Cardiac Action Potential By Jenny R. Sheriff, EMT-P, I/c

Action Potential The ability of a muscle cell to depolarize Happens in 3 stages Stimulation Depolarization Repolarization

Regulatory Properties Chronotropic state Rate of contraction Dromotropic state Rate of electrical conduction Inotropic state Strength of contraction (contractility)

Regulatory Properties Controlled by the brain Autonomic NS Sympathetic Parasympathetic Endocrine system Insulin Sympathetic nervous system – fight or flight. Major neurotransmitters are epinephrine and norepinephrine, which are classified as catecholemines.These are made in the adrenal glands which lie on top of the kidneys and secreted by the adrenal medulla. Parasympathetic NS – feed or breed. Neurotransmitter is acetylcholine, which is produced in the brain and nerves. Glucose plays a part in production of acetylcholine. Acetylcholine is classified as a cholinergic.

Receptors Triggered by changes in chemicals or pressure Are found throughout the body

Baroreceptors Sense changes in pressure and send signals to the brain. Aortic Arch Carotid Sinus Changes in pressure against the walls of the aorta (particularly in the arch) and the carotid sinus cause the baroreceptors to signal the brain to act. For example, if the blood pressure drops, baroreceptors signal the brain. The brain then sends a signal to the parasympathetic NS via the vagal nerve to inhibit acetylcholine, which increases the availability of catecholemines, resulting in increased HR, peripheral vasoconstriction, and increased BP.

Chemoreceptors Sense chemical changes Acid/base balance Chemoreceptors can be found throughout the vascular system and in the brain. They sense chemical changes, like CO2 and O2, and try to make the body compensate for the changes.

Adrenergic Receptors Alpha 1 Alpha 2 Beta 1 Beta 2 Peripheral vasoconstriction Alpha 2 Peripheral vasodilation Beta 1 Increases HR, conductivity, automaticity, and contractility Beta 2 Bronchodilation In cases of acidosis, the chemoreceptors in the brain identify increased levels of CO2 and trigger the sympathetic nervous system. RR increases, while sympathetic outflow to the heart is also triggered, resulting in increased HR, via the vagus nerve and adrenergic receptors on the heart.

Ions Na is the major extracellular ion K is the major intracellular ion Ca is an extracellular ion necessary for neurologic and muscle function Ca can enter cells through calcium channels and into the sarcoplasmic reticulum of cardiac muscle fibers, resulting in muscle contraction. Ca is regulated through the parthyroid hormone.

Cardiac Action Potential Electrical Potential Polarized state Depolarization Repolarization

Electrical Potential The difference between the electrical charges on opposite sides of a cell wall.

Polarized State Resting phase The inside of the cell has a negative charge in relation to the outside of the cell The polarized state is the electrical charge of the cell when it’s at rest. This is true for all muscle cells.

Depolarization Muscle contraction The charge inside the cell becomes positive in relation to the outside of the cell During depolarization, neurons stimulate fast NA channels open, allowing an influx of sodium to enter the cell. Calcium also enters the cell, and small amounts of potassium leave the cell. The calcium acts on actin and myosin filaments, causing muscle contraction.

Repolarization Sodium/potassium pump Muscle begins to relax Refractory period Absolute refractory period Relative refractory period In repolarization, ATP is used to pump potassium and sodium against the concentration gradient. That is, potassium is pumped back into the cell and sodium is pumped back out through the sodium/potassium pump.

Scenario Your 60 yof patient is c/o chest pain and flu like symptoms. She states she has been taking calcium supplements because she is afraid of osteoporosis. She’s been taking these for about a year. She states severe diarrhea for the last week, with some vomiting. She also feels weak and seems lethargic.

Scenario Your patient is a 54 yom c/o chest pain. Hx of diabetes, CAD, HTN, lung CA.

Questions??