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Anatomy of Cardiovascular System

Physiology & Anatomy of  Cardiovascular System , Heart Physiology Lecture Notes

بروفسيورعلي التميمي ,استاذ الباثوفسيولوجي والتشريح المرضيPhysiology & Anatomy of Cardiovascular System , Heart Physiology Lecture Notesالبروفسیورعلي التمیمي

Heart  Physiology

  1. Cardiac Muscle 

Cardiac Muscle Cells

fairly short

semi-spindle shape

branched, interconnected

connected (intercalated discs)

electrical link (gap junction)

common contraction (syncytium)

1 or 2 central nuclei

dense “endomysium”

high vasculature

MANY mitochondria (25% space)

almost all AEROBIC (oxygen)

myofibers fuse at ends

  1. Mechanism of Contraction of Contractile Cardiac Muscle Fibers
  2. Na+ influx from extracellular space, causes positive feedback opening of voltage-gated Na+ channels; membrane potential quickly depolarizes (-90 to +30 mV); Na+ channels close within 3 ms of opening.
  3. Depolarization causes release of Ca++ from sarcoplasmic reticulum (as in skeletal muscle), allowing sliding actin and myosin to proceed.
  4. Depolarization ALSO causes opening of slow Ca++ channels on the membrane (special to cardiac muscle), further increasing Ca++ influx and activation of filaments. This causes more prolonged depolarization than in skeletal muscle, resulting in a plateau action potential, rather than a “spiked” action potential (as in skeletal muscle cells).

Differences Between Skeletal & Cardiac MUSCLE Contraction

  1. All-or-None Law – Gap junctions allow all cardiac muscle cells to be linked electrochemically, so that activation of a small group of cells spreads like a wave throughout the entire heart. This is essential for “synchronistic” contraction of the heart as opposed to skeletal muscle.
  1. Automicity (Autorhythmicity) – some cardiac muscle cells are “self-excitable” allowing for rhythmic waves of contraction to adjacent cells throughout the heart. Skeletal muscle cells must be stimulated by independent motor neurons as part of a motor unit.
  1. Length of Absolute Refractory Period – The absolute refractory period of cardiac muscle cells is much longer than skeletal muscle cells (250 ms vs. 2-3 ms), preventing wave summation and tetanic contractions which would cause the heart to stop pumping rhythmically.

III.       Internal Conduction (Stimulation) System of the Heart

  1. General Properties of Conduction
  2. heart can beat rhythmically without nervous input
  3. nodal system (cardiac conduction system) – special autorhythmic cells of heart that initiate impulses for wave-like contraction of entire heart (no nervous stimulation needed for these)
  4. gap junctions – electrically couple all cardiac muscle cells so that depolarization sweeps across heart in sequential fashion from atria to ventricles
  5. “Pacemaker” Features of Autorhythmic Cells
  1. pacemaker potentials – “autorhythmic cells” of heart muscle create action potentials in rhythmic fashion; this is due to unstable resting potentials which slowly drift back toward threshold voltage after repolarization from a previous cycle.

Theoretical Mechanism of Pacemaker Potential:

  1. K+ leak channels allow K+ OUT of the cell more slowly than in skeletal muscle
  2. Na+ slowly leaks into cell, causing membrane potential to slowly drift up to the threshold to trigger Ca++ influx from outside (-40 mV)
  3. when threshold for voltage-gated Ca++ channels is reached (-40 mV), fast calcium channels open, permitting explosive entry of Ca++ from of the cell, causing sharp rise in level of depolarization
  4. when peak depolarization is achieved, voltage-gated K+ channels open, causing repolarization to the “unstable resting potential”
  5. cycle begins again at step a.
  1. Anatomical Sequence of Excitation of the Heart

Autorhythmic Cell Location & Order of Impulses

(right atrium)               sinoatrial node (SA) ->

(right AV valve)          atrioventricular node (AV) ->

atrioventricular bundle (bundle of His) ->

right & left bundle of His branches ->

Purkinje fibers of ventricular walls

(from SA through complete heart contraction = 220 ms = 0.22 s)

  1. sinoatrial node (SA node) “the pacemaker” – has the fastest autorhythmic rate (70-80 per minute), and sets the pace for the entire heart; this rhythm is called the sinus rhythm; located in right atrial wall, just inferior to the superior vena cava
  1. atrioventricular node (AV node) – impulses pass from SA via gap junctions in about 40 ms.; impulses are delayed about 100 ms to allow completion of the contraction of both atria; located just above tricuspid valve (between right atrium & ventricle)
  1. atrioventricular bundle (bundle of His) – in the interATRIAL septum (connects L and R atria)
  1. L and R bundle of His branches – within the interVENTRICULAR septum (between L and R ventricles)
  1. Purkinje fibers – within the lateral walls of both the L and R ventricles; since left ventricle much larger, Purkinjes more elaborate here; Purkinje fibers innervate “papillary muscles” before ventricle walls so AV can valves prevent backflow
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