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bioRxiv2025

Motor unit behaviour adaptations across the lifespan: sex differences in young, middle-aged and old adults

Ageing is associated with neuromuscular decline, and emerging evidence suggests that sex may influence the time course of motor unit adaptations. This study examined age- and sex-related differences in motor unit firing behaviour across young (YG), middle-aged (MA), and older adults (OLD), by integrating high-density EMG motor unit analysis with muscle morphology and daily physical activity measurements. The analysis of single motor unit activity during submaximal isometric contractions of the vastus lateralis revealed that older adults had lower firing rates and a reduced capacity to modulate discharge frequency during force-increasing contractions. In the YG and MA groups, females showed higher motor unit firing rates and variability than males, while in OLD these sex differences were no longer present. Females also demonstrated a steeper decline in firing rate modulation between MA and OLD. Reductions in muscle cross-sectional area and thickness were similar between sexes. Physical activity levels declined with age in both sexes. These findings reveal distinct, sex-specific trajectories of neuromuscular ageing, with females showing greater motor neuron function decline between MA and OLD, in the absence of sex-related differences in the rate of morphological deterioration. The attenuation of sex differences in older age suggests a convergence of neuromuscular profiles with ageing. While physical activity may contribute to the observed sex-specific patterns, other mechanisms related to hormonal shifts warrant further investigation. These insights underscore the importance of considering age and sex in the study of motor control and in the development of targeted interventions to preserve muscle function across the lifespan.

neuroscience↗

bioRxiv2021

Landscape dynamics promoted the evolution of mega-diversity in South American freshwater fishes

Landscape dynamics and river network rearrangements are widely thought to shape the diversity of Neotropical freshwater fishes, the most species-rich continental vertebrate fauna on Earth. Yet the effects of hydrogeographic changes on fish dispersal and diversification remain poorly understood. Here we integrate an unprecedented occurrence dataset of 4,967 South American freshwater fish species with a species-dense phylogeny to track the evolutionary processes associated with hydrogeographic events over 100 Ma. Net lineage diversification was heterogeneous through time, across space, and among clades. Three abrupt shifts in diversification rates occurred during the Paleogene (between 63 and 23 Ma) in association with major landscape evolution events, and net diversification accelerated from the Miocene to the Recent (c. 20 - 0 Ma). The Western Amazon exhibited the highest rates of in situ diversification and was also the most important source of species dispersing to other regions. All regional biotic interchanges were associated with documented hydrogeographic events and the formation of biogeographic corridors, including Early Miocene (c. 20 Ma) uplift of the Serra do Mar, and Late Miocene (c. 10 Ma) uplift of the Northern Andes and formation of the modern transcontinental Amazon River. Reciprocal mass dispersal of fishes between the Western and Eastern Amazon coincided with this phase of Andean uplift. The Western Amazon has the highest contemporary levels of species richness and phylogenetic endemism. Our results support the hypothesis that landscape dynamics were constrained by the history of drainage basin connections, strongly affecting the assembly and diversification of basin-wide fish faunas. Significance StatementDespite progress in mapping geographic distributions and genealogical relationships, scientists have few clear answers about the origins of South American freshwater fishes, the most diverse vertebrate fauna on Earth. Here we used the most complete dataset of geographic distributions and evolutionary relationships of South American fishes to track how the geological history of river dynamics influenced the origin, extinction, and interchange of species over the past 100 Ma. We found abrupt increases of species origination between 66 and 23 Ma, coinciding with repeated uplifts of the Andes. The Western Amazon region served as source of freshwater fishes to other regions, as a place where species tended to persist over longer historical periods, and where species originations occurred with higher frequency.

ecology↗