The Conduction System Flashcards
describe mechanical cells
- also known as myocardial cells
- primary fx is contraction. contract by receiving electrical stimulus from electrical cells
- make up bulk of heart tissue
define extensibility
ability of muscle fibers to stretch
describe electrical cells and their function
- pacemaker cells
- generate an electrical stimulus or impulse and conduct it down the conduction system
what properties do electrical cells have?
RACER
1) rhythmicity
2) automaticity
3) conductivity
4) excitability
5) refractoriness
define automaticity
ability of pacemaker cell to generate an electrical impulse spontaneously without external stimulation
define excitability
ability of a pacemaker cell to depolarize in response to an electrical stimulus
define refractoriness
period during which pacemaker cells are unresponsive to any stimulus, regardless of strength
define rhythmicity
ability of pacemaker cells to fire at regular intervals
define conductivity
the spread of electrical activity from one specialized pacemaker cell to another
what does contractility do for mechanical cells?
enables them to shorten and return to original length. it is dependent on the relationship between electrical and mechanical cells
excitation-contraction coupling
myocyte electrical activation leads to mechanical contraction
what is the purpose of the cardiac conduction system?
to maintain a regular and orderly sequence of electrical events that result in myocardial contraction
what does the conduction system consist of?
SA node, internodal tracts/pathways, AV node, bundle of His, R+L bundle branches, Purkinje fibers
SA node
- function
- location
- rate of firing
- primary pacemaker of the cell
- in the superior portion of R atrium close to where SVC enters
- electrical impulses begin here at rate of 60-100x/min
what are important properties of electrical cells in the SA node?
automaticity and rhythmicity
internodal tracts
- location
- function
- 3 pathways in R atrium connecting SA node to AV node
- anterior, posterior and middle tract
- allow rapid conduction of impulses from SA node to AV node
AV node
- location
- function
- on floor of R atrium above tricuspid valve
- delays conduction of impulses from SA node allowing atria to contract and atrial kick to occur
- cannot generate impulses, but in junctional tissue b/w AV node and bundle of His are some cells with electrical impulse forming properties of automaticity
what happens if SA node fails?
junctional pacemaker cells can take over as secondary pacemaker site, but only fire at 40-60x/min
bundle of His
- arises from AV junction and passes in interventricular septum, dividing into bundle branches
bundle branches
- R bundle branch travels down right side of septum and branches over R ventricle, terminating in purkinje fibers on surface of R ventricle
- L bundle branch travels down L side of septum and divides into 2 branches/fascicles (anterior and posterior) and terminates in purkinje fibers
purkinje fibers
- form a network across R+L ventricles
- pacemaker cells in network passes electrical impulse-forming properties or automaticity
- will fx as pacemakers if both SA node and junctional tissue pacemaker sites fail
- rate of firing would be 20-40x/min
action potential
a rapid sequence of changes in voltage across a cellular membrane
a higher concentration of ____ ions are found in the intracellular space. A higher concentration of ____ ions are found in the extracellular space.
K+
Ca & Na
polarization
- resting state during which no electrical activity occurs in cardiac cells
describe the cell in a polarized state and why
inside is more (-) than outside because
1) K+ leaks out of cell taking positive charges away from ICS
2) Na/K pump takes 3 Na ions out of the cells but only returns 2 K ions, leaving negative charge in ICS
3) concentration of negatively charged organic protein molecules is higher in ICS than ECS
depolarization
- begins when cardiac cell receives a stimulus
- Na ions rush through open Na membrane channels into cell, followed by Ca ions
- as positive ions enter cell, inside becomes more positive and outside negative making cell in a depolarized/stimulated state
SA node is a pacemaker site. therefore, it has the ability to ________
depolarize without an external stimulus, a property of automaticity unique to pacemaker cells
what happens when the SA node depolarizes?
depolarization travels down conduction system, and when it reaches mechanical cells stimulating cardiac muscle fibers, myocardial contraction occurs (excitation-contraction coupling)
repolarization
- results from efflux of K outside of cell through K channels
- when inside of cell becomes more +, it triggers efflux of K ions
- for a cell to be able to conduct another electrical impulse/depolarize, it must return to resting state
- Na/K pump restores Na and Ca ions outside of cell and K into cell, making negative charge again so cell is ready to depolarize
what is the clinical significance of action potentials?
1) ECG shows action potentials in heart
2) helps one understand actions of cardiac meds
what are refractory periods and what are the phases?
- inability to respond to a stimulus
- absolute, relative and supernormal/vulnerable
absolute phase
- occurs during depolarization and early repolarization
- can’t respond to any stimulus regardless of strength
relative phase
- occurs at end of repolarization
- cell can respond to a stronger than normal stimulus bc some cells are already restored to their resting state while others remain depolarized
- dysrhythmias can arise
supernormal/vulnerable phase
- during middle phase of repolarization and extends all the way to the resting state
- minimal stimulus can elicit a response in this phase
- dysrhythmias can arise
what are the coronary arteries?
1) RCA (right coronary artery)
2) LDA (left descending artery)
3) circumflex
RCA: what portion of the myocardium is supplied? conduction system?
- r atrium and r ventricle
- SA node in 50-60% of population, 90% of AV node
LDA: what portion of the myocardium is supplied? conduction system?
- anterior wall of L ventricle and septal wall
- bundle branches
circumflex: what portion of the myocardium is supplied? conduction system?
- l atrium and posterior wall of l ventricle
- SA node in 40-50% of population, 10% AV node
