20mm long and 2-3 mm wide
• Intrinsic rate: 60-100 bpm
• Primary pacemaker due to fastest firing rate
• Other pacemakers take over if SA node fails to fire, fires too slowly, or fails to activate surrounding atrial myocardium

Impulse Pathways/Internodal Pathways

• Leads to almost simultaneous contraction of R and L atria
• Fibrous skeleton separates atria from ventricles
• 3 internodal pathways (anterior, middle, posterior) exist before atrial depolarization is complete
• Pathways consist of working myocardial cells and specialized conducting pathways
• Anterior internodal pathway: Bachmann's bundle conducts impulses to the left atrium
• Middle internodal pathway: Wenckebach's bundle
• Posterior internodal pathway: Thorel's pathway
• Internodal pathways gradually merge with cells of the AV node

Atrioventricular (AV) Junction

• Includes AV node and nonbranching portion of Bundle of His
• Specialized conduction tissue provides electrical links between atria and ventricles

Accessory Pathway

• Abnormal route; bypasses AV junction
• Extra bundle of working myocardial tissue; connects atria and ventricles outside of normal conduction system

Atrioventricular Node

• A group of cells in floor of R atrium behind tricuspid valve and near opening of coronary sinus
• Depolarization and repolarization are slower, vulnerable to AV blocks
• Supplied by R coronary artery in 85-90% of people
• Others are supplied by left circumflex artery
• Sympathetic and parasympathetic nerve fibers supplied
• Atrial impulse to AV node delays impulse to ventricles
• Fibers in AV junction are smaller than atrial with fewer gap junctions
• The delay is for atria and ventricles to contract at different times so atria can empty blood into ventricles before next ventricular contraction
• Increases blood volume in ventricles, increasing SV
• Divided into 3 functional regions according to APs and electrical/chemical stimulation

AV Node Functional Regions

• Atrionodal (AN) region: Upper regional region, AKA Transitional zone
• Nodal (N) region: Midportion of AV node
• Nodal-His (NH): Lower junctional region where fiber of AV node merge gradually with bundle of His
• Primary delay occurs in AN and N areas

Bundle of His

• After passing AV node → bundle of His
• Located in upper portion of interventricular septum
• Bundle of His pacemaker cells discharge at intrinsic rate of 40-60 bpm
• Conducts electrical impulse → R and L bundle branches
• Normally the only electrical connection between atria and ventricles
• Receives dual blood supply from branches of left anterior and posterior descending coronary arteries, which are less vulnerable to ischemia

His-Purkinje System

• His-Purkinje network
• Bundle of His + Bundle branches + Purkinje fibers

Right & Left Bundle Branches

• Right Bundle Branch innervates R ventricle.
• Left Bundle Branch spreads impulse to interventricular septum and L ventricle (L ventricle thicker than R ventricle)
• Left Bundle Branch divides into 3 divisions (fascicles)

Fascicles

• Three small bundles of nerve fibers
• Anterior fascicle spreads impulses to anterior portions of L ventricle
• Posterior fascicle relays impulses to posterior portions of L ventricles
• Septal fascicle relays impulses to the mid septum

Purkinje Fibers

• Bundle Branches divide into smaller branches, then into Purkinje fibers
• Fibers spread from interventricular septum → papillary muscles → down to apex of heart
• Elaborate web penetrates half way into ventricular muscle mass
• Fibers continuous with muscle cells of R and L ventricles
• Pacemaker cells fire at 20-40 bpm
• Electrical impulse spreads rapidly → R/L Bundle Branch → Purkinje → ventricular muscle
• Endocardium → myocardium → epicardial surface
• Ventricular walls stimulated to contract in twisting motion to wring blood out of ventricles, which forces them into the arteries

Summary of Pacemaker Sites

• SA node: Right atrial wall - just inferior to superior vena cava; primary pacemaker; impulse to atria at 60-100 bpm
• AV junction: Floor of right atrium behind tricuspid valve and near the coronary sinus; impulse from SA node is delayed and relayed to Bundle of His so atria contract before ventricles - 40-60 bpm
• Bundle of His: Superior portion of interventricular septum; receives impulse from AV, relays to R and L Bundle Branches
• Bundle Branches: Interventricular septum; receives impulse from Bundle Branches and relays to Purkinje fibers
• Purkinje fibers: Ventricular myocardium; receives from Bundle Branches and relays to ventricular myocardium - 20-40 bpm

ECG Intervals

• PR interval: 0.12-0.20 sec (3-5 small blocks)

Sinus Rhythm Characteristics

• QRS complex/interval: narrow complexes with uniform shape; regular spacing; less than 0.12 seconds
• P wave: upright and rounded, married to QRS
• PR interval: 0.12-0.20 sec, constant from beat to beat.
• HR: 60-100.

Bipolar Leads

• Standard limb leads using positive and negative electrodes
• Record the difference in electrical potential between 2 selected electrodes
• Require right arm, left arm, and left leg.

Bipolar Leads and Limbs

• Lead I: Difference between left arm (+) and right arm (-)
• Lead II: Difference between left leg (+) and right arm (-)
• Lead III: Difference between left leg (+) and left arm (-)
• Limb leads are ROMAN numerals, precordial are ARABIC numerals.

Precordial Leads

• 6 unipolar leads that see heart from horizontal plane; all positive

Precordial Lead Locations

• V1: right of sternum
• V2: left of sternum
• V3: In between V2 and V4
• V4: 5th intercostal, mid-clavicle
• V5: Anterior axillary line
• V6: Past the anterior axillary line

Leads for Continuous Monitoring

• Most commonly used:
  • Lead II has positive electrode on left abdomen, negative electrode on right shoulder and ground electrode on left shoulder.
  • MCL₁ has positive electrode at 4th ICS RSB, negative electrode on left shoulder and ground electrode on right shoulder.
  • MCL₁ is modified chest lead 1 and is like V1.

Electrocardiographic Truths

• Positive QRS is from impulse traveling towards positive electrode
• Negative QRS is from impulse traveling away from the positive electrode
• Isoelectric QRS is from an impulse traveling perpendicular to the positive electrode (no electrical activity)
• Flat line is when there is no impulse at all

Explore neuron signal transmission: structure sequence, Nodes of Ranvier, and myelin sheath damage. Understand synapses, graded responses, and cardiac action potential phases, including the role of the SA and AV nodes.