A model of the mammalian muscle spindle

Proprioceptors such as muscle spindles and Golgi tendon organs provide the\ncentral nervous system with sensory feedback for motor control and kinesthesia. It is\ndifficult to record afferent activity from such receptors during motor behavior, so theories\nof motor control usually depend on implicit or explicit assumptions about such activity.\nThe muscle spindle is the most important proprioceptor, playing a dominant role\nin kinesthesia and in reflexive adjustments to perturbations. Each muscle spindle\naccurately senses and encodes length and velocity information of the extrafusal muscle\nfibers over a wide range of movements despite the relatively restricted dynamic range of\nfiring rates for action potentials. It does this by shifting the relative importance and\nsensitivity to length and velocity in response to specialized fusimotor efferents (gamma\nmotoneurons), at the cost of complicating the interpretation of their signals by the\nnervous system. We have constructed a physiologically realistic model of the spindle that\nis composed of mathematical elements closely related to the anatomical components\nfound in the biological spindle. The spindle model consists of three nonlinear intrafusal\nfiber models: bag1, bag2 and chain. The bag1 fiber model is the only one that receives\ndynamic fusimotor control and is primarily responsible for velocity sensitivity of the\nspindle. The bag2 and chain receive ?static fusimotor control and contribute mainly to\nlength sensitivity. All three fiber types give rise to primary afferent activity, while only\nbag2 and chain to secondary afferent activity. In the case of the primary afferent, the\nmodel incorporates the experimentally observed effect of partial occlusion, where\nprimary afferent activity results from a competition between two impulse generator sites,\none located on the bag1 and other on bag2 and chain fibers. When both sites are active,\nthe dominant generator wins and suppresses all activity in the weaker generator by resetting\nits spike generator. While that results in total occlusion, the mechanism\nresponsible for partial occlusion observed in the case of primary afferent is believed to\ninclude electrotonic current spread between the suppressed and dominant generator,\nresulting in increased impulse generation at the dominant site. The model also\nincorporates the appropriate temporal properties of three types of intrafusal fibers during\nstatic or dynamic fusimotor stimulation. The advantage of including these properties is\ndemonstrated by comparing model simulations with and without these properties to data\nfrom recently published experiments in which both fusimotor efferent and spindle\nafferent activity were recorded simultaneously during decerebrate locomotion in the cat\n(Taylor et al., J Physiol 529.3: 825-836, 2000). We have inverted the spindle model in\norder to use it as a tool to better understand fusimotor control in natural tasks. By\nsupplying the inverted model with records of afferent activity and kinematics during\nnatural tasks, the inverted model can be used to infer the underlying fusimotor drive.\nOnce the principles of fusimotor control are understood, it should be possible to apply the\nspindle model to predict more accurately the activity of spindle afferents and their role in\ncontrol of motor tasks.