Muscle spindle and fusimotor activity in locomotion

Abstract

Mammals may exhibit different forms of locomotion even within a species. A particular form of locomotion (e.g. walk, run, bound) appears to be selected by supraspinal commands, but the precise pattern, i.e. phasing of limbs and muscles, is generated within the spinal cord by so-called central pattern generators. Peripheral sense organs, particularly the muscle spindle, play a crucial role in modulating the central pattern generator output. In turn, the feedback from muscle spindles is itself modulated by static and dynamic fusimotor (gamma) neurons. The activity of muscle spindle afferents and fusimotor neurons during locomotion in the cat is reviewed here. There is evidence for some alpha-gamma co-activation during locomotion involving static gamma motoneurons. However, both static and dynamic gamma motoneurons show patterns of modulation that are distinct from alpha motoneuron activity. It has been proposed that static gamma activity may drive muscle spindle secondary endings to signal the intended movement to the central nervous system. Dynamic gamma motoneuron drive appears to prime muscle spindle primary endings to signal transitions in phase of the locomotor cycle. These findings come largely from reduced animal preparations (decerebrate) and require confirmation in freely moving intact animals.

Gamma motor neurons survive and exacerbate alpha motor neuron degeneration in ALS

“…We focused on the small γ fusimotor neurons (γ-MNs), which represent approximately one third of all MNs in most limb-innervating motor pools and lack direct excitatory inputs from proprioceptive sensory neurons (5). γ-MNs are distinct from large, force-generating alpha MNs (α-MNs) in that they selectively innervate intrafusal fibers of the muscle spindle and control the sensitivity of spindle afferent discharge (15); beta (β) skeletofusimotor neurons innervate both intra-and extrafusal muscle (16). In addition to morphological differences, distinct muscle targets, and the absence of primary afferent (I A ) inputs on γ-MNs, these functional MN subtypes also differ in their trophic requirements, and γ-MNs express high levels of the glial cell line-derived neurotropic factor (GDNF) receptor Gfrα1 (17).…”

Muscle Spindles and Locomotor Control-An Unrecognized Falls Determinant?

“…Muscle spindles, the most commonly observed sensory structures located in mammalian muscles [3] appear to be highly significant in all phases of the locomotor cycle. They appear to provide an effective source of sensory signals that can influence several features of normal locomotion [2,40] .…”

“…That is, once one movement in the gait cycle has occurred, the muscle afferents can respond by helping to initiate a subsequent movement that is consistent with the magnitude of force needed to ensure stability, safety and efficiency. Work by Prochazka et al [6] has shown that fusimotor drive during locomotion can be altered during unpredictable and novel movements or if the organism inadvertently deviates from their stereotypical stepping cycle, for example if they encounter a slippery surface [3] and there is a need to avert falling, slipping, or tripping. That is, the muscle spindle afferents can help recalibrate the locomotor system accordingly.…”

“…Their ability to respond differentially to muscle length variations, as well as velocity signals is also consistent with their documented role in the control of human walking [10] , as is their ability to 'smooth' out irregularities in movement trajectories by permitting symmetric and consistent responses, which do not yield following initial stretch [11] . Able to provide sensory input via two types of afferent nerve fibres to the spinal cord and central nervous system [3] , and to receive motor inputs from the central nervous system and spinal cord via gamma motoneurone, the muscle spindle pathways appear to provide a unique neural mechanism for controlling bipedal locomotion [3] .…”

“…Their sensitivity in turn can be modulated by ongoing inputs from supraspinal or central neuronal mechanisms, making them ideally suited to enable desirable gait adaptations in response to changing environmental contexts [2] . To further examine the degree to which muscle spindles and their sensory connections are implicated in locomotion and its control, many studies have examined the role of muscle spindle inputs in modulating spinal cord central pattern generator outputs to primary muscles, as well as their synergists [3,4] . Others have focused on examining the role of muscle spindles in adaptive walking in the face of varying external perturbations [5] .…”