Plasticity in the Cerebellum and Primary Somatosensory Cortex Relating to Habitual and Continuous Slender Branch Climbing in Laboratory Mice (Mus musculus)

Abstract

The origin of the mammalian order Primates is nested within a Euarchontan ancestry that was probably exploiting the fine branch arboreal niche in a facultative way. A putative transition into this habitat may have begun with a more generalized small-bodied mammal that lacked climbing specializations for grasping hands and feet. Here, we investigate whether mice exhibit central nervous system (CNS) plasticity associated with learning to grasp/climb proficiently. House mice were used to study phenotypic plasticity within the cerebellum and primary somatosensory cortex associated with the fine branch niche. This experimental treatment has previously been shown to influence skeletal plasticity in part because climb-training encourages tail use and facultative pedal grasping. The CNS necessary to coordinate and control these locomotor behaviors was investigated in a standard mouse model (N 5 10 male CD-1/ICR mice), and plasticity was detected by histomorphometric and immunohistologic changes within the cerebellum and cerebrum. The climbing group had a significantly smaller relative granule cell layer in cerebellar lobule 1-3 than the control group (P < 0.10), but increased nerve growth factor immunoreactivity in white matter tracts of these lobules (P < 0.05). Qualitative observations in the primary somatosensory cortex revealed greater pyramidal/stellate cell counts in climbers. We suggest that coordinated tail and hindlimb learning within the arboreal milieu is facilitated by increased growth factor expression and neuronal alterations in the CNS. These findings suggest that mammals with a generalized Euarchontogliran body plan were capable of facultative pedal grasping and tail use so as to exploit the terminal branch niche. Anat Rec,

Mouse Hind Limb Skeletal Muscle Functional Adaptation in a Simulated Fine Branch Arboreal Habitat

“…Previous work by Byron and colleagues using a similar mouse model of fine‐branch arboreal climbing demonstrated considerable skeletal adaptation over the same time course (Byron et al, ). Additionally, the Byron laboratory demonstrated significant central nervous system plasticity in their climbing mice, as the granular cell layers of cerebellar lobules responsible for coordinating muscle function of the tail are thicker, suggesting more muscle coordination (Byron et al, ). We expected to see adaptation of the hind limb skeletal musculature in the present study.…”

“…Animals were raised in one of two custom enclosures: an experimental enclosure and a control enclosure. The experimental enclosure is a 2 ft 3 Plexiglas box traversed by multiple 2.25 mm diameter steel wires that create a three-dimensional climbing substrate approximating a fine branch arboreal niche (Byron et al, 2009(Byron et al, , 2011(Byron et al, , 2013. Wires are oriented either horizontal to the bottom of the enclosure or at 458 relative to horizontal.…”

“…For example, climbing mice raised in a simulated fine branch arboreal habitat exhibit increased femoral neck angles, larger patellar groove indices, more circular talar heads, increased curvature of the distal third metacarpal, relatively longer caudal vertebral transverse processes, and more robust first metatarsals (Byron et al, ). Additionally, these mice were characterized by thicker granular cell layers in the cerebellar lobes, areas of the brain responsible for coordination of muscle function in the tail (Byron et al, ).…”

Mouse hallucal metatarsal cross‐sectional geometry in a simulated fine branch niche

“…It has been shown that standard laboratory rodents as well as the European Red Squirrel locomote on slender substrates with adaptive behaviors that promote stability (Schmidt and Fischer, ). Recently, Byron et al () also promoted the use of a mouse model system that simulates a fine‐branch arboreal habitat. This is because pedal (specifically hallucal) prehension along with tail coordination are observed to help counteract the body's tendency to pitch and roll mediolaterally on narrow substrates (Fig.…”

“…). The development of this skill for fine branch arboreality is evident at a cerebral and cerebellar somatotopic level (Byron et al, ). The habitual use of hallucal grasping commits the mouse to foot postures that place the substrate between the first and second digits.…”