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George E. Carvell, PhD, PT   University of Pittsburgh
 
PERIPHERAL INFLUENCES: Dorsal Root Afferent Input
Large dorsal root afferents enter the dorsal column and branch. Large fiber branches enter the gray matter to terminate on neurons in the dorsal horn, intermediate gray and some even terminate on neurons in the ventral horn (fig 5-1). In addition, many large fibers have ascending axonal projections to the dorsal column nuclei in the medulla. A considerable degree of  'sensory' integration occurs in the dorsal horn which, in turn, influences spinal segmental motor centers. Ascending projections provide the major input for 'conscious' discriminative somatic sensation and 'feedback' for control of simple to complex behaviors.
The spinal gray dorsal horn contains projection neurons whose axons ascend in the dorsal, lateral or ventral funiculus. For example, Spinothalamic Tract Neurons live in the dorsal horn.  Local excitatory and inhibitory interneurons modulate the discharge properties of these projection neurons. Both types of neurons are influenced by peripheral afferent input and by descending pathway axons. Small fiber dorsal root afferents enter the Tract of Lissauer, branch, and then terminate in the dorsal horn or possibly the intermediate gray.
Medial Division = Large Fibers
(Aa  & Ab)
Lateral Division = Small Fibers
(Ad & C)
Group I
(A Alpha)
Lateral Division = Small Fibers
(Ad & C)