Receptor apparatus and pain perception
Primary perception of damaging stimuli is carried out by specialized sensory endings known as nociceptors.
They are widely distributed in the body and are present in:
- skin;
- skeletal muscles;
- joint capsules;
- periosteum;
- internal organs.
Nociceptors are capable of responding to various types of stimuli: mechanical (pressure, impact), thermal (extreme temperatures), and chemical.
Biochemical mechanisms and vanilloid receptors
Vanilloid receptors play a crucial role in the perception of pain signals.
For this receptor to be excited and trigger a nerve impulse, specific activators are required. According to physiological mechanisms, such activators include:
- Bradykinin — a tissue hormone actively released upon injury.
- Protons (H+) — hydrogen ions whose concentration rises when the environment becomes acidic (e.g., during ischemia or hypoxia).
- ATP — a molecule normally found inside the cell, but released extracellularly when the cell membrane is disrupted.
- Vanilloids — specific substances, the most famous of which is capsaicin (an alkaloid found in chili peppers).
The phenomenon of sensitization
During any tissue injury (whether trauma, ischemia, hypoxia, or classic inflammation), endogenous substances — inflammatory mediators — accumulate in the lesion. These include:
- histamine;
- serotonin;
- prostaglandins;
- bradykinin.
These substances cause the phenomenon of sensitization. They significantly increase the sensitivity of nociceptors to any external stimuli. Consequently, a stimulus that would normally not cause pain (such as a light touch) begins to be perceived as painful. In some cases, especially with pronounced inflammation, these mediators can independently, without external stimuli, cause a persistent sensation of pain.
Afferentation: pathways of impulse conduction
Once the receptor is excited, the signal must reach the central nervous system. The transmission of the pain impulse (afferentation) occurs via two main types of nerve fibers:
- C-fibers — they lack a myelin sheath, and therefore conduct impulses very slowly (forming dull, aching pain).
- Aδ-fibers — thin myelinated fibers that transmit signals rapidly (forming sharp, primary pain).
The primary signal processing unit in the CNS is located in the spinal cord. The pain impulse enters the dorsal horns of the spinal cord, where it is relayed to subsequent nerve pathways via a complex system of interneurons.
Spread of excitation in the spinal cord
Upon entering the dorsal horns, the excitation does not stop, but is distributed along reflex arcs. At the spinal cord level, two major propagation pathways are distinguished:
- Motor pathway (to the ventral horns).
Here, the signal is relayed to motor neurons. As a result, a rapid protective motor reflex occurs — a sharp contraction of skeletal muscles takes place (a classic example: the reflex withdrawal of a limb from a hot object).
- Autonomic pathway (to the lateral horns).
In the lateral horns, the impulse is relayed to neurons of the sympathetic division of the autonomic nervous system. This provides the autonomic component of the pain response (stimulation of the sympathetic system prepares the body for stress and potential combat).
Briefly: answers to topical questions
Which tissue and plasma alogens are released during cell damage and inflammation?
- Inflammatory mediators — histamine, serotonin, prostaglandins, and bradykinin; they increase receptor sensitivity and in some cases independently cause pain.
- Pain mediators — prostaglandins in combination with histamine, serotonin, bradykinin, and substance P.
- Pro-inflammatory vasodilator peptides — calcitonin gene-related peptide (CGRP), neurokinin A, and substance P.
Through which ascending pathways of the spinal cord is the pain impulse transmitted to the brain?
- Lateral spinothalamic tract (tractus spinothalamicus lateralis) — conducts pain and temperature to the brain.
- Spinothalamic tracts (tractia spinothalamica) — ascending pathways running through the lateral funiculi to the thalamus.
What is the role of substance P in the synaptic transmission of the pain signal?
- Substance P — a neuropeptide involved in the excitatory transmission of the pain signal at the terminals of peripheral nociceptors near second-order neurons.
- Substance P is part of the complex of pain mediators along with prostaglandins, histamine, serotonin, and bradykinin.
- Upon activation of the trigeminovascular system, substance P is released from trigeminal nerve fiber endings.
Which structures of the central nervous system form the endogenous antinociceptive system?
- Periventricular gray matter and periaqueductal gray matter — the anatomical substrate of the analgesic effect.
- Pontine tegmentum.
- Amygdala.
- Hippocampus.
- Reticular formation.
- Individual cerebellar nuclei.
- At the synaptic level, inhibition occurs in the dorsal horns of the spinal cord and nuclei of the raphe obscurus/medius of the medulla oblongata.
Which parts of the brain are responsible for forming the emotional-affective component of pain?
- Cerebral cortex — the highest center of perception and emotional evaluation of pain; selection and integration of information occur in the cortex, transforming the sensation of pain into suffering and shaping conscious pain behavior.
- Limbic system — participates in altering the emotional evaluation of pain and forming the emotional component of suffering.
It is easy to remember the impulse distribution in the spinal cord using the first letters: Posterior horns — Receive the signal (input), Anterior horns — Move (muscle contraction), Lateral horns — Anxious (autonomic sympathetic response).
Class and exam questions
Where are pain-sensing receptors located?
Nociceptors are located in the skin, skeletal muscles, joint capsules, periosteum, and internal organs.
What is nociceptor sensitization?
This is a process in which inflammatory mediators (histamine, serotonin, prostaglandins) increase receptor sensitivity. As a result, they begin to respond even to weak stimuli, and during severe inflammation, they can trigger pain on their own.
Which fibers are responsible for conducting pain signals?
Pain impulses are transmitted via slow unmyelinated C-fibers and fast myelinated Aδ-fibers.
What effect is caused by the transition of a pain signal into the lateral horns of the spinal cord?
Impulse switching in the lateral horns stimulates neurons of the sympathetic nervous system, triggering the autonomic component of the pain response.
What else to study on this topic
- Mechanisms of ischemic and hypoxic pain formation
- The role of prostaglandins in maintaining the inflammatory process
- Pharmacological targets: how drugs block vanilloid receptors
- Differences in conduction velocity between C- and Aδ-fibers
- Central mechanisms of pain processing (above the spinal cord level)
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