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23 Cards in this Set
- Front
- Back
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• Put heart rate in its proper homeostatic context. What is the role of heart rate in the grand scheme of things?
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o Heart rate is a means to an end, not an end in and of itself.
o Heart rate is not a variable under homeostatic control. The body does not vigorously attempt to maintain a constant heart rate. o Heart rate is a component of cardiac output and is altered in order to maintain cardiac output. Cardiac output is important only insomuch as it is an important determinant of blood pressure. o Blood pressure IS a variable under homeostatic control. It is an end in and of itself. The body fights to keep blood pressure constant so that blood is continuously delivered to the tissues. o To summarize: blood pressure is a variable under homeostatic control, and heart rate, as a component of cardiac output, is modified (increased or decreased) only to maintain blood pressure homeostasis. o Heart rate is a means to an end, not an end in and of itself. o Heart is a pawn in a game, with the ultimate objective being the maintenance of blood pressure. |
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• What sets the normal resting heart rate?
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o Spontaneous depolarization of the SA node, which occurs about 100 times per minute.
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• What system modifies the rate of sinus node depolarization and thus controls the heart rate under physiologic conditions?
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o Autonomic nervous system
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• ANS control of heart rate is run through a “center” in the CNS. This center receives input from various detectors and then runs corresponding output through sympathetic and parasympathetic tracts to the heart.
• What is this center called? |
Cardiovascular center
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• Where is the cardiovascular center located?
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o Medulla oblongata
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• What type of input does the cardiovascular center receive?
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o Input about the chemical composition of blood detected by chemoreceptors
o Input about blood pressure detected by baroreceptors |
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• What types of detectors provide input to the cardiovascular center?
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o Chemoreceptors, which detect chemical changes in the blood
o Baroreceptors, which detect changes in blood pressure |
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• What types of situations, then, is heart rate altered in response to?
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o Increased/decreased oxygen demand of tissue
o Increased/decreased blood pressure |
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What specifically is modified in order to modify the heart rate?
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The frequency of spontaneous SA node depolarization (automaticity) is modified in order to modify heart rate.
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Describe on a molecular level the spontaneous depolarization of a pacemaker cell in the SA node.
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...
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How, specifically, is the frequency of spontaneous SA node depolarization altered?
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Alteration of the probability that the pacemaker channels will be open
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What is the autorhythmic rate of SA node depolarization?
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Approx. 100 bpm
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If the SA node spontaneously depolarizes around 100 times per minute, why is a normal resting heart rate not 100 bpm?
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A continually shifting balance exists between sympathetic and parasympathetic stimulation of the heart.
At rest, parasympathetic stimulation predominates. The resting heart rate is therefore lower than the autorhythmic rate of the SA node. |
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Describe, in general, the anatomic location of the outflow tract of the sympathetic branch of the ANS.
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Preganglionic neurons have cell bodies located in the lateral horns of the gray matter in the 12 thoracic segments and the first two (and sometimes three) lumbar segments of the spinal cord.
For this reason, the sympathetic division is also called the thoracolumbar division. |
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Describe, in general, the anatomic location of the outflow tract of the parasympathetic branch of the ANS.
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Cell bodies of preglanglionic neurons of the parasympathetic division are located in the nuclei of four cranial nerves in the brain stem (III, VII, IX, and X) and in the lateral gray matter of the second through fourth sacral segments of the spinal cord.
Hence the parasympathetic division is also known as the craniosacral division and the axons of the parasympathetic preganglionic neurons are referred to as the craniosacral outflow. |
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Describe the sympathetic innervation of the heart.
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Sympathetic neurons extend from the thoracic region of the spinal cord (which nerve roots?) to the SA node, AV node, and most portions of the myocardium.
The sympathetic neurons extending from the thoracic spinal cord to the heart are called the cardiac accelerator nerves. |
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What type of adrenergic receptor do cardiac myocytes possess?
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Beta 1 adrenoreceptors
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Describe the parasympathetic innervation of the heart.
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Parasympathetic nerves reach the heart via the right and left vagus nerves.
Vagal axons terminate in the SA node, AV node, and atrial myocardium. |
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Name an important difference between sympathetic and parasympathetic innervation of the heart.
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Sympathetic neurons innervate the SA and AV nodes as well as the atrial and ventricular myocardium.
Parasympathetic neurons innervate the SA and AV nodes and the atrial myocardium but NOT the ventricular myocardium. As such sympathetic stimulation can increase contractility, while parasympathetic stimulation has little effect on ventricular contractility. |
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Which type of cholinergic receptor is present in cardiac myocytes?
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Muscarinic cholinergic receptor
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What effect does increased body temperature have on heart rate?
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Increases heart rate by causing the SA node to discharge impulses more rapidly.
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What effect does decreased body temperature have on heart rate?
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Decreases heart rate by causing the SA node to discharge impulses less frequently.
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A patient's heart rate is elevated. Describe an approach to the potential causes of this tachycardia.
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1) Physiologic/compensatory
-1a. raised in response to decrease in blood pressure -the decrease in blood pressure may itself result from decreased stroke volume, decreased intravascular volume, or decreased peripheral resistance -1b. the compensation may also be in response to increased O2 demand 2) Raised in response to systemic catecholamines released by adrenal medullae 3) Raised as a result of increased body temperature 4) Raised in response to inappropriate levels of thyroid hormone 5) Drugs? 6) Mental state? |
