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Control of the Cardiac Cycle
I). Cardiac Cycle
A). First the atriums go through systole and diastole together
B). Then the ventricles go through systole and diastole together
II). Control of the cardiac cycle
Intrinsic: Internal control
Extrinsic: External control
The heart functions in response to a combination of factors both within the heart muscle itself andoutside of the cardiovascular system.
Intrinsic control is due to specialized cells that initiate and distribute
electrical impulses in an orderly manner through out the heart.
A). Unique Characteristics of Cardiac Muscles
2). The heart contracts as a unit
3). Long refractory period
B). Action Potential of Pacemaker Cells
1).Decrease in K+ permeability but a continuous Na+ permeability that upsets the Na+ and K+ balance.
2). Ca++ channels open allowing Ca++ in until the action potential is reached.
3). K+ cannel open allowing for repolarization and than the K+ channels are closed.
C). Sequence of Excitation
1). The SA (sinoatrial) node
2) AV (atrioventricular) node
3). AV node to the bundle of His
4). left and right branches.
5). Purkinje fibers to the ventricular muscles.
Check this out
1). There are 3 distinguishable deflection waves.
i). P wave
depolarization from SA node
ii). QRS wave
from ventricular depolarization
repolarization or relaxation of the ventricles
2). ECG Tracings
i). Normal sinus rhythm.
ii). Atrial Flutter
iii). Prolonged QRS Complex
iv). Ventricular fibrillation
IV). Extrinsic Control of the Cardiac Cycle
A). autonomic nervous system.
1). sympathetic system enhances activity
2). The parasympathetic system inhibits activity
Thus extrinsic control is the result of a balance between both the parasympathetic and the sympathetic nervous system.
B). During times of physical or emotional stress:
The sympathetic system releases norepinephrine and epinephrine allowing the heart to reach threshold faster and the SA node to fire faster.
C). When the stress is over or during depression:
The parasympathetic system releases acetylcholine which hyperpolarizing the heart and thus the SA node does not fire as often.
D). Other extrinsic controls on the cardiac cycle:
1). Pressoreceptors that detect blood pressure.2). Hormones such as epinephrine and thyroxine3). Age4). Exercise5). Body temperature