The spinoreticular tract (also paleospinothalamic pathway, or indirect pathway of the anterolateral system) is a partially decussating (crossed-over) four-neuron sensory pathway of the central nervous system. The tract transmits slow nociceptive/pain information (but thermal, and crude touch information as well) from the spinal cord to reticular formation which in turn relays the information to the thalamus via reticulothalamic fibers as well as to other parts of the brain (as opposed to the spinothalamic tract - the direct pathway of the anterolateral system - which projects from the spinal cord to the thalamus directly without such "layovers"). Most (85%) second-order axons arising from sensory C first-order fibers ascend in the spinoreticular tract - it is consequently responsible for transmitting "slow", dull, poorly-localised pain. By projecting to the reticular activating system (RAS), the tract also mediates arousal/alertness (including wakefulness) in response to noxious (harmful) stimuli. The tract is phylogenetically older than the spinothalamic ("neospinothalamic") tract.

Anatomy

Origin

Axons of sensory group C nerve fibers first synapse with interneurons in the substantia gelatinosa of Rolando (lamina II), and lamina III of the posterior grey column of the spinal cord. These interneurons then synapse with second-order neurons in laminae V-VIII Their axons then ascend in the spinal cord near the lateral spinothalamic tract. A minority of second-order axons of the spinoreticular tract bypass the reticular formation, and project directly to the intralaminar thalamic nuclei..

Pathway

The tract is bilateral: its fibers ascend predominately ipsilaterally, but a minority decussate in the anterior white commissure to ascend contralaterally. Second-order axons of this tract terminate by forming multiple synapses in the nuclei of the medullary, pontine, and mesencephalic reticular formation. The nuclei of the reticular formation lack somatotopic organisation (consequently, sensory stimuli conveyed via this pathway are indistinctly localised). The reticular formation in turn conveys the tract to: