how does the action potential stimulating heart muscle cells differ from other action potentials, and why? How does this affect the refractory period of heart muscle cells?
After initiation of an action potential, the refractory period is defined two ways: The absolute refractory period coincides with nearly the entire duration of the action potential. In neurons, it is caused by the inactivation of the Na+ channels that originally opened to depolarize the membrane. These channels remain inactivated until the membrane hyperpolarizes. The channels then close, de-inactivate, and regain their ability to open in response to stimulus.
The relative refractory period immediately follows the absolute. As voltage-gated potassium channels open to terminate the action potential by repolarizing the membrane, the potassium conductance of the membrane increases dramatically. K+ ions moving out of the cell bring the membrane potential closer to the equilibrium potential for potassium. This causes brief hyperpolarization of the membrane, that is, the membrane potential becomes transiently more negative than the normal resting potential. Until the potassium conductance returns to the resting value, a greater stimulus will be required to reach the initiation threshold for a second depolarization. The return to the equilibrium resting potential marks the end of the relative refractory period.
The refractory period in cardiac physiology is related to the ion currents that, in cardiac cells as in nerve cells, flow into and out of the cell freely. The flow of ions translates into a change in the voltage of the inside of the cell relative to the extracellular space. As in nerve cells, this characteristic change in voltage is referred to as an action potential. Unlike that in nerve cells, the cardiac action potential duration is closer to 100 ms (with variations depending on cell type, autonomic tone, etc.). After an action potential initiates, the cardiac cell is unable to initiate another action potential for some duration of time (which is slightly shorter than the "true" action potential duration). This period of time is referred to as the refractory period, which is 250ms in duration and helps to protect the heart.
In the classical sense, the cardiac refractory period is separated into an absolute refractory period and a relative refractory period. During the absolute refractory period, a new action potential cannot be elicited. During the relative refractory period, a new action potential can be elicited under the correct circumstances.
The cardiac refractory period can result in different forms of re-entry, which are a cause of tachycardia.Vortices of excitation in the myocardium (autowave vortices) are a form of re-entry. Such vortices can be a mechanism of life-threatening cardiac arrhythmias. In particular, the autowave reverberator, more commonly referred to as spiral waves or rotors, can be found within the atria and may be a cause of atrial fibrillation.
how does the action potential stimulating heart muscle cells differ from other action potentials, and why?...
Which statements describe the action potential in cardiomyocytes (heart muscle cells)? Depolarization stimulates voltage-gated, slow calcium channels to open. The absolute refractory period can last for about 200 ms. Rapid sodium influx causes the membrane to rapidly depolarize. Repolarization occurs immediately after the initial depolarization.
Diagram how an action potential generates contraction (excitation-contraction coupling) in a cardiac muscle cell. Compare and contrast cardiac muscle cell action potentials (both autorhythmic and contractile cells) and contraction with those in skeletal and smooth muscles. (be sure to discuss refractory periods & summation/tetanus in each type of muscle).
Action potentials from different axons differ in amplitude. What does the amplitude predict about the axon's diameter and why?
7. Correlate heart sounds with heart action The first heart sound results from the closing of which valves? The second heart sound results for the closing of which valves? 8. Describe the basic contraction characteristics of the heart. Contraction of the heart muscle is referred to as Relaxation of the heart muscle is referred to as Remember the ventricles contract 'upward from the apex to the base-forcing blood out of the heart, and that the atria contract downward' from the...
Answer following questions. (a)How are smooth muscle cells different from skeletal muscle fibers and cardiac muscle cells? (b)How are skeletal and cardiac muscle tissues similar to each other? How do these two types of muscle tissue differ from each other?
16. Which of the following explains the action of Epinephrine at the SA node? a) It binds to Beta 1 receptors on the SA node increasing the strength of cardiac muscle contraction b) It binds to muscarinic receptors on the SA node prolonging repolarization c) It binds to beta 1 receptors on the SA node causing rapid opening of HCN causing the membrane increasing the frequency of action potentials 17. Which of the following describes the action of a cholinergic...
Neuron Signaling and Muscle Contraction 1) “Dissect” the various parts of an action potential by describing the status (active or nonactive) of the voltage-gated sodium channel, voltage-gated potassium channel, sodium/potassium pump, and overall voltage and/ or voltage range (in millivolts, or mV) for each of the following. A) Resting membrane (prior to the initiation of an action potential) B) just before threshold to just after threshold (Depolarization) C) Rising phase of the action potential D) Falling phase of the action...
1. Use the Hodgkin-Huxley model (equations and parameters from Hoppensteadt) for the nerve action potential to estimate the following: The voltage threshold for excitation The current threshold for excitation The refractory period for stimulation with a current of 10 microAmperes/cm2 and duration 1 ms The relationship of the frequency of action potentials to the strength of a continuous stimulatory current.
1. Use the Hodgkin-Huxley model (equations and parameters from Hoppensteadt) for the nerve action potential to estimate the following: a. The voltage threshold for excitation b. The current threshold for excitation c. The refractory period for stimulation with a current of 10 microAmperes/cm2 and duration 1 ms d. The relationship of the frequency of action potentials to the strength of a continuous stimulatory current.
QUESTION 35 Which of the following is NOT true regarding the propagation of action potentials O A. All of these statements are true B. The magnitude of the action potential stays the same as it travels down the axon. C. Voltage-gated Na+ channels open only when the threshold potential is reached. OD. Each segment of the axon prevents the adjacent segments from firing, E. The refractory period allows the impulse to travel in only one direction OF. None of these...