The nervous system is divided into the central nervous system, so the brain and spinal cord, and the peripheral nervous system, which includes all the nerves that connect the central nervous system to the muscles and organs.
The peripheral nervous system can be divided into the somatic nervous system, which controls voluntary movement of our skeletal muscles, and the autonomic nervous system, which is further divided into the sympathetic and the parasympathetic, and controls the involuntary movement of the smooth muscles and glands of our organs.
The sympathetic and parasympathetic nervous systems have opposite effects on the body.
The sympathetic nervous system controls functions like increasing the heart rate and blood pressure, as well as slowing digestion. All of this maximizes blood flow to the muscles and brain, and can help you either run away from a threat or fight it, which is why it’s also called the fight-or-flight response.
The parasympathetic nervous system instead slows the heart rate and stimulates digestion - the effects can be summarized as 'rest and digest'.
Now, neurons are the main cells of the nervous system. They’re composed of a cell body, which contains all the cell’s organelles, and nerve fibers, which are projections that extend out from the neuron cell body. These are either dendrites that receive signals from other neurons, or axons that send signals along to other neurons.
Where two neurons come together is called a synapse, and that’s where one end of an axon sends neurotransmitters to the dendrites or directly to the cell body of the next neuron in the series.
Now the autonomic nervous system - so both the sympathetic and parasympathetic nervous system - is made up of a relay that includes two neurons. And when there’s a group of neuron cell bodies located next to each other in the central nervous system, the whole thing is called a nucleus, while a group of neuron cell bodies located outside the central nervous system is called a ganglion.
We’ll focus on just the parasympathetic nervous system.
Signals for the autonomic nervous system start in the hypothalamus, which is a portion of the brain located at the base of the brain.
Some hypothalamic neurons have relatively short axons that synapse with preganglionic neurons in the brainstem nuclei.
The preganglionic neurons make up cranial nerve III which is the oculomotor nerve, cranial nerve VII which is the facial nerve, cranial nerve IX which is the glossopharyngeal nerve, and cranial nerve X which is the vagus nerve.
The rest of the hypothalamic neurons have really long axons - up to 1.4 meters or 4.5 feet - that synapse with preganglionic neuron cell bodies all the way down at the second, third, and fourth sacral spinal cord segments, which is S2, S3, and S4.
From there, the signal travels down the preganglionic neurons and exits the brainstem or spinal cord, and goes to the parasympathetic ganglia, which are collections of postganglionic neuron cell bodies.
The postganglionic neurons exit the parasympathetic ganglion and extend all of the way to the target cell.
The vagus nerve and the nerves that arise from the sacral spinal cord segments S2, S3, and S4 - also called the pelvic splanchnic nerves, have their parasympathetic ganglia located directly in their target organs.
More specifically, the vagus nerve synapses at postganglionic ganglia on the esophagus, heart, lungs, liver, and most of the gastrointestinal tract, up to the transverse colon.
The pelvic splanchnic nerves synapse at postganglionic ganglia on the reproductive organs and the bladder.
In contrast, the oculomotor, facial, and glossopharyngeal nerves synapse in cranial ganglia that aren’t directly in the target organ, but are nearby.
The oculomotor nerve synapses with postganglionic neurons of the ciliary ganglia located right behind the eye, and it innervates the pupils.
The facial nerve synapses with postganglionic neurons of two cranial ganglia, the pterygopalatine ganglion which is located in the pterygopalatine fossa behind the maxilla on each side, as well as with the submandibular ganglion which is located above the submandibular salivary glands.
The facial nerve innervates the sublingual and submandibular salivary glands, the lacrimal glands, and glands in the nasal cavity.