Sensory information is required for the control of the simplest of movements. One sensor of particular importance for movement control is the muscle spindles. Its exact role is still unknown but is investigated in this project.
Muscle spindles are found in a large number in practically all human skeletal muscles. They are known to elicit the so-called short-latency stretch reflex and there are good reasons to believe they are important for our position sense. But if the short-latency stretch reflex is important for stabilizing muscles it is surprisingly weak and our position sense is certainly not exclusively dependent on inputs from the muscle spindles. In this project several alternative hypotheses are investigated: that they inform the brain about future muscle states, that they assist in identifying the mechanical properties of unexpected loads, and that they convey time critical information about ongoing movements.
Muscle spindles fulfill critera for forward sensory models: their responses are shaped by both the current kinematic state (i.e., length, velocity, and acceleration of their parent muscles) and by an efferent command related to the α-motor command (i.e., the fusimotor drive). Prelminary analyses of single muscle spindle afferents recorded with microneurography as well as ensemble responses indicate that muscle spindle also full the critical critera of forward sensory models, i.e., they predicted the future state of the their parent muscles.
The adequate response when a limb is subjected to an unexpected perturbance depends in part on the mechanical nature of the load. If the load is very stiff (e.g., a tree trunk) it is futile to increase the muscle force whereas this may be appropriate if the load is compliant (e.g., a tree branch). The ability of humans to detect and respond appropriately to loads of different stiffness will be investigated in behavioral studies and the dependence of muscle spindle responses on the mechanial properties of loads will be investigated in neurophysiological experiments.
The control of several types of motor tasks are known to depend on temporaly discrete sensory events. Muscle spindles respond with a burst of action potentials when their parent muscles are elongated while suddenly relaxing, an event that occur during several types of movements. The role of such burst will be investigated by injecting spurious muscle spindle activity at different phases of ongoing movements to determine its efficacy in affecting the behavior.