Electrical Properties of the Heart Chapters 9 and 10

Review of Heart Muscle Has actin and myosin filaments Cardiocytes, myocardium Branched cells Intercalated discs- (desmosomes) and electrical junctions (gap junctions). Has actin and myosin filaments Has low resistance (1/400 the resistance of cell membrane) Atrial syncytium Ventricular syncytium Fibrous insulator exists between atrium and ventricle (what would this do to any electrical activity trying to go through?) Figure 9-1; Guyton & Hall

If the electrical signals from the atria were conducted directly into the ventricles across the AV septum, the ventricles would start to contract at the top (base). Then the blood would be squeezed downward and trapped at the bottom of the ventricle. The apex to base contraction squeezes blood toward the arterial opening at the base of the heart. The AV node also delays the transmission of action potentials slightly, allowing the atria to complete their contraction before the ventricles begin their contraction. This AV nodal delay is accomplished by the naturally slow conduction through the AV node cells. (Why are they slow conductors? Small diameter cells, fewer channels. Refer to text)

Fibers within the heart Specialized Fibers are the fibers that can spontaneously initiate an AP all by themselves! The AP will spread to all other fibers via gap junctions AKA “leading cells” But they are also muscle, so they do contract, albeit feebly! They are not nerves!!!! Contractile Fibers These maintain their RMP forever, unless brought to threshold by some other cell They cannot generate an AP by themselves AKA “following cells” But they do have gap junctions, so once they’re triggered, they will help spread the AP to neighbors.

Pathway of Heartbeat Begins in the sinoatrial (S-A) node Internodal pathway to atrioventricular (A-V) node Impulse delayed in A-V node (allows atria to contract before ventricles) A-V bundle takes impulse into ventricles Left and right bundles of Purkinje fibers take impulses to all parts of ventricles KEY Red = specialized cells; all else = contractile cells

Sinus Node Specialized cardiac muscle connected to atrial muscle. Acts as pacemaker because membrane leaks Na+ and membrane potential is -55 to -60mV When membrane potential reaches -40 mV, slow Ca++ channels open causing action potential. After 100-150 msec Ca++ channels close and K+channels open more thus returning membrane potential toward -55mV.

A-V Node Internodal Pathways Delays cardiac impulse Transmits cardiac impulse throughout atria Anterior, middle, and posterior internodal pathways Anterior interatrial band carries impulses to left atrium. A-V Node Delays cardiac impulse Most delay is in A-V node Delay AV node---0.09 sec. Delay AV bundle--0.04 sec.

Purkinje System A-V Bundles Only conducting path between atria and ventricles Divides into left and right bundles Time delay of 0.04sec Fast conduction; many gap junctions at intercalated disks

Time of Arrival of Cardiac Impulse (0.22) SA Node AV Bundle H (0.19) (0.0) T (0.03) (0.12) Left Bundle Branch (0.16) (0.19) (0.18) AV Node Right Bundle Branch (0.21) Main Arrival Times S-A Node 0.00 sec A-V Node 0.03 sec A-V Bundle 0.12 sec Ventricular Septum 0.16 sec Base 0.22 sec (0.17) (0.18) Copyright © 2006 by Elsevier, Inc.

How can these Specialized fibers spontaneously “fire?” Can’t hold stable resting membrane potential Potentials drift (gradual depolarization) –”prepotential” or “pacemaker potential” During this time, they have a gradually increasing perm to Na+ and less leaky to K+ (more “+” inside causes cell to depolarize, remember?) Na+

Specialized fibers Notice slow rise from rest to threshold. This is called the “prepotential” or “pacemaker potential” Only specialized fibers of the heart can do this. This is what gives the heart it’s rhythm.

Membrane Potential (mV) Rhythmical Discharge of Sinus Nodal Fiber Slow Ca++ Channels Open Sinus Nodal Fiber Ventricular Muscle fiber K+ Channels Open more +20 Threshold -20 -40 Membrane Potential (mV) } -60 “Pre- Potential” -80 Na+ Leak And less leaky to potassium -100 1 2 3 4 Seconds

Specialized fibers of conductive system Each region generates its own rhythm. If cells didn’t touch, then…. Faster at SA vs AV node, etc. SA is faster than AV- “pacemaker” SA 60-80 depol/min AV 40-60 depol/min Purkinje 15-30 depol/min Draw it! SA AV Pur Because SA node has the highest intrinsic rhythm, it is called the cardiac pacemaker. What if damaged…? Time (min)

Specialized fibers of conductive system These rhythms can ALSO be modified by the ANS NTS can change slope of prepotentials…faster or slower rise to threshold (bringing them closer or further from threshold.) by altering ion permeability. ACh (psymp postganglionic); NE (symp postganglionic) K+ efflux

Sympathetic and Parasympathetic Sympathetic – speeds heart rate by  Ca++ & Na+ channel influx and  K+ permeability/efflux (positive chronotropy) Parasympathetic – slows rate by  K+ efflux &  Ca++ influx (negative chronotropy) Figure 14-17: Modulation of heart rate by the nervous system

