Publisher Summary This chapter describes the autonomic nervous system (ANS). The ANS is divided into three divisions that include the sympathetic, parasympathetic, and enteric. The sympathetic division regulates the use of metabolic resources and coordinating the emergency response of the body to potentially life-threatening situations. The parasympathetic division usually presides over the restoration of metabolic reserves and the elimination of wastes. The sympathetic nervous system consists of nuclei in the brain stem that connects to preganglionic neurons in the interomedial gray in the spinal cord. The preganglionic fibers exiting the cord through the ventral root reach the paravertebral ganglia over the white rami communicans. The paravertebral ganglia form a sympathetic chain on each side of the cord. Preganglionic fibers synapse with neurons in the paravertebral ganglion at the same level of the cord, ascend the chain to make synapses above, descend the chain to make synapses below, or travel through the ganglion to prevertebral ganglia. The receptors at the preganglionic parasympathetic synapse are nicotinic and those at the postganglionic synapse are muscarinic. The pupillary light reflex involves special retinal cells that sense light but do not contribute to image formation. It is found that these intrinsically photosensitive retinal ganglion cells use a different chromophore—melanopsin—to detect light.