Pain pathway to the CNS

Pain fibres enter the spinal cord in the medial portion of the dorsal roots of spinal nerves. They enter in the dorsilateral funiculus, just dorsal to dorsal grey horn. Descending collaterals travel for one to two cord segments in the dorsilateral funiculus as the tract of Lissauer. The main group of fibres ascend in the tract of Lissauer for two cord segments before entering the dorsal horn to synapse in cells in laminae II, III, IV and V on a group of neurons called the nucleus proprius. The Rexed laminae II and III correspond to the substantia gelatinosa. A delta fibres synapse in laminae IV and V, and C fibres in laminae I and II.

The axons of the cells in the nucleus proprius travel across the midline of the cord in the ventral grey and white commissure just below the central canal of the spinal cord. They pass to the ventro-lateral portion of the contralateral lateral funiculus, where they form the ventral and lateral spinothalamic tracts. The lateral spinothalamic tract is topographically laminated. The neurotransmitters involved predominantly are:

A delta sensory afferents: excitatory amino acids (aspartate and glutamate) and ATP;

C afferents: substance P.

The descending pain inhibitor system comprises the periaqueductal grey matter, the rostral ventro-medial medulla, and the dorsal lateral pontine tegmentum.

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