Gamma loop

The gamma loop is a feedback mechanism which allows the nervous system to maintain muscle contraction and proprioception [1]. Gamma motor neurons in the spinal cord excite intrafusal muscle fibers in muscle spindles, causing them to contract [2]. This stretches the central portions of the intrafusal fibers, stimulating the Ia sensory afferents that wrap around them [1]. Excitation of the Ia afferents activates alpha motor neurons to contract the extrafusal muscle fibers, completing the gamma loop [1][2].

The gamma loop allows the nervous system to control muscle spindle sensitivity independently of alpha motor neuron activation. Co-activation of alpha and gamma motor neurons allows the nervous system to sense limb position and velocity during active movement [4]. The sensitivity of the muscle spindle response is modulated based on background force levels, explaining adaptation to unusual force environments like spaceflight [5]. Specific patterns of gamma and alpha activation are integrated with signals from Golgi tendon organs to compute limb configuration [4]. Dysregulation of the gamma loop can contribute to improper muscle tone and proprioceptive deficits [3][6].

Sources:

  1. Goldstein, E. B. (Ed.). (2015). Encyclopedia of perception. Sage Publications.
  2. Kiernan, J., & Rajakumar, R. (2013). Barr’s the human nervous system: An anatomical viewpoint (10th ed.). Wolters Kluwer.
  3. Mason, P. (2011). Medical neurobiology. Oxford University Press.
  4. Mason, P. (2011). Medical neurobiology. Oxford University Press.
  5. Kiernan, J., & Rajakumar, R. (2013). Barr’s the human nervous system: An anatomical viewpoint (10th ed.). Wolters Kluwer.
  6. Tilney, F., Tilney, L., DeCarli, G. M., & Sarwar, M. (1992). Atlas of neuroanatomy and neurophysiology. Saunders.