A gamma motor neuron (γ motor neuron), also called gamma motoneuron, or fusimotor neuron, is a type of lower motor neuron that takes part in the process of muscle contraction, and represents about 30% of (Aγ) fibers going to the muscle. Like alpha motor neurons, their cell bodies are located in the anterior grey column of the spinal cord. They receive input from the reticular formation of the pons in the brainstem. Their axons are smaller than those of the alpha motor neurons, with a diameter of only 5 μm. Unlike the alpha motor neurons, gamma motor neurons do not directly adjust the lengthening or shortening of muscles. However, their role is important in keeping muscle spindles taut, thereby allowing the continued firing of alpha neurons, leading to muscle contraction. These neurons also play a role in adjusting the sensitivity of muscle spindles.
The presence of myelination in gamma motor neurons allows a conduction velocity of 4 to 24 meters per second, significantly faster than with non-myelinated axons but slower than in alpha motor neurons.
General background of muscles
Muscle spindles
Muscle spindles are the sensory receptors located within muscles that allow communication to the spinal cord and brain with information of where the body is in space (proprioception) and how fast body limbs are moving with relation to space (velocity). They are mechanoreceptors in that they respond to stretch and are able to signal changes in muscle length. The sensitivity of detecting changes in muscle length are adjusted by fusimotor neurons – gamma and beta motor neurons. Muscle spindles can be made up of three different types of muscle fibers: dynamic nuclear bag fibers (bag1 fibers), static nuclear bag fibers (bag2 fibers), and nuclear chain fibers.
Types of lower motor neurons
Muscle spindles are innervated by both sensory neurons and motor neurons in order to provide proprioception and make the appropriate movements via firing of motor neurons. There are three types of lower motor neurons involved in muscle contraction: alpha motor neurons, gamma motor neurons, and beta motor neurons. Alpha motor neurons, the most abundant type, are used in the actual force for muscle contraction and therefore innervate extrafusal muscle fibers (muscle fibers outside of the muscle spindle). Gamma motor neurons, on the other hand, innervate only intrafusal muscle fibers (within the muscle spindle), whereas beta motor neurons, which are present in very low amounts, innervate both intrafusal and extrafusal muscle cells. Beta motor neurons have a conduction velocity greater than that of both other types of lower motor neurons, but there is little currently known about beta motor neurons. Alpha motor neurons are highly abundant and larger in size than gamma motor neurons.
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 maintains the tautness of muscle spindles and is called alpha gamma co-activation. The nuclei of spindle muscle cells are located in the middle of these spindles, but unlike extrafusal muscle fibers, the myofibril contractile apparati of spindle fibers are located only at both ends of spindle. Efferent stimulation of the spindle by gamma motor neurons contracts the myofibrils, tautening the central region of spindle—which maintains the muscle spindle's sensitivity to muscle's length change.
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 and both alpha and gamma neurons firing, muscle fibers within the muscle spindles are pulled parallel to the extrafusal contraction causing the muscle movement. The firing of gamma motor neurons in sync with alpha motor neurons pulls muscle spindles from polar ends of the fibers as this is where gamma motor neurons innervate the muscle. 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.