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What do the autorhythmic cardiac muscle fibers do?
They generate action potentials that trigger heart contractions, automatically. They act as a pacemaker and form conduction systems.
Where is the sinoatrial (SA) node and what does it do?
-right wall of right atrium, medial to superior vena cava
-electrically signals both atria to begin contraction
Where is the atrioventricular (AV) node and what does it do?
-right atrium, medial to right AV valve
-electrically signals both ventricles to begin contraction
Where is the atrioventricular bundle and what does it do?
-extends from AV node, passes through fibrous skeleton and branching into interventricular septum towards the apex
-conducts action potentials
What are Purkinje fibers and what do they do?
-terminal branches of AV bundle branches extending up lateral walls of myocardium
-produce depolarization of myocardium
What happens after the SA node is innervated?
The charge travels to the atrioventricular (AV) node in interatrial septum.
What happens after the AV node is innervated?
The charge enters the atrioventricular (AV) bundle, also known as the Bundle of HIS.
What happens after the AV bundle is innervated?
The charge enters the right and left bundle branches which extends through interventricular septum, towards the apex.
What is the last step of charge innervation?
The Purkinje fibers conduct action potentials to remainder of ventricular myocardium.
What modifies the strength of each heartbeat?
Nerve impulses from the autonomic nervous system (ANS) and hormones modify timing and strength of each heartbeat (Do NOT establish a fundamental rhythm).
What is the process of action potential movement of the heart?
What happens during depolarization?
Contractile fibers have stable resting membrane potential. Voltage-gated fast Na+ channels open- Na+ flows in. Then deactivate and Na+ inflow decreases.
What channels are open during depolarization?
Ca2+ channels (slow). Ca2+ moved from the interstitial fluid into cytosol. Triggers contraction.
How is depolarization maintained?
Through voltage-gated K+ channels balancing Ca2+ inflow with K+ outflow.
What happens during repolarization?
Additional voltage-gated K+ channels open, and the outflow of K+ restores negative resting membrane potential. Calcium channels close.
What is the refractory period?
The time interval during which second contraction cannot be triggered (takes longer than the contraction itself).
What is the P wave?
The excitation of the SA node (the beginning of the action potential). Atrial depolarization.
What is the QRS complex?
The action potential found in the AV bundle and our over the ventricles. Masks atrial repolarization.
What is the S-T segment?
It beings shortly after the QRS complex, and is the contraction of the ventricles, ventricular systole.
What happens during relaxation?
Both the atria and the ventricle are relaxed. In each cycle, atria and ventricles alternately contract and relax.
Where does the sound of the heartbeat come from?
The blood turbulence caused by the closing of heart valves.
What is Cardiac Output (CO?)
The volume of blood ejected from the left (or right) ventricle into the aorta (or pulmonary trunk) each minute.
What is the Frank-Starling law of the heart?
The more the heart fills with blood during diastole, the greater the force of contraction during systole.
What two factors influence EDV?
The duration of ventricular diastole, and the venuous return (volume of blood returning to the right ventricle).
What is contractility?
The strength of contraction at any given preload. Ability of heart to stretch, generate tension.
What increases contractility?
Positive inotropic agents (increase stroke volume, promote Ca2+ inflow, epinephrine, norepinephrine, digitalis.
What decreases contractility?
Negative inotropic agents (anoxia, acidosis, some anesthetics, and increased K+ in interstitial fluid)
What is afterload?
Pressure that must be overcome before a semilunar valve can open; how much pressure has to be generated after a contraction to get the blood out.
What does an increase in afterload lead to?
It causes stroke volume to decrease (blood remains in ventricle at the end of systole).
What aids in the regulation of the heartbeat?
Cardiac output depends on heart rate and stroke volume, adjustments in the heart rate are important short term, and the autonomic nervous system and epinephrine/norepinephrine are MOST important.
What part of the brain regulates autonomic regulation of the heart?
The medulla oblongata, in the cardiovascular center.
How does norepinephrine control heart rate?
In SA and AV node speeds; rate of spontaneous depolarization and in contractile fibers which enhance Ca2+ entry, increasing contractility.
How can the heart rate be slowed down?
By the release of acetylcholine, which slows the rate of spontaneous depolarization. Released by parasympathetic nerves.
How can the heart rate be changed by the ANS?
Increases or decreases in frequency of nerve impulses in both sympathetic and parasypmathetic branches.
What hormone, besides epinephrine and norepinephrine, increase heart rate?
Thyroid hormones increase heart rate and contractility as well.
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