Control of Heart Rate.

• The heart is made of cardiac muscle. Cardiac muscle is myogenic, contraction occurs due to electrical impulses generated from the heart muscle.
• 2 regions of the heart are able to generate electrical impulses.

  Sino-Atrial Node- in the walls of the right atrium.

  Atrioventricular Node- in the walls at the base of both atria.
• Bundle of His: Conducting (of electrical impulse) tissue.

  In the septum.
• Purkyne (Purkinje) Fibres: Outer walls of ventricles.
• Electrical impulses generated by S.A.N. initiates the heartbeat. This causes the atria to contract.

  This does not cause the ventricle to contract because of collagen fibres.

  Impulses reach the A.V.N. and this signal is delayed before impulses are sent along the bundle of his.

  When impulses reach the apex of the heart it travels along purkyne fibres.

  This causes ventricular contraction from the bottom up.
• Non-conducting tissue. Collagen fibres between atria and ventricles.
• Why do collagen fibres ensure the ventricles contract after the atria?


  It allows the ventricles to fill with blood before contraction.
• Why do the ventricles contract from the bottom to top?


  To make sure that all the blood leaves the ventricle.
• Why does the right side contract fractionally before the left?

  Right atrium is under impulse first, by the S.A.N.
• Autonomic Nervous System: the part of the nervous system responsible for control of bodily functions not consciously directed.
• Changes to the heart rate are controlled by a region of the brain called medulla oblongata within the medulla oblongata is a cardiac centre that is linked to the SAN of the heart.
• More impulses sent along the sympathetic nerve to SAN result in an increase in SAN activity and therefore an increase in heart rate and stroke volume.
• More impulses sent along the parasympathetic nerve to SAN result in a decrease in SAN activity and therefore a decrease in heart rate and stroke volume.
• Receptors sensitive to chemicals and pressure send impulses via sensory neurons to the medulla.
• Chemoreceptors: sensitive to CO2, pH, lactic acid.
• Baroreceptors: sensitive to blood pressure.
• Chemoreceptors are found in the wall of the carotid artery, aorta and in the medulla.
• Chemoreceptors. An increase in respiration rate raises the CO2 & lactic acid concentration of the blood, increasing H+ concentration and lowering pH. Carotid artery increases action potentials to medulla. Medulla increases action potentials across sympathetic nerve and decreases action potentials across parasympathetic nerve to the SAN. Blood pressure increases, SAN electrical impulses and cardiac output increase, to lungs which release more CO2.
• Baroreceptors are found in the wall of the carotid arteries, aorta and vena cava and are sensitive to stretch.
• Baroreceptors. When you suddenly stand up BP decreases, this results in baroreceptors in the vena cava to stretch less decreasing action potentials to the medulla. More action potentials sent to SAN via sympathetic nerve and less action potentials sent via the parasympathetic nerve. SAN sends more electrical impulses increasing heart rate. Resulting in a higher cardiac output and increase in BP.
• Why lower blood pressure?


  To prevent blood pressure increasing too high in response to exercise or CO2.