Levels of physiological response formation
The response to a painful stimulus is formed in stages, involving various levels of the nervous system:
- Peripheral level. It all begins with the irritation of primary afferent fibers. At this stage, the axon reflex is triggered, which is realized without the participation of CNS neuron cell bodies. Its main effect is local dilation of blood vessels. This increases local blood and oxygen inflow, leading to enhanced oxidative processes (tissue respiration).
- Spinal level. The dorsal horns of the spinal cord act as the first relay station. Here, primary signal processing and the generation of an ascending excitation flow take place. Simultaneously, segmental reactions are triggered — rapid activation of skeletal musculature (reflexive withdrawal of the limb) for the immediate elimination of the harmful factor.
- Brainstem and reticular formation (RF). The midbrain RF ensures the generalization of excitation. Ascending pathways activate the cortex and sensory systems (vision, hearing) to assess the situation. Descending pathways (according to H. Magoun) exert a facilitatory influence on spinal reflexes, increasing the readiness of the motor system.
- Thalamus. This is the main collection station, receiving impulses from the lemniscal and extralemniscal pathways. The thalamus forms sensations associated with epicritic and protopathic pain, determining the nature, severity, and exact localization of the damage.
- Hypothalamus. The involvement of this structure is accompanied by complex autonomic and hormonal shifts. Together with the limbic system, the hypothalamus forms a pronounced negative emotional state.
Components of the systemic pain reaction
According to L.V. Kalyuzhny's classification, several relatively independent components are distinguished in the body's systemic reaction:
- Perceptual component: the pain sensation proper, arising upon the excitation of mechano- and chemo-nociceptors.
- Reflex protective motor reaction: realized at the spinal cord level.
- Pain activation: ensured by the excitation of the reticular formation.
- Negative emotion: the result of the activity of the hypothalamo-limbic-reticular complex.
- Motivation to eliminate sensations: activation of the frontal and parietal cortical areas to form purposeful behavior (wound treatment, threat avoidance).
- Activation of memory mechanisms: retrieval of past experience regarding escape from a damaging factor.
Important rule of specificity: in this integration, only one component is specific — the perceptual one. All other links (emotions, autonomic responses, movements) are nonspecific, as they can manifest in response to any other stressors.
Pain from the perspective of functional systems theory
Within this concept, pain is viewed as a negative biological need that mobilizes the organism. The process includes the following stages:
- Incoming signals (Etiology): External or internal damaging impact.
- Reception and transmission: Irritation of nociceptors and transmission of nociceptive afferentation to the CNS.
- Central processing: The process of generating a local pain sensation in the somatic analyzer under the mandatory control of the antinociceptive system.
- Effector reactions: Three key adaptation mechanisms are triggered: behavioral elimination of the factor, function limitation (sparing the diseased organ), and compensatory mobilization of defensive forces.
- Final useful result: Complete restoration of normal functioning of damaged organs and tissues.
Briefly: answers to topical questions
What structures make up the antinociceptive system?
The antinociceptive system has a hierarchical structure and consists of cortical, hypothalamic, and bulbospinal levels.
- Cerebral cortex — the highest center, specifically the secondary somatosensory cortical area, which maintains the constant activity of the system.
- Thalamus — a collector of sensory information, relaying impulses to the cortex.
- Hypothalamus — triggers antinociceptive reactions and exerts descending influences.
- Midbrain — includes the periaqueductal gray matter.
- Medulla oblongata — contains nuclei of the brainstem reticular formation: the nucleus raphe magnus and the locus coeruleus (or blue spot).
- Spinal cord — the level of the dorsal horns, where urgent blockade of impulse conduction occurs.
Which neurotransmitters mediate the transmission of nociceptive signals in the dorsal horns of the spinal cord?
The transmission of nociceptive signals in the dorsal horns of the spinal cord is mediated by excitatory amino acids and neuropeptides released in the area of peripheral nociceptor terminals. The main excitatory pain mediators:
- Glutamate and aspartate — excitatory amino acids.
- Substance P (Substance P) — a neuropeptide that exerts a modulatory amplifying effect on the transmission of nociceptive impulses.
- Neurokinins — neuropeptides.
At the level of the first relay station (the spinal cord), many other substances also act: histamine, neurotensin, angiotensin, cholecystokinin, oxytocin, enkephalins, GABA, norepinephrine, and serotonin.
Through which ascending pathways are pain impulses transmitted from the spinal cord to the thalamus?
Pain impulses are transmitted from the spinal cord to the thalamus via the lateral spinothalamic tract and the pathways of the extralemniscal system.
- Lateral spinothalamic tract — conducts pain and temperature; its fibers terminate in the ventrolateral nucleus of the thalamus.
- Extralemniscal system participates in the conduction of pain impulses and includes the spinoreticular tract, spinotectal tract, and spinobulbothalamic tract; it engages nonspecific thalamic nuclei and participates in the formation of protopathic pain.
To memorize the components of the L.V. Kalyuzhny reaction, use the abbreviation PROMAP: Perception (sensation), Reflex (spinal cord), Ogative (negative) emotion, Motivation (cortex), Activation (RF), Pemory (memory).
Class and exam questions
What is the physiological significance of the axon reflex in pain?
It causes local vasodilation without CNS participation. This increases oxygen inflow and enhances oxidative processes (tissue respiration) in the focus of damage.
Which structure determines the severity and character of the pain sensation?
Thalamus. Being the main relay station for pain impulses, it is responsible for the formation of epicritic and protopathic pain.
How does the reticular formation (RF) affect the spinal cord?
The RF exerts facilitatory descending influences (described by H. Magoun) on spinal motor reflexes, increasing the readiness of muscles for a protective reaction.
Are all components of the pain response specific?
No, only the perceptual component (the sensation itself) is specific. Emotional, autonomic, and behavioral reactions are nonspecific and can occur during any stress.
What else to study on this topic
- Epicritic and protopathic pain: differences and conduction pathways
- Mechanisms of mechano- and chemo-nociceptor function
- Structure and functions of the antinociceptive system
- Facilitatory and inhibitory influences of the reticular formation according to H. Magoun
- The role of the hypothalamo-pituitary system in the hormonal component of pain
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