Hsf: Cardiac Excitation-Contraction Coupling

Front 1

Central Task of Both Skeletal and Cardiac Muscle

Back 1

-To produce mechanical force

Front 2

Similarities in Skeletal and Cardiac EC coupling

Back 2

-Cardiac ventricular myocytes contain T-tubules and highly develop intracellular SR networks similar to skeletal muscle
-AP provide excitation stimulus used to activate plasma membrane Ca2+ channels and action of sarcolemmal DHPRs triggers the opening of nearby SR Ca2+ release channels
-Resulting elevation in myoplasmic Ca2+ activates the contractile machinery and initiates crossbridge cycling in a manner similar to that observed in skeletal muscle

Front 3

Differences in Skeletal and Cardiac EC coupling

Back 3

-Skeletal muscle contraction is primarily voluntary, initiated by neural activation, and its magnitude is primarily regulated by extrinsic factors
-Cardiac muscle contraction is involuntary, occurs continuously without interruption, is self-initiated, and its magnitude is regulated largely by instrinsic factors
-Electrical impulse that initiates contraction in skeletal muscle is much briefer

Front 4

Categories in Which Skeletal and Cardiac EC Coupling

Back 4

-Initiated
-Accomplished
-Regulated

Front 5

Terminal SR membranes typically only make ______ junctional contacts with the T-tubule in cardiac muscle.

Back 5

-Single (diads)

Front 6

In Cardiac muscle, SR membranes make functional contacts with the _____ ____ that do not occur in skeletal muscle.

Back 6

-Subsarcolemmal Cisterna=surface membrane

Front 7

Mitochondria (Cardiac Muscle)

Back 7

-Larger and more plentiful=to provide a sufficient supply of ATP to fuel the continuous contractile activity of cardiac muslce

Front 8

What type of tissues is in cardiac muscle?

Back 8

-Contractile (atrial and ventriculate muscle cells)
-Specialized Automatic (sino-atrial and atrioventricular nodes)
-Conductive fibers
-Adjacent cardiac cells
(All in syncytium of many heart cells electrically connected at intercalated discs by gap junctions)

Front 9

Mechanism of EC coupling in cardiac muscle

Back 9

-Extracellular calcium and calcium flux across the sarcolemma (through voltage-gated calcium channels) is an absolute requirement for initiation of cardiac contraction
-Involves "calcium-induced calcium release" (CICR)

Front 10

Voltage-gated L-type Ca2+ channels are activitated during the _____ ______ of the long-lasting ventricular muscle action potential.

Back 10

-Plateau Phase

Front 11

Where does calcium come from for EC coupling of cardiac muscle?

Back 11

-Extracellular space
-SR Ca2+ stores

Front 12

What does Ca2+ released during EC coupling interact with?

Back 12

-Troponin C

Front 13

Troponin C (Cardiac Muscle)

Back 13

-Initiates crossbridge cycling

Front 14

When does contraction stop?

Back 14

-When myoplasmic Ca2+ levels are reduced back to resting levels

Front 15

How are resting levels restored in the cardiac muscle?

Back 15

-Through plasma membrane and SR Ca2+ ATPase pumps
-Sodium-calcium exchangers in the sarcolemma
-Mitochondria Ca2+ uptake

Front 16

What do the mechanisms to restore the cardiac muscle act to ensure?

Back 16

-To ensure cytoplasmic/SR Ca2+ levels are restored
-Excess Ca2+ is removed from the cell
-Ca2+-dependent mitochondrial enzymes are activated to produce sufficient ATP levels required for the crossbridge cycle

Front 17

4 Factors that Determine Skeletal Contractile Strength

Back 17

1. Number of myofibrils per muscle fiber
2. Length-tension relationship
3. *Motor unit recruitment
4. *Temporal summation of neural input
*most important for acute regulation of skeletal muscle contractile strength

Front 18

What does cardiac contractility primarily involve?

Back 18

-Stretch-induced optimization of the length-tension relationship
-Degree of sympathetic tone to the heart

Front 19

What does sympathetic stimulation of cardiac contractile cells activate?

Back 19

-Sarcolemmal Beta-adrengergic receptors (primarily Beta-1 receptors)

Front 20

Beta-adernergic receptors (primarily Beta-1 receptor in cardiac muscle)

Back 20

-Increases contractile strength (inotropy) and abbreviate contractile relaxation (lusitropy) and the duration of the myoplasmic Ca2+ transient