(ANS) A component part of the PERIPHERAL NERVOUS SYSTEM, the ANS was originally considered primarily an effector or motor system, but afferent axons from the VISCERA and central autonomic pathways were also included as elements of the system. The term ‘autonomic’ was coined by J.N.Langley (1852–1925) at the turn of the twentieth century to reflect the reflex nature and relative independence from conscious control of this component of the nervous system. The ANS exerts largely involuntary control of the activity of viscera, glands and smooth muscle but CONDITIONING experiments and BIOFEEDBACK training demonstrate that limited voluntary control can be achieved over visceral functions. Classically, the ANS consists of two main divisions: the SYMPATHETIC NERVOUS SYSTEM and the PARASYMPATHETIC NERVOUS SYSTEM. A third component known as the ENTERIC NERVOUS SYSTEM is usually considered separately because of its association with the gastrointestinal tract.
Most viscera and glands receive a dual, functionally reciprocal innervation from the sympathetic and parasympathetic divisions of the ANS. The sympathetic nervous system is dominant in situations involving stress (FIGHTOR-FLIGHT situations) while the parasympathetic system is more active during rest and the conservation and restoration of energy. The two divisions are tonically active and their actions tend to produce opposite effects such that a balance exists between them with the relative dominance of one or the other depending on circumstances. Some organ systems are regulated by only one division, or are regulated by a complementary rather than an antagonistic relationship. The ANS is a highly dynamic, continuously active system which affects virtually every aspect of maintenance of the internal milieu of the body, including adjustments to blood pressure in response to changing POSTURE, THERMOREGULATION, DIGESTION and a constellation of major physiological responses to states of EMOTION and PAIN.
The basic organization of the ANS is a two-neuron chain consisting of a PREGANGLIONIC NEURON in the central nervous system that projects to a POSTGANGLIONIC NEURON in a peripheral autonomic ganglion. The sympathetic or thoracolumbar preganglionic neurons are located in the LATERAL HORN and the intermediomedial nucleus of the thoracic and upper segments of the lumbar SPINAL CORD (T1–L3). They project via ventral roots to peripheral sympathetic ganglia located near the vertebral column (paravertebral and cervical ganglia of the sympathetic trunk) and in neural plexuses of the abdomen (prevertebral sympathetic ganglia). Long unmyelinated post-ganglionic sympathetic axons then project to the eyes, lacrimal glands, salivary glands, mucous membranes of the head, and to the respiratory, cardiovascular, digestive, urinary and reproductive systems. Sympathetic axons are closely associated with blood vessels which they also innervate, having a vasoconstrictor function. Sympathetic axons in the skin innervate blood vessels, sweat glands and the arrector pili smooth muscle of hair follicles.
Parasympathetic or craniosacral preganglionic neurons are found in the brainstem and sacral spinal cord (S2–S4). Parasympathetic ganglia are found in or near the target organs. Brainstem preganglionic axons leave the brain via four CRANIAL NERVES (third, seventh, ninth and tenth) and project long, myelinated axons to autonomic ganglia in the head. Short, unmyelinated postganglionic axons innervate the eyes, lacrimal glands, salivary glands and mucous membranes. The tenth cranial nerve (the VAGUS NERVE) projects to autonomic ganglia closely associated with the respiratory system, heart, gastrointestinal tract, liver and kidneys.
ACETYLCHOLINE is the neurotransmitter for pre- and postganglionic parasympathetic neurons and for postganglionic sympathetic neurons innervating sweat glands, while NORADRENALINE is the transmitter for all other postganglionic sympathetic neurons. In addition, many neurons of the sympathetic division of the ANS have one or more NEUROPEPTIDES colocalized with their classical neurotransmitters.
Afferent information from the viscera and blood vessels is carried to the spinal cord by axons that travel in autonomic nerves of the viscera, in somatic nerves and in the GLOSSOPHARYNGEAL NERVE and vagus nerve to modulate visceral reflexes and to be relayed to higher levels of the NEURAXIS. Most of the detailed incoming information regarding the state of the viscera is not consciously perceived. Visceral sensations of fullness, motility and pain can be appreciated but not with the precise localization characteristic of somatic sensation. Visceral and somatic pain are among the most powerful activators of the ANS.
Ascending and descending central autonomic neural pathways integrate autonomic, ENDOCRINE and behavioural responses against a background of HOMEOSTATIC and non-homeostatic mechanisms and ongoing MOTIVATION and EMOTION. Central autonomic pathways are characterized by reciprocal connections and redundancy. The NUCLEUS OF THE SOLITARY TRACT and the PARABRACHIAL NUCLEI of the PONS, nuclei that receive afferent visceral information either directly or indirectly from the cranial nerves or spinal cord, are the main ascending waystations of central autonomic pathways. Ascending projections also relay visceral afferent information to the HYPOTHALAMUS, LIMBIC SYSTEM and CEREBRAL CORTEX (insular and prefrontal). Important descending central autonomic pathways arise from the cerebral cortex, AMYGDALA, hypothalamus and periaqueductal GREY MATTER to act upon autonomic centres and preganglionic neurons of the BRAINSTEM and spinal cord.
DAVID A.HOPKINS
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