Introduction to ANS 1

Anatomy and General Functions of the Autonomic and Somatic Motor Nervous Systems

The autonomic nervous system, also called the visceral, vegetative, or involuntary nervous system, is distributed widely throughout the body and regulates autonomic functions that occur without conscious control. In the periphery, it consists of nerves, ganglia, and plexuses that innervate the heart, blood vessels, glands, other visceral organs, and smooth muscle in various tissues.

Differences between Autonomic and Somatic Nerves

The efferent nerves of the involuntary system supply all innervated structures of the body except skeletal muscle, which is served by somatic nerves. The most distal synaptic junctions in the autonomic reflex arc occur in ganglia that are entirely outside the cerebrospinal axis. These ganglia are small but complex structures that contain axodendritic synapses between preganglionic and postganglionic neurons. Somatic nerves contain no peripheral ganglia, and the synapses are located entirely within the cerebrospinal axis. Many autonomic nerves form extensive peripheral plexuses, but such networks are absent from the somatic system. Whereas motor nerves to skeletal muscles are myelinated, postganglionic autonomic nerves generally are nonmyelinated. When the spinal efferent nerves are interrupted, the denervated skeletal muscles lack myogenic tone, are paralyzed, and atrophy, whereas smooth muscles and glands generally retain some level of spontaneous activity independent of intact innervation.

Visceral Afferent Fibers

The afferent fibers from visceral structures are the first link in the reflex arcs of the autonomic system. With certain exceptions, such as local axon reflexes, most visceral reflexes are mediated through the central nervous system (CNS).

Information on the status of the visceral organs is transmitted to the CNS through two main sensory systems: the cranial nerve (parasympathetic) visceral sensory system and the spinal (sympathetic) visceral afferent system (Saper, 2002). The cranial visceral sensory system carries mainly mechanoreceptor and chemosensory information, whereas the afferents of the spinal visceral system principally convey sensations related to temperature and tissue injury of mechanical, chemical, or thermal origin. Cranial visceral sensory information enters the CNS via four cranial nerves: the trigeminal (V), facial (VII), glossopharyngeal (IX), and vagus (X). These four cranial nerves transmit visceral sensory information from the internal face and head (V), tongue (taste, VII), hard palate, upper part of the oropharynx, and carotid body (IX), and lower part of the oropharynx, larynx, trachea, esophagus, and thoracic and abdominal organs, with the exception of the pelvic viscera (X). The pelvic viscera are innervated by nerves from the second through fourth sacral spinal segments.

The visceral afferents from these four cranial nerves terminate topographically in the solitary tract nucleus (STN) (Altschuler et al., 1989). The most massive site for termination of the fibers from the STN is the parabrachial nucleus, which is thus the major relay station. The parabrachial nucleus consists of at least 13 separate subnuclei, which in turn project extensively to a wide range of sites in the brainstem, hypothalamus, basal forebrain, thalamus, and cerebral cortex. Other direct projections from the STN also innervate these brain structures.

Sensory afferents from visceral organs also enter the CNS via the spinal nerves. Those concerned with muscle chemosensation may arise at all spinal levels, whereas sympathetic visceral sensory afferents generally arise at the thoracic levels where sympathetic preganglionic neurons are found. Pelvic sensory afferents from spinal segments S2–S4 enter at that level and are important for the regulation of sacral parasympathetic outflow. In general, visceral afferents that enter the spinal nerves convey information concerned with temperature as well as nociceptive visceral inputs related to mechanical, chemical, and thermal stimulation. The primary pathways taken by ascending spinal visceral afferents are complex and controversial (Saper, 2002). Most probably converge with musculoskeletal and cutaneous afferents and ascend via the spinothalamic and spinoreticular tracts. Others ascend via the dorsal column. An important feature of the ascending pathways is that they provide collaterals that converge with the cranial visceral sensory pathway at virtually every level (Saper, 2000). At the brainstem level, collaterals from the spinal system converge with the cranial nerve sensory system in the STN, the ventrolateral medulla, and the parabrachial nucleus. At the level of the forebrain, the spinal system appears to form a posterolateral continuation of the cranial nerve visceral sensory thalamus and cortex (Saper, 2000).

The neurotransmitters that mediate transmission from sensory fibers have not been characterized unequivocally. Substance P and calcitonin gene–related peptide, which are present in afferent sensory fibers, in the dorsal root ganglia, and in the dorsal horn of the spinal cord, are leading candidates for neurotransmitters that communicate nociceptive stimuli from the periphery to the spinal cord and higher structures. Other neuroactive peptides, including somatostatin, vasoactive intestinal polypeptide (VIP), and cholecystokinin, also have been found in sensory neurons (Lundburg, 1996; H?kfelt et al., 2000), and one or more such peptides may play a role in the transmission of afferent impulses from autonomic structures. Adenosine triphosphate (ATP) appears to be a neurotransmitter in certain sensory neurons, including those that innervate the urinary bladder. Enkephalins, present in interneurons in the dorsal spinal cord (within an area termed the substantia gelatinosa), have antinociceptive effects that appear to arise from presynaptic and postsynaptic actions to inhibit the release of substance P and diminish the activity of cells that project from the spinal cord to higher centers in the CNS. The excitatory amino acids glutamate and aspartate also play major roles in transmission of sensory responses to the spinal cord.