Ch 9.2A -- Electrical Activity of the Heart -- VTPP 435

Ch 9.2A -- Electrical Activity of the Heart -- VTPP 435

Autorhythmicity or Automaticity

-ability of an excitable cell to rhythmically initiate its own action potentials
-method how heart beats

Cardiac Contractile Cells

-do mechanical work of pumping
-do no initiate action potentials
-99% of cardiac muscle cells

Cardiac Autorhythmic Cells

-initiate and conduct action potentials
-do not contract themselves
-display pacemaker activity

Authorhymic Cells Behavior

-has a slow drift to threshold called pacemaker potential
-action potentials spread throughout heart to trigger rhythmic beating without nervous stimulation

Funny or If Channels

-Na+ channels that open when potential becomes more negative
-open when one action potential ends

T-type Ca2+ Channels

-Ca2+ voltage gated channel
-brings in Ca2+ briefly to depolarize membrane
-open when If channels close
-close at threshold

Membrane Clock Mechanism

-permeability changes in surface membrane ion channels
-cyclically brings membrane of autorhythmic cells to threshold

Ca2+ Clock Mechanism

-acts concurrently with membrane clock mechanism
-means of self-depolarizing membrane of autorhythmic cells to threshold
-depends on local Ca2+ recycling within pacemaker cell

L-Type Ca2+ Channel

-type of Ca2+ voltage gated channel
-opens when threshold is reached
-brings large Ca2+ influx
-closes at peak potential

Steps of Pacemaker Potential in Autorhythmic Cells

1) If channels open and K+ channels close
2) T-Type Ca2+ channels open and If channels close; just before threshold
3) Threshold reached, L-type Channels open and T-type Channels close
4) Peak potential reached, K+ channels open & L-type channels close

4 Types of Autorhythmic Cardiac Cell Regions

1) Sinoatrial (SA Node) -- fastest and lead rate; 70-80 APs/min
2) Atrioventricular Node (AV Node); 40-60 APs/min
3) Bundle of His or Atrioventricular bundle; 20-40 APs/min
4) Purkinje Fibers; 20-40 APs/min

Sinoatrial Node (SA Node)

-small region in right atrium wall near superior venae cavae opening
-pacemaker of heart due to fastest action potential and makes whole heart beat at its pace

Atrioventricular Node (AV Node)

-at base of right atrium near septum just above atria and ventricle junction
-only site of electrical contact between atria & ventricles
-takes over if SA node fails

Bundle of His or Atrioventricular Bundle

-starts in AV node and extends past pulmonary valve down middle of septum
-rapidly transmits an action potential down septum

Purkinje Fibers

-extend from bundle of His and spread throughout ventricular myocardium walls

Complete Heart Block

when conducting tissue between atria & ventricles is damaged and becomes nonfunctional

Ectopic Focus

-abnormally excitable area that initiates premature action potential that spreads around heart
-depolarizes quicker than SA node and becomes main driver of heart rate
-causes Premature Ventricular Contraction (PVC)

For efficient cardiac function, spread of excitation should satisfy 3 criteria

(1) Atrial excitation & contraction finish before onset of ventricular contraction
(2) Excitation of cardiac muscle fibers should be coordinated to ensure each chamber contracts as a unit
(3) Pair of atria and pair of ventricles should contract simultaneously

Fibrillation

-random uncoordinated excitation and contraction of cardiac cells
-ventricular fibrillation worse than atrial and causes death