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Physiostudy2 - The Heart as a Pump The cardiac action...

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Unformatted text preview: The Heart as a Pump The cardiac action potential Vocab • Atherosclerosis: narrowing of coronary artery Key Concepts 1. In a healthy heart, the SA node determines the intrinsic frequency of cardiac cantraction Anatomy of Heart • Pericardium: fibrous sac that encases heart • Atrioventricular valves: 1 way valve – – • • • Tricuspid: right Bicuspid/mitral: left Chordae Tendinae: attached to AV valves muscular projections to prevent their prolapse Papillary Muscles: attach chordae tendinae on inside wall of ventricles Semi-lunar vavles: separate the ventricles from their downstream arteries to prevent reflux of blood back into heart during relaxation Cardiac Blood Supply • • • • Heart receives NO nourishment from blood inside the heart chambers Blood supplied via the coronary arteries = 1st branches of aortic arch Atherosclerosis can result in angina or myocardial infarction Blood flow to heart = 4% of resting CO Cardiac Muscle • Straited: regular arrangement of thick myosin • Contain only few nuclei • Much shorter than skeletal muscle • Show branching • Adjacent cells joined end-end by interculated discs • • • Innervation by ANS effector neuron: both symp and parasymp Contraction of myocytes both autorythmic and spontaneous = does NOT require innervation (other just control rate) Comparison of Muscle Types • Only cardiac – Branched cells – – Speed of force generation: slow Not capable of sustained force – – • Has intercalated discs Produce ANP Cardiac & skeletal – – Have sacromeres (banding) Have T-tubules Cardiac & smooth -Source of Ca++: SR & extracellular -Few nuclei -Short fibers -ANS innervation -Have gap junctions -Spontaneous contraction -Effected by hormones -Stimulatory/inhibitory neural input Moderate SR – • • All -Have thick and thin filaments Cardiac Conduction System • Efficient pumping requires coordinated wave of contraction – • • Beginnning in right/left atria ventricles Gap junctions connect muscle cells and allow AP to proceed from one to next Conducting system also exists consisting of specialized myocytes (permits orderly depolarization): – SA node – AV node – Bundle of His – R/L bundle branches Conduction system The Cardiac Conduction System cient pumping by the heart requires a ordinated wave of contraction to occur ginning in the right and left atria and ceedingSA node ventricles. Gap through the ctions connect cardiac muscle cells and AV node ow this cardiac action potential to proceed Bundle of His m one cell to the next. To permit orderly polarization of the heart, a conducting R/L bundle stem also exits in the heart consisting of ecialized branches cardiac myocytes. These are: bas e Purkinje fibers Sinoatrial (SA) Node Atrioventricular (AV) Node Bundle of His Right & Left Bundle Branches Purkinje Fibers apex Vander!s 11th, figure 12-10 Orderly Depolarization of the Heart 1. SA node: 100 -depolarizations/min 2. Atrial muscle 3. AV node: -0.1 sec delay 4. Bundle of His: -20-40dep/min 5. Purkinje Fibers 6. Ventricular Muscle Equilibrium Potentials in Cardiac Myocytes • Excitable cells = polarized in relaxed state • Resting potential ~ -90mV • Ions: – – K+: -90mV / high in ICF – • Na+: equil. Pot. = +60mV / low in ICF Ca2+: positive / very low in ICF Driven by: ion pumps & cation exchangers that generate electrochemical gradients across plasma membrane – Na+/K+ ATPase: K in / Na out – Ca++ ATPase: Ca out – Molecular Basis of the Cardiac AP • Depolarization of plasma membrane involve: – • Ion channel present in unique combos in different cardiac cell types When an ion channel opens: – Membrane potential tends to approach the equilibrium for that ion the membrane potential tends t for that particular ion. For exam c i nd [ c + ] Hogh uKtance would give: - Events in AP of a contractile ventricular myocyte K++ K 1. 2. Resting state 3. 2. 3. drives resting membrane potential = -90mV (a) (b) 10 Outward K+ current balanced by equivalent inward Ca2+ membrane potential near 0 mV - P Na+ P Ca2+ (L) P K+ 0 0.15 due to slow opening of Long Lived (L-type) Ca2+ channels & 0.1 leakage thu K channels 10 0 0N P.15a+ Cells eventually repolarize - -50 RAPID membrane depolarization. SAME TIME: K+ begin closing -50 K+ channels never completely close - Here is the sequence of events ventricular myocyte beginning -100 0 0.15 0.30 - 1.0 Membrane potential held near 0mV for extended period -100 1. 5. - K+ Arrival of AP opens rapid voltage gated Na+ channels 0 1. 4. (a) 0 High [K+] K+ channels open 1. Rb l v b eme ane Po e l t a Relative Membrane Permeaeliaittiy e MemMranbrPermeatbinitiy l (mV) Membrane Potential (mV) Lo e K+] Herw i[s the sequence of +vents e ventricular myocyte beginning - 0.30 (b) 0.30 Time (sec) P P holds Ca2+ (L) current = K+ th Vander !s 11 , figure 12-12 1.0 - CV.12 0.1 0 0.15 0.30 Time (sec) th - K+ > Na+ + Na 2+ Ca IC MembraneFPhases Phase 0: – – – • Na+ open K+ (IR) closing K(V) & Ca closed Phase 1: – K (IR) leaky – – • Ca opening Na & K(V) closed Phase 2: – – K(IR) leaky Ca open Membrane Potential • 1 2 0 potential K + “Leaka K IR C han Channel 3 Phase 0 Phase 1 0 Phase 2 -50 4 Phase 3 Phase 4 -100 Time: 0 0.15 0.30 sec The action potential of Atrial Muscle Cells is sim potential shown above, except that it precedes th about 0.1 sec and the atrial action potential has a Clinical Correlate: • Calcium channel blockers of the DHP cla decrease Ca2+ conductance, shortening the and reducing cardiac contractility . These d Clinical Correlate: Calcium channel blockers • • Decrease Ca2+ conductance shortens length of plataue phase reducing contractility Useful for treating hypertension & angina pectoris • Autorhythmic Cells: SA & AV Nodes Display GRADUAL depolarization = pacemaker potential • Triggers rhythmic Aps whenever threshold is reached • Ion channels responsible: – – K(IR) channels (GIRKs): close slowly & reopen after membrane depolarizes Slow I(F) Na channels: display atypical V gated pattern – only open K efflux induces hyperpolarization = Hyperpolarization-activated cAMP-gated Na channels (HCN) – – • • • Transient (T-type) Ca channels: boost pacemaker potential Long lived (L-type) Ca channels: only open when threshold reached Nodal cells lack the rapid voltage gated Na channels T-Type Ca channels have lower voltage gating threshold than LType Overall duration of AP in nodal cells is LESS than contractile cells ...
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