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 wall of the effector organ.
This system is not a single continuous network. It forms separate, functionally isolated areas tied to specific body systems. The enteric (in the intestinal wall), cardiac (in the heart), respiratory (in the airways), and other areas are distinguished. According to available data, these nerve plexuses work locally and most likely have no direct functional connections with each other.
The fundamental structural and functional unit of each such area 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: Dogiel's classification
Neurons of intramural ganglia are typically classified according to the morphology of their processes (Dogiel's classification) and their function in the reflex arc.
- Sensory neurons (Type II cells / equidenritic). Unlike pseudounipolar receptor neurons of spinal ganglia, these cells are multipolar (having several dendrites). Their dendrites form receptors directly within the organ tissue. The main signal transmission pathway from them leads to association cells of the same ganglion. There is also a hypothetical alternative pathway — direct signal transmission to effector neurons forming a two-neuron peripheral reflex arc.
- Association neurons (Type III cells). They act as an intercalated link. They receive information from sensory neurons and transmit it further via their axons: either to effector neurons within the same ganglion or by sending processes to neighboring ganglia, providing important interganglionic connections.
- Efferent neurons (Type I cells / long-axon). They are the final effector link. Their long processes transmit impulses directly to the working tissue of the organ.
Organ autonomy and dual innervation
Efferent neurons of the metasympathetic system (Type I cells) play a unique role — they act as a "common final pathway" for signals coming from different sources. These cells receive dual innervation:
- From the metasympathetic system itself (via local association cells).
- From the parasympathetic nervous system (its preganglionic fibers terminate on them).
This architecture is critical for organ life support. In the event of a blockade of external influences (both sympathetic and parasympathetic), the organ does not completely lose innervation. Due to its own closed reflex arcs, the metasympathetic system retains the ability to autonomously maintain the basic functioning of the organ.
Histological picture of the intramural ganglion
The morphology of intramural ganglia in histology courses is often examined using the wall of the urinary bladder as an example (they are localized in its muscular tunic).
In a histological slide, ganglion neurons stand out due to their large size. They are characterized by pronounced basic staining (basophilia) of the cytoplasm and markedly light nuclei. Neuron bodies are densely surrounded by supporting elements: satellite glial cells and connective tissue structures.
Important practical note: when examining routine hematoxylin and eosin-stained histological slides, it is visually impossible to distinguish Dogiel cell types (I, II, or III) from one another. All three cell types look morphologically similar without the use of special staining methods for processes.
Briefly: answers to topical questions
What specific impregnation methods are used to visualize processes and differentiate Dogiel cells?
To visualize neuronal processes, sources cite silver impregnation: silver nitrate or ammoniacal silver impregnation. In this process, cells and fibers of the nervous system are selectively stained black, the nucleus remains lighter, and neuronal processes are clearly outlined. For Dogiel cells, sources indicate that their classification is based on the morphology of processes, but on routine slides stained with hematoxylin and eosin, it is impossible to distinguish the cell types from one another.
Are all areas of the metasympathetic system limited to the enteric, cardiac, and respiratory areas, and what others exist?
The areas of the metasympathetic nervous system are not limited only to the enteric, cardiac, and respiratory areas, as they form in the walls of various internal organs. The innervation zone of this division is strictly limited exclusively to those organs that possess their own automatism — the ability for rhythmic motor activity without external influences. In addition to the heart and intestines, such organs also include the ureter and uterus.
What neurotransmitters do efferent neurons of the metasympathetic nervous system release in synapses on smooth muscles?
Sources indicate the following mediators for the metasympathetic division of the ANS: acetylcholine, noradrenaline, VIP, ATP, ADP, and adenosine. It is also indicated that the activation of purinergic neurons reduces the tone of the stomach and intestines. There is no direct indication of nitric oxide as a mediator of efferent neurons of the metasympathetic system in these sources.
The "common pathway" rule for Type I cells: effector neurons receive signals from two sides simultaneously — from the local arc (via Type III cells) and from the central nervous system (preganglionic parasympathetic fibers).
Class and exam questions
Can Dogiel cells be distinguished from each other under a microscope during class?
No, on standard slides (hematoxylin-eosin staining) it is impossible to distinguish sensory, association, and effector neurons. Special impregnation methods are required for this.
What is the main morphological difference of sensory cells of the metasympathetic system?
Sensory cells (Dogiel Type II) in intramural ganglia are multipolar, whereas sensory neurons in spinal ganglia are pseudounipolar.
What will happen to an organ if sympathetic and parasympathetic nerves are blocked?
The organ will not lose its ability to function. The metasympathetic system will ensure its autonomous operation through its own intramural reflex arcs.
What else to study on this topic
- Organization of a hypothetical two-neuron peripheral reflex arc
- Structure of interganglionic connections and the role of Type III cells
- Morphology of satellite glial cells in intramural ganglia
- Interaction of the metasympathetic system with preganglionic parasympathetic fibers
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