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George E. Carvell, PhD, PT   University of Pittsburgh
The Spinal Cord has gray matter where neurons live and work (communicating and integrating information sent to, and received from the Brain,  the Periphery, and other Spinal Cord segments). This circuitry incorporates Relay (Projection) Neurons, Propriospinal Neurons (spinospinal connections) and local interneurons (excitatory and inhibitory) and                                                        motoneurons to innervate muscle. White matter surrounds the spinal gray to provide highways for Action Potential signals among segments of the Spinal Cord, for signals that ascend from the Spinal Cord to the Brain (Ascending Tracts/ Pathways), and for descending signals from the Brain to the Spinal Cord (Descending Tracts/ Pathways).
KEY: MZ = Marginal Zone
          SG = Substantia Gelatinosa
          NP = Nucleus Proprius
The Spinal Gray:Dorsal horn neurons are typically associated with sensory function and ventral horn with motor. While this distinction is generally true normal function may draw no such distinct structural                         boundaries. The Spinal White surrounds the gray.
It is divided into three funiculi: dorsal, lateral and ventral (anterior). Axons in the white matter provide continuity between spinal segments (Propriospinal Tract) and between the brain and spinal cord levels of the CNS (ascending and descending tracts). In addition, peripheral (sensory) afferents enter the spinal white at the dorsal root entry zone.
Dorsal Root Entry
Ventral Root Exit