Other effect of ANS Symp (NE) also affects inotropy in ALL fibers, specialized and contractile Inotropy is the “force of contraction” or the tension development in the muscle fiber (strength of the contraction.) Sympathetic firing causes positive inotropy! (the pounding heart) Parasympathetics have little effect on inotropy Terminology: Chrono, Inotropy Symp (+,+) Parasym ( -, ) http://www.phschool.com/science/biology_place/biocoach/cardio1/electrical.html

Regulators of the Heart: Reflex Controls of Rate Your HR at any moment is the balance between symp and parasym discharge rates. (“tone”/ reserve) Tonic discharge How to speed up? Two ways (faucet analogy) How to slow down? Two ways Range: about 50 – near 200 Typical resting HR: near 70 --SA would normally beat at 60-80 bpm- but vagal tone slows it down. Parasympathetic slows-down (20bpm or even stop)- Sympathetic speeds-speed up (230bpm) K+

Contractile Fibers of Heart Bulk of heart mass Review APs Still need calcium to initiate contraction Differences from Skeletal Muscle and neurons Nature of AP Source of calcium (EC vs. SR) Duration of contraction Resting potential

EC Coupling – how it works (skeletal muscle) AP Sequence of Events: AP moves along T-tubule The voltage change is sensed by VONa+ Channels Is communicated to the (VOCC) (voltage operated calcium channel; VOCR) how much calcium released depends on voltage Contraction occurs. Calcium is pumped back into SR. Calcium binds to calsequestrin to facilitate storage. Contraction is terminated. AP Ca2+ pump calsequestrin

EC Coupling – Cardiac Muscle AP Sequence of Events: Ca2+/Na+ exchanger (Ca2+ out / Na+ in) Sequence of Events: 1. AP moves along T-tubule. 2. Activation – voltage sensors that release a small amount of Ca into the fiber. 3. Ca then binds to a receptor which opens, releasing a large amount of Ca. (Calcium Activated Calcium Release) How much calcium released depends on how much calcium gets through cell membrane 4. Calcium is pumped (a) back into SR, and (b) back into T tubule. 5. Contraction is terminated. AP Ca2+ pump requires ATP calsequestrin

Slow Ca++ Channels open and decreased K+ permeability Ventricular Muscle Action Potential-RMP -85mV; Slow Ca++ Channels open and decreased K+ permeability Fast Na+ close K+ Channels Open 1 2 3 4 phase +20 -20 Membrane Potential (mV) -40 -60 -80 Fast Na+ Channels Open -100 1 2 3 4 phase 0- Fast Na+ channels open phase 1- Fast Na+ channels close phase 2- slow Ca++ open and decreased K+ permeability phase 3- K+ channels open phase 4- Resting membrane potential Seconds Copyright © 2006 by Elsevier, Inc.

Important things to consider Cardiac muscle cells have a long absolute refractory period Twitches can not summate Tetanus not possible (this is good!) If average heart beats 72bpm; what does the heart do for the rest of the time? Answer : It “rests” and fills

Spread of Depolarization

Direction of Depol Resting Cell + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

Direction of Depol + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Stim microelectrode

Direction of Depol + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Depolarizing Current! Stim microelectrode

Direction of Depol + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Depolarizing Current! Stim microelectrode

Direction of Depol + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Depolarizing Current! Stim microelectrode

Direction of Depol - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Stim microelectrode

Direction of Depol - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + Stim microelectrode

Direction of Depol - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + Stim microelectrode

Direction of Depol - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + Stim microelectrode

Direction of Depol - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + Stim microelectrode

Direction of Depol - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + Stim microelectrode

Direction of Depol - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + Stim microelectrode

Direction of Depol - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Stim microelectrode

Direction of Depol - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Stim microelectrode

Direction of Depol - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Depol = spread of surface NEG charge Stim microelectrode

Direction of Repolarization Begin Repolarization - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Stim microelectrode

Direction of Repolarization - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Stim microelectrode

Direction of Repolarization + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Stim microelectrode

Direction of Repolarization + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Stim microelectrode

Direction of Repolarization + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - Stim microelectrode

Direction of Repolarization + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - Repolarization= spread of positive surface charge Stim microelectrode

Direction of Repolarization + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + ++ + Stim microelectrode

Direction of Repolarization + + + + + + + + + + + + + + + + + + + + + + + + + + + + + ++ + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + ++ + Repolarization= spread of POS surface charge Stim microelectrode

Depolarization/Repolarization Cycle in the Atria

Depolarization Begins =Resting cell =Depol cell

=Resting cell =Depol cell

=Resting cell =Depol cell

=Resting cell =Depol cell

Depolarization Complete =Resting cell =Depol cell

Repolarization Begins =Resting cell =Depol cell

=Resting cell =Depol cell

=Resting cell =Depol cell

Repolarization Complete =Resting cell =Depol cell

Depolarization/Repolarization Cycle in the Ventricles

Depolarization Begins =Resting cell =Depol cell

=Resting cell =Depol cell

=Resting cell =Depol cell

=Resting cell =Depol cell

Depolarization Complete =Resting cell =Depol cell

Repolarization Begins =Resting cell =Depol cell

=Resting cell =Depol cell

=Resting cell =Depol cell

Repolarization Complete =Resting cell =Depol cell