3.6.1.3 Control Of Heart Rate

Question 1

Control of the cardiac cycle

Answer 1

• SAN spontaneously depolarises, producing wave of excitation across atria -> atrial contraction
• non-conductive tissue in atria-ventricular septum prevents wave of excitation passing directly to ventricles
• AVN causes pause in contraction, allowing time for atria to empty
• wave of excitation passes from AVN down bundle of HIS & out into purkyne fibres
• causing contraction of ventricles from apex upwards

Question 2

Role of the autonomic nervous system

Answer 2

Controls involuntary responses in the body
- sympathetic - increases activity e.g. HR (flight or flight responses)
- parasympathetic - reduce activity e.g. HR (rest, refuel and reproduce responses)

Question 3

Which area of the body controls changes to heart rate

Answer 3

Controlled by cardiovascular centre in the medulla oblongata:

• increase in action potentials along accelerator nerve (SNS) causes increase in rate of SAN depolarisation
• increase in action potentials along decelerator / vagus nerve (PNS) causes decrease in rate of SAN depolarisation.

Question 4

Role of chemoreceptors in controlling heart rate during exercise

Answer 4

Chemoreceptors in walls of aorta and carotid artery are sensitive to changes in PH -> changes in Co2 conc
- fall in PH, increase Co2 conc
- increase action P along sensory neurone to medulla O.
- increase in action P along accelerator nerve
- increased cardiac output and excretion of Co2
- as PH increases, the frequency of action P along sensory neurone decreases

Question 5

Role of baroreceptors in controlling heart rate

Answer 5

Baroreceptors in walls of aorta + carotid artery
- if pressure increases - increase in action P to decelerator region of medulla O - results in more PNS activity, decreases HR + cardiac output
- if pressure decreases - increase in action P to accelerator region of medulla O - results in more SNS activity, increased HR + cardiac output