19.2 Cardiac Muscle and Electrical Activity

what type of muscle is cardiac muscle similar to

both skeletal and smooth muscle

what is autorhythmic in cardiac muscle

ability of cardiac muscle to initiate an electrical potential at a fixed rate that spreads rapidly from cell to cell to trigger the contractile mehanism

is the heart rate of cardiac muscle fixed due to autorhythmicity

heart rate is modulated by the endocrine and nervous system

since cardiac muscle has autorhythmicity, that means it can

contract and pump blood on its own

what are the 2 major types of cardiac muscle cells

myocardial contractile cells and myocardial conducting cells

what is the function of myocardial contractile cells

conduct impulses and are responsible for contractions that pump through the body

what percentage of cells in the atria and ventricles are myocardial contractile cells

99%

what is the function of myocardial conducting cells

initiate and propagate the action potential (electrical impulse) that triggers the contractions of the heart

how do myocardial conducting cells differ from contractile cells

conducting cells are generally smaller, have fewer myofibrils and filaments, and are specialized to initiate and propagate electrical impulses rather than contract

how do cardiac muscle cells (cardiomyocytes) compare in size to skeletal muscle cells

shorter with much smaller diameters compared to skeletal

what are cardiac muscle cells called

cardiomyocytes

do cardiac muscle cells have striations

they demonstrate striations which are the alternating pattern of dark A bands and light I bands

what is the function of T tubules in cardiac muscle cells

T tubules allow the electrical impulse to penetrate from the surface plasma membrane to the interior of the cell

where are T tubules located in cardiac muscle cells

T tubules are located at the Z discs in cardiac muscle cells

how does the sarcoplasmic reticulum in cardiac muscle cells differ from that in skeletal muscle cells

stores fewer calcium ions, so most of the calcium ions must come from outside the cells

what is notable about the mitochondria in cardiac muscle cells

plentiful in cardiac muscle cells, providing energy for the contractions of the heart

how many nuclei do cardiac muscle typically have

single, central nucleus, but some cells may have two or more nuclei

how do cardiac muscle cells connect to each other

branch freely and connect to each other through structures called intercalated discs

what is the function of intercalated discs in cardiac muscle

help support the synchronized contraction of the muscle by binding adjacent cells together

what types of junctions are found in intercalated discs

consists of desmosomes, tight junctions, and large numbers of gap junctions

what is the role of gap junctions in intercalated discs

allow the passage of ions between cells, helping to synchronize the contraction of cardiac muscle

why is it important for cardiac muscle cells to be strongly bound together

the strong binding is necessary to withstand the forces exerted by contraction

what type of respiration does cardiac muscle undergo

aerobic respiration

what substances does cardiac muscle primarily metabolize

lipids and carbohydrates

what substances are stored within the cytoplasm of cardiac muscle cells

myoglobin, lipids, and glycogen

what type of contractions do cardiac muscle cells undergo

twitch type contractions

why is the refractory period in cardiac muscle cells long

to prevent tetany

what is tetany

condition in which muscle remains involuntarily contracted

why is tetany not compatible with life in the heart

would prevent the heart from pumping blood, making it incompatible with life

what happens when embryonic heart cells are separated and kept alive

each embryonic heart cell is capable of generating its own electrical impulse followed by contraction

what happens when two independently beating embryonic cardiac muscle cells are placed together

the cell with the higher inherent rate sets the pace, and the impulse spreads from the faster to the slower cell to trigger a contraction

what determines the heart rate when multiple cardiac muscle cells are joined together

the fastest cell continues to assume control of the rate

what is the cardiac conduction system

a group of cells that generate and transmit electrical impulses (action potential) to trigger heart contractions

what are the components of the cardiac conduction system

sinoatrial node, atrioventricular node, atrioventricular bundle, atrioventricular bundle branches, and purkinje cells

what is the sinoatrial (SA) node

specialized group of myocardial conducting cells located in the right atrium that initiates the normal electrical pattern of the heart

