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Pain origin

Following tissue aggression, independently of its type, nociceptive messages are transmitted from various nociceptors towards spinal cord roots through small sensitive calibrated nervous fibres. Many algogen products are implicated in pain genesis: bradykinin, H⁺ or K⁺ ions, histamine, serotonin, various derivatives of arachidonic acid (prostaglandins, prostacyclins, leucotriens).

Prostaglandins sensitise receptors to many algogen compounds. Therefore, the use of anti-inflammatory drugs, aspirin or glucocorticosteroids has a positive effect.

Transmission within the spinal cord

The dorsal horn of the spinal cord is the first step for the transmission and modulation of pain. Nociceptive afferent neuronal fibres transmit information to converging neurons, then to the ascending spinothalamic tract.

There, many peptides are involved in pain transmission: substance P, vasoactive intestinal peptide (VIP), ocytocin, galanine, angiotensin 2, dynorphine, enkephalin.

The classical theory considers that all of these chemical molecules are a gate filtrating pain transmission towards the superior centres. Pain is transmitted:

either if it provokes stimulation that is too important to be inhibited at medullar level,

or if the medullar filter is altered.

Thus two different types of pain are considered: nociception pain (too much stimuli) or deafferentation pain (no inhibition), with different therapeutic attitudes.

The presence of such a medullar filter allows local medullary treatment (peridural injection, intrathecal injection), particularly by morphine or its derivatives, in order to reduce the general dosing and diminish side effects.

Transmission towards the brain

Pain is then transmitted through spinothalamic tracts (anterolateral medullar chord) towards brainstem reticular formations and ventral thalamic nuclei.

The lateral thalamus discriminates sensitive information to precisely locate its origin.

The central thalamus has less systematised information responsible for motor reactions and the emotional components of pain.

From the thalamus, the pain information is transmitted to the ascending parietal area (precisely discriminating pain location) as well as towards the limbic system provoking emotional and behavioural reactions and towards the hypothalamus inducing neurovegetative and endocrine manifestations.