Abstract: For decades, researchers have known that when neurons lose their activating inputs, they can atrophy and even die. For example, removing sensory nerve inputs from the arm to the spinal cord after amputation or cutting the spinal nerves can shrink the post-synaptic target zones (e.g., Woods et al., 2000). From touch receptors for the hand and arm, primary nerve afferents enter the spinal cord, and axons travel in the dorsal columns to target the cuneate nucleus (Cu) of the brainstem on the same side of the body (Figure 1). When sensory loss is unilateral, the overwhelming result is for the associated primary target zone to shrink in total size (cross-sectional area), as compared to the size of the opposite side. While this concept is widely accepted, many details about the factors that influence the shrinkage are not known. What are the effects of different types of sensory loss on the relative shrinkage? Are the effects different whether the sensory loss occurred during development or adulthood? What are the implications of size reduction due to sensory loss? Qi et al. (2023) examined the histological consequences of sensory loss of the hand and arm on the Cu in nonhuman primates (Figure 2A and B). Historically over 40 years, Kaas and colleagues reported on the plasticity in the cortex of nonhuman primates that experienced experimental and therapeutic unilateral sensory loss (for specific references, see Qi et al., 2023). In many of the same cases that were studied for cortical effects, tissue sections of the brainstem were histologically processed for cytochrome oxidase, revealing cytoarchitecture of the Cu in 122 cases, which included 37 Old World macaque monkeys. The remaining 85 cases included prosimian Galagos and New World monkeys. The sizes of left and right Cu were measured for each brainstem section in the histological series, and the ratio of the sizes in cases with and without unilateral sensory loss was calculated. Types of sensory loss studied included the transection of one or more peripheral sensory nerves with or without manipulations of regeneration, therapeutic amputations ranging from digits to the forelimb, and central spinal cord injury of the primary somatosensory dorsal column pathway.