The autonomic nervous system is a dynamic system intimately involved with the function of every organ in the body. In addition, it plays a key role in integrative function, such as control of the circulation and regulation of body temperature. Its motor (efferent) components, consist of the parasympathetic nervous system with a cranial and sacral spinal outflow, and the sympathetic nervous system with a thoraco-lumbar spinal outflow (Fig. 52.1). However, there is interaction at various levels of the neural axis. Thus, virtually every afferent in the body, through relays at a cerebral or spinal level, influences function of the autonomic nervous system. Centres in the brain can activate autonomic pathways directly or by stimulating spinal autonomic centres. There are multiple neurotransmitters at different synapses and ganglia that are better defined in the periphery than centrally (Fig. 52.2). Complex processes at parasympathetic and sympathetic nerve terminals influence the synthesis, release and re-uptake of various transmitters (Figs 52.3 and 52.4). The autonomic supply to the gut and pelvic viscera (enteric nervous system) additionally is richly endowed with peptides, amines and purines involved in neurotransmission and neuromodulation; they also have direct effects, especially upon the gastrointestinal tract and splanchnic circulation.
Classification of autonomic disorders
An outline classification is provided (Table 52.1). There are primary disorders where the etiology is not known; examples are pure autonomic failure (PAF) and multiple system atrophy (MSA). A large number of secondary autonomic disorders may be hereditary, associated with disease (such as diabetes mellitus), due to a specific deficit (dopamine beta-hydroxylase deficiency) or the result of trauma. A variety of drugs, poisons and toxins cause autonomic dysfunction by directly influencing sympathetic or parasympathetic activity, or by causing an autonomic neuropathy. In neurally mediated syncope, autonomic function is intermittently abnormal with either overactivity (such as increased vagal tone causing bradycardia) or underactivity (sympathetic withdrawal causing hypotension); the most common is vasovagal syncope. A recently described disorder is postural tachycardia syndrome (PoTS).
In many of these autonomic disorders there is involvement of different organs or systems. Some are localized, predominantly affecting one organ, area or system, such as the pupil in the Holmes–Adie syndrome, the face in Horner's syndrome and sweat glands in essential hyperhidrosis.
Characteristic features follow dysfunction affecting the sympathetic and parasympathetic nervous systems.