Orbeli–Ginetsinsky phenomenon
The classic proof of the adaptive-trophic influence is the experiment on striated muscle demonstrating the Orbeli–Ginetsinsky phenomenon.
The experiment proceeds in two key phases:
- Development of fatigue. The muscle is stimulated continuously and rhythmically via its motor nerve. As a result, fatigue naturally sets in—the amplitude of muscle contractions decreases.
- Sympathetic stimulation. Without stopping the stimulation of the motor nerve, the experimenters additionally apply an electrical current to the postganglionic fibers of the sympathetic nerve innervating the same muscle.
Result: the contraction amplitude of the fatigued muscle is restored.
The main conclusion of this experiment is that the effect is caused by the direct action of the sympathetic system on the metabolism of muscle tissue. To prove that the recovery is not related to a simple improvement in blood flow (vascular influence), the researchers pre-filled the muscle's vascular bed with petroleum jelly. Despite the complete blockade of blood flow, sympathetic stimulation still restored muscle performance.
Mechanism of action: trophic function and trophogens
Trophic (nourishing) action is characteristic of all nerves, but it is most pronounced in the sympathetic system. This process is mediated by the release of specific substances—trophogens—from synaptic terminals.
Several groups of substances with adaptive-trophic action are distinguished:
- Basic trophogens: nucleotides, selected amino acids, prostaglandins, complex lipids, and gangliosides. This category also includes classical neurotransmitters (acetylcholine, serotonin, catecholamines), as well as substance P and CGRP (calcitonin gene-related peptide).
- Oligopeptides: liberins and statins, endorphins and enkephalins, bradykinin, cholecystokinin (CCK), neurotensin, oxytocin, and ACTH fragments.
- Growth factors (released upon stimulation of postganglionic fibers): nerve growth factor (NGF), epithelial growth factor (EGF), insulin-like growth factor (IGF), and tumor necrosis factor (TNF).
Sympathoadrenal system and metabolic effects
The sympathetic nervous system is closely linked with endocrine regulation. The sympathoadrenal system is a functional complex consisting of sympathetic fibers and the adrenal medulla cells they innervate. Its main physiological role is the global mobilization of the body; it is a crucial stress-response system.
The adrenal medulla represents a modified sympathetic ganglion (chromaffin tissue). Preganglionic sympathetic neurons form excitatory cholinergic synapses on these cells. Upon excitation, the adrenal glands secrete 90% adrenaline and 10% noradrenaline into the blood.
Metabolic effects of catecholamines: Acting as metabolic hormones, they trigger energy-supplying processes:
- Glycogenolysis: acceleration of carbohydrate (glycogen) breakdown.
- Lipolysis: enhanced release of fatty acids from subcutaneous adipose tissue.
The binary effect of adrenaline at the cellular level depends on the receptor type:
- Interaction with $\alpha_2$-adrenergic receptors causes inhibition of cellular processes.
- Interaction with $\beta$-adrenergic receptors activates the enzyme adenylate cyclase, leading to the accumulation of the intracellular messenger — cAMP (cyclic adenosine monophosphate).
Briefly: answers to topical questions
Which enzymes inactivate catecholamines after their action on receptors?
Catecholamines are inactivated by:
- Catechol-
O -methyltransferase (COMT, catechol-ortho-methyltransferase). - Monoamine oxidase (MAO).
For noradrenaline, sources indicate sequential degradation by COMT and MAO. Catecholamine metabolites are excreted by the kidneys.
To easily remember the Orbeli–Ginetsinsky phenomenon, imagine a marathon runner: muscles are stiff, legs are heavy (fatigue from rhythmic work). Suddenly, a dog jumps out of the bushes (sympathetic activation). The runner sharply accelerates. This is not just a blood rush; the sympathetic system has "injected" metabolic fuel into the muscles, giving a second wind.
Class and exam questions
How was it proven that the recovery of a fatigued muscle is not related to blood vessel dilation?
During the experiment, the vascular bed of the muscle was filled with petroleum jelly, completely ruling out the vascular factor. The effect of restoring contraction force upon sympathetic nerve stimulation was nevertheless preserved.
What is the anatomy of the adrenal medulla from the perspective of the autonomic nervous system?
This is a modified sympathetic ganglion consisting of chromaffin tissue. Preganglionic sympathetic fibers approach it, forming excitatory cholinergic synapses.
What is the binary mechanism of action of adrenaline?
The action of adrenaline depends on the target receptor: via alpha-2 adrenergic receptors, it exerts an inhibitory effect, whereas via beta adrenergic receptors, it activates adenylate cyclase, causing the accumulation of cAMP.
What else to study on this topic
- Classification and biochemical nature of synaptic terminal trophogens
- The role of oligopeptides (enkephalins, bradykinin, neurotensin) in trophic regulation
- Growth factors (NGF, EGF, IGF) and their effects on target tissues
- Cholinergic synapses of preganglionic fibers on adrenal cells
- Biochemical cascades of lipolysis and glycogenolysis during sympathoadrenal system activation
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