Lecture 19/ Human circulation

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ii.sympathetic nervous system (norepinephrine) - increases rate of K+ channel closing

In contrast, epinephrine and norepinephrine stimulate the closing of the K+ channelsin the SA node cells; this allows them to reach threshold and depolarize sooner and speeds up HR. In addition, these molecules increase the opening of Ca++ channels, which causes a stronger, faster muscle contraction.

b. Hormones
- epinephrine increases both HR and SV
c. Temperature
- increase of 1ºC causes increase in HR of 10 beats/min.
d. Exercise
e. Some antibiotics & antihistamines increase the chance of cardiac arhythmias because they block one class of K+ channel.

F. Kinetics of action potential
Earlier, I diagramed the action potential of the SA node (pacemaker) cells. Their whole function is to generate electrical activity, like a neuron. The atrial and especially the ventricular cells also contract. Their electrical activity is a bit more complicated, with several types of ion channels involved.

In response to depolarization, voltage-gated Na+ channels open, allowing a rapid depolarization, like a neuron. K+ channels open, and the cell starts to repolarize, but this is countered by an influx of Ca++. This creates a plateau phase, before increased flow of K+ out repolarizes the cell.

IMAGE imgs/220C_19Lecture06.gif

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	ii.sympathetic nervous system (norepinephrine) - increases rate of K+ channel closing
In contrast, epinephrine and norepinephrine stimulate the closing of the K+ channelsin the SA node cells; this allows them to reach threshold and depolarize sooner and speeds up HR. In addition, these molecules increase the opening of Ca++ channels, which causes a stronger, faster muscle contraction. b. Hormones - epinephrine increases both HR and SV c. Temperature - increase of 1ºC causes increase in HR of 10 beats/min. d. Exercise e. Some antibiotics & antihistamines increase the chance of cardiac arhythmias because they block one class of K+ channel. F. Kinetics of action potential Earlier, I diagramed the action potential of the SA node (pacemaker) cells. Their whole function is to generate electrical activity, like a neuron. The atrial and especially the ventricular cells also contract. Their electrical activity is a bit more complicated, with several types of ion channels involved. In response to depolarization, voltage-gated Na+ channels open, allowing a rapid depolarization, like a neuron. K+ channels open, and the cell starts to repolarize, but this is countered by an influx of Ca++. This creates a plateau phase, before increased flow of K+ out repolarizes the cell.


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