Cardiac M for physiology

functional syncytium

• a group of individual, separate cells that are connected to work together as a single, unified unit

• connected by intercalated disks

  • gap junctions, adhering junctions, desmosomes

how are cardiac myocytes arranged

• in series and parallel

2 syncytium:

• atria

• ventricles

action potential

• avg. 105 millivolts each beat

• 0.2 sec depolarization (15x longer than skMSC)

Phase 0 of depolarization

• depolarizes to -20 from -80mv

• by voltage activated fast sodium channels

  • open for 1/1000th of a second

  • same for Sk m.

• L Type ca channels (calcium channels)

  • open for longer (1/10th of a second)

  • slower

Phase 1: initial repolarization

• fast sodium channels close, Ca channels stay open, and K leaves the cell

Phase 2: plateau

• Ca remains open

• K channels close

• reduced permeability of membrane to K

• Reduction of efflux of K prevents early repolarization

Phase 3: Repolarization

• Calcium channels close

• Calcium ceases to enter cell

• Membrane permeability to K is restored, channels re-open, and cells rapidly repolarize Membrane permeability

Phase 4:

• Resting membrane potential

explain this chart

explain this chart and how it correlates to other charge

Na is phase 0

ca is phase 2

K+ balanced out after decreases is phase 3

Velocity of action potential

• 0.3-0.5 m/sec

Refractory in nature?

• Effective refractory period (ERPA)

  • from phase 0-2, cell cannot depolarize again until repolarization

• 0.23-0.30 sec

Relative refractory period

• typically occur in phase 2

• +0.05 sec

• if stimulus is strong enough u can overcome RRP to get an early premature contraction

  • only during RRP

explain

can u get premature contractions to make wave summation?

no

Excitation-contraction coupling and relaxation in cardiac muscle: explain

• AP travels along membrane to transverse (T) tubules

• Activation of voltage-dependent Ca channels

• Travels along longitudinal sarcoplasmic tubules

• Release of calcium from SR via ryanodine receptor (RyR) channels

• Calcium in sarcoplasm binds troponin

• Activation of actin-myosin complex

• Relaxation occurs via Ca ATPase (SERCA) and Ca/Na exchanger

T tubules:

• Ca required from T tubules for full contraction strength.

• T tubules retain calcium through electronegative mucopolysaccharides.

• Diameter 5x and volume 25x compared to SkMsc.

No ca what?

NO CA no HEAWRT BEAT

Sarcomere

• basic contractile unit of the myocyte

Myosin combines (what is combined myosin called) o form?

• combined myosin is called filament

• combines to form thousands of cross bridges with actin

myosin is made of?

• 2 heavy and 4 light chains

• head has 2 flexible hinges at either side of the arm

  • contains ATPase enzyme

• anchored by titin

actin contains:

2 F actin and 2-tropomyosin molecules

• F actin contains G actin + ADP (active site)

• Tropomyosin lies on top of active sites at rest

structure of actin and myosin?

both have twisted helix structure

troponin complex

• troponin I has high affinity for actin

• troponin T has high affinity for tropomyosin

• Troponin C has high affinity for calcium

what happens at rest on troponin-tropomyosin complex

• there is an inhibitory effect

• 4 Ca ions induce a conformational change and the troponin complex pulls tropomyosin away from active sites

• Active sites bind to myosin cross-bridges and contraction occurs

• Cross-bridges are independent thus number in contact with actin filament correlates to force of contraction

specific sequence of propagation of cardiac action pot.?

Propagation of cardiac action potential is simultaneous: T/F

• False

Influence of voltage-gated Na channels absent in SA and AV node: T/F?

T

which node is the primary pacemaker?

• SA Node

• connects directly to the atrial muscle fibers

• sets normal sinus rhythm

• 
  

phases in SA node :

• Phase 0: Depolarization

  • L-type Ca channels open

• Phase 1-2: Absent in SA

• Phase 3: Repolarization

  • Ca channels close and K channels open

• Phase 4: Return to resting membrane

can SA node self excitate?

• yes,

• Resting potential is less negative

• Leaky to sodium (funny currents) and calcium

• Slight delay with K channels remaining open (hyperpolarization)

what effect does neural control of SA node have

• alters rate of depolarization, HR

• sympathetic control

  • (mostly release NE, activates beta1 adrenergic receptors, increases rate of depolarization (note: circulating Epi also activates beta1-AR)

• Parasympathetic control

  • Right = SA and Left = AV

  • Release Ach, activates muscarinic receptors and decreases rate of depolarization

Neural control is needed to initiate action potential?

FALSE

AP spreads thru atrial muscle cells and atria contract?

True

what phases do atrial m AP phases exhibit?

• typical phases of cardiac muscle

Basal level of neural control

• Strong vagal stimulation can decrease the strength of heart muscle contraction by 20% to 30%.

• Can stop heart beat for a few seconds

• Vagal fibers are distributed mainly to the atria so primary effect is on heart rate

Contraction movement of ventricle

• by purkinje system and left ventricle rotation

• Purkinje System

• Causes synchronous contraction of the ventricular muscle

• Rapid conduction results in almost all parts of the ventricles contracting within a narrow time

• First to last muscle fiber only 0.03-0.06 sec apart

• Functional syncytium

• Run ~1/3 of the way into ventricular muscle

• Merge with ventricular muscle fibers

• Muscle fibers conduct the action potential to the rest of the ventricular muscle mass

Left ventricular rotation

• During systole, the left ventricle contracts in a twisting motion

• Counterclockwise rotation of the apex of the heart

• Clockwise rotation of the base of the left ventricle during the ejection phase

• Pulls base downward towards apex during systole (wrings it out)

• Like a loaded spring, recoils (untwists) during diastole

Pressure across the Cardiac Cycle

• Pressure changes in the aorta (top dotted), left ventricle (red), and left atrium (bottom dotted)

• Iso = equal or same

• Atrial contraction usually causes an additional 20% filling of the ventricles

• Approximately 60% of the blood in the ventricles at the end of diastole is ejected during systole

systole vs diastole

• Systole:

  • 60% volume ejected during period of rapid ejection (first 1/3 systolic time)

  • 30% volume ejected during period of slow ejection (last 2/3 systolic time)

• Diastole

  • Ventricle fillin