Final answer:
Kehr's sign refers to referred pain in the left shoulder following a ruptured spleen, explained by overlapping sensory innervation in the spinal cord, with the diaphragm also influencing pain perception due to its proximity to the spleen.
Step-by-step explanation:
Referred pain is the phenomenon where visceral sensations are perceived in regions of the body that are different from the source of the stimulus. This can occur with a ruptured spleen, presenting as Kehr's sign, where the felt pain is in the left shoulder, chest, and neck rather than the actual location of the spleen. The underlying reason for this misdirected sensation is that the visceral sensory fibers and the somatosensory fibers corresponding to the upper-left abdominopelvic quadrant and the shoulder and neck enter the spinal cord at similar levels. This causes the brain to misinterpret the origin of the pain.
The diaphragm also plays a crucial role in referred pain due to its proximity to the spleen. When the spleen ruptures and blood spills into the region, pressure on the diaphragm results in pain perceived in regions corresponding to diaphragmatic innervation, leading to Kehr's sign. However, this referred pain is not entirely understood, and the prevertebral ganglia and connections of autonomic and somatosensory fibers complicate the precise mechanism behind the phenomenon.