Cardiac Muscle, Electrical Conduction - Lecture 11

Chapter Links and Quizzes

Origin of heartbeat

Electrocardiogram

Electrical Conduction of the Heart, Electrocardiogram

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I. Anatomy of the heart

1. Gross Anatomy

a. 4 chambers - 2 atria, 2 ventricles

b. 2 sets of valves - 2 A-V valves, 2 aortic pulmonic (semilunar) valves

60 beat/min; 31 billion times/year; 2 trillion times/70 years

The heart sits in the thoracic cavity

Walls of the heart are composed of muscle and are called the myocardium

The heart is covered by a fibrous sac called the pericardium

The human heart is divided into right and left halves, each consisting of an atrium and ventricle

Between the atrium and ventricle are the atrioventricular valves (A-V valves)

Right heart----tricuspid

Left heart-----Mitral

The valves at the opening to the pulmonary artery and aorta are the pulmonary valve and the aortic valve

The valves are attached to papillary muscle by chordae tendineae

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2. Microanatomy

composed of short branched cells

connected through gap junctions - intercalated disks
-rapid movement of ion - low electrical resistance

also desmosomes -- hold cells together to resist mechanical stress

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3. Conduction system

-some groups of cells in the heart exhibit spontaneous electrical activity and are designed for rapid conduction of action potentials.

a. Sino-atrial node (SA node)

1. a group of cells located in right atria near point of entry of vena cava

2. primary pacemaker

 

b. Atrio-ventricular node (AV node)

1. Located in atria near junction of ventricles

2. secondary pacemaker

c. Bundle of His

1. Group of modified, i.e. large muscle, cells, that travel from AV node across the junction between atria and ventricle- travels down septum - wall between ventricles - to apex of heart

2. Conduct electricity very rapidly because of large size

d. Purkinje Fibers

-fibers that spread through ventricle

II. Origin of Heartbeat

A. Membrane potential

more permeable to Na⁺ than in nerve cell -- or less permeable to K⁺

Cardiac muscle: Em = -90mV --- less permeable to Na⁺

pacemaker: Em = -60mV

B. Autorhythmicity - heart placed in physiological salt solution will continue to beat

1. This is due to changes in membrane permeability to K⁺

Ca⁺⁺ in during the slow rising phase is through T channels (transient) - Ca⁺⁺ in during the fast rising phase is through L channels (long lasting)

true for cells in SA, AV, bundle of His and Purkinje fibers

Ca⁺⁺ is very important

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C. Coordination

-Cells may have different rates but because they are coupled (gap junctions) - take on rate of faster cell

-fastest cell generates AP that is transmitted to all the other cells and generates an AP

-faster cell drives the slower cell - SA action potential is propagated to other cells and brings them to threshold -- other cells can then not fire because they are in refractory period

D. Sequence of events

1. SA node

2. atrial muscle

3. AV node

4. Bundle of His

5. Purkinje system

6. ventricular fibers

 

E. Ectopic focus

--if the pacemakers should be damaged - unexcitable - i.e. disease, heart attack, infarct

- a new region initiates AP

- the site is called an ectopic focus

person that eats lots of bacon, etc, coronary artery may become blocked - cardiac tissue will be destroyed

-if S.A.- another site takes over (AV node - secondary pace maker)

-if both SA and AV node - another site takes over (maybe along normal conduction pathways - maybe not)

-if a lot of the heart (arrhythmia or maybe even fibrillation)

• if ectopic foci fires even when normal pacemaker is functioning, get disorganization of heart firing, - fibrillation - if not corrected, this leads to death

• Shock -- coordinate pacemakers

REVIEW QUESTIONS

1. Which of the following does not describe the correct movement of a rbc through the heart? A rbc passes through the ______ before is passes through the _______.
   1. RV, LA
   2. tricuspid valve, mitral valve
   3. pulmonic valve, aortic valve
   4. pulmonary artery, pulmonary vein
   5. pulmonic valve, RV
2. The secondary  pacemaker is the
   1. SA node
   2. AV node
   3. Bundle of His
   4. Purkinje fibers
3. The group of modified cells that travel from the down the septum to apex of heart is called the
   1. SA node
   2. AV node
   3. Bundle of His
   4. Purkinje fibers
4. Cardiac cells are able to beat in a rhythmic fashion because
   1. the Em is -60 mV in pacemaker cells
   2. the cardiac cells are connected via gap junctions
   3. at rest the cardiac cell is more permeable to K⁺ than Na⁺
   4. All of the above are correct.
5. Membrane depolarization in a pacemaker cell is due to
   1. increased permeability to Na⁺
   2. decreased permeability to Na⁺
   3. increased permeability to K⁺
   4. increased permeability to Ca⁺⁺
   5. decreased permeability to Ca⁺⁺
6. If the AV node takes over for the SA node because of tissue damage, the Heart Rate will
   1. increase slightly
   2. decrease slightly
   3. not change
   4. stop
7. A drug (like acetylcholine) that decreases the firing of the SA node
   1. increases the closing of Na⁺ channels
   2. decreases the closing of Na⁺ channels
   3. increases the closing of Ca⁺⁺ channels
   4. increases the closing of K⁺ channels 
   5. decreases the closing of K⁺ channels

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