14- Control of Heart Rate

What are the two divisions of the autonomic nervous system?

• The sympathetic nervous system

• The parasympathetic nervous system

Which response is coordinated by the sympathetic nervous system?

The fight or flight response

Which response is coordinated by the parasympathetic nervous system?

The rest and digest response

Why can the sympathetic and parasympathetic nervous systems be described as antagonistic?

They oppose one another’s actions

What is the name for the muscle of the heart?

Cardiac muscle

What does myogenic mean?

Contraction is initiated from within the muscle itself, rather than from external nerve impulses

What is an example of a type of myogenic muscle?

Cardiac muscle

Where does the initial stimulus for contraction of the heart originate from?

The sinoatrial node (SAN)

What else can the sinoatrial node (SAN) be referred to as, and why?

• The pacemaker of the heart

• It determines the heartbeat

Outline the sequence of events involved in controlling basic heart rate. (6 marks)

• A wave of electrical excitation spreads out from the sinoatrial node across both atria, causing them to simultaneously contract

• A layer of non-conductive tissue, known as the atrioventricular septum, initially prevents the wave crossing to the ventricles

• The wave of excitation enters the atrioventricular node (AVN), which lies between the atria

• The atrioventricular node, after a short delay, conveys a wave of electrical excitation between the ventricles along the Bundle of His

• The Bundle of His conducts the wave to Purkyne tissue at the base of the ventricles

• 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 upwards

What is the resting heart rate of a typical adult human?

around 70 beats per minute

What are changes to the heart rate controlled by?

The medulla oblongata

What are the two centres of the medulla oblongata, and how are they linked to the SAN?

• A centre that increases heart rate, linked via the sympathetic nervous system

• A centre that decreases heart rate, linked via the parasympathetic nervous system

Which two types of stimuli may affect heart rate?

• Chemical changes in the blood

• Pressure changes in the blood

Where is the SAN located within the heart?

Within the wall of the right atrium

Where is the AVN located within the heart?

Lies between the atria

Where can chemoreceptors be found?

In the wall of carotid arteries

What are carotid arteries?

The arteries that serve the brain

What do chemoreceptors detect?

Changes in the pH of the blood that result from changes in carbon dioxide concentration

Outline the sequence of events when blood pH is lower than normal. (6 marks)

• When the blood has a higher carbon dioxide concentration than normal, its pH is lowered

• Chemoreceptors in the walls 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

• The centre increases the frequency of impulses sent to the SAN via sympathetic neurones. The SAN increases heart rate by increasing the rate of production of electrical waves

• Increased blood flow leads to more carbon dioxide being removed by the lungs- carbon dioxide concentration returns to normal

• pH of blood returns to normal and chemoreceptors reduce the frequency of nerve impulses to the medulla oblongata

• Medulla oblongata reduces frequency of impulses to the SAN, leading to a reduction in heart rate

Outline the sequence of events when blood pressure is higher than normal. (3 marks)

• Pressure receptors transmit more impulses to the centre of the medulla oblongata that decreases heart rate

• Centre sends more impulses to the SAN via parasympathetic neurones

• Leads to a decrease in heart rate

Outline the sequence of events when blood pressure is lower than normal. (3 marks)

• Pressure receptors transmit more impulses to the centre of the medulla oblongata that increases heart rate

• Centre sends more impulses to the SAN via sympathetic neurones

• Leads to an increase in heart rate