All living organisms interact continuously with their environment. They receive multiple signals from the environment via their sensory systems and respond by way of their somatomotor system. Both sensory processing and motor actions are entirely under the control of the central nervous system. The brain represents the extracorporeal space, the somatic body domains, the executive motor programs and programs for the diverse patterns of behavior initiated from higher centers. It generates complex motor commands on the basis of these central representations leading to movements of the body in its environment against different internal and external forces. The tools for performing these actions are the effector machines, the skeletal muscles and their controlling somatomotoneurons.

The peripheral autonomic nervous system supplies each group of target tissues by one (sometimes two) pathway(s) each consisting of sets of pre- and postganglionic neurons with distinct patterns of reflex activity as established for the lumbar sympathetic outflow to skin, skeletal muscle and viscera, for the thoracic sympathetic outflow to the head and neck and for some parasympathetic pathways. The principle of organization into functionally discrete pathways is the same in both the sympathetic and the parasympathetic nervous system, the only difference being that some functional targets of the sympathetic system are widely distributed (e.g., muscle blood vessels, skin blood vessels, sweat glands, etc.). Experimental investigations in humans support the idea of functionally discrete sympathetic pathways innervating skin or skeletal muscle developed in animal studies. The reflex patterns observed in each group of autonomic neurons are the result of integrative processes in the spinal cord, brain stem and hypothalamus. The concept that the sympathetic nervous system operates in an "all-or-none" fashion, without distinction between different effector organs, is not valid. The same applies to the idea of a functional antagonism between the sympathetic and parasympathetic nervous systems.

Upper brain stem, hypothalamus and cerebral hemispheres contain the representations of elementary motivational behaviors. These representations receive multiple afferent feedback and are responsible for the integration of regulation of autonomic, neuroendocrine and somatomotor systems. The dorsolateral, lateral and ventrolateral cell columns in the periaqueductal gray of the mesencephalon contain the neural circuits representing the autonomic and somatomotor components of the defense behaviors, confrontation, flight and quiescence. These circuits are quickly activated by the cortex during dangerous situations and represent the basic neural machinery for active and passive coping. Coordinated autonomic responses are quickly generated by signals from the telencephalon during diving, freezing, tonic immobility, exercise, etc. These autonomic responses occur in anticipation of the somatomotor responses demonstrating that the cortical signals have direct access to the autonomic centers. The basic emotions in humans are accompanied by autonomically mediated response patterns characteristic for each emotion. The hypothalamus contains the neural structures that integrate and coordinate autonomic, neuroendocrine and somatomotor responses to basic behaviors such as defensive, reproductive, nutritive, drinking, thermoregulation and sleep-waking behavior.

The sympathetic and parasympathetic nervous systems are defined anatomically based on the levels of outflow from the spinal cord and brainstem. The sympathetic system originates from the thoracic and upper lumbar spinal segments and is therefore called the thoracolumbar system. The parasympathetic system originates from the brain stem and sacral spinal cord and is called the craniosacral system. Both systems consist of chains of preganglionic and postganglionic neurons, which are synaptically connected in autonomic ganglia. Sympathetic ganglia are situated away from their targets and organized bilaterally in the sympathetic chains and in the prevertebral ganglia. Parasympathetic ganglia are situated close to the target organs. Most autonomic target tissues react under physiological conditions to only one of the autonomic systems. The widely propagated idea of the antagonism between sympathetic and parasympathetic nervous systems is misleading. The adrenal medulla is an endocrine gland made up of cells releasing either adrenaline or noradrenaline. Postganglionic neurons of autonomic pathways contain combinations of neuropeptides colocalized with acetylcholine or noradrenaline. The principal organization of the peripheral autonomic nervous system in submammalian vertebrate groups is highly conserved in evolution over about 500 million years.