where is the SA node located

in the superior and posterior walls of the right atrium, near the orifice of the superior vena cava

why is the SA node known as the pacemaker of the heart

highest inherent rate of depolarization, making it the natural pacemaker that sets the heart rate

what is sinus rhythm

normal electrical pattern followed by contraction of the heart, initiated by the SA node

what is the pacemaker

cluster of specialized myocardial cells known as the SA node that initiates the sinus rhythm

what does depolarization mean

electrical trigger that makes the heart cell “fire” and contract

how does the electrical impulse from the SA node spread throughout the atria

through specialized internodal pathways to the atrial myocardial contractile cells and the atrioventricular node

what are internodal pathways

three specialized bands: anterior, middle, and posterior, that lead directly from the SA node to the atrioventricular node

what are the three specialized bands of the internodal pathways

anterior, middle, and posterior

how long does it take for the impulse to travel from the SA node to the atrioventricular node

the impulse takes approximately 50 milliseconds to travel between the SA node and the atrioventricular node

what is the Bachmann’s bundle or interatrial band

specialized pathway that conducts the impulse directly from the right atrium to the left atrium

the bachmann’s bundle is also known as

interatrial band

why is the pathway of impulse transmission through the internodal pathways debated

the impulse could reach the atrioventricular node through the contractile cells of the myocardium in the atrium without the need for specialized pathways

what prevents the impulse from spreading into the myocardial cells in the ventricles

the connective tissue of the cardiac skeleton prevents the impulse from spreading into the myocardial cells in the ventricles except at the atrioventricular node

how does the electrical signal from the SA node control and coordinate the heartbeat

electrical signal from SA node travels through specific pathways to reach the next part of the heart’s electrical system, allowing the heartbeat to be controlled and coordinated

what is the 1st step in cardiac conduction

SA node and the remainder of the conduction system are at rest

what is the 2nd step in cardiac conduction

SA node initiates the action potential, which sweeps across the atria

what is the 3rd step in cardiac conduction

after reaching the atrioventricular node, there is a delay of approximately 100 ms that allows the atria to complete pumping blood before the impulse is transmitted to the atrioventricular bundle

what is the 4th step in cardiac conduction

following the delay, the impulse travels through the atrioventricular bundle and bundle branches to the purkinje fibers, and also reaches the right papillary muscle via the moderator band

what is the 5th step in cardiac conduction

impulse spreads to the contractile fibers of the ventricles

what is the 6th step in cardiac conduction

ventricular contraction begins

what triggers muscular contraction in the heart

the electrical event, the wave of depolarization, triggers muscular contraction

where does the wave of depolarization begin

in ra

how does the impulse spread across the atria

the impulse spreads across the superior portions of both atria and then down through the contractile cells

what is the sequence of contraction in the atria

the contractile cells begin contraction from the superior to inferior portions of the atria

what is the result of the atrial contraction sequence

efficiently pumps blood into the ventricles

what is the atrioventricular (AV) node

group of specialized myocardial conductive cells located in the inferior portion of the right atrium within the atrioventricular septum

what is the function of the AV node in the cardiac conduction system

AV node delays the transmission of the impulse to the ventricles, allowing the atria to complete their contraction before the ventricles contract

why is there a delay in transmission of the impulse at the AV node

the delay is due to the small diameter of the cells of the node and less efficient conduction between nodal cells

how long does it take for the impulse to pass through the AV node

100 ms for the impulse to pass through the AV node

what is the importance of the delay at the AV node

allows the atrial cardiomyocytes to complete their contraction and pump blood into the ventricles before they contract

what determines the maximum heart rate in a healthy young individual

the maximum heart rate is determined by the AV node’s ability to transmit impulses maximally at 220 per minute

what happens if the heart rate exceeds the maximum reate

the heart can no longer effectively pump blood

what is the atrioventricular bundle also known as

bundle of His

what is the atrioventricular bundle (bundle of His)

