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Gamma motor neurons, like all cells, express specific genetic markers at birth. Muscle spindle derived GDNF neurotrophic factors must also be present for postnatal survival. [10] Wnt7A is a secreted signaling molecule selectively in gamma motor neurons by embryonic day 17.5 of mice.
Gamma motor neurons innervate intrafusal muscle fibers found within the muscle spindle. They regulate the sensitivity of the spindle to muscle stretching. With activation of gamma neurons, intrafusal muscle fibers contract so that only a small stretch is required to activate spindle sensory neurons and the stretch reflex.
The motor part of the spindle is provided by motor neurons: up to a dozen gamma motor neurons also known as fusimotor neurons. [4] These activate the muscle fibres within the spindle. Gamma motor neurons supply only muscle fibres within the spindle, whereas beta motor neurons supply muscle fibres both within and outside of the spindle.
A muscle spindle, with γ motor and Ia sensory fibers. A type Ia sensory fiber, or a primary afferent fiber, is a type of afferent nerve fiber. [1] It is the sensory fiber of a stretch receptor called the muscle spindle found in muscles, which constantly monitors the rate at which a muscle stretch changes.
They constitute the muscle spindle, [2] and are innervated by both sensory (afferent) and motor (efferent) fibers. [3] Intrafusal muscle fibers are not to be confused with extrafusal muscle fibers, which contract, generating skeletal movement and are innervated by alpha motor neurons. [4]
A secondary set of neurons also causes the opposing muscle to relax. Gamma motoneurons regulate how sensitive the stretch reflex is by tightening or relaxing the fibers within the spindle. There are several theories as to what may trigger gamma motoneurons to increase the reflex's sensitivity.
Gamma motor neurons, unlike alpha motor neurons, are not directly involved in muscle contraction. The nerves associated with these neurons do not send signals that directly adjust the shortening or lengthening of muscle fibers. However, these nerves are important in keeping muscle spindles taut. [8]
Alpha motor neurons and gamma motor neurons do not merely differ in their postsynaptic targets. The physiological differences between these two classes are significant. The axonal diameter of gamma-motor neurons is half of that of alpha-motor neurons, resulting in a higher cytoplasmic resistance and therefore a slower signal propagation velocity.