Neurocardiology: a neurobiologist's perspective
Preand postganglionic neurones of the autonomic nervous system are functionally differentiated according to their target tissues. Individual sympathetic preand postganglionic neurones can be activated or inhibited reflexly by appropriate physiological stimuli as has been shown in anaesthetized animals (mainly cats and rats) for neurones of the lumbar sympathetic outflow to skeletal muscle, skin and pelvic viscera and for neurones of the thoracic sympathetic outflow to the head and neck (Janig, 2006; Janig McLachlan, 2013) and in humans for postganglionic neurones projecting in muscle or skin nerves (Janig Habler, 2003; Wallin, 2013) The reflexes correspond to the effector responses that are induced by changes in activity of these neurones. The reflex patterns are characteristic and therefore represent the physiological 'fingerprints' for each type of sympathetic pathway. They are the functional expression of the neural circuits in spinal cord, brain stem and hypothalamus connected to the peripheral sympathetic pathways. The same types of reflex patterns have been observed in both preganglionic and postganglionic neurones. The neurones in most of these pathways (e.g. the vasoconstrictor, sudomotor, motility regulating pathways, and other pathways) have ongoing activity whereas the neurones in other pathways are normally silent in anaesthetized animals (e.g. pilomotor and vasodilator pathways, pathways to sexual organs) It is likely that other target cells are similarly innervated by functionally distinct groups of sympathetic neurones that have not been systematically studied so far. These include heart (pacemaker, myocytes, coronary arteries) kidney (blood vessels, juxtaglomerular cells, tubules) urogenital tract, hindgut, spleen, and brown and white adipose tissue (Verberne Sartor, 2010; Morrison, 2011, 2013) Several systematic studies have been made on the functional properties of parasympathetic preand postganglionic neurones. The principle of organization into functionally discrete pathways is likely to be the same as in the sympathetic nervous system, the main difference being that some targets of the sympathetic system are widely distributed throughout the body (e.g. blood vessels, sweat glands, erector pili muscles, fat tissue) whereas the targets of the parasympathetic pathways are more restricted (Janig, 2006; Janig McLachlan, 2013) The centrally generated signals are faithfully transmitted from the preganglionic neurones to the postganglionic neurones in the autonomic ganglia and from the postganglionic neurones to the effector tissues at the neuroeffector junctions or at the sites where the varicosities of the postganglionic axons are in close proximity with the tissue (e.g. non excitable tissues) This signal transmission is function specific and the basis for the precise regulation of autonomic effector tissues by the brain. A few effector tissues also contain intrinsic neuronal networks that include the components for reflex control within the periphery. The most prominent is the enteric nervous system, which contains afferent neurones, interneurones and motoneurones that regulate various target tissues (smooth musculature, secretory epithelia, endocrine cells) and even modulate postganglionic sympathetic neurones in prevertebral ganglia. The preganglionic parasympathetic and sympathetic pathways regulate the activity of these intrinsic pathways in an integrative manner to determine organ function. Some peripheral afferents from the visceral organs project as far as prevertebral sympathetic ganglia and amplify postganglionic activity (e.g. Szurszewski, 1981; Janig McLachlan, 1987; McLachlan Meckler, 1989) Peripheral neuronal reflex systems have been identified in the heart and pancreas, and possibly other organs. However, other than the important control of peristalsis and secretion in the gut (Furness, 2006; Furness et al. 2014) the operation of these peripheral reflex pathways in determining organ function remains largely unknown and needs further investigation. The conclusions to be drawn from these neurophysiological studies are important for the discussion of the neural regulation of the heart (Janig, 2006; Janig McLachlan, 2013)
The Journal of physiology