Core Concept of the Gate Control Theory
The theory proposed by Melzack & Wall describes spinal reciprocal mechanisms of interaction between various neuronal groups. The main principle relies on competition between nerve cells that rapidly conduct excitation and neurons that transmit pain via C-afferents.
The operation of this mechanism («the gate») involves two states:
- Pain Inhibition ("Gate Closed"): Transmission of nociceptive signals is attenuated or completely prevented by a barrage of impulses from large-diameter afferent fibers. These fast-conducting structures mediate tactile sensation. A classic clinical example of this mechanism is the relief of pain by vigorously rubbing an injured area, where tactile stimulation suppresses pain.
- Pain Facilitation ("Gate Open"): If input traffic from small-diameter fibers (responsible for slow pain conduction) predominates, signal transmission to the central nervous system is markedly amplified.
- Inhibitory interneurons serve as the physiological and anatomical substrate functioning as the gate.
Peripheral Formation of Pain Excitation
Before a signal reaches the spinal cord, it must be generated in the tissues. The primary process is triggered by tissue damage, leading to cell membrane disruption and the massive release of endogenous algogens—pain-producing substances. These compounds activate or sensitize chemo-nociceptors.
Major endogenous algogens include:
- Potassium ions ($K^+$). According to physiologist H.N. Kassil, these ions directly alter nerve cell membrane excitability.
- Substance P.
- Bradykinin and prostaglandins.
- Serotonin.
Additionally, metabolic factors of hypoxia are recognized universal algogens. The mechanical component of inflammation contributes further: tissue destruction is accompanied by edema, causing overstretching of visceral capsules or direct mechanical pressure on afferent nerves. Any of these influences recruit slow-conducting group C fibers, leading to prolonged activation of central structures and creating ideal conditions for severe chronic pain. Notably, polymodal C-nociceptors are activated primarily by acetylcholine, norepinephrine, and serotonin.
Nociceptor Activation Pathway
Excitation in a specific nociceptor follows a strictly defined sequence. The sensory neuron soma synthesizes neurotransmitters. Axonal transport then distributes the mediator in two directions: via the central process into the CNS and via the peripheral process to the nerve endings themselves.
The activation process unfolds as follows:
- Stimulation: A strong mechanical stimulus directly impacts tissue.
- Mediator Release: The stimulus triggers the release of preformed neurotransmitter from peripheral nerve endings.
- Reception: The released substance binds to receptors on the membrane of its own nerve endings.
- Excitation Generation: An action potential (impulse) is generated and travels along the sensory neuron to the central nervous system.
- Inactivation: Specific enzymes degrade the neurotransmitter, terminating stimulation.
Neurochemistry of the Spinal Dorsal Horn
The dorsal horns of the spinal cord function as the first relay station along the pain pathway, executing complex neurochemical modulation. Substance P plays a crucial role, exerting a powerful excitatory effect on nociceptive transmission. Additionally, somatostatin is localized in specific populations of C-fibers and is closely linked to pain excitation.
Receptors for a vast array of substances have been identified at the spinal cord level:
- Histamine, neurotensin, and angiotensin.
- Cholecystokinin and oxytocin.
- Enkephalins.
- Glutamate and GABA (gamma-aminobutyric acid).
- Serotonin and norepinephrine.
This demonstrates that nociceptive excitation across various CNS levels is mediated by an intricately complex array of chemical compounds. Nevertheless, the organism possesses protective mechanisms against excessive stimulation: the antinociceptive system—a complex of endogenous pathways aimed at suppressing or reducing pain perception.