Hierarchy levels of the antinociceptive system
The antinociceptive system has a heterogeneous structure and includes three functional levels:
- Brainstem/Spinal (first level). Encompasses the structures of the spinal cord, medulla oblongata, and midbrain. Responsible for the immediate blockade mechanism, interrupting the pain signal directly at the site of stimulus application.
- Hypothalamic (second level). Represented by the hypothalamic nuclei. Includes a short-duration mechanism that directs descending signals to the relay nuclei of the thalamus and spinal cord neurons.
- Cortical (third level). Localized in the secondary somatosensory cortex. Provides a tonic mechanism that maintains continuous baseline activity of the entire pain control system.
Mechanisms of descending inhibition
The basis of the analgesic effect lies in descending inhibition. Signals from higher centers (hypothalamus, cortex) arrive at brainstem structures, such as the periaqueductal gray matter (Periaqueductal gray matter), raphe nuclei (Raphe nuclei), and locus coeruleus (Locus coeruleus).
From monoaminergic brainstem cells, descending fibers travel down through the dorsolateral funiculus into the spinal cord. In the dorsal horns, these fibers directly or via inhibitory interneurons block the transmission of excitation from primary sensory neurons. Thus, the conduction of the pain impulse is attenuated or completely interrupted at the level of the first synapse («gate control»).
Neurochemistry of pain suppression
Pain suppression is mediated through several neurochemical systems:
- Opioid system. Endogenous opioids (endorphins and enkephalins) bind to opioid receptors, reducing the sensitivity of second-order neurons to pain mediators (glutamate, substance P). Opioid deficiency leads to spontaneous pain, which underlies the withdrawal syndrome in drug addiction.
- Serotonergic system. A powerful non-opioid mechanism associated with the activity of the nucleus raphe magnus. It exerts a direct descending inhibitory effect on the spinal cord. Unlike opioid analgesia, the effect of serotonin is not blocked by naloxone.
- Non-opioid peptides. Substances such as neurotensin (whose analgesic activity is hundreds of times greater than that of enkephalins), oxytocin, and cholecystokinin are also involved in antinociception.
- Adrenergic system. Mediated by the hypothalamus and reticular formation. Responsible for stress-induced analgesia, when strong emotions (fear, anger) block pain sensations.
Briefly: answers to topical questions
What substances belong to endogenous opioid peptides?
Endogenous opioid peptides include three main classes of oligopeptides produced in the body.
- Endorphins — include α-, β-, and γ-endorphin fractions.
- Enkephalins — have a wider localization in the central nervous system compared to endorphins.
- Dynorphins — serve as endogenous ligands for kappa-opioid receptors.
What types of antinociceptive mechanisms are distinguished by their activation method?
Within the hierarchical control structure, three main mechanisms of action of the antinociceptive system are distinguished.
- Immediate blockade mechanism — functions at the spinal and brainstem levels, blocking impulse conduction upon direct exposure to a painful stimulus.
- Short-duration mechanism — implemented at the hypothalamic level via descending influences on the spinal cord.
- Tonic mechanism — provided by the somatosensory cortex to maintain constant system activity.
Class and exam questions
What is the essence of the «gate control» theory of pain?
This is a mechanism for blocking or attenuating the pain impulse at the level of the first synaptic switch in the spinal cord via descending inhibitory influences from the brainstem.
Why does severe pain occur upon drug withdrawal?
Chronic opiate intake activates endopeptidase enzymes. Upon sudden cessation of the exogenous drug, these enzymes begin to degrade the body's own endogenous opioids, depriving it of natural pain defense.
How does stress affect pain sensitivity?
Extreme emotional stress or severe negative emotions trigger the adrenergic mechanism of antinociception, leading to stress-induced analgesia — the suppression of pain.
What else to study on this topic
- Mechanisms of pre- and postsynaptic inhibition in the dorsal horns of the spinal cord
- Specific mechanism of action of different types of opioid receptors (κ, μ, σ, δ)
- Interaction of the serotonergic and opioid systems during acupuncture
- Differences between hyperalgesia and spontaneous pain
- The role of excitatory amino acids and neuropeptides in nociception
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