Morphology of cells:Varies with location and function SA Node AV Node Purkinje Fibers Myocyte Cells in SA and AV node:pacemaker cells that are not contractile. Purkinje fibers:conduction fibers that speed the conduction of activity but which are not contractile. Myocytes:Contractile cells made up of sarcomeres.These cells do the work of the heart. (Compared to skeletal muscle fibers,Cardiac myocytes are: .shorter, .somewhat narrower. .electrically and mechanically coupled to one another via intercalated disks
The heart is a syncytium.Action potentials are conducted from muscle cell to muscle cell in the heart.Unlike skeletal muscle,no nerves are involved in conduction of activity through the heart.Cells are electrically coupled to each other through intercalated discs,which contain gap junction channels Intercalated disc Gap junction (nexus) they bifurcate:each cell is Y-shaped,and contacts more than one downstream myocyte
2.Cardiac Action Potentials are based on different types of channels than nerves or skeletal muscle.This figure:Ventricular Action Potential. Ventricular Myocyte Action Potential +50 0 金 0 3 -50 ICa 4 4 -100 INa lKR
Channels Contributing to Action Potential in SA Node Na".K" Co 2. K DD(diastolic depolarization) Threshold for opening of L- type Ca channels
Control of i Control Sympathetic stimulation Parasympathetic stimulation If current through the HCN channel.The HCN channel is gated by both voltage and cyclic nucleotides.This slide illustrates how the current is modified by the cyclic nucleotide cyclic AMP cause by activation of the sympathetic or parasympathetic nervous system. Sympathetic stimulation raises the intracellular concentration of cAMP molecules; Parasympathetic stimulation reduces cAMP and decreases the heart rate by the opposite action
Channel mechanisms in SA nodal cells 2:GIRK channels (G-protein coupled Inward-Rectifying K(GIRK)channels/ M2 muscarinic receptors) Outside cell eurotransmitter GPCR CR SGPCR GIRK Inside cell GIRK channels are ACH-gated K channels found in cells of the SA node Parasympathetic neurons release acetylcholine,which acts on M2 muscarinic receptors, and GIRK channels are gated open.They allow an outward current of potassium, which counteracts If and slows the heart
Refractory Periods in Ventricular Myocytes Absolute Relative 40 人 Vm 20 (m) 40 -60 品 -80 .100 c 100 msec E Injected current needed to trigger action potential
Cardiac action potential:variation throughout the heart SA Noda Purkinje Fibe Septum LV Slow depolarization is referred to as a pacemaker potential. Pacemaker potentials are observed only in cells of the SA node,the AV node,and in Purkinje cells.The rate of depolarization of the SA nodal cells is fastest,meaning that they set the heart rate
Conduction Pathways in Heart Sinus A-V node node A-V bundle Left Internodal bundle pathways branch Right bundle branch
Comparison of AP in Ventricle and SA Node Fast-response action Slow-response action potentials potentials Phase 2 50 Phase 4 Phase 4 00 0.15 03 0.15 0.30 Time(s) Time(s)
