Control of Heart Rate

Question 1

what does the autonomic nervous system do?

Answer 1

controls the involuntary activities of internal muscles and glands

Question 2

what are the 2 divisions of the autonomic nervous system?

Answer 2

• sympathetic nervous system
• parasympathetic nervous system

Question 3

what does the sympathetic nervous system do?

Answer 3

• stimulates effectors, speeds up any activity
• helps us cope with stressful situations, preparing us for activity (fight or flight)

Question 4

what does the parasympathetic nervous system do?

Answer 4

• inhibits effectors, slows down any activity
• controls activities under normal resting conditions
• conserves energy and replenishes the body’s reserves

Question 5

what is the heart muscle called?

Answer 5

cardiac muscle

Question 6

what are some features of the cardiac muscle?

Answer 6

• myogenic = its contraction is initiated from within the muscle rather than by nervous impulses

Question 7

what is in the right atrium of the heart?

Answer 7

• sinoatrial node (SAN)

Question 8

what is the sinoatrial node(SAN)?

Answer 8

• initial stimulus for contraction originates
• SAN has a basic rhythm of stimulation that determines the beat of the heart
• often referred to as pacemaker

Question 9

what are the sequence of events than controls the basic heart rate?

Answer 9

• a wave of electrical excitation spreads out from the sinoatrial node across both atria, causing them to contract
• a layer of non-conductive tissue (atrioventricular septum) prevents the wave crossing to the ventricles
• the wave of excitation enters a second group of cells called atrioventricular node (AVN) which lies between the atria
• the atrioventricular node, after a short delay cconveys a wave of electrical excitation between the ventricles along a series of specialised muscle fibres (Purkyne tissue) which created a structure called the bundle of His

Question 10

what does the bundle of His do?

Answer 10

• conducts the wave through the atrioventricular septum to the base of the ventricles, where the bundles branches into smaller fibres of Purkyne tissue
• the wave of excitation is released from the Purkyne tissue, causing the ventricles to contract quickly at the same time, from the bottoms of the heart upwards

Question 11

what is the typical adult human resting heart rate?

Answer 11

70 beats per minute

Question 12

where is the changes in heart rate controlled?

Answer 12

medulla oblongata

Question 13

what are the 2 centres of the medulla oblongata called and what do they do?

Answer 13

• a centre that increases the heart rate, which is linked to the sinoatrial node by the sympathetic nervous system
• a centre that decreases the heart rate, which is linked to the sinoatrial node by the parasympathetic nervous system

Question 14

what are the centres stimulated by?

Answer 14

• nerve Impulses they receive from the two types of receptors
  1) chemical
  2) pressure changes in the blood

Question 15

what are chemoreceptors?

Answer 15

• found in the wall of the carotid arteries
• sensitive to changes in the pH of the blood that results from changes in CO2 concentration
• in solution, CO2 forms an acids and therefore lowers pH

Question 16

what is the process of control by chemoreceptors?

Answer 16

• when the blood has a higher than normal concentration of CO2, its pH is lowered
• chemoreceptors in the wall of carotid arteries and the aorta detect this and increase the frequency of nervous impulses to the centre in the medulla oblongata that increases heart rate
• this centre increases the frequency of impulses by the sympathetic nervous system to the sinoatrial node
• this increases the rate of production of electrical waves by the sinoatrial node and increases heart rate
• the increased blood flow this causes leads to more CO2 being removed by the lungs so CO2 concentration of the blood returns back to normal
• as a consequence the pH of blood rises to normal and chemoreceptors in the wall of the carotid arteries and aorta reduce the frequency of nerve Impulses to the medulla oblongata
• the medulla oblongata reduces the frequency of impulses to sinoatrial node, which reduces heart rate

Question 17

where are pressure receptors?

Answer 17

• pressure receptors occurs within walls of the carotid arteries and the aorta

Question 18

what is the process of control by pressure receptors when the blood pressure is higher than normal?

Answer 18

• 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 sinoatrial node of the heart
• this leads to a decrease in the heart rate

Question 19

what is the process of control by pressure receptors when the blood pressure is lower than normal?

Answer 19

• pressure receptors transmits more nervous impulses to the centre of the medulla oblongata that increases heart rate
• this centre sends impulses via the sympathetic nervous syetm to the sinoatrial node which increases the heart rate