Spinotectal tract

Sources may distinguish between a distinct spinotectal tract which projects to the visual reflex system, and spinomesencephalic tract which projects to structures involved in pain processing.

Anatomy

The ST/SM tract mostly consists of myelinated fibers. It conveys stimuli from disparate sensory afferents (low/high threshold, small/large receptive fields, wide dinamic range).

Origin

The ST/SM tract arises mostly in the laminae I and V (but, to a lesser extent, also laminae IV and VI-VIII) of the spinal cord. It arises in same region of the spinal cord grey matter as the spinothalamic tracts.

Course

It mostly crosses over (decussates) to ascend contralaterally, but uncrossed ipsilaterally ascending fibers are prominent in the upper cervical levels.

In the spinal cord, it ascends in its anterolateral quadrant - alongside the spinothalamic tract and spinoreticular tracts.

Projections

Note: rightward arrows indicate subsequent functionally relevant "downstream" projections/pathways of some targets of the ST/SM tract.

• Periaqueductal gray (PAG) → nucleus raphe magnus and gigantocellular reticular nucleus of the reticular formation of the medulla oblongata (enkephalinergic excitatory synapse) - modulates nociception through the serotonergic-opioid peptide descending pain-inhibiting system. Stimulation of the area of PAG into which the ST/SM tract elicits excruciating pain.
• Nuclei of the reticular formation
  • Mesencephalic raphe nuclei - modulate nociception through the serotonergic-enkephalinergic opioid peptide descending pain-inhibiting system.
  • Parabrachial nucleus → amygdala (the latter being involved in processing affective responses).
• Superior colliculus → tectospinal tract (mediates reflex movement of the head and eyes in the direction of origin of a noxious sensation).
• Pretectum
• Nucleus of Darkschewitsch
• Interstitial nucleus of Cajal