A nuclear bag fiber is a type of intrafusal muscle fiber that lies in the center of a muscle spindle. Each has a large number of nuclei concentrated in bags and they cause excitation of both the primary and secondary nerve fibers.
There are two kinds of bag fibers based upon contraction speed and motor innervation.
Both bag types extend beyond the spindle capsule.
These sense dynamic length of the muscle. They are sensitive to length & velocity.
|Nuclear bag fiber|
|Anatomical terms of microanatomy|
A gamma motor neuron (γ motor neuron), also called gamma motoneuron, 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.