Area of innervation and the concept of automatism
The metasympathetic division is localized exclusively in the walls of hollow internal organs. A key condition for the presence of the metasympathetic network is the organ's ability to exhibit automatism. Automatism refers to the organ's ability to contract rhythmically, maintaining a set frequency and amplitude without external influences. It is the microganglionar formations of the MNS that serve as the substrate for this property.
The MNS encompasses only organs with motor activity. Specific regions are distinguished:
- Cardiometasympathetic region (heart).
- Enterometasympathetic region (gastrointestinal tract).
The system also innervates the bronchi, uterus, urinary bladder, gallbladder, and biliary tract.
Morphology and structure of microganglia
Structurally, the system is a complex of microganglia. In their internal organization, they are strikingly similar to the nuclear formations of the central nervous system.
Features of tissue organization:
- Isolation: microganglia are reliably separated from surrounding tissues by specialized barriers.
- Compactness: the extracellular space within the ganglia is extremely minimal.
- Glia: a large number of glial elements are present to support neuronal function.
Inside the microganglia, there are three types of neurons: afferent (sensory), interneurons, and efferent (motor). The neurons possess a spontaneous impulse activity generator. Their cell bodies are densely covered with synapses, and their processes store a huge number of vesicles containing various neurotransmitters.
Interaction with the ANS and neurochemistry
The metasympathetic system exhibits the highest degree of autonomy: its activity is least dependent on the somatic nervous system compared to the classic sympathetic and parasympathetic systems. At the same time, it is not in an antagonistic relationship with them.
The MNS receives inputs from the sympathetic and parasympathetic divisions, and ganglionar neurons are equipped with receptors for acetylcholine and noradrenaline.
Features of neurotransmitter systems:
- Purinergic transmission. Neurons secrete ATP and adenosine. Their main physiological role is to exert inhibitory (suppressive) influences, opposing activating ones. For instance, activation of purinergic neurons leads to a decrease in the tone of the stomach and intestines.
- Postganglionic fiber transmitters. A wide spectrum of biologically active substances is released in the synapses: in addition to acetylcholine and noradrenaline, serotonin, dopamine, adrenaline, histamine, and others operate here.
Briefly: answers to topical questions
What Dogel cell types form the morphological basis of the intramural ganglia of the MNS?
The morphological basis of the intramural ganglia is formed by three functional types of neurons classified according to Dogel.
- Sensory neurons (Type II cells) — equibranched (multipolar) cells whose dendrites form receptors in the organ tissue.
- Associative neurons (Type III cells) — interneurons that receive a signal from sensory neurons and transmit it to effector neurons of the same or neighboring ganglia.
- Effector neurons (Type I cells) — long-process cells that serve as the final link in transmitting the impulse to the working tissue ("final common path").
What local reflex arcs are closed at the level of the metasympathetic nervous system?
At the level of the metasympathetic nervous system, local reflex arcs are closed, comprising sensory, associative, and effector neurons.
The main pathway of signal transmission: sensory neurons transmit the signal to associative cells, most often within the same ganglion; axons of associative neurons project to effector neurons of the same or neighboring ganglia.
An alternative option, though debated, is:
- Two-neuron peripheral reflex arc — direct signal transmission from sensory neurons to effector neurons.
To remember the organs with metasympathetic innervation, recall the rule of "hollow motor sacs and tubes": heart, bronchi, GI tract, uterus, gallbladder, and urinary bladder. If a hollow organ can contract rhythmically on its own, it definitely contains the MNS.
Class and exam questions
What is automatism in the context of the metasympathetic nervous system?
This is the ability of an organ to contract rhythmically with a specific frequency and amplitude without external commands. This property is entirely determined by the activity of the MNS microganglionar formations.
Is the metasympathetic system an antagonist of the sympathetic system?
No, the MNS is not in an antagonistic relationship with other divisions. It receives input signals from them and possesses receptors for both noradrenaline and acetylcholine.
What does the activation of purinergic neurons lead to?
Purinergic transmission (via ATP and adenosine) exerts inhibitory influences. In practice, this manifests, for example, in a decrease in the muscle tone of the stomach and intestines.
What else to study on this topic
- Structural organization of microganglia and their similarity to CNS nuclei
- Mechanisms of the spontaneous impulse activity generator
- The role of glial elements and barriers in the isolation of metasympathetic ganglia
- Specifics of purinergic transmission and ATP/adenosine receptors
- Comparative characteristics of the autonomy of somatic, sympathetic, and metasympathetic systems
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