Internal Conduction (Stimulation) System of the Heart

1. Heart can beat rhythmically without nervous input
2. 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)
3. Gap Junctions - electrically couple all cardiac muscle cells so that depolarization sweeps across heart in sequential fashion from atria to ventricles

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.

• 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

Special Considerations of Wave of Excitation

• initial SA node excitation causes contraction of both the R and L atria
• contraction of R and L ventricles begins at APEX of heart (inferior point), ejecting blood superiorly to aorta and pulmonary artery
• the bundle of His is the ONLY link between atrial contraction and ventricular contraction; AV node and bundle must work for ventricular contractions
• since cells in the SA node has the fastest autorhythmic rate (70-80 per minute), it drives all other autorhythmic centers in a normal heart
  
• arrhythmias - uncoordinated heart contractions
• fibrillation - rapid and irregular contractions of the heart chambers; reduces efficiency of heart
• defibrillation - application of electric shock to heart in attempt to retain normal SA node rate
• ectopic focus - autorhythmic cells other than SA node take over heart rhythm
• nodal rhythm - when AV node takes over pacemaker function (40-60 per minute)
• extrasystole - when outside influence (such as drugs) leads to premature contraction
• heart block - when AV node or bundle of His is not transmitting sinus rhythm to ventricles

• heart can beat without external innervation
• external innervation is from AUTONOMIC SYSTEM
  

• parasympathetic - (acetylcholine) DECREASES rate of contractions ;cardioinhibitory center (medulla) -> vagus nerve (cranial X) -> heart

• sympathetic - (norepinephrine) INCREASES rate of contractions ; cardioacceleratory center (medulla) -> lateral horn of spinal cord to preganglionics T1-T5 -> postganlionics cervical/thoracic ganglia -> heart

Category: Physiology Notes