Control of heart rate

Question 1

What is the autonomic nervous system and its two divisions?

Answer 1

• The autonomic nervous system controls involuntary activities of internal muscles and glands, it has two divisions:
• The sympathetic nervous system = this stimulates effectors and speeds up activity. Controls effectors when we exercise and copes with stressful situations
• The parasympathetic nervous system = inhibits effectors and slows down any activity. Controls activity under restful conditions

Question 2

Describe cardiac muscle

Answer 2

• cardiac muscle is myogenic which means contractions within the heart muscle are initiated within itself rather than outside the heart muscle

Question 3

Describe the SAN?

Answer 3

• the sinoatrial node.
• this is where initial stimuli for contraction orginates
• The SAN has a basic rhythm that determines the beating of the heart, therefore it is called a pacemaker

Question 4

What is the sequence of events that controls heart rate?

Answer 4

• a wave of electrical impulses from the SAN across both atria, therefore the SAN acts as a pacemaker
• this results in the atria contracting
• A layer of non conducting tissue called the atrioventricular septum prevents the wave of electricity from reaching the ventricles
• the electrical signals then enter a second group of cells called the atrioventricular node (AVN) which lies between the atria
• The AVN has a small delay and then conveys the wave of electrical excitation between the ventricles along a series of specialised muscle fibres called purkyne tissue which collectively form a structure called the bundle of his
• the wave of excitation is released from the purkyne tissue, causing the ventricles to contract quickly at the same time, from the bottom of the heart upward, forcing blood out of the body

Question 5

How can heart rate be modified?

Answer 5

• resting heart rate may need to be modified to meet varying demands for oxygen
• this is controlled by the medulla oblongata which has two control centres
• a centre that increases heart rate which is linked to the sinoatrial node by the sympathetic nervous system
• a centre that decreases heart rate which is linked to the sinoatrial node by the parasympathetic node
• which centre that is stimulated depends on whether the nerve impulse that they recieve from receptors

Question 6

What are the two receptors that detect changes and change heart rate?

Answer 6

• chemoreceptors
• baroreceptors

Question 7

Where are chemoreceptors located and what are sensitive to?

Answer 7

• they located in the walls of the carotid artery
• sensitive to changes in PH

Question 8

What is the process in which chemoreceptors control heart rate?

Answer 8

• when blood has a higher than normal concentration of carbon dioxide, its PH is lowered
• the chemoreceptors in the wall of the carotid arteries detect this and increase the frequency of nervous impulses to the centre of the medulla oblongata that increases heart rate
• The centre increases the frequency of impulses via the sympathetic nervous system to the sinoatrial node which therefore increases heart rate
• this increased heart rate increases blood flow which leads to more carbon dioxide being removed so levels of carbon dioxide return to normal
• ## as a consequence, PH rises to normal levels and chemoreceptors in the wall of the carotid arteries and aorta reduce the frequency of impulses to the sinoatrial node, leading to a reduction in heart rate

Question 9

How do baroreceptors detect a higher blood pressure than normal?

Answer 9

• when pressure is higher than normal, pressure receptors transmit more nervous impulses to the centre in the medulla oblongata that decreases heart rate
• this centre sends impulses via the parasympathetic nervous system to the SAN of the heart, leading to a decrease in heart rate

Question 10

How do baroreceptors respond to a lower blood pressure than normal?

Answer 10

• when pressure is lower than normal, pressure receptors transmit more nerve impulses to the centre of the medulla oblongata that increases heart rate
• this centre sends impulses via the sympathetic nervous system to the SAN which leads to an increase in heart rate