Cardiac Muscle Physiology Faisal Mohammed MD Ph D

Cardiac Muscle Physiology Faisal Mohammed, MD, Ph. D 1

Objectives: By The end of this lecture students should be able to: n Distinguish the cardiac muscle cell microstructure n Describe cardiac muscle action potential n Point out the functional importance of the action potential n Follow the cardiac muscle mechanism of contraction n Delineate cardiac muscle energy sources n Outline the intracellular calcium homeostasis n Explain the relationship between muscle length and tension of cardiac muscle (Frank-Starling law of the heart) 2

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Cardiac Muscle Vs Skeletal Muscle v Syncytium structure v Gap Junction (electrical coupling) low resistance area v Poorly developed Sarcoplasmic reticulum (SR) v Transverse (T)Tubule on Z-line (i. e. One T-tubule per sarcomere) v Rich in mitochondria v Low in nuclei 8

Permeability Changes and Ionic Fluxes During an Action Potential (skeletal Muscle) 9

1 0 2 3 4 10

The Action Potential in Skeletal and Cardiac Muscle 11

(inactivation gate) h Gate (activation gate) m Gate 12

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Mechanism of Cardiac Muscle Excitation, Contraction & Relaxation 15

Intracellular Calcium Homeostasis… 1 16

Intracellular Calcium Homeostasis… 1 17

Intracellular Calcium Homeostasis… 2 18

FORCE ACTION POTENTIAL EFFECTS OF Ca++ CHANNEL BLOCKERS AND THE CARDIAC CELL ACTION POTENTIAL CONTROL 30 10 10 CONTROL 30 TIME DILTIAZEM 10 u. Mol/L 30 u. Mol/L

Cardiac Muscle action potential Vs. Skeletal Muscle Ø Phase 0 –Depolarization phase (Na+ influx) Ø Phase 1 partial repolarization (Not in skeletal) Ø Phase 2 Plateau (depolarization not in skeletal) slow calcium channels Ø Phase 3 fast repolarization phase (K+ efflux Ø Phase 4 resting membrane potential 20

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1 2 0 3 4 25

The Action Potential in Skeletal and Cardiac Muscle 26

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Cardiac Muscle contraction Vs. Skeletal Muscle ± Sliding filament hypothesis ± No tetany (Long refractory period because of plateau) ± Fatty acids main source of energy unlike skeletal muscle (Anaerobic and Aerobic) ± Attachment and detachment cycle and ATP dependence is the same 34

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v Active tension cannot be measured directly v What can be measured? (1) passive tension - tension required to extend a resting muscle v (2) total tension - active tension and passive combined v Active is calculated from 1 & 2 v (AT = TT – PT) v Note that active tension falls away linearly with increasing length v Tension (%age max contraction) Length-Tension Relation for Skeletal Muscle Normal operating range 100 total tension 50 active tension passive tension 0 1. 0 2. 0 Length (proportion of resting length) 37

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Isometric Contraction 40

SERIES ELASTIC ELEMENTS CONTRACTILE COMPONENT PARALLEL ELASTIC ELEMENTS (ACTIVE TENSION) (PASSIVE TENSION) TOTAL TENSION

Cardiac Muscle length-tension relationship FCardiac muscle works at much less than its maximum length in contrast to skeletal FTotal, Active and Passive length-tension relationship differ FFrank-Starling law of the heart 42

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Thank You 44