By Bhushan, Vikas
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Co-activation, which is such a regular feature of work on human muscle spindles in voluntary contraction , is not a useful description of what is seen here. 28 ORAL PHYSIOLOGY & OCCLUSION Parallel work  on the rhesus monkey has used animals trained to accept head fixation and biting on foods of various consistencies. The more complicated jaw movements of this species have necessitated recording in two dimensions. No distinction could be made between primary and secondary type activity but most units paused during fast active muscle shortening.
Jerge, C. Organization and function of the trigeminal mesencephalic nucleus. J. Neurophysiol, 26: 379-392, 1963. Joyce, G. C , Rack, P. M. , and Westbury, D. R. The mechanical properties of cat soleus muscle during controlled lengthening and shortening movements. J. , 204: 461-474, 1969. Kuffler, S. , Hunt, C. C , and Quilliam, J. P. Function of medullated small-nerve fibres in mammalian ventral roots: Efferent muscle spindle innervation. J. , 14: 29-54, 1951. Luschei, E. , and Goodwin, G. M. Patterns of mandibular movement and jaw muscle activity during mastication in the monkey.
None of this proves there are no muscle spindles in digastric but it does make it rather unlikely. I don't know why this should be, but one might suppose that the jaw opening movements do not need to be well controlled but are in the nature of a quick jerk pulling the jaw down. This could just be an impulsive movement so that under normal conditions these animals do not need any more sophisticated control, I just don't know why, but that's the way it stands. If somebody were willing to do a very careful histological study again (and above all to do careful recordings from dissected filaments of digastric nerve, while stretching the muscle), I think this is what is needed to establish whether there are any receptors there or not.
Anatomy by Bhushan, Vikas