The heart is innervated by parasympathetic and sympathetic fibers. The medulla is the primary site in the brain for regulating sympathetic and parasympathetic outflow to the heart and blood vessels. The hypothalamus and higher centers modify the activity of the medullary centers and are particularly important in regulating cardiovascular responses to emotion and stress (eg, exercise, thermal stress).
The vagus nerve has 3 nuclei in the central nervous system (CNS) associated with cardiovascular control: (1) the dorsal motor nucleus, (2) the nucleus ambiguus, and (3) the solitary nucleus. The parasympathetic output to the heart comes mainly from neurons in the nucleus ambiguus and to a lesser extent from the dorsal motor nucleus (see the image below).
The solitary nucleus, being an integrating hub for the baroreflex, receives sensory input about the state of the cardiovascular system.
Schematic illustration of arterial baroreceptor reflex. SNS = sympathetic nervous system.
The afferent fibers of the autonomic nervous system of the heart share the same pathway with gastrointestinal, genitourinary, baroreceptors, and chemoreceptors and transmit signals to the medulla by cranial nerves X and IX. The nucleus tractus solitarius (NTS) of the medulla receives sensory input from baroreceptors and chemoreceptors (see the image above).
Autonomic outflow from the medulla is divided principally into sympathetic and parasympathetic branches (see the image below). All the fibers forming the different cardiac plexus present synapse with the cervical plexus, brachial plexus, and intercostal nerves through communicating branches. These synapses have a great importance in the presence of pain in certain cardiac pathologies.
Sympathetic and parasympathetic efferents.