Notes

It is the basic principle of organization of the somatosensory system. Information from each class of receptors reaches a different group of neurons in the CNS and these neurons project to higher levels along segregated "parallel" pathways. This segregation begins with the place of termination of different classes of afferent axon in the spinal cord. It continues with two major ascending pathways:

• The Dorsal Column -- Medial Lemniscal Pathway
• The Spino -- Thalamic Pathway (pain & temperature information)

Segregation in Spinal Cord

• The spinal grey matter is divided into a number of layers (laminae); laminae I -- VI comprise the dorsal horn.
• Aδ and C fibers terminate predominantly in layer I (the marginal layer) and layer II (the substantia gelatinosa).
• Aδ axons bifurcate, one branch ascending in the dorsal columns, the other terminating on cells in the deeper dorsal laminae.