BSI: Rhythmic Excitation of the Heart

SA node

Internodal pathways

• AV node
• -Transmission through AV node is slower due to smaller fibers and fewer gap junctions
• - Delay through AV node allows atria to contract before the ventricles contract

• Bundle of His
• - Conductance of action potential very fast

Right and left bundle branches

Perkinje fibers

Transmission through AV node is slower due to smaller fibers and fewer gap junctions.

The delay through AV node is important as it allows atria to contract before the ventricles contract.

• The SA node is the natural pacemaker of heart:
• 1) Specialized muscle tissue with no contractile proteins
• 2) Fibers of SA Node connect directly to regular myocardial fibers of atria
• 3) Action potential that is generated in SA node fibers travels immediately to atrial fibers to cause atrial depolarization
• 4) Autorhythmicity
• - Fibers of SA Node fire action potentials in rhythmic pattern 60-100 beats/min via self excitation
• - Action potential of autorhymic cell

• Mechanism of self-excitation of autorhythmic cell:
• 1) Fibers of SA node leaky to Na⁺
• 2) Resting membrane potential less negative (-55 mV) in comparison to other fibers (-90 mV)
• 3) Causes resting potential to creep up toward threshold (-40 mV)

Heart Rate is set by the SA Node.

Although the heart rate is set by SA node, the rate and contractility can be regulated by the Autonomic Nervous System (ANS), which includes the Sympathetic (SNS) and the Parasympathetic (PNS).

Our heart rhythms are controlled by a number of factors but most powerfully by the interplay between the SNS and PNS branches of the autonomic nervous system. The SNS accelerates the heart; whereas the PNS acts as a brake a decelerates the heart.

When the two systems are working in proper balance, the heart becomes exquisitely responsive to the body’s ever changing needs and yet maintains a strong underlying stability.

When the two systems are not working well together, the heart shows a poor response to changing body needs and less stability in heart rate at rest. Chronic stress leads to autonomic imbalance with increased SNS activity and reduced PNS influence (Note: Imagine a powerful car with a very sensitive and twitchy gas pedal and very poor brakes.) and shows up as increased resting heart rate and reduced heart rate variability.

• The SNS increases heart rate and contractility:
• 1) Release of Norepinephrine (also called Noradrenaline, secreted by the adrenal medulla and the nerve endings of the SNS) increases Na⁺ and Ca⁺⁺ permeability.
• 2) In SA node, Na⁺ increases resting membrane potential and increases rate of drift towards threshold.
• 3) Ca⁺⁺ increases contractility of muscle fibers.

• The PNS decreases heart rate and contractility:
• 1) Vagus nerve releases acetylcholine.
• 2) Increased permeability to K+ causes hyperpolarization.
• 3) The rate of SA node firing decreases.
• 4) Decreased excitability of AV junctional fibers slows the transmission of electrical impulse.

Norepinephrine (NE), also known as noradrenaline is involved in increasing the heart rate and contractility. NE is released from the adrenal medulla and the nerve endings of the SNS.

NE affects the leakiness of Na⁺ channels, increasing the spontaneous firing of action potentials and therefore increasing HR.

Acetylcholine is involved in decreasing the heart rate and contractility. The vagus nerve releases acetylcholine.

Acetylcholine affects permeability to K+ channels, hyperpolarizing the cell and taking the resting membrane potential further away from threshold.

NE affects the leakiness of Na⁺ channels, increasing the spontaneous firing of action potentials and therefore increasing HR.

Acetylcholine affects permeability to K⁺ channels, hyperpolarizing the cell and taking the resting membrane potential further away from threshold.

An action potential of a contractile myocyte leads to Cardiac Muscle Contraction.

• The contraction process is very similar to skeletal muscle. However, there are some differences:
• 1) Action potential can propogate from fiber to fiber via gap junctions.
• 2) Long action potential with plateau.
• 3) Ca²⁺ comes from SR and extracellular fluid.

• Neuronal cell depolarization:
• 1) Ligand-gated Na⁺ open and bring the cell toward threshold.
• 2) Once the cell reaches threshold, a huge influx of voltage-gated Na⁺ channels open and depolarize the cell.
• 3) Na⁺ channels close and voltage gated K+ channels open and repolarize the cell.

• Cardiac myocyte depolarization:
• 1) Leaky Na⁺ channels bring the cell towards threshold.
• 2) Once the cell reaches threshold, voltage gated Na⁺ channels open and the cell is rapidly depolarized.
• 3) Na⁺ channels close and K⁺ channels open.
• 4) Slow "L-gated"Ca²⁺ channels open and cause a slow plateau unique to cardiac myocyte.
• 5) Ca²⁺ channels close and the rest of K⁺ channels open.

The SA node causes ions (Na⁺ and Ca²⁺) to diffuse to neighboring cardiac muscle cells.

The cell starts to depolarize, and neighboring cells allow ions to flow in from cell to cell, causing a wave of positive depolarization.

• Phase 0
• The opening of voltage gated Na⁺ channels causing rapid depolarization.

• Phase 1
• Na⁺ channels close, and K⁺ channels open.

• Phase 2
• Plateau Phase that is UNIQUE to cardiac cells. Ca²⁺ channels open SLOWLY because they are "L-type" Ca²⁺ channels.

