in vivo, control of the pacemaker rate is mediated peripherally via the autonomic nervous system. Central control of pacemaker rate lies in parasympathetic and sympathetic nuclei of the medulla. These are responsible for cardiovascular reflexes, and are influenced by higher centres including the posterior hypothalamus and cortical areas. Relevant autonomic pathways may be categorized as follows:
• Parasympathetic—parasympathetic fibres to the heart originate in the dorsal motor nucleus of the vagus and the nucleus ambiguous in the medulla oblongata. These parasympathetic fibres travel to the heart via the right and left vagus nerves. During embryological development the right vagus nerve becomes connected to the SA node, while the left vagus supplies the AV node. Vagal stimulation (parasympathetic activity) decreases the slope of phase 4 and also increases hyperpolarization, thus, slowing the heart rate
• Sympathetic—sympathetic innervation to the heart originates in the sympathetic chain (T1-T5 fibres) passing via the stellate ganglion to all parts of the heart, with a strong representation to the ventricular muscle. The right side distributes to the SA node, while the left side primarily supplies the AV node. Sympathetic activity increases the slope of phase 4 and, hence, heart rate. This effect occurs in exercise, anxiety and febrile illness
• Mixed—direct interconnections exist between the parasympathetic and sympathetic cardiac supplies which enable both systems to inhibit each other directly
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