University:
Glendale Community College
Course:
BIO160 | Introduction to Human Anatomy and Physiology
Academic year:

2019
Views:

487

Pages:

14
Author:

acaronada09

Sympathetic Nervous System • Cardioacceleratory Center – stimulates sympathetic nerves to SA node, AV node and myocardium – these nerves secrete norepinephrine, which binds to -adrenergic receptors in the heart – CO normally peaks at HR of 160 to 180 bpm – Sympathetic n.s. can  HR up to 230 bpm, (limited by refractory period of SA node), but SV and CO  (less filling time) Parasympathetic Nervous System • Cardioinhibitory Center – stimulates vagus nerves • right vagus nerve - SA node • left vagus nerve - AV node – secrete ACh (acetylcholine), binds to muscarinic receptors • opens K+ channels in nodal cells, hyperpolarized, fire less frequently, HR slows down – vagal tone: background firing rate holds HR to sinus rhythm of 70 to 80 bpm • severed vagus nerves results in the SA node firing at its intrinsic rate of about 100bpm • maximum vagal stimulation  HR as low as 20 bpm Structure of Cardiac Muscle • Short, thick, branched cells with central nuclei. • Sarcoplasmic reticulum is less developed than in skeletal muscle. – cardiac cells must get more Ca2+ from extracellular fluid during excitation • T-tubules are invaginations of sarcolemma (muscle cell plasma membrane) that carry depolarization into the cells. • Intercalated Discs, join cells end to end. – interdigitating folds -  surface area for strength and communication – mechanical junctions tightly join cells • adherent junctions and desmosomes – electrical junctions • gap junctions form channels that allow ions to flow directly from cell to cell to synchronize contractions Intercalated Disk Cardiac Muscle Histology Metabolism of Cardiac Muscle • Fatigue resistant aerobic respiration • Cells have abundant myoglobin – most tissues extract about 25% of the oxygen from the blood as it circulates, but cardiac muscle extracts about 65% of the oxygen • Abundant, large mitochondria • Organic fuels: fatty acids, glucose, glycogen, ketones Cardiac Conduction System • Cardiac Conduction System is Myogenic - heartbeat originates within heart • Cardiac Conduction System is Autorhythmic – heartbeats are regular and spontaneous Cardiac Conduction System • Cardiac Conduction System Components: – Cardiac Conduction System is composed of Nodes and Bundles of specialized cardiac cells. – Sinoatrial (SA) Node is the pacemaker that initiates heartbeat and sets heart rate. – Atrioventricular (AV) node is an electrical relay to the ventricles – AV bundle (bundle of His) is a pathway for signals from the AV node to muscles of ventricles. – Right and left bundle branches are divisions of the AV bundle that enter the interventricular septum and descend to the apex. – Purkinje Fibers project upward from apex and spread throughout ventricular myocardium. – Moderator Band is a muscular bundle of heart tissue crossing the ventricular cavity from the interventricular septum to the base of the paillary muscles of the right ventricle. – fibrous skeleton of the heart is connective tissue that insulates atria from ventricles. Cardiac Conduction System Impulse Conduction • SA node signal travels at 1 m/sec across the atria • AV node slows signal to 0.05 m/sec – composed of thinner myocytes with fewer gap junctions – delays signal 100 msec, allows ventricles to fill • AV bundle and purkinje fibers speeds up signal to ventricles at 4 m/sec • Papillary muscles get the signal first, and their contraction stabilizes the AV valves • Ventricular systole begins at apex, progresses up – spiral arrangement of myocytes twists ventricles slightly during contraction which increases the efficiency of blood ejection from the chambers Cardiac Rhythm • Sinus Rhythm is set by the SA node – adult sinus rhythm at rest is 70 to 80 bpm • Nodal Rhythm is set by the AV node – adult nodal rhythm is 40 to 50 bpm but it is normally overridden by the sinus rhythm • Ectopic Foci are regions of spontaneous firing outside of the SA node – ectopic foci can be caused by hypoxia, caffeine, nicotine, electrolyte imbalance, drugs • Arrhythmia – any abnormal cardiac rhythm – heart block: failure of conduction system due to disease • bundle branch block (caused by damage to bundle branches) • total heart block (caused by damage to AV node)