21 Rhythmic excitation of the heart Flashcards
Trace the pathway of the heartbeat
1) begins at the SA (sinoatrial) node
2) intermodal pathway to atrioventricular (AV) node
3) Impulse is delayed in AV node and bundle (allows atria to contract before the ventricles)
4) AV bundle takes the impulse into the ventricles
5) Left & right bundles of purkinje fibers take the impulses to all parts of the ventricles
o Specialized cardiac muscle connected to atrial muscle
o Acts as a pacemaker because membrane leaks Na+ and membrane potential is -55 to -60 mV
o When membrane potential reaches -40mV (threshold of cardiac muscle), slow Ca++ channels open causing an action potential
o After 100-150 msec Ca++ channels close and K+ channels open more thus returning membrane potential to -55mV
Sinus node
Review all graphs
Do it
o Transmits cardiac impulse throughout atria
o Anterior, middle and posterior ___
o Anterior interatrial band carries impulses to left atrium
Internodal pathways
o Delays cardiac impulse (so that atria and ventricles contract separately)
o Most delay is in the ___
o Delay AV node –0.09 sec
o Delay AV bundle –0.04 sec
AV node
o Normally one-way conduction
o The only conducting path between atria and ventricles is AV node & the ___
o Divides into the left and right bundles
o Transmission time between ___ and last of ventricular fibers is 0.06 seconds (QRS)
AV bundles
o Fibers lead from AV node through AV bundle into ventricles
o Fast conduction; many gap junctions at intercalated disks
Purkinje System
The cardiac pacemaker. The normal rate of discharge is 70-80/min, AV node: 40-60/min, Purkinje fibers: 15-40/min
Sinus node
o This is pathogenic (abnormal). This is a portion of the heart with a more rapid discharge than the sinus node
o Also occurs when transmission from the sinus node through the AV node is blocked (AV block)
Ectopic pacemaker
T/F. Ca++ influx through the T tubules (DHP receptor) during an AP is a very important source of Ca++
True
o Norepinephrine is released at the sympathetic postganglionic synapse
o Causes increased sinus node discharge
o Increases the rate of conduction of impulses
o Increases the force of contraction in the atria and ventricles
Sympathetic effects on heart rate
o Norepinephrine binds to the beta 1 transmembrane receptor
o The receptor binds to the alpha stimulatory subunit in the intracellular domain
o Activated alpha subunit binds to adenylyl cyclase
o Adenylyl cyclase converts ATP to cAMP
o cAMP phosphorylates protein kinase A (pkA)
o Funny channels (Na+ influx) and T-type calcium channels are phosphorylated and open
Slope of the pacemaker potential increases (due to funny channel phosphorylation)
Threshold is reached sooner
Intrinsic firing rate increases
o Cationic loading refers to the influx of cations as a result of sympathetic NS stimulation
Effects of sympathetic NS on SA node cells
o Vagal nerves, which release acetylcholine at their postganglionic endings, innervate SA node and AV junctional fibers proximal to AV node
o Causes hyperpolarization b/c of increase K+ permeability in response to acetylcholine
o This causes decreased transmission of impulses maybe temporarily stopping heart rate
o Ventricular escape occurs
Parasympahetic effects on heart rate
o Acetylcholine binds to muscarinic receptors in cardiac muscle
o Alpha subunit is activated
o Alpha subunit activates adenylyl cyclase
o Adenylyl cyclase converts ATP to cAMP, which phosphorylates protein kinase A
Protein kinase A prevents phosphorylation of the Ca+ channels (inhibits Ca++ influx)
o Beta and gamma subunits bind to the K+ receptor, leading to to cationic deloading
Effects of parasympathetic NS on SA node cells
T/F. The left bundle of purkinje fibers conducts the impulse after the right bundles
False. Left bundle of purkinje fibers conduct the impulse before the right bundles (by 0.01 seconds)
