The parasympathetic nervous system is a fundamental component of the body’s involuntary control system. It operates largely without conscious thought, managing crucial internal processes. Within this system, “preganglionic neurons” serve as initial command centers, relaying signals from the central nervous system to other nerve cells that influence various organs.
The Autonomic Nervous System’s Branches
The parasympathetic nervous system is one of the two primary divisions of the autonomic nervous system (ANS), which regulates involuntary bodily functions. The ANS oversees processes such as heart rate, digestion, respiration, and pupillary response.
The two main branches of the ANS are the sympathetic and parasympathetic nervous systems, which generally have opposing effects to maintain bodily equilibrium. The sympathetic nervous system is often associated with the “fight-or-flight” response, preparing the body for stressful situations. In contrast, the parasympathetic nervous system is commonly described as the “rest and digest” system, promoting energy conservation and recovery.
Specific Locations of Parasympathetic Preganglionic Neurons
Parasympathetic preganglionic neurons originate from two distinct regions within the central nervous system, collectively referred to as the “craniosacral outflow.” Their cell bodies are located either in the brainstem or in the sacral spinal cord segments S2 to S4. These neurons have long axons, extending from the central nervous system to ganglia located near or within their target organs.
In the brainstem, parasympathetic preganglionic neurons are found within the nuclei of specific cranial nerves: the oculomotor nerve (III), facial nerve (VII), glossopharyngeal nerve (IX), and vagus nerve (X). The oculomotor nerve’s preganglionic fibers project to the ciliary ganglion, influencing pupillary constriction and lens accommodation. Preganglionic neurons associated with the facial nerve innervate ganglia that control lacrimal glands (tear production) and salivary glands (submandibular and sublingual). The glossopharyngeal nerve’s preganglionic fibers target the otic ganglion, which influences the parotid salivary gland. The vagus nerve is particularly significant, carrying approximately 75% of all parasympathetic fibers and supplying organs in the thorax and abdomen, including the heart, lungs, and most of the gastrointestinal tract.
The sacral spinal cord, specifically segments S2, S3, and S4, is the other origin point. The cell bodies of these neurons reside in the lateral horn of the spinal cord. Their axons form the pelvic splanchnic nerves, which extend to ganglia located in the pelvic cavity. These sacral preganglionic neurons primarily innervate organs in the lower abdomen and pelvis, such as the distal colon, rectum, bladder, and reproductive organs.
How These Neurons Function
Parasympathetic preganglionic neurons play a central role in orchestrating the body’s “rest and digest” activities. These neurons transmit signals through long axons that travel to peripheral ganglia. Unlike sympathetic ganglia, which are often located close to the spinal cord, parasympathetic ganglia are typically found near or directly within the walls of the target organs they regulate.
At these ganglia, the preganglionic neurons release a specific chemical messenger called acetylcholine. This acetylcholine then acts on nicotinic receptors on the postganglionic neurons, stimulating them. The postganglionic neurons, which have very short axons, then release acetylcholine onto muscarinic receptors on the target organs, eliciting a parasympathetic response. This two-neuron pathway effectively relays the command from the central nervous system to the effector cells.
The physiological impacts of this signaling pathway are widespread and facilitate energy conservation and bodily restoration. For instance, these neurons slow heart rate, decrease blood pressure, and reduce respiratory rate. They also stimulate various digestive processes, including increased gastrointestinal activity, secretion from salivary and digestive glands, and relaxation of sphincters in the digestive tract, aiding in food processing and nutrient absorption. Additionally, parasympathetic activity contributes to bladder contraction for urination and sexual arousal.
Developmental Formation of These Neurons
The developmental formation of parasympathetic preganglionic neurons is a complex process that occurs during embryogenesis. The nervous system, including these neurons, primarily develops from the ectoderm, one of the three primary germ layers in the early embryo. A specialized region of the ectoderm, the neuroectoderm, forms a structure called the neural tube, which eventually gives rise to the brain and spinal cord.
The preganglionic neurons of the parasympathetic system are derived from specific regions of this developing neural tube. Those forming the cranial outflow originate from the hindbrain region of the neural tube, which differentiates into the brainstem. The neurons of the sacral outflow develop from the sacral region of the spinal cord. While many neurons of the peripheral nervous system originate from neural crest cells, a population of migratory stem-like cells that detach from the neural tube, the development of parasympathetic neurons has revealed a more intricate mechanism.
Recent research suggests that immature glial cells, specifically Schwann cell precursors, play a significant role in generating parasympathetic neurons and ganglia. These precursors, which are neural crest derivatives, travel along existing peripheral nerves to reach distal locations in the embryo. Once at their target sites, these Schwann cell precursors invade the area, proliferate, and differentiate into the parasympathetic neurons that form ganglia near or within organs. This process highlights a unique method of populating peripheral organs with parasympathetic neurons, especially those that develop later in embryonic stages.