Synaptic transmission from preganglionic to postganglionic neurons in autonomic ganglia is cholinergic nicotinic. Most autonomic ganglia transmit the central message with high accuracy to the postganglionic neurons. This concurs with the idea that the target tissues are predominantly under the control of the central nervous system. A small number of sympathetic preganglionic neurons connect to a large number of postganglionic neurons. This divergence is primarily for distribution. Additionally, postganglionic neurons receive convergent synaptic input from several preganglionic neurons. Neurons in sympathetic paravertebral and parasympathetic ganglia and some neurons in prevertebral ganglia receive one or two suprathreshold preganglionic synaptic inputs, the rest being subthreshold. Discharges in these postganglionic neurons are generated by strong synaptic inputs but not by summation of weak synaptic inputs. Some neurons with non-vascular functions in sympathetic prevertebral ganglia receive, in addition to preganglionic inputs, cholinergic synaptic inputs from peripheral intestinofugal neurons. In many sympathetic neurons of the prevertebral ganglia, and in some parasympathetic ganglia, the central synaptic input is weak or even may play a subordinate role. Nicotinic synaptic transmission in vasoconstrictor neurons in sympathetic ganglia can be enhanced by muscarinic and non-cholinergic mechanisms.

Visceral organs are innervated by vagal and spinal visceral afferent neurons. Of the axons in the vagal nerves, 85% are afferent and have their cell bodies in the nodose or jugular ganglion. Vagal afferents are involved in autonomic reflexes and regulation, and in visceral sensations but not pain. They project viscerotopically to the nucleus tractus solitarii. Spinal visceral afferent neurons have their cell bodies in the dorsal root ganglia. They are involved in organ reflexes, organ regulation (pelvic organs), extraspinal "peripheral" reflexes, protective "axon reflex"-mediated effector reactions, non-painful visceral sensations and visceral pain. Thoracolumbar spinal visceral afferent neurons are polymodal and activated by mechanical and chemical stimuli. Sacral visceral afferent neurons are involved in specific organ regulation, and sacro-lumbar reflexes. Spinal visceral afferents project to lamina I, lamina V and deeper laminae of the spinal gray matter. All spinal neurons receiving synaptic input from spinal visceral afferents are convergent viscero-somatic neurons. In primates, lamina I neurons project topographically to the posterior part of the ventromedial nucleus of the thalamus. This nucleus projects topographically to the dorsal posterior insula, which is the primary interoceptive cortex and represents sensations related to the states of the body tissues.

Sympathetic preganglionic neurons are located in the intermediate zone of the thoraco-lumbar spinal cord, those supplying somatic tissues being located laterally in the intermediolateral nucleus and those supplying viscera more medially. Sacral parasympathetic preganglionic neurons are located in the sacral intermediate zone. Parasympathetic preganglionic neurons in the brain stem are located in special nuclei. The nucleus tractus solitarii (NTS) receives peripheral synaptic input from gustatory afferents (rostral half) and from gastrointestinal, respiratory and cardiovascular afferents (caudal half). The NTS neurons project to nuclei in the brain stem, hypothalamus and forebrain, and receive synaptic input from these brain sites. Several groups of autonomic interneurons lie in the thoracolumbar and sacral spinal cord. Sympathetic premotor neurons that project to the preganglionic neurons are situated in the ventrolateral medulla, ventromedial medulla, caudal raphe nuclei, A5 area, medial hypothalamus and lateral hypothalamus. Parasympathetic premotor neurons that project to preganglionic neurons innervating heart, pancreas, trachea or salivary glands are located in the ventral medulla in the same nuclei as sympathetic premotor neurons but additionally in the periaqueductal gray. Neurons antecedent to autonomic premotor neurons are also present in various nuclei of the pons, periaqueductal gray, hypothalamic nuclei and nuclei of the telencephalon.