group of specialized fibers that arise from the AV node and proceed through the interventricular septum

what are the atrioventricular bundle branches

the divisions of the atrioventricular bundle that supply the left and right ventricles

what is the atrioventricular bundle branches also known as

left and right bundle branches

what is the function of the bundle brances

transmit the impulse to the ventricles, allowing them to contract

what is the difference in size between the left and right bundle branches

left is larger than the right

why is the left bundle branch larger than the right

left ventricle is much larger than the right

what is the role of the moderator band in the right ventricle

moderator band carries portions of the right bundle branch and supplies the right papillary muscles, allowing them to contract simultaneously with the rest of the ventricular muscle

why is it important for the papillary muscles to contract simultaneously with the ventricles

it allows tension to develop on the chordae tendineae prior to ventricular contraction

how long does it take for the impulse to travel through the bundle branches

25 ms for the impulse to travel through the bundle branches and reach the purkinje fibers

what purkinje fibers

specialized myocardial conductive fibers that spread the impulse to the myocardial contractile cells in the ventricles

what is the function of purkinje fibers

transmit the electrical impulse to the ventricular muscle cells, allowing them to contract

how do purkinje fibers spread the impulse in the ventricles

extend throughout the myocardium from the apex of the heart toward the atrioventricular septum and the base of the heart

how long does it take for the electrical impulse to reach all ventricular muscle cells through purkinje fibers

75 ms

where does ventricular contraction begin, and which direction does it travel

at the apex of the heart and travels toward the base

what is the benefit of the contraction sequence in the ventricles

the sequence allows blood to be pumped out of the ventricles and into the aorta and pt

how long does it take for the impulse to travel from the SA node to the ventricles

225 ms

what is spontaneous depolarization (prepotential depolarization)

slow influx of sodium ions into cardiac conductive cells that causes the membrane potential to rise, leading to depolarization

what is spontaneous depolarization also known as

prepotential depolarization

what causes the membrane potential to rise in cardiac conductive cells

a slow influx of sodium ions through sodium ion channels causes the membrane potential to rise

what happens when the membrane potential reaches approximately -40 mV in cardiac conductive cells

calcium ion channels open, and Ca 2+ enters the cell, further depolarizing it at a more rapid rate

what occurs when the membrane potential reaches approximately +15 mV in cardiac conductive cells

the calcium ion channels close, and K+ channels open, allowing outflux K+ and resulting in repolarization

what happens when the membrane potential reaches approximately -60 mV in cardiac condcutive cells

the K+ channels close, and Na+ channels open, starting the prepotential phase again

why dont cardiac conductive cells have a stable resting potential

cardiac conductive cells have a continuous influx of sodium ions, leading to spontaneous depolarization and a lack of a stable resting potential

what is the electrical pattern of cardiac contractile cells

consists of rapid depolarization, followed by a plateau phase, and then repolarization

why do cardiac contractile cells have long refractory periods

allow cardiac muscle cells to pump blood effectively before they are capable of firing for a 2nd time

do cardiac contractile cells initiate their own electrical potential

no, they typically wait for an impulse to reach them

what is the resting membrane potential of cardiac contractile cells in the atria and ventricles

the resting membrane potential is approximately -80 mV for cells in the atria and -90mV for cells in the ventricles

what causes the rapid depolarization in cardiac contractile cells

the rapid depolarization is caused by the opening of voltage gated sodium channels

what is the plateau phase in cardiac contractile cells

period of slow decline in membrane potential primarily due to the opening of slow Ca 2+ channels and few K+ channels being opne

how long does the entire electrical event in cardiac contractile cells last

between 250-300ms

what are the two critical roles of calcium ions in cardiac muscle physiology

contribute to the prolonged plateau phase and absolute refractory period, and they also play a role in muscle contraction by binding to troponin

how do calcium ions contribute to muscle contraction in cardiac muscle

Ca ions bind to troponin, removing the inhibition that prevents myosin heads from forming cross-bridges with actin, allowing contraction to occur