Cardiac Muscle and Intrinsic Conduction Flashcards
How does cardiac muscle contract
the sliding filament mechanism - sarcomeres
Cardiac myocytes
shorter and fatter than skeletal muscle cells - also more branched and interconnected
push and pull on the cardiac skeleton
How many cardiac myocyte are centrally located nuclei
1 or 2
Large mitochondria accounts for how much cardiac myocyte nuclei
25%-35%
Intercalated Discs
the plasma membranes of adjacent cardiac myocytes interlock at junctions
Desmosomes
prevent separation during contraction
Gap junctions
allow ions to pass from cell to cell - transmitting current across the entire heart
Sarcomere
Thick (Myosin)
Thin (Actin)
Unlike skeletal muscles, cardiac muscle cell sarcomeres vary greatly in diameter and branch extensively
Calcium delivery
fewer, wider t tubules - 1 per sarcomere - regulate calcium concentration
Similarities between skeletal and cardiac
both muscle types are contractile tissues - contractions are preceded by depolarization in the form of action potentials
transmission of an action potential across the t tubules tiriggers the release of calcium from the sarcoplasmic reticulum
calcium binds to toponin, moves tropomyosin, allows cross bridge cycling to begin
Differences between skeletal and cardiac muscle (Self Excitability)
some cardiac muscle cells are self-excitable - these are specific, noncontractile cells called pacemaker
self- generated depolarization travel throughout the heart via gap junctions
no neural input is needed for cardiac myocytes
Automaticity/Autorhythmicity
the ability to spontaneously depolarize
Differences between skeletal and cardiac muscle (Functional Syncytium)
cardiac muscle cells are tied together to form a functional syncytium
either all cardiac myocytes contract together, or the heart doesn’t contract
skeletal muscles contract via motor unit recruitment
Differences between skeletal and cardiac muscle (Release of calcium)
in skeletal muscle, depolarization causes release of calcium from the sarcoplasmic reticulum
in cardiac muscle, depolarization opens special slow-flow calcium channels in the cell membrane - the combination of extracellular calcium and calcium from the SR allows contraction
Differences between skeletal and cardiac muscle (Tetany)
in skeletal muscle, the refractory period is shorter than contraction allowing for summation
in cardiac muscle, the refractory period is longer than contraction preventing tetany
Absolute refractory period
the period during an AP when an additional AP cannot be generated
Differences between skeletal and cardiac muscle (Aerobic Respiration)
cardiac myocytes are dense in mitochondria reflecting a great dependence on oxygen
cardiac muscles is more adaptable to using different nutrient sources as fuel
Intrinsic Conduction System
noncontractile cells specialized to initiate and distribute impulses throughout the heart
Cardiac Pacemaker Cells
the 1% of cardiac myocytes that are autorhythmic - able to depolarize spontaneously - and set the pace of the heart
found in the sinoatria and atrioventricular nodes
Initiation of an Action Potential (Pacemaker Potential)
K+ channels are closed, slow Na+ channels are open, the cell’s interior becomes more positive (-60mV to -40mV)
Initiation of an Action Potential (Depolarization)
Calcium channels open (around -40mV), calcium influxes leading to an AP
Initiation of an Action Potential (Repolarization)
K+ channels open, K+ effluxes, cell’s interior becomes more negative
What is the order that cardiac pacemaker cells pass impulses across the heart
Sinoatrial (SA) Node
Atrioventricular (AV) Node
Atrioventricular Bundle
Right and Left Bundle Branches
Purkinje Fibers
The Sinoatrial (SA) Node
“The pacemaker”
Crescent shaped, located in the right atrial wall - just below the entrance of the SVC
generates impulse 75 times/minute and sets the pace for the heart (“Sinus Rhythm”)
The Atrioventricular (AV) Node
from the SA Node, the impulse travels down the intranodal pathway to the AV Node
located in the inferior portion of the interatrial septum - just above the tricuspid valve
at the AV Node, the impulse is delayed .1s allowing the atria to complete their contraction
the AV Node has smaller diameter fibers and fewer gap junctions than the other places on the intrinsic conduction pathway
The Atrioventricular (AV) Bundle
starts in the superior part of the interventricular septum
the AV Bundle is the only electrical connection between the atria and the ventricles
the cardiac skeleton is nonconducting
Right and Left Bundle Branches
AV Bundles splits into 2 pathways - the Right and Left Bundle Branches
these bundles branches course along the interventricular septum towards the heart’s apex
Purkinje Fibers
long strands of barrel-shaped cells with few myofibrils
these fibers complete the pathway through the interventricual septum, penetrate the heart’s apex, and turn superiorly into the ventricular walls
provide the impulse for the bulk of ventricular depolarization
there is a more elaborate network of pukinje fibers on the left side of the heart
Ejection of Blood
ventricular contraction almost immediately follows ventricular depolarization
the heart contracts with a “wringing” motion starting at the apex
the wave of the contraction moves toward the atria
blood is “ejected” superiorly in the great vessels
What would happen without input from the SA Node
the AV Node would depolarize about 50 times/minute
What would happen without the input of the AV Node
the pacemakers of the AV Bundle of the Purkinje Fibers would depolarize about 30 times/minute
When do slow pacemakers dominate
when the fast pacemakers stop working
Arrythmia
an irregular heart rhythm resulting from a defect in the intrinsic conduction system
Fibrillation
rapid or irregular contractions of the heart
control of the heart by the SA Node is disrupted
fibrillating ventricle are not useful pumps
Defibrillation
electricity shocking the heart to depolarize the entire myocardium - ideally, the SA Node begins to function normally, and sinus rhythm is restored
Implanatable Cardioverter Defibrillators (ICDs)
devices that continually monitor heart rhythms; they will slow tachycardia and emit an electrical shock in the event of fibrillation
Ectopic Pacemaker
an abnormal pacemaker
if the AV Node takes over, the heart rate will be slower (40-60 bpm), still adequate for tissue perfusion/circulation
Premature Ventricular Contraction (PVC) (extrasystole)
small regions of the heart become hyper excitable and the heart prematurely contracts
after PVC, the heart has a slightly longer time to fill, and the next normal contraction feels like a “thud”
typically, an occasional PVC is harmless
Damages to what structures will prevent impulses from reaching the ventricles (Heart Block)
AV Node > AV Bundle > R/L Bundle Branches
Total Heart Block
no impulses get though, the ventricles beat at their own intrinsic rate - too slow for adequate tissue perfusion
Partial Heart Block
only some impulses get through
Artificial Pacemakers
medical devices to recouple the atria and ventricles - pacemakers can be reprogrammed to change with changing energy demands and interrogated to with symptoms appear
What can sympathetic branch do?
Accelerate heart rate and increases contractility
What can parasympathetic branch do?
decelerate heart rate
Where are cardiac centers located
Medulla Oblongata
Cardioacceleratory Center
sends impulses through the sympathetic trunk - stimulates the SA Node, AV Node, myocardium, and the coronary arteries to increase heart rate and contractility
Cardioinhibitory Center
sends impulses through the vagus nerve to decrease heart rate
