6.1.3 Control of heart rate Flashcards
Myogenic Stimulation of the Heart
The heart cycles through systole and diastole at a steady rate, without requiring an external stimulus.
The SAN acts as the pacemaker, setting the heart rate.
It is this rate of contraction that can change in response to external stimuli.
How is the cardiac cycle initiated and coordinated?
Sinoatrial node (SAN) initiates heartbeat by sending waves of electrical activity across atria, causing atrial contraction.
Non-conducting tissue prevents the waves of electrical activity reaching the ventricles.
Electrical activity passes through atrioventricular node (AVN), which delays the excitation from reaching the bundle of His. During this delay the atria empty and ventricles fill.
AVN sends wave of electrical activity down the bundle of His to the Purkyne fibres at the apex of the heart, causing the ventricles to contract from the base up.
The autonomic nervous sytem:
The nervous system can be divided into central (CNS) and peripheral. The peripheral can be divided into somatic (sensory neurones) and autonomic (motor neurones). The autonomic nervous system can be divided into sympathetic and parasympathetic.
Sympathetic generally increases activity (fight or flight responses).
Parasympathetic generally decreases activity (rest and digest responses).
Medulla oblongata (in the brain) has cardiac acceleratory centre to increase heart rate, and cardiac inhibitory centre to decrease heart rate.
Cardiac acceleratory centre uses the sympathetic nervous system (cardiac accelerator neuron) with noradrenaline at the synapse with the SAN.
Cardiac inhibitory centre uses the parasympathetic nervous system (vagus nerve) with acetylcholine at the synapse with the SAN.
How does exercise cause an increase in heart rate?
Increased rate of respiration leads to increase in carbon dioxide concentration, which causes blood pH to fall.
Chemoreceptors in carotid arteries detect increase in acidity, and in response increase the frequency of impulses they send to the medulla.
Cardiac centre increases the frequency of impulses along sympathetic neurones to the SAN, and decreases the frequency of impulses along parasympathetic neurones to the SAN.
How does a decrease in metabolic rate lead to a decrease in heart rate?
Lower metabolic rate means less carbon dioxide in the blood, causing an increase in pH which is detected by chemoreceptors in the carotid artery, resulting in fewer impulses to the medulla, which sends more impulses along parasympathetic neurones and fewer impulses along sympathetic neurones to the SAN.
How does a rise in blood pressure change the heart rate?
Baroreceptors are found in the wall of carotid arteries and aorta.
STIMULUS: rise in blood pressure
RECEPTOR: increased stimulation of baroreceptors, more impulses down sensory neurone (toward medulla)
COORDINATOR: medulla cardiac inhibitory centre increases frequency of impulses down parasympathetic neurone
EFFECTOR: SAN stimulated by acetylcholine
RESPONSE: SAN reduces frequency of waves of excitation across atrial walls, lowering heart rate.
How does a fall in blood pressure change the heart rate?
Baroreceptors are found in the wall of carotid arteries and aorta.
STIMULUS: fall in blood pressure
RECEPTOR: decreased stimulation of baroreceptors, fewer impulses down sensory neurone (toward medulla)
COORDINATOR: medulla cardiac acceleratory centre increases frequency of impulses down sympathetic neurone
EFFECTOR: SAN stimulated by noradrenaline
RESPONSE: SAN induces frequency of waves of excitation across atrial walls, raising heart rate.
