heart conduction

subendocardial conducting network

(SCN) spread superiorly through ventricle walls -nerve like processes that begin at apex, turn and spread superiorly thru each ventricle

pacemaker physiology

**look at graph in slides -pacemaker potential=gradual increase in cell charge, below threshold (slow Na+ inflow) -when reach threshold, calcium channels open and rapid inflow of positively charged ions (depolarization), leads electrical impulse to be released -repolarization- internal cell charge back towards baseline -AP= rapid depol and repolarization (fast Ca2+-Na+ inflow, fast K+ outflow)

sympathetic innervation

-(originate) from cardioacceleratory center (region of medulla oblongata) -from this center, sympathetic fibers travel down spinal cord, exit thru lateral horn, synapse in SCG (superior cervical ganglion, image) in sympathetic trunk, postsynaptic ganglion exit SCG as cardiac nerves -cardiac nerves innervate SA and AV nodes, cardiac muscle (cells), and great/coronary arteries -(effect of symp innerv=) increase heart rate (increases depolarization of both nodes which makes depol happen quickly) and contractile strength (causes contractile cells to contract more forcefully), induce vasoconstriction (causes smooth muscles of blood vessels to contract, causing) -increase activity of heart ➡️increased cardiac output and increased BP

cardiomyocyte contraction- skeletal vs cardiac muscle

-AP happens slightly before contraction (mechanical activity) bc AP is a stimulus leading to contraction (in both) **look at diagrams on slides -skeletal=rapid depolariz and repolarization and fairly rapid contraction of muscle cells -cardiac muscle=rapid depol, then period of plateau where charge of cell remains high, before repolarization allows muscle cell to relax (longer period of contraction)

since pacemaker cells are noncontractile,

-SA and AV nodes are noncontractile so instead of depolarization leading to contraction, depolarization initiates an electrical discharge (send out electrical impulse which spreads to other tissues of the heart, what causes initiation of each heartbeat) -heartbeat!

what is special about the SA node in particular

-SA node is our pacemaker, sets rhythm for depolarization of other heart tissues

cardiac rhythm

-a single sequence of atrial and ventricle contraction is a heartbeat (contraction, relaxation) -sinus rhythm -ectopic focus

basics of muscle cell contraction

-cardiomyocytes (cardiac cells)=modified muscle cells -at baseline, most cells have interior environment more negatively charged than exterior environment -to start, a stimulus triggers inflow of positively charged ions into cell (as this happens, internal environment increases in cell charge=depolarization/becomes more positive) -depolarization causes cell to contract -after depolarization, positively charged ions start to leave cell -as interior cell becomes more negative=repolarization -repolarization allows muscle cell to relax -1 full cycle of depol and repol= 1 action potential

pacemaker cells

-cells making AV node and SA node and cells of conducting system are special in that they are noncontractile cells and also do not require external signal (provide own stimulus) -more negative internal environment, over time Na+ leak into cell slowly, very gradual change/increase in cell charge=pacemaker potential (happens spontaneously bc of slow Na+ inflow) -pacemaker potential leads to depolarization of SA node -once reaches threshold, opens Ca2+ channels within cell, fast inflow of positive calcium ions (rapid increase in cell charge which depolarizes the cell) -depolarize themselves rather than depolarized by external stimulus -when depolarization is complete, K+ channels open and K+ exit the cell, making internal charge increasingly negative until returns to baseline=repolarization

fibrillation

-condition that occurs when normal SA node activity is disrupted by other areas of heart -rapid, irregular contractions -atrial/ventricular -defibrillation shocks heart with electricity to depolarize entire myocardium at same time (hope is that SA node will kick back in and start to regulate heart activity)

cardiac innervation

-conduction system is autorhythmic (pacemaker cells are self depolarizable so do not require nervous system to function) -autonomic innervation modifies heartbeat (autonomic NS influences activity of heart, doesn't cause heartbeat but can regulate it) -originate from cardiac centers in medulla oblongata (brainstem)

cardiomyocytes AP

-do require external stimulus to initiate AP (=electrical impulse produced by pacemaker cells) -rapid inflow of Na+ into cell is depolarization -external stimulus can be received directly from our conduction system but not all cardiac cells are connected to it so can go from cell to cell so may receive from cell adjacent to them as well -depolarization of majority of cardiomyocytes causes contraction (also opens calcium channels and see slow inflow of positive calcium ions ) -repolarization= K+ ions exit cell, makes interior cell charge increasingly negative, allows for relaxation

parasympathetic innervation

-from cardioinhibitory center (region of medulla obl) -prasympathetic fibers leave brainstem with CN 10 (vagus nerve) -vagus nerves /nerve fibers innervated by para (only pacemaker cells) and innervate SA and AV nodes -reduce rate of depolarization of pacemaker cells which reduces heart rate

Atrial vs ventricular fibrillation

-heart efficiency greatly reduced during atrial fibrillation (not completely lost, bc majority of movement of blood atria to ventricles is through gravity) -heart function stops during ventricular fibrillation (much larger problem, heart stops working essentially; heart is useless as a pump, no coordianated activity of muscle cells and no ejection of blood from the heart)

sinus rhythm

-heart rate set by SA node -70-80 bpm is typical -typical heart rhythm, occuring as expected -most heart rates, regardless of how fast, are considered this

nodal/junctional rhythm ECG

-missing P wave bc SA node not functioning so not getting depolarization of atria -bc AV node taking over, still get depolarization and repolarization of ventricles

ventricular fibrillation ECG

-no coordinated activity of heart -complete loss of coordination so don't see any of expected landmarks

sinus rhythm ECG

-normal -regular intervals

plateau- cardiomyocytes

-slow inflow of Ca2+ delays repolarization -extends period of contraction (which facilitates blood ejection) -slow inflow of positive calcium ions that keeps charge of cell elevated and delay in repolarization extends period of contraction for muscle cell, facilitates ejection of blood from heart

heart conduction

-system in the heart responsible for initiation and propagation for electrical impulses responsible for each heartbeat -internal pacemaker and series of nerve conduction pathways that coordinate and synchronize heart muscles -important to happen in proper order

atrial fibrillation ECG

-uncoordinated -some electrical activity from atria unlike junctional but not coordinated -P wave wacky -T wave little wacky too -still get QRS

conduction (pathway/order)

