Lecture 8

Question 1

Factors that influence heart rate, and in turn, cardiac output

Answer 1

1- Parasympathetic NS
2- Sympathetic NS
3- Hormones
4- Ions

Question 2

Parasympathetic Nervous System

Answer 2

Negative chronotropic & dromotropic effects

Question 3

Negative chronotropic effect

Answer 3

• Decreases HR by decreasing the rate of phase 4 depolarization
• Fewer action potentials occur per unit time because the threshold potential is reached more slowly and, therefore, less frequently
• Sympathetic Nervous System
• The mechanism of the negative chronotropic effect is decreased If, the inward Na+ current that is responsible for phase 4 depolarization in the SA node

Question 4

Negative dromotropic effect

Answer 4

• Decreases conduction velocity through the AV node

* PQ segment of ECG is increased

Question 5

Sympathetic Nervous System

Answer 5

• Positive chronotropic effect

- Positive dromotropic effect

Question 6

o Positive chronotropic effect

Answer 6

• Increases HR by increasing the rate of phase 4 depolarization
• More action potentials occur per unit time because the threshold potential is reached more quickly and, therefore, more frequently
• The mechanism of the positive chronotropic effect is increased If, the inward Na+ current that is responsible for phase 4 depolarization in the SA node

Question 7

o Positive dromotropic effect

Answer 7

• Increases conduction velocity through the AV node

Question 8

o Hormones

Answer 8

• Epinephrine / NE from the adrenal medulla exert similar effects as the SNS; these tend to be important during exercise, stress and excitement
• Thyroid hormones (T3 & T4) can elevate HR at rest
Increased the number of b-receptors

Question 9

o Ions

Answer 9

-For example, hyperkalemia & hypokalemia can both decrease HR (via different MOAs)
• Age, gender, physical fitness and body temperature also influence resting HR

Question 10

Other effects on resting HR

Answer 10

• Age
• Gender
• Physical fitness
• Body temperature