5 Control (EC Coupling) copy

5 Control (EC Coupling) copy - Control of the Heart...

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Unformatted text preview: Control of the Heart Excitation-Contraction (E-C) Coupling In order to meet the metabolic needs of the body the heart must be controlled . This involves initiation of the contraction which implies heart rate. Also in order to increase cardiac output the heart also needs to contract with varying force. This all comes under the category of E-C Coupling which we have learned in skeletal muscle. But there are differences between skeletal muscle and heart muscle . Let us describe these differences. Similarities between Skeletal and Cardiac Muscle 1. Striated . There are sarcomeres in both, however in cardiac muscle the sarcomeres are not in perfect register. 2. Action potential initiates contraction by triggering release of Ca from the S.R. system aided by T -Tubules allowing Ca to bind to troponin uncovering myosin binding sites and initiating cross bridge cycling. Differences between Skeletal and Cardiac Muscle 1. Cardiac cells are electrically coupled via gap junctions (channel like communications between cells through which current can Fow. This means that an action potential initiated at the pacemaker can be conducted from cell to cell rapidly so that they all contract relatively simultaneously. Notice that the striations are not quite as aligned as skeletal muscle. Also, there are intercalated discs (gap junctions) linking each cell electrically 2. Cardiac cells have different action potentials than skeletal muscle. It is prolonged (avg. - 300 msec) and produces 4 phases seen below (0 - upstroke; 1 - max amplitude; 2 - plateau; 3 - repolarization; 4 - resting potential. Because of this prolongation cardiac action potentials cannot be tetanized. 3. The heart initiates its own action potential ( myogenic ) via a pacemaker within the R.A. unlike skeletal muscle which receives a synaptic input to initiate each action potential ( neurogenic ). The ANS modulates the rate of the action potentials but does not initiate them . In other words the heart will beat on its own if removed from neural inputs (eg. - transplants or an Arnold Schwarzenegger movie!) :-) 4. Varying Contractile Force (to increase amount of blood ejected and therefore Cardiac Output ) - The heart cannot recruit varying numbers of ¡bers or tetanize as skeletal muscle does. It varies force by: a. Varying Initial Fiber Length b. Varying “Contractility ” a. Varying Initial Fiber Length (Starling’s Law of the Heart) Starling’s Law of the Heart- The heart pumps out what it receives . i.e. - if it receives more it pumps more and if it receives less it pumps less. Sounds like a good thing... but how does the heart do it? Answer- The Length - Tension Curve that we learned during skeletal muscle mechanical function. Cardiac Length - Tension Curve The curve on the right shows that as the length (flling volume or pre-load) increases the Force oF contraction increases....
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5 Control (EC Coupling) copy - Control of the Heart...

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