• Phase 3
• Ca²⁺ channels close and the rest of K⁺ channels open.

• Phase 4
• Na⁺/K⁺ pump brings the cell back to equilibrium resting conditions.

Intracellular Ca²⁺ comes from the SR and extracellular fluid.

An ectopic pacemaker describes other autorhythmic pacemakers of the heart other than the SA node. Ectopic pacemakers can result in an abnormal sequence of contraction.

The normal pacemaker of the heart is via the SA node (60-100 beats/min). Compare that rate to abnormal pacemakers (ectopic pacemakers):

• 1) AV node (40-60 beats/min)
• 2) Perkinje fibers (15-40 beats/min)
• 3) Atrial or ventricular muscle tissue (~20 beats/min)

Records electrical activity within myocardial muscle fibers (not the conductive system).

A normal EKG consists of a P-QRS-T complex.

• X axis: Time
• - Each small box (1 mm) = 0.04 seconds
• - Each large box = 0.20 seconds

• Y axis: Voltage (mV)
• - Each small box = 0.10 mV
• - Each large box = 0.5 mV

A P wave represents atrial depolarization.

Normally is less than < 0.12 seconds (less than three small boxes).

The QRS complex represents ventricular depolarization.

Normally is 0.08 to 0.10 seconds (about two or two and half small boxes).

• Q wave: first downward deflection
• R wave: first upward deflection
• S wave: downward deflection following R wave

No. The Q wave, which represents the first downward deflection on an EKG is often not present.

The T wave represents ventricular repolarization.

Generally, a T wave should go in the same direction as the QRS complex.

The P-Q or P-R interval represents the time it takes from the beginning of P wave (atrial depolarization) to the beginning of QRS complex (ventricular depolarization).

Normally is 0.12 to 0.20 seconds (three to five small boxes)

Normally is 0.12 to 0.20 seconds (three to five small boxes). This is important to know because an unusual time could signify an AV block.

The Q-T interval represents the time between the beginning of ventricular depolarization and the end of ventricular repolarization.

Normally is < 0.45 seconds (a little more than two large boxes).

Normally is < 0.45 seconds (a little more than two large boxes). This is important to know because this is the time it takes for the ventricles to contract.

The S-T segment represents the time between the end of ventricular depolarization and beginning of ventricular repolarization.

Normally is around 0.32 seconds (almost two large boxes).

A wave of depolarization causes a wave of positive charges within the myocardial muscle fibers.

A wave of repolarization causes a wave of negative charges inside of myocardial muscle fibers

Neither...they are equally important. Relaxation is just as important as contraction so that the ventricles have time to fill.

The absolute refractory period is extremely important because it allows the heart muscle to relax so that the heart can fill.

Tetanic contraction, which is very bad and will lead to a myocardial infarction (heart attack) if not treated immediately.

These waves can be detected and recorded by electrodes placed on the skin.

As a wave of positive charge (depolarization) moves toward positive electrode, get an upward or positive deflection on EKG.

If wave of depolarization moves away from positive electrode, get downward or negative deflection on EKG.

The wave returns to baseline when there is no difference in charge (complete depolarization or repolarization).

A wave of depolarization is a wave of positive charges.

If a positive depolarization wave is going towards a positive electrode, then there will be a positive or upward deflection on the EKG.

If opposite charges are going towards each other, such as in a negative repolarization wave is going towards a positive electrode, then there will be a negative or downward deflection on the EKG.

• Six limb leads:
• 1) Lead I
• 2) Lead II
• 3) Lead III
• 4) Lead AVR
• 5) Lead AVL
• 6) Lead AVF

• Six chest leads:
• 1) V₁
• 2) V₂
• 3) V₃
• 4) V₄
• 5) V₅
• 6) V₆

For all of the six limb leads, electrodes are placed on the right arm, left arm, and left leg to form a triangle.

For the three bipolar limb leads, each side of the triangle is formed by two electrodes; each pair of electrodes is a lead.

• The bipolar limb leads:
• Lead I: right arm (-) to left arm (+)
• Lead II: right arm (-) to left leg (+)
• Lead III: left arm (-) to left leg (+)

For all of the six limb leads, electrodes are placed on the right arm, left arm, and left leg to form a triangle.

For the unipolar limb leads, each side of the triangle is formed by three electrodes; each triplet of electrodes is a lead.

• The unipolar limb leads:
• Lead AVF: left foot (+) and both left arm & right arm electrodes are common ground (-)
• Lead AVL: left arm is (+) and right arm & left foot common ground(-)
• Lead AVR: right arm is (+) and left arm & left foot common ground(-)

In all six chest leads (V₁-V₆), each chest electrode is positive (+).

• The six chest leads are placed at six positions around the chest.
• 

A depolarization wave moving toward a given chest electrode produces a positive (upward) deflection.

Chest leads are projected through the AV node towards the patient’s back, which is the negative end of each chest lead.

The plane of the chest leads cuts the body into superior and inferior halves and is called horizontal plane.

• V₁: the QRS will be mostly negative
• V₅ and V₆: the QRS will be mostly positive
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• As progress from V₁ to V₆, the QRS will progressively become more positive because wave of depolarization through ventricle is most directly moving toward V₅ and V₆ and moving away from V₁.
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By superimposing the directionality of all six limb leads, we can get a pinwheel which you should memorize!