Control Of Heart Rate

Question 1

Why is the heartbeat described as myogenic?

Answer 1

Produces its own electrical impulses without being stimulated by nerves

Question 2

What produces the electrical impulses in the heart?

Answer 2

SAN

Question 3

How is the heart-rate controlled at rest?

Answer 3

1. SAN in the wall of the right atrium produces regular bursts of electrical impulses (waves of depolarisation)
2. The waves spread across both atria causing them to contract together
3. Impulses from the SAN reach the AVN, where there is a slight delay to allow for the atria to empty and ventricles to fill before the AVN relays the impulses
4. The AVN sends impulses along the purkyne fibres, down the septum to the apex the up through the ventricular walls
5. This causes the ventricles to contract from the apex upwards, ensuring blood is pushed up and out of the arteries

Question 4

Why are impulses passed through the two atrial walls but not the ventricular muscle?

Answer 4

The wall between the two atria is conducting tissue, but the septum wall between the atria and ventricles is made of non conducting tissue.

Question 5

Where are purkyne fibres found?

Answer 5

Bundle of His

Question 6

What controls the change in heart rate?

Answer 6

Nerve impulses from the Automatic Nervous System (ANS)

Question 7

What makes up the ANS?

Answer 7

Sympathetic Nervous System
—> higher frequency action potentials along the sympathetic motor neurones which release noradrenaline

Parasympathetic Nervous System
—> higher frequency action potentials along the parasympathetic motor neurones which release acetylcholine

Question 8

Describe the coordinator and receptors for the control of the heart rate.

Answer 8

Coordinator:
- Cardiac centre found in the medulla

Receptors:
- pressure receptors in the aorta and carotid arteries detect changes in blood pressure
- chemoreceptors in the aorta, carotid arteries and medulla detect changes in blood CO2 level

Question 9

How does an increased in carbon dioxide in the blood during exercise lead to an increased heart rate?

Answer 9

1. Exercise increases rate of respiration so blood CO2 increases
2. Chemoreceptors in the aorta, carotid arteries and medulla detect the change
3. Impulses travel along the sensory neurone to the cardiac centre in the medulla (CAC)
4. Higher frequency of impulses travel down the sympathetic motor neurones to SAN
5. Noradrenaline is released at the end of the motor neurone
6. SAN send out impulses (waves of depolarisation) at a faster rate so heart rate increases

Question 10

How does an increase in blood pressure in the blood after exercise lead to a decreased heart rate?

Answer 10

1. When exercise stops, blood pressure is increased due to lots of muscle contraction
2. Pressure receptors in the aorta and carotid arteries detect change in blood pressure
3. Impulses travel along the sensory neurone to the cardiac centre in the medulla (CIC)
4. Higher frequency of impulses travel down the parasympathetic motor neurones to the SAN
5. Acetylcholine is released at the end of the neurone
6. SAN sends impulses out at a decreased rate so the heart rate decreases