Embryonic development of the Peripheral Nervous System:
The peripheral nervous system (PNS) is primarily derived from neural crest precursor cells that begin migration on E8 of mouse embryonic development, and then undergo multiple processes, until E17 or even postnatally, toward PNS establishment. During this period, ganglion formation and neuroblast proliferation, axon growth and preganglionic synaptogenesis, dendrite formation and target innervations take place. In the mouse peripheral nerve development process, neural crest cells generate myelinating and non-myelinating glial cells in a process that parallels that observed in the central nervous system (CNS), but differs in the duration of the phase dedicated to Schwann cell precursor and immature Schwann cell generation. The transition from precursors to Schwann cells is complete by E16.
Cells from the margins of the dorsal neural tube migrate ventrally to form a column of sympathetic ganglion primordial near the dorsal aorta. There, they, undergo specification, and begin to acquire noradrenergic properties. The neuroblasts then coalesce to form the definitive sympathetic ganglia, migrate rostrally to establish the superior cervical ganglion (SCG) and ventrally to form prevertebral ganglia, while the remainder of the column becomes the sympathetic chain.
Sacral neural crest cells leave the neural tube between E9 and E9.5 and reach in the hindgut approximately 12 hours late, where upon the cells undergo massive proliferation, followed by differentiation into glial cells or into one of the many different types of enteric neurons.
The PNS is the collective term for all the nervous structures outside of the CNS, including the cranial, and spinal nerves, sensory receptors, enteric plexuses, and ganglia. The peripheral nervous system can be categorized in accordance with its functional components: the autonomic nervous system (ANS), the somatic nervous system (SNS), and the enteric nervous system (ENS).
The ANS is comprised of both sensory and motor components, which involuntarily tend to homeostasis. Autonomic sensory neurons transmit information to the CNS, via autonomic sensory receptors which are primarily situated, in the visceral organs (smooth muscle organs in the thorax, abdomen, and pelvis). Autonomic motor neurons can be subcategorized to sympathetic and parasympathetic classes, which typically induce opposing effects. The two neurons types convey information from the CNS to smooth muscle, cardiac muscle, and glands, leading to muscle contraction and inducing glandular activity. The sympathetic autonomic motor neurons support exercise or emergency responses, "fight-or-flight" responses, while the parasympathetic division regulates "rest-and-digest" activities.
The SNS consists of somatic sensory neurons, which transmit stimuli from the sensory receptors in the skin, skeletal muscles, joints, and the special senses, to the CNS. In parallel, SNS motor neurons, called somatic motor neurons, voluntarily convey information from the CNS to the skeletal muscles.
The ENS spans the entire length of the gastrointestinal tract (GIT) and is comprised of over 100 million neurons, which include both sensory and motor components, whose function is involuntary and CNS-independent. The ENS sensory neurons monitor both chemical and mechanical modifications within the GIT, while the motor neurons control GIT smooth muscle contraction, underlying passage of food through the GIT. These neurons are also responsible for regulating gastric acid secretion and secretion of endocrine cell-derived hormones.