However, unlike other types of peripheral mechanoreceptors, the spindle organs have their own motor supply in the form of γ motor (‘fusimotor’) neurons ( Barker and Chin, 1961 Matthews, 1972). These response patterns reflect the general view of spindles, which says that type Ia firing encodes static muscle length and the velocity of stretch, and type II encode static muscle length. That is, under passive conditions, primaries can be considered to have both a good dynamic and fairly good static muscle-length sensitivity, whereas type II from passive muscle represent good static length sensitivity but a poorer dynamic sensitivity ( Edin and Vallbo, 1990a). When imposing a ramp-and-hold stretch of the relaxed muscle, type Ia from this muscle are most responsive during muscle stretch, are sensitive to the rate of change of length (i.e., velocity), may encode static length but are silent during muscle shortening. Specifically, there are two main types of muscle spindle receptors, the primary and the secondary, which give rise to the primary (type Ia) and secondary (type II) afferents, respectively ( Boyd and Davidson, 1997). In the relaxed muscle of the unengaged human, the characteristics of imposed muscle stretch are rather faithfully encoded by the signals of muscle spindle afferents. of the mechanoreceptor under in vivo efferent control) has remained unclear(for a recent comprehensive review see Macefield and Knellwolf, 2018). Still, the role of muscle spindle organs in their entirety (i.e. An interesting recent proposition is that the mechanoreceptive part of spindles responds best to force-related variables, as shown in relaxed muscles ( Blum et al., 2017). Spindles have been proposed to play a basic, low-level role in reflex motor control ( Houk, 1976) and proprioception ( Goodwin et al., 1972), and their malfunction has been linked to impaired motor coordination ( Sainburg et al., 1993). Kruse and Poppele, 1991 Bewick and Banks, 2015 Woo et al., 2015). Previous work and more recent studies using genetic manipulation methods have added a great deal of knowledge about the molecular mechanisms of mechanotransduction (e.g. Spindles are generally believed to be basic mechanoreceptors that encode muscle stretch and provide reliable information about actual limb posture and movement kinematics. Most of our skeletal muscles contain a large collection of muscle spindle organs. Incorporation of advanced signal-processing at the periphery may well prove a critical step in the evolution of sensorimotor control theories. Such studies have so far shown that spindle tuning enables the independent preparatory control of reflex muscle stiffness, the selective extraction of information during implicit motor adaptation, and for segmental stretch reflexes to operate in joint space. This role is compatible with previous findings and supported by recent studies with naturalistically active humans. That is, I propose that spindles play a unique overarching role in the nervous system: that of a peripheral signal-processing device that flexibly facilitates sensorimotor performance, according to task characteristics.
A new model is presented, viewing γ motor activity as an intermediate coordinate transformation that allows multimodal information to converge on spindles, creating flexible coordinate representations at the level of the peripheral nervous system.
Spindle organs can be tuned by spinal γ motor neurons that receive top-down and peripheral input, including from cutaneous afferents. Here, I argue that the traditional view of spindles is outdated. Prevalent models of sensorimotor control assume the role of spindles is to reliably encode limb posture and movement. Muscle spindles are encapsulated sensory organs found in most of our muscles.
0 kommentar(er)
