Lecture 10: Cardiac Muscle and the Cardiac Cycle

contractile and pacemaker cells

what are the two cell types of cardiac muscle?

contractile cells

produce 99% of contractions of the heart via sliding actin and myosin filaments

pacemaker cells

produce 1% of heart contractions; control and coordinate contractile cells by spontaneously depolarizing

short, striated, 1 central nucleus, many mitochondria, intercalated discs

characteristics of cardiac muscle cells

intercalated discs

structures that connect cardiac muscle cells that allow for stronger and easier transmitted electrical signals

desmosomes and gap junctions

what structures make up intercalated discs?

desmosomes

anchors ends of the cardiac muscle fibers together

gap junctions

allow for direct communication/transmission of electrical signals between cells

spontaneous depolarization

what does the pacemaker cell undergo to generate an AP?

cardiac/intrinsic conducting system

interconnected pacemaker cells that the AP travels through to the contractile cells

voltage gated Na, Ca, and K channels, and non-selective cation channels

what ion channels are in cardiac muscle cells?

slow repolarization period (absolute refractory)

why can’t cardiac muscle achieve tetanus?

rapid depolarization, initial repolarization, plateau phase, and repolarization phase

what are the four steps of contractile cell AP?

-85mV

resting membrane potential of contractile cell AP

Na+ channels open

what channels are open during the rapid depolarization phase?

Na+ inactivated and some K+ open

what channels are open during the initial repolarization phase?

Ca2+ open

what channels are open during the plateau phase?

Na+ and Ca2+ closed, K+ open

what channels are open during the repolarization phase?

Ca2+ is entering as K+ exits

what causes the plateau phase in contractile cell AP?

autorhythmic

cardiac cells do not maintain a stable resting membrane potential; giving them pacemaker potential as the membrane can spontaneously drift towards threshold

slow initial depolarization, full depolarization, repolarization, and minimum potential

what are the steps of a pacemaker cell AP?

non-specific cation channels

what channels are open during the slow initial depolarization phase?

cations leak out and the membrane slowly depolarizes to threshold

what are the function of non-specific cation channels during the slow initial depolarization phase?

voltage gated Ca2+

what channels are open during the full depolarization phase?

Ca2+ close, K+ open

what channels are open during the repolarization phase?

K+ open, nonspecific cation channel activated

what channels are open during the minimum potential phase?

-60mV

what is the resting membrane potential of a pacemaker cell?

sinoatrial node, atrioventricular node, AV bundle, and purkinge fibers

what are the four structures of the cardiac conduction system?

sinoatrial (SA) node

pace of 80-100x/min; pacemaker of the heart

atrioventricular (AV) node

40-60x/min

AV bundle

20-40x/min; only electrical connection between atria and ventricles

purkinje fibers

20-40x/min; serves ventricle muscles

SA node, AV node, AV bundle, bundle branches, and purkinje fibers

sequence of excitation in the heart

electrocardiography (EKG)

measures all of the electrical activity in the heart using electrodes; 3 distinct waves

P, QRS, and T

what are the 3 waves of an EKG?

P wave

depolarization of the atria; atrial systole

QRS wave

depolarization of ventricles; ventricular systole

T wave

repolarization of ventricles; relaxation period

cardiac cycle

the events in the heart during one complete heartbeat

systole and diastole

what are the two parts of the cardiac cycle?

systole

contraction periods; atrial and ventricular

diastole

relaxation periods; atrial and ventricular

high to low pressure

how does blood flow?

isovolumetric contraction

AV valve closes

ventricular ejection

SL valves open

isovolumetric relaxation

SL valves close

ventricular filling

AV valve open

atrial contraction

what pressure/volume is associated with atrial systole?

isovolumetric contraction and ventricular ejection

what volume/pressure is associated with ventricular systole?

isovolumetric relaxation and ventricular filling

what pressure/volume is associated with relaxation period?

heart beats

the sound of valves closing

heart murmurs

caused by leaking valves or unusual blood flow through the heart