Physiology & Anatomy of Cardiovascular System , Heart Physiology Lecture Notes
بروفسيورعلي التميمي ,استاذ الباثوفسيولوجي والتشريح المرضيPhysiology & Anatomy of Cardiovascular System , Heart Physiology Lecture Notesالبروفسیورعلي التمیمي
- Cardiac Muscle
Cardiac Muscle Cells
connected (intercalated discs)
electrical link (gap junction)
common contraction (syncytium)
1 or 2 central nuclei
MANY mitochondria (25% space)
almost all AEROBIC (oxygen)
myofibers fuse at ends
- Mechanism of Contraction of Contractile Cardiac Muscle Fibers
- 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.
- Depolarization causes release of Ca++ from sarcoplasmic reticulum (as in skeletal muscle), allowing sliding actin and myosin to proceed.
- 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
- 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.
- 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.
- 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
- General Properties of Conduction
- heart can beat rhythmically without nervous input
- 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)
- gap junctions – electrically couple all cardiac muscle cells so that depolarization sweeps across heart in sequential fashion from atria to ventricles
- “Pacemaker” Features of Autorhythmic Cells
- 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:
- K+ leak channels allow K+ OUT of the cell more slowly than in skeletal muscle
- Na+ slowly leaks into cell, causing membrane potential to slowly drift up to the threshold to trigger Ca++ influx from outside (-40 mV)
- 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
- when peak depolarization is achieved, voltage-gated K+ channels open, causing repolarization to the “unstable resting potential”
- cycle begins again at step a.
- 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)
- 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
- 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)
- atrioventricular bundle (bundle of His) – in the interATRIAL septum (connects L and R atria)
- L and R bundle of His branches – within the interVENTRICULAR septum (between L and R ventricles)
- 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