General Principle of Structural Organization
The distinctive and most important feature of the metasympathetic division is its strict intramural localization. This means that absolutely all elements of the neural network (afferent, interneurons, and efferent neurons) are located directly within the thickness of the working organ's wall.
This system does not form a single continuous network. It creates separate, functionally isolated regions tied to specific body systems. These include the enteric (in the intestinal wall), cardiac (in the heart), respiratory (in the airways), and other regions. According to available data, these nerve plexuses work locally and most likely lack direct functional connections with each other.
The fundamental structural and functional unit of each such region is the intramural (intraorgan) ganglion. It contains all three functional types of neurons necessary to form a complete local reflex arc.
Cellular Composition of Ganglia: Dogel Classification
Neurons of intramural ganglia are typically classified by the morphology of their processes (Dogel classification) and their function within the reflex arc.
- Sensory neurons (Type II cells / equal-branched cells). Unlike the pseudounipolar sensory neurons of spinal ganglia, these cells are multipolar (having multiple dendrites). Their dendrites form receptors directly in the organ tissue. The primary signal transmission pathway from them goes to associative cells of the same ganglion. A hypothetical alternative pathway also exists—direct signal transmission to effector neurons, forming a two-neuron peripheral reflex arc.
- Associative neurons (Type III cells). Act as interneurons. They receive information from sensory neurons and transmit it via their axons either to effector neurons within the same ganglion or to adjacent ganglia, providing vital interganglionic connections.
- Effector neurons (Type I cells / long-axon cells). Serve as the final executive link. Their long processes transmit impulses directly to the working tissue of the organ.
Organ Autonomy and Dual Innervation
Effector neurons of the metasympathetic system (Type I cells) play a unique role—they act as a "common final pathway" for signals arriving from different sources. These cells receive dual innervation:
- From the metasympathetic system itself (via local associative cells).
- From the parasympathetic nervous system (where its preganglionic fibers terminate).
This architecture is critical for organ survival. In the event of a blockade of external influences (both sympathetic and parasympathetic), the organ does not completely lose its innervation. Through its own closed reflex arcs, the metasympathetic system maintains the ability to autonomously support baseline organ function.
Histological Appearance of the Intramural Ganglion
The morphology of intramural ganglia in histology courses is often studied using the urinary bladder wall as an example (they are localized in its muscular coat).
In histological sections, ganglion neurons stand out due to their large size. They are characterized by prominent cytoplasmic basophilia and noticeably pale nuclei. Neuron cell bodies are densely surrounded by supportive elements: satellite glial cells and connective tissue structures.
Important practical note: When examining routine histological preparations stained with hematoxylin and eosin, it is visually impossible to distinguish Dogel cell types (Type I, II, or III) from one another. All three cell types appear morphologically similar without specialized staining methods for neuronal processes.