Chapter 20 Lecture 4 Flashcards by Katie Hostetter

Conduction System of Heart

Coordinates contraction of heart muscle.

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Autorhythmic Cells

Cells fire spontaneously, act as pacemaker and form conduction system for the heart

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SA node

cluster of cells in wall of Rt. Atria
begins heart activity that spreads to both atria
excitation spreads to AV node

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AV node

in atrial septum, transmits signal to bundle of His

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AV bundle of His

the connection between atria and ventricles

divides into bundle branches & purkinje fibers, large diameter fibers that conduct signals quickly

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Timing of Atrial & Ventricular Excitation

SA node setting pace since is the fastest
In 50 msec excitation spreads through both atria and down to AV node
100 msec delay at AV node due to smaller diameter fibers- allows atria to fully contract filling ventricles before ventricles contract
In 50 msec excitation spreads through both ventricles simultaneously

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Physiology of Contraction

Depolarization, plateau, repolarization

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Depolarization

Cardiac cell resting membrane potential is -90mv
excitation spreads through gap junctions
fast Na+ channels open for rapid depolarization

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Plateau Phase:

250 msec (only 1msec in neuron) 
slow Ca+2 channels open, let Ca +2 enter from outside cell and from storage in sarcoplasmic reticulum, while K+ channels close
Ca +2 binds to troponin to allow for actin-myosin cross-bridge formation &amp; tension development

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Repolarization:

Ca+2 channels close and K+ channels open & -90mv is restored as potassium leaves the cell

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Refractory period:

very long so heart can fill

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action potential in cardiac muscle:

changes in cell membrane permeability

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EKG (Electrocardiogram)

Action potentials of all active cells can be detected and recorded

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P wave

atrial depolarization

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P to Q interval:

conduction time from atrial to ventricular excitation

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T wave

ventricular repolarization

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Cardiac Cycle:

At 75 beats/min, one cycle requires 0.8 sec

systole (contraction) and diastole (relaxation) of both atria, plus the systole and diastole of both ventricles

End diastolic volume (EDV)

volume in ventricle at end of diastole, about 130ml

End systolic volume (ESV)

volume in ventricle at end of systole, about 60ml

Stroke volume (SV)

the volume ejected per beat from each ventricle, about 70ml

SV = EDV - ESV

Phases of Cardiac Cycle:

Isovolumetric relaxation
Ventricular filling
Ventricular systole

Isovolumetric:

brief period when volume in ventricles does not change– ventricles relax, pressure drops and AV valves open

Ventricular filling

as blood flows from full atria
diastasis: as blood flows from atria in smaller volume
atrial systole pushes final 20-25 ml blood into ventricle

Ventricular systole

isovolumetric contraction

ventricular ejection: as SL valves open and blood is ejected

Fibrillation

erratic heartbeat, uncoordinated contractions.

Atrial (AF, A-fib) =

interference with AV node  erratic, incomplete atrial contractions. Often secondary to other heart problem, and rarely life-threatening. - Treatment: drug/electro-cardioconversion; anti-coagulants - Symptoms: none, palpitations, fainting, nausea, chest pain; can lead to stroke and congestive heart failure.

Ventricular fibrillation (VF, V-fib) =

erratic, incomplete ventricular contractions. Immediately life-threatening due to lack of somatic, pulmonary, and cardiac circulation; often secondary to other heart disease. - Symptoms: sudden collapse; death often first “symptom.” - Treatment: electric cardioconversion, then treat underlying issue

Tachycardia:

rapid (especially ventricular) heartbeat. - Symptoms: range from faintness, short-of-breath to sudden death, depending location and cause. - Treatment: drug or electric cardioconversion, then treat underlying issue

bradycardia:

slow heartbeat

Blood pressure in aorta

is 120mm Hg

Blood pressure in pulmonary trunk is

30mm Hg

The volume of blood ejected from each ventricle is

70ml (stroke volume)

To increase: | More blood/beat

Upper limit = size of ventricle and time allowed to fill.

To increase:More beats/min

Upper limit = need to reinitiate beat (refractory period)

1. Nervous System -Sensors:

: limbic system, baroreceptors, chemoreceptors, proprioreceptors. Feed into medulla’s cardiovascular center (medulla). Control through ANS.

1. Nervous System - Sympathetic NS:

nerves connect with SA & AV nodes and myocardium. increases rate of autorhythmic SA firing and uptake of Ca++ ( contractility) by all muscle cells.

1. Nervous System - Parasympathetic NS:

nerves that connect with SA & AV nodes and myocardium of atria. Decreases rate of autorhythmic firing with ACh.

2. Endocrine System Control:

Adrenal medulla (responding to hypothalamus) releases epinephrine and norepinephrine = increase of autorhythmic rate and contractility.

3. Cation Control:

Why is KCl included in lethal injection cocktails? Increased K+ prevents action potential Increased Na+ blocks Ca++ entry Increased Ca++ boost contractility, rate

4. Metabolic Control

Increased H+ (acidosis) or OH– decrease heart rate