1. 3.3.2 Control of the cardiac cycle

2. Lesson 3. Control of the Cardiac Cycle The cardiac cycle is started by specialised cardiac muscle tissue in the wall of the atrium called the sino-atrial node (SAN). This is sometimes called our ‘pacemaker’. The cells of the SAN set the rhythm of the heart by sending out electrical impulses to the rest of the atria. This makes the muscle cells of the atria contract . They contract in time with the impulses from the SAN. Collagen fibres prevent the impulses reaching the ventricles, otherwise they would contract before the atria had finished contracting. A second node , the atrio-ventricular node (AVN) picks up the impulses and generates its own electrical impulse. These travel down specialised muscle fibres called Purkinje fibres. The fibres in the right and left ventricle walls are called the Bundle of His. The impulses are carried rapidly to the apex of the ventricles. This causes the cardiac muscle in the ventricles to contract simultaneously from the bottom. Blood is squeezed up and out through the arteries.

3. Sino-atrial node (SAN) • Atrio-ventricular node (AVN) • Bundle of His • Left bundle branch • Right bundle branch • Purkinje fibres 6

4. Modifying the heart beat • Although the SAN normally initiates the heart beat, the rate can be influenced by a number of factors, due to the changing needs and activities of the body. For example exercise and stress. Control of the heart rate is achieved by a combination of nervous and hormonal mechanisms. • The SAN and AVN are supplied with nerve endings from the autonomic nervous system. This system is part of the peripheral nervous system and controls internal activities such as heart rate and sweating, which are normally involuntary. It consists of neurones which penetrate the muscle of internal organs and is divided into 2 parts • The sympathetic system. • The parasympathetic system.

5. The major differences between the 2 systems as they relate to the control to the heart

6. These two opposing systems work on a negative feedback principle involving two centres in the medulla of the brain. • The cardiac acceleratory centre – linked by the sympathetic nervous system to the SAN. • The cardiac inhibitory centre is linked to the vagus nerve of the parasympathetic nervous system to the SAN, AVN and the Bundle of His What controls these two opposing systems? How do you think this feedback work, use exercise as an example? Stimulation of cardiac output: Raised carbon dioxide levels and lowered pH. Depression of cardiac output: Fall in carbon dioxide levels and a rise in pH.

7. Stretch receptors in walls of aorta and carotid artery BP BP Medulla sends impulses along sympathetic nerves to heart and via parasympathetic nerves to arterioles Cardiac-inhibitory centre stimulated and cardiac acceleratory centre inhibited Impulses sent from medulla along sympathetic nerves to arterioles causing vasodilation

8. Plenary 1: Copy and complete these sentences: Stimulation of the sympathetic nerve causes the heartbeat rate and stroke volume to ____________ Parasympathetic stimulation causes the heartbeat rate and stroke volume to _______________ The two opposing hormones are ____________ for the sympathetic and _______________ for the parasympathetic. Cardiac output = _____________ X ______________

9. PLENARY • What is the blue region? • What is the green region? • What is this structure called? • Where is the a-v septum? (1-5)? • What are these structure called? • Where are the Purkyne fibres? (1-5)? • What part is composed of non-conducting collagen tissue (1-5)? • Briefly explain how the action potential spreads across the atrium to the lower chambers of the heart 2 1 5 3 4

10. Task to investigate how the cardiac cycle is controlled Keywords : Cardiovascular control centre:- (SAN, AVN, Bundle of His, Network of Purkinje fibres). Sympathetic and parasympathetic Use internet and A level Biology exercise books to find out more on how the cardiac cycle is controlled Find a diagram of the Heart which indicate the structures within the heart that are responsible for the control of the cardiac cycle. Then produce a flow diagram to show the sequence with the heart. Notes on sympathetic and parasympathetic control of the heart. Reference to exercise