Control of Heart Rate

Question 1

What does myogenic mean?

Answer 1

That contraction is initiated from within the muscle rather than nervous impulses from outside/

Question 2

Where is the sinoatrial node (SAN)?

Answer 2

It’s positioned at the top of the right atrium.

Question 3

Describe heartbeat control.

Answer 3

1). Wave of depolarisation spreads from SAN across both atria, causing them to contract.
2). Once atria fully contracted, wave of depolarisation enters atrioventricular node (AVN), which releases wave of depolarisation.
3). Non-conducting tissue ensures depolarisation goes down bundle of His. (Ensures atria fully contracted and blood pumped into ventricles).
4). Short delay then AVN conveys a wave of electrical excitation down the septum along specialised muscle fibres (Purkyne tissue).
5). Bundle of His conducts wave of depolarisation through septum to base of the ventricle where bundles branch into smaller non-insulated fibres.
6). Wave of excitation released from Purkyne fibres causing ventricles to contract quickly at the same time from the apex, squeezing blood up and out of the arteries.

Question 4

Response to high blood pressure (heart rate).

Answer 4

• Detected by baroreceptors (wall of artery is stretched if high bp, triggers action potential along sensory neurone)
• More electrical impulses sent to medulla oblongata + then impulses sent via parasympathetic nervous system to SAN to decrease frequency of electrical impulses
• Blood pressure lowered due to decreased heart rate

Question 5

Response to low blood pressure (heart rate).

Answer 5

• Detected by baroreceptors (wall of artery not stretched)
• More electrical impulses sent to medulla oblongata + then impulses sent via sympathetic nervous system to SAN to increase frequency of electrical impulses
• Blood pressure increased due to increased heart rate

Question 6

Response to decrease in pH (heart rate).

Answer 6

• Decrease in blood pH during times of high respiratory rate (exercise) due to production of CO2 and lactic acid
• Detected by chemoreceptors (in wall of artery)
• Excess acid must be removed from blood rapidly to stop enzymes + proteins like haemoglobin from denaturing
• More electrical impulses sent to medulla oblonga + then impulses sent via sympathetic nervous system to SAN to increase frequency of electrical impulses
• Increased heart rate to deliver blood to lungs to rapidly remove CO2