Imaging of cardiac adrenergic innervation.

Cardiac myocytes and the coronary circulation are innervated by both sympathetic and parasympathetic fibres which constitute the autonomic nervous system. 2 The autonomic outflow is controlled by regulatory centres in the midbrain, hypothalamus, pons, and medulla which integrate inputs from other brain areas as well as afferent stimuli from the periphery. The efferent signals follow descending pathways in the lateral funiculus of the spinal cord that terminate on cell bodies in the intermediolateral and intermediomedial columns. Sympathetic fibres leave the spinal cord at T1–L2-3. These myelinated preganglionic fibres synapse in the paravertebral ganglia while small unmyelinated postganglionic fibres connect with body organs. Sympathetic innervation to the heart is provided by fibres originating from a series of ganglia which constitute the cardiac plexus. These fibres branch and terminate as sympathetic nerve endings in atrial and ventricular tissue. The main neurotransmitter of the sympathetic system is noradrenaline (norepinephrine) that, after its release by sympathetic nerve terminals, can bind to a series of different postsynaptic receptors (α and β) whose activation determine the stimulatory and inhibitory effects of the system. Under normal circumstances sympathetic activation results in an increased heart rate (chronotropic effect), a more forceful contraction (inotropic effect), and enhanced atrioventricular conduction (dromotropic effect). Dysfunction of the sympathetic nervous system is thought to play a pathogenetic role in a number of cardiac diseases, including arrhythmias, dilated and hypertrophic cardiomyopathies, 6 postinfarction remodelling, and congestive heart failure.