Term
| What are the different layers in cardiac muscle? |
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Definition
Epicardium - visceral pericardium, denser connective tissue covered with mesothelium
Myocardium - thick muscle layer in the middle (thicker in left ventricle)
Endocardium - inner layer, endothelium underlain by loose connective tissue |
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Term
| How are myocardial fibers arranged? |
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Definition
| Single band of muscle in a helical coil (ventricular) |
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Term
| What are the main tissue features of cardiac muscle? |
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Definition
Branched fibers - cause irregular shape
Central nuclei
Cross-striations
intercalated disks - where the cells join
big intercellular spaces - accommodate capillaries |
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Term
| How do atrial and ventricular fibers differ? |
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Definition
Atrial are smaller cells with fewer T-tubules.
They also contain small dense granules which contain atrial natriuretic peptide |
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Term
| What is the function of atrial natriuretic peptide? |
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Definition
It is used to decrease blood volume
It increases the amount of Na+ excreted to the urine and the change in solute conc. causes water to flow as well and decrease blood volume
The heart can sense when pressure is too high from increased blood volume |
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Term
| What are Purkinje fibers? |
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Definition
Modified cardiac muscle fibers
sparse, peripherally located, and contain myofilaments
part of the conducting system
located between ventricular endocardium and contractile muscle
there is more space between cells than in normal cardiac fibers |
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Term
| What are some special features of cardiac muscle when compared to skeletal muscle? |
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Definition
irregular myofibrils
myofibril bands often out of register
abundant mitochondria
organelles collect at nuclear poles
intercalated disks join cells end to end |
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Term
| What is the structure/composition of an intercalated disk? |
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Definition
Fascia Adherens - patch-like junction anchored with actin filaments, similar to the ZA but more plate-like as opposed to belt-like
Macula Adherens - spot-like junction anchored with desmin intermediate filaments
Gap Junction - allow ions to pass, transmits contractile stimulus, critical for conduction |
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Term
| Explain the membrane system of cardiac muscle |
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Definition
The T-tubules are at the Z-lines
The terminal cisternae are small
The sarcoplasmic reticulum is smaller than in skeletal muscle
There are dyads as opposed to triads |
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Term
| What is the origin of cardiomyocytes? |
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Definition
| They come from myoblasts and angiogenic cell clusters in mesoderm that surround the neural plate |
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Term
| How does the contraction mechanism of cardiac muscle compare to skeletal muscle? |
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Definition
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Term
| What is the fiber type of cardiac muscle |
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Definition
Type 1, slow oxidative
abundant mitochondria
very slow to fatigue |
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Term
| Explain the sequence of the impulse pathway in cardiac muscle |
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Definition
Sinoatrial node (the "pacemaker")
Atrial cardial cardiac muscle
internodal fibers
AV node
AV Bundle
Right and left bundle branches
purkinje fibers
ventricular cardiac muscle |
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Term
| Explain the importance of the SA node |
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Definition
Cardiac muscle cells contract at own intrinsic rate.
Each contraction generates an action potential, transmitted to attached cells through gap junctions. Fastest rules.
Specialized cardiac muscle cells in the SA node have the fastest intrinsic rhythm (100-110/min) and therefore serve as pacemaker.
Normal vagal tone drops resting rate to 60-80/min
Normal “sinus rhythm” considered to be 60-100/min. Faster = Tachycardia; Slower = Bradycardia |
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Term
| How does sympathetic modulation of cardiac muscle work? |
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Definition
Speed up through the use of NE of EPI
short preganglionic: Lateral Cell Column
long postganglionic: Sympathetic Chain
Target: SA & AV Node, Atrial & Ventricular Cardiac Muscle |
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Term
| How does parasympathetic modulation of cardiac muscle work |
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Definition
Slow down contraction through the use of ACh on muscarinic receptors
long preganglionic: Medullary Nuclei
Vagus Nerve
short postganglionic: Cardiac Ganglia
Target: SA & AV Node, Atrial Muscle |
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Term
| How does innervation of cardiac muscle work? |
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Definition
Axons of autonomic neurons may contact the sarcolemma (en passage), but do not form specialized motor end plates
The axons release the NT's into space |
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Term
| What are the characteristics of blood supply to cardiac muscle? |
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Definition
Cardiac muscle is the most richly vascularized tissue in the body (has to do the most work)
Blood delivered by coronary arteries very richly oxygenated
Intense oxidative metabolism requires abundant oxygen, very sensitive to decreases
There is lots of space between cells in order to make room for the many capillaries |
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Term
| Explain an ischemic heart attack |
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Definition
Sudden blockages (or vascular spasms) in coronary Vessels rob myocardium of O2 supply, disrupt contractile function and lead to cell death.
The body can better adjust to blockages that occur over time as opposed to sudden ones
The left anterior descending coronary artery (aka the widow-maker) is an artery that is affected a lot. This is important because a blockage will affect both ventricles |
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Term
| How does hypoxia affect cardiac muscle? |
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Definition
Hypoxia (lack of oxygen) damages mitochondria, causes cytochrome C release, initiates apoptosis
Since cardiac muscle cells are terminally differentiated and cannot divide to replace dead cells, fibroblasts invade site and form scars. |
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Term
| How does reperfusion affect cardiac muscle? |
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Definition
Reperfusion yields O2 radicals that further damage membranes. Ca++ influx and release from SR & mitochondria further stimulate apoptosis
Reperfusion occurs when there is blockage that has been removed and there is a massive increase in oxygen. This hyperoxia is what damages the lipids in the membrane. |
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Term
| What are the distinguishing features of cardiac muscle? |
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Definition
branched, cylindric cells
one or two central nuclei
filament ratio of 6 thin/1 thick
less organized sarcoplasmic reticulum than skeletal muscle, not nearly as many distinct myofibrils
T-tubules form dyads at Z-lines
no motor end plates
involuntary contraction
striated
intercalated disks at cell-cell junctions - a major defining feature |
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