Gamma Motoneuron - Alpha Gamma Co-Activation

Alpha Gamma Co-Activation

When the central nervous system sends out signals to alpha neurons to fire, signals are also sent to gamma motor neurons to do the same. This process is called alpha gamma co-activation, which is what maintains tautness of the muscle spindles. It is important to note that nuclei (plural for singular nucleus) of spindle muscle cells are located in the middle of these spindles and intrafusal muscle fibers do not have myofibrils, the contractile elements of muscles. Furthermore, the lack of these elements means that the equatorial region of fibers do not contract.

Without gamma motor neurons, muscle spindles would be very loose as the muscle contracts more. This does not allow for muscle spindles to detect a precise amount of stretch since it is so limp. However, with alpha gamma co-activation where both alpha neurons and gamma neurons are present, muscle fibers with the muscle spindles are pulled parallel to the extrafusal contraction causing the muscle movement. The firing of gamma motor neurons in synch with alpha motor neurons pulls muscle spindles from polar ends of the fibers as this is where gamma motor neurons innervate the muscle. This part of the spindle is innervated by type Ia sensory fiber that go on to synapse with alpha motor neurons, completing the gamma-loop. The parallel pulling keeps muscle spindles taut and readily able to detect minute changes in stretch.