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2 Cards in this Set
- Front
- Back
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Describe the origin of the contraction signal and how it is conducted through the heart.
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Origin: A pacemaker cell, usually in the SA node, but can be in the AV node, will depolarize spontaneously to threshold via If channels and eventually through Ca channels during phase 0 depolarization. Often, it starts from one pacemaker cell that spreads to the rest of the surrounding cells quickly via gap junctions in the intercalated disks.
Conduction: The impulse initiated at the SA node will propogate to the AV node quickly (utilizes fast-opening Na channels) thus allowing for atrial contraction. The impulse is delayed at the AV node (lower max diastolic potential, fewer gap junctions, and smaller diameter of cells), thus allowing for the atria to have enough time to dump its contents into the ventricle. (protection via refractory period) If there is a problem with the SA node, the AV node may become the origin of contraction. The impulse will then proceed through the His Bundle system which is part of the larger Purkinje network. Impulses proceed very quickly through this network bc of fast Na-channels, large fiber diameter, and the abundant gap junctions. Furthermore, impulse is propogated from the endocardium to the epicardium pretty much simultaneously. Like the AV node, impulses may be started at the ventricle, but at a very slow rate (15-40 beats/min) |
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Describe how the ANS controls HR and signal conduction.
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ANS works through dumping Ach to muscarinic receptors on the M2 postganglionic neurons in the nodal cells. This will work through the Gi pathway to decrease activity of adenyl cyclase which will decrease cAMP which will decrease pKa. Decrease in pKa will decrease activation of Ca channels, thereby decreasing signal conduction by decreasing slope of phase 0 depolarization. Additionally, the If current will be slowed (slows phase 4 depolarization) and the KACh channels will be stimulated (reducing max diastolic potential, effectively reducing HR)
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