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. General background of muscles 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. 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. Upper motor neurons, most of which have fibers in the corticospinal tract, can control activity in both alpha and gamma motor neurons.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. Less activity in the gamma motor neurons decreases the sensitivity of the annulospiral ring. This increased tension on the ring makes it more sensitive to any further change in length caused by contraction or stretching of the extrafusal muscle fibers. As gamma motor neurons become active, they cause greater contraction of intrafusal fibers and greater stretching of the annulospiral ring. Innervation of the muscle spindle by gamma motor neurons. Ia afferents from annulospiral stretch receptors A number of descending pathways indirectly influence motor activity through control of the drive on the gamma loop system, thus making muscles more or less susceptible to myotactic reflex responses. Like alpha motor neurons, the gamma motor neurons are located within the ventral horn of the spinal cord they also receive substantial input from higher motor centers. The role of the gamma system is to allow the spindle to maintain high sensitivity over a wide range of muscle lengths during contraction and relaxation. The motor neurons innervating the muscle spindle fibers are not the large alpha motor neurons that innervate the main muscle mass but a different set of smaller cells, the gamma motor neurons. A correction for this potential problem is achieved through motor innervation of the small intrafusal muscle fibers themselves, specifically near the ends of the fiber, so that a minimum amount of tension can be maintained on the fibers within the capsule at all times. To provide information about static or changing muscle length, the annulospiral ring must be under some minimal amount of tension otherwise, the stretch-sensitive channels remain closed, and no action potentials are generated. This system operates to maintain tension on the intrafusal muscle fibers. The muscle spindle not only serves as a feedback system to the alpha motor neuron, but has within it a separate feedback system of its own called the gamma loop system.
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