Ulnar nerve innervation patterns and muscle fiber type composition of intrinsic hand muscles in mice

The motor function of the hand is crucial in daily life, with fine motor abilities relying on intrinsic hand muscles. Ulnar nerve injury causes atrophy of the intrinsic hand muscles it innervates, severely impacting patients' daily activities. However, current understanding of the innervation patterns and muscle fiber composition of these muscles is limited, hindering the development of effective therapies for restoring hand function after such injuries. In this study, we investigated the anatomical basis of ulnar nerve innervation of the lumbrical and interosseous muscles in mice, a common neuromuscular research model. Our findings revealed that the ulnar nerve innervates the third and fourth lumbrical muscles and all interosseous muscles in mice. Six months after median nerve transection, motor endplate (MEP) areas in the ulnar-innervated fourth lumbrical muscle (152.200 ± 10.63 μm², n = 6) and dorsal interosseous muscles (e.g. third dorsal interosseous muscles: 171.100 ± 10.380 μm², n = 6) were preserved, whereas MEPs in the median-innervated first and second lumbricals were nearly abolished (e.g. LM1: 16.600 ± 1.126 μm², p < 0.001, n = 6).​ The third dorsal interosseous muscles predominantly consist of myosin heavy chain (MHC) -I (11.45% ± 1.92%, n = 6) and MHC-IIa (88.55% ± 1.92%, n = 6) fibers, whereas the lumbrical muscles mainly comprise MHC-IIa (31.67% ± 7.31%, n = 6) and MHC-IIb (64.44% ± 7.17%, n = 6) fibers. These results provide an essential anatomical and histological foundation for using mice to study the functional recovery of intrinsic hand muscles following ulnar nerve injury.