1) SA node fires (for each heart beat, start when SA node fires and initiates heart beat) 2) excitation spreads thru atria (electrical signal spread thru cells of atria, each atria contracts as a result) 3) (signal reaches AV node) AV node fires/transmits signal after short delay. 4) Excitation spreads down AV bundle and branches (excitatory signals spreads down thri structures of conducting system within IV septum) 5) SCN fibers distribute excitation through ventricles (when signal reaches apex of heart, SCN fibers distribute signal, start and base, turn superiorly, continue to transmit signal thru rest of heart; once signal spread, 2 ventricles contract simultaneously)

impulse conduction to myocardium

1) SA node spontaneously depolarizes 2) depolarization and contraction of atria 3) signal reaches AV node 4) depolarization wave continues down interventricular septum 5) depolarization and contraction of ventricles *entire process takes about 200 milliseconds at rest in resting heart rate

the conduction system (pieces of)

1) sinoatrial (SA) node 2) atrioventricular (AV) node 3) atrioventricular (AV) bundle 4) right and left bundle branches 5) subendocardial conducting network

step 4 and 5 impulse conduction to myocardium

4)depolarization wave continues down IV septum 5) depolarization and contraction of ventricles -continue to spread signal, cells of walls of ventricle contract starting at apex to base (upward direction helps more blood from base to semilunar valves) -myocardiac vortex (spiral arrangement) -as makes way superior, muscles contract in spiral pattern -spiral contraction helps move blood out of heart, allows ventricles to be more efficient in contraction and facilitates ejection of blood

Junctional Rhythm

AV node sets the heart rhythm (if SA not fxn, then AV node can take over) -40 to 60 bpm (a little slower; can survive but not lower than this)

P-Q segment

Atrial systole (Atria contract)

at rest...

SA depolarization every 0.8 seconds= 75x/min, 75 bpm heartrate (average resting heart rate)

P wave

atrial depolarization (electrical activity) -electrical signal given off by atria when depolarize -signal by SA node (initiation of each heartbeat, leads to depolarization of rest of atrium)

atrioventricular (AV) bundle

connection from AV node (down into the structures of) to IV septum (separates from L and R ventricles, where find lot of structures of electrical system) -splits into 2 branches

heartbeat contraction and relaxation

contraction=systole and relaxation=diastole

pacemaker potential

gradual increase in cell charge from slow inflow of Na+ -happens spontaneously bc of slow Na+ inflow

electrocardiogram

graphic representation of heart electrical activity -ECGs measure electrical activity, not mechanical activity so what is happening in heart just before chambers contract -used to diagnose pathologies, if something not working properly, what region is not working -number of landmarks that give info about heart function

ectopic focus

heart rate/rhythm of heart set by area/region of heart other than SA node (pacemaker) -typically SA node=pacemaker bc reinitiates AP much more quickly so sets rhythm for other structures (has fasted depolarization rate) -junctional rhythm -extrasystole

pacemakers, etc are

modified cardiomyocytes

ECG landmarks

p wave, QRS complex, T wave, PQ segment, ST segment

sinoatrial (SA) node

pacemaker of the heart -group of modified cardiac muscle cells in R atrium -initiates each heartbeat and determines the heart rate

Extrasystole

premature depolarization/contraction of overexcited cardiomyocytes -occasionally, regions of cardiomyocytes get overexcited and start to depolarize faster than SA node, start to fire before SA node sets rhthym -may be caused by hypoxia or electrolyte imbalances

QRS complex

represents ventricular depolarization -electrical impulse spreading thru heart tissues -muscles of ventricles have greater mass (bigger spike) -when atria contracting

T wave

represents ventricular repolarization -don't have corresponding for atrial bc expect to occur at same time of QRS complex (so gets overshadowed by QRS complex)

S-T segment

represents ventricular systole/ventricular contraction -entire ventricular myocardium is depolarized??yesorno?

why short delay before AV node fires

short delay (0.1 secs) which gives 2 atria time to finish contraction before signal sent to rest of heart

step 3 impulse conduction to myocardium

signal reaches AV node (when signal reaches AV node, it will depol) -delayed transmission allows atria time to complete contraction and emit blood/transmit signal -then signal goes to other parts of heart

SA node spontaneously depolarizes and what happens next

step 1 of impulse conduction to myocardium -at beginning of each heartbeat, SA node spontaneously depolarizes which initiates electrical impulse which triggers depolarization of adjacent cells, depolarization spread thru tissues of atria and atria contract (2nd step=depolarization and contraction of atria)

pacemaker cells vs the pacemaker

the pacemaker is the SA node; pacemaker cells encompass ...

right and left bundle branches

transmit signal down IV septum -travel thru IV septum to apex of heart (then to other branches)

atrioventricular (AV) node

transmits signal to ventricles -bundle of modified cells -picks up signal generated by SA node and transmits to ventricle -only way for this electrical signal to pass from atria into the ventricles