This process maintains the tautness of muscle spindles and is called 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. Alpha motor neurons are highly abundant and larger in size than gamma motor neurons. 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. 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. 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). There are three types of lower motor neurons involved in muscle contraction: alpha motor neurons, gamma motor neurons, and beta 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. 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. These neurons also play a role in adjusting the sensitivity of muscle spindles. However, their role is important in keeping muscle spindles taut, thereby allowing the continued firing of alpha neurons, leading to muscle contraction. Unlike the alpha motor neurons, gamma motor neurons do not directly adjust the lengthening or shortening of muscles. Their axons are smaller than those of the alpha motor neurons, with a diameter of only 5 μm. They receive input from the reticular formation of the pons in the brainstem. Like alpha motor neurons, their cell bodies are located in the anterior grey column of the spinal cord. 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.
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