Where is the first synapse in the spinal cord?
There is a differential projection of small-diameter and large-diameter primary afferent fibers to the spinal cord dorsal horn. The largest diameter Ia primary afferents arise from muscle spindles and make monosynaptic connection with motoneurons in the ventral horn. Large-diameter, non-nociceptive primary afferents synapse on neurons in lamina III and lamina IV that are at the origin of the spinocervical tract and on wide dynamic range neurons (see Question 7) in lamina V. Small-diameter nociceptive A delta and C fibers arborize most densely in the superficial dorsal horn. The C fibers predominantly synapse with neurons in lamina I; they also synapse upon dorsally directed dendrites of neurons located more ventrally (e.g., in lamina V). In addition, there are connections with interneurons in the substantia gelatinosa. Many A delta nociceptors terminate in lamina V.
