The parasympathetic nervous system is primarily active during rest and digest activities.
It is also referred to as the cranial sacral division of the autonomic nervous system.
Structure and Origin
The parasympathetic nervous system arises from the brain stem nuclei.
A nucleus is defined as a collection of cell bodies located in the central nervous system (CNS).
Key control centers located in the brain stem include:
Cardiovascular function
Vasomotor tone: the regulation of blood vessel constriction or relaxation
Respiratory centers
Overall, these centers regulate many automatic functions through the autonomic nervous system.
Pathway of Neurons
Parasympathetic nervous system pathways consist of two neurons:
First Order Neuron: Originates from brain stem nuclei and travels through cranial nerves.
Typically represented in purple in textbooks.
First order neurons leave through cranial nerves III (oculomotor), VII (facial), IX (glossopharyngeal), and X (vagus).
Known as preganglionic neurons because they synapse close to the target organ.
Second Order Neuron: Known as postganglionic neurons, synapse with the first order neurons at specific ganglia before innervating target organs.
Preganglionic cells are also referred to as presynaptic cells, while postganglionic cells are referred to as postsynaptic cells.
Neuron Characteristics
First order neurons are myelinated type B fibers, which are important for faster conduction along the axon.
The length of the pregaglionic neuron is typically long to reach the target organs.
The synapse between the preganglionic and postganglionic neurons occurs close to the target organ, in nearby ganglia.
The approach of comparing these ganglia to stardust helps visualize the arrangement of ganglia in relation to organs.
Synapse and Neurotransmission
The synaptic transmission involves the following:
The presynaptic neuron communicates with the postsynaptic neuron by releasing neurotransmitters at the synapse.
Postsynaptic neurons have specific receptors that bind to these neurotransmitters.
After synapse, the postganglionic neuron (or second order neuron) innervates the target organ directly.
These neurons can be referred to interchangeably as postganglionic neurons or, less commonly, ganglionic neurons.
Postganglionic neurons are typically unmyelinated and of shorter length. They are classified as type C fibers.
Functional Implications
The function of the parasympathetic nervous system is primarily motor, controlling smooth muscle, cardiac muscle, and glandular secretion.
An example of this involvement is the pancreas, where postganglionic neurons stimulate cells to release digestive juices in the duodenum during digestion.
Each neuron will relay impulses and signals necessary for the proper function of organs during rest and digestive activities.