Principles of Autonomic Control
Most internal organs are controlled according to the principle of dual innervation. They receive fibers from both divisions of the autonomic nervous system (ANS), which exert opposing—antagonistic—effects on the tissues.
- The sympathetic nervous system triggers ergotropic regulation. It is aimed at active behavior, mobilization of all resources, and rapid energy expenditure.
- The parasympathetic nervous system implements trophotropic regulation. Its primary task is to counterbalance powerful sympathetic effects, ensuring rest, normal digestion, and restoration of altered organ functions.
It is important to understand that despite opposing effects, both divisions always act cooperatively. This functional antagonism allows for fine-tuning of internal organ activity. The final outcome of their intervention directly depends on the functional state of the tissue itself and the current excitability of its receptors.
Innervation Features: Exceptions to the Rule
Although dual control is a classic rule for the ANS, important exceptions exist. Some organs are controlled predominantly by only one division, while others have a more complex regulatory system.
- Predominantly parasympathetic control is characteristic of the urinary bladder.
- Organs with dominant sympathetic influence include:
- Sweat glands
- Arrector pili muscles (mm. arrectores pilorum)
- Spleen
- Adrenal glands
Additionally, the smooth muscle of the gastrointestinal tract features a unique triple innervation. Its regulation involves not only sympathetic (adrenergic) and parasympathetic (cholinergic) fibers, but also serotonergic neurons.
Fate of Neurotransmitters and Endocrine Support
Autonomic influence can be either local or generalized. In local action, released neurotransmitters bind to the postsynaptic membrane of target cells and are then rapidly degraded by enzymes.
Generalized effects occur when neurotransmitters enter the systemic circulation (especially if degrading enzymes are blocked) and act on multiple organs possessing appropriate receptors. The fate of the main ANS neurotransmitters in the blood differs:
- Norepinephrine has a prolonged effect. It diffuses from nerve terminals and is constantly present in blood plasma along with dopamine and epinephrine.
- Acetylcholine is completely absent from the blood because it is destroyed by enzymes lightning-fast.
The dual influence of nerve fibers is reinforced by the endocrine system. Antagonistic hormones from endocrine glands enhance autonomic effects. For example, during an acute stress response, epinephrine from the adrenal medulla and insulin from the pancreas are simultaneously released into the blood.
Levels of ANS Division Interaction
Optimal balance of tone between the sympathetic and parasympathetic systems is a complex process ensured by continuous signal integration at four anatomical and functional levels:
- Effector cell level. Interaction occurs directly where excitation reaches the synapses of the working tissue.
- Nerve terminal level. The mechanism of reciprocal inhibition between different types of endings is implemented here.
- Autonomic ganglion level. Signals are integrated through contacts between neurons within the ganglia themselves.
- Central nervous system (CNS) level. Higher control, where various CNS structures interact to ensure global mobilization of the organism when needed.