Several ascending tracts are responsible for transmitting nociceptive impulses from the dorsal horn to supraspinal targets. Of these, the spinothalamic tract is considered the primary perception pathway.
The descending pathways originate in the somatosensory cortex, which relays to the thalamus and the hypothalamus. Thalamic neurons descend to the midbrain. There, these neurons synapse on ascending pathways in the medulla and spinal cord and inhibit ascending nerve signals, producing an analgesic effect which comes from the stimulation of endogenous endorphins, dynorphins, and enkephalins. The extent of autonomic responses to pain (tachypnea, tachycardia, hypertension, diaphoresis, etc.) can be depressed in the cortex through descending pathways. Of interest, the influences of the descending pathways may also be responsible for psychogenic pain (pain perception that has no obvious physical cause).
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