Anatomical site of action
To understand how pain is suppressed at the spinal level, one must turn to the anatomy of neural connections. The primary action of opioids unfolds in the dorsal horns of the spinal cord. This is precisely where crucial information transfer takes place: the endings of primary afferents (fibers carrying pain signals from the periphery) meet interneurons. This synaptic connection serves as the main "checkpoint" for the pain impulse heading toward the brain. Opioids affect this junction from both sides simultaneously—at the presynaptic and postsynaptic levels—ensuring a reliable blockade of pain signaling.
Presynaptic inhibition: cessation of neurotransmitter release
The first stage of defense against pain is the effect on the presynaptic membrane, specifically on the terminals of primary afferent fibers.
Under normal conditions, upon arrival of a pain signal, these terminals actively release specific "nociceptive" pain mediators into the synaptic cleft. The main ones include:
- Glutamate;
- Neurokinins;
- Substance P.
The development of presynaptic inhibition under the influence of opioids leads to a sharp decrease in the release of these substances. As the concentration of glutamate, neurokinins, and substance P in the synaptic cleft drops, their activating effect on the subsequent interneurons in the chain also diminishes. The pain signal is literally deprived of its chemical "transport".
Postsynaptic inhibition: membrane hyperpolarization
The second stage is a direct effect on the receiving side, namely on the interneurons themselves located in the dorsal horns of the spinal cord.
This process is called postsynaptic inhibition. Its essence lies in changing the electrical charge of the cell wall. Hyperpolarization of the interneuron membranes occurs. The state of hyperpolarization makes the cell extremely unresponsive to any excitatory stimuli. Thus, the activity of interneurons is profoundly suppressed.
The combined action of presynaptic (lack of neurotransmitters) and postsynaptic (membrane unresponsiveness) inhibition yields the main outcome: a complete blockade of pain impulse transmission to higher CNS centers.
Role of descending pathways and additional systems
The spinal mechanism is not limited solely to local synapses of the dorsal horns. A huge role in the antinociceptive (pain-relief) system is played by descending inhibitory pathways originating from supraspinal structures.
These descending pathways are formed by axons of strictly three types of neurons:
- Noradrenergic;
- Serotonergic;
- Purinergic.
In addition, it is important to note the contribution of other receptor systems. Specifically, the cannabinoid system plays a significant role. The activation of cannabinoid receptors makes a substantial additional contribution to the development of the overall analgesic effect by enhancing the suppression of pain signals at the spinal level.
To remember the three types of descending inhibitory pathways, use the mnemonic: "Reliably Hidden Pathway" (Noradrenergic, Serotonergic, Purinergic).
Class and exam questions
Where exactly in the spinal cord is the action of opioids implemented?
The main target is the dorsal horns of the spinal cord, specifically the synapses between primary afferent terminals and interneurons.
Which pain mediators stop being released during presynaptic inhibition?
The release of glutamate, neurokinins, and substance P decreases, depriving interneurons of the activating stimulus.
What happens to the interneuron membrane at the postsynaptic level?
Membrane hyperpolarization develops. This suppresses neuronal activity and makes further impulse transmission impossible.
How many types of neurons make up the descending inhibitory pathways?
They are formed by axons of three types of neurons: noradrenergic, serotonergic, and purinergic.
What else to study on this topic
- Mechanism of action of glutamate and substance P on specific receptors
- Ionic mechanisms of interneuron membrane hyperpolarization
- Anatomy and physiology of descending inhibitory pathways
- Synergism of the opioid and cannabinoid systems in pain suppression
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