The enteric nervous system (ENS) is an autonomic nervous system in its own right. It can function independently of the central nervous system. The neurons of the ENS are located the ganglia of the myenteric or submucosal plexus and consist of intrinsic primary afferent neurons, interneurons and motor neurons innervating various effectors. The primary transmitter in most excitatory enteric neurons is acetylcholine. Inhibitory motor neurons use several cotransmitters. Afferent neurons, interneurons and motor neurons form reflex circuits that underlie the neural regulation of motility, secretion, reabsorption, local blood flow and in protective reactions of the gastrointestinal tract. Motility patterns are mainly directed by the myenteric plexus. The neural basis of peristalsis consists of the coordinated activation of ascending and descending reflex pathways. The circular muscles are additionally influenced by a descending inhibitory reflex pathways. Inhibitory and excitatory reflex circuits are organized and coordinated with pacemaker activity of the interstitial cells of Cajal (ICCs) to generate the different movement patterns. The networks of ICCs are also responsible for oral-aboral contraction. Neural regulation of fluid and electrolyte transport is controlled through the submucosal plexus. The brain modulates the functions of the ENS via the parasympathetic and sympathetic nervous systems.

The spinal cord and brain stem are connected to the autonomic target cells by two-neuron chains of the peripheral sympathetic and parasympathetic nervous systems. These chains consist of populations of preganglionic neurons and postganglionic neurons that are synaptically connected in the autonomic ganglia. They transmit messages from the central nervous system to the target cells are called "final autonomic pathways." These pathways are the building blocks of the peripheral autonomic nervous system. The main difference between the final somatomotor pathways and the final autonomic pathways is that the central messages may undergo quantitative changes in the autonomic ganglia, and that some effector cells are innervated by more than one type of functional autonomic pathway. The impulse pattern transmitted by peripheral autonomic pathways to the target cells is the result of integration in the spinal cord, brain stem, hypothalamus and telencephalon. Reflex patterns that are generated by afferent stimuli in peripheral autonomic neurons may serve as physiological markers to analyze the functional structure of the autonomic circuits in the neuraxis. Using this approach of neurophysiological recording from single autonomic neurons in vivo, detailed knowledge has accumulated about the organization of the autonomic nervous system in animals and humans.

The spinal cord is an autonomic integrative organ in its own right. Preganglionic neurons, autonomic interneurons and primary afferent neurons form spinal autonomic reflex circuits, which are integrated into the regulation of preganglionic activity by supraspinal centers. Autonomic premotor neurons projecting from the brain stem and hypothalamus to the spinal cord synapse with preganglionic neurons and autonomic interneurons. The discharge pattern in the different types of sympathetic neuron is associated with integration into the spinal cord, brain stem and hypothalamus. Reflex integration in the spinal cord is related to distinct afferent inputs from skin, deep somatic tissues and viscera. Spinal interneurons and preganglionic neurons constitute spinal autonomic motor programs that are integrated into the regulation of autonomic target organs. Spinal circuits, spinal afferent inflows and descending influences from the brain stem and hypothalamus work together and determine the activity of the preganglionic sympathetic neurons. This organization is exemplified in the regulation of evacuation and continence of urinary bladder and colon-rectum and in the regulation of the sequence of erection, emission and ejaculation of male reproductive organs. The functioning of these pelvic organs depends on multiple sacral and sacro-lumbar reflex circuits and several supraspinal integrative centers that interact with them.

Regulation of the cardiovascular system, the respiratory system and the gastrointestinal tract (GIT) is represented in the lower brain stem. These control systems require precise coordination and are closely integrated, which is reflected in the anatomy and physiology of the neural substrates of these control systems. Included in this integration are the final autonomic pathways, the enteric nervous system and the spinal autonomic circuits. The circuits in the medulla oblongata are under the control of the upper brain stem, hypothalamus and telencephalon. Neurons involved in regulation of arterial blood pressure and respiration are situated in rostrocaudally organized columns of neurons in the ventrolateral medulla (VLM). The rostral VLM is a sympathetic cardiovascular premotor nucleus mediating reflexes to sympathetic cardiovascular preganglionic neurons such as arterial baroreceptor, arterial chemoreceptor and other reflexes. The caudal raphe nuclei of the medulla oblongata are involved in thermoregulation and regulation of energy balance. The respiratory pattern in cardiovascular neurons is generated by the "common cardiorespiratory network" in the VLM. Neural control of the GIT by the lower brain stem is exerted by multiple reflex circuits consisting of vagal afferents from the GIT, neurons in the nucleus tractus solitarii and parasympathetic preganglionic neurons projecting to the GIT.