o Two specialized types of cardiac muscle cells Contractile cells 99 of cardiac

O two specialized types of cardiac muscle cells

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o Two specialized types of cardiac muscle cells: Contractile cells 99% of cardiac muscle cells Do mechanical work of pumping Normally do not initiate their own action potentials Autorhythmic cells Do not contract Specialized for initiating and conducting action potentials responsible for contraction of working cells o Locations of noncontractile cells capable of autorhythmicity: Sinoatrial node (SA node) Specialized region in the right artrial wall near opening of the superior vena cava This is the pacemaker of the heart Atrioventricular node (AV node) Small bundle of specialized cardiac cells located at the base of the right atrium near the septum Bundle of His (Atrioventricular bundle) Cells originate at the AV node and enter the interventricular septum Divides to form the right and left bundl branches which travel down the septum, curve around the tip of the ventricular chambers, and travel back toward the atria along the outer walls Purkinje Fibers Small, terminal fibers that extend from the bundle of His and spread throughout the ventricular myocardium Specialized conduction system of the heart o FIGURE 9.8
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Electrical activity of the heart cont. o Cardiac impulses originate at the SA node o Action potential spreads throughout the right atria and left atria o Impulse passes from atria into the ventricles through the AV node (only point of electrical contact between the chambers) o Action potential is briefly delayed at the AV node (ensures atria contraction precedes ventricular contraction to allow complete ventricular filling) o Impulse travels rapidly down interventricular septum by means of the Bundle of His o Impulse rapidly disperses throughout the myocardium by means of the Purkinje Fibers o The rest of the ventricular cells are activated by cell-to-cell spread of the impulse through the gap junction o Atria contract as a single unit followed by a brief delay by a synchronized ventricular contraction Pacemaker Cells o Do not have a steady resting membrane potential (never stops) o Spontaneously depolarizes the cell Cardiac Pacemaker Cell Action Potential o Cardiac pacemaker cells spontaneously depolarize because of the activity of the “Funny” channels o Funny channels are activated by depolarization o Funny channels=HCN channels (HNC =hyperpolarized-activated and cyclic nucleotide gated channels) o Voltage gated K channels close “Funny” channels (let Na in, K out) open PNa high PK low o “Funny” channels close T-type voltage gated Ca channels open PNa low PCa high Purpose of T-type voltage gated K channels is to increase rate of the potential to reach threshold o Once threshold is reached, voltage gated L-type channels open and voltage-gated Na channels open Extremely rapid depolarization PCa really high PNa high o Voltage gated K channels open; voltage gated L-type Ca channels close; voltage gated Na channels inactivate PK high PCa low PNa low
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