Parasympathetic pathway

Overview of the Parasympathetic Nervous System

• 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.