Stretch reflex When a muscle is stretched, primary type Ia sensory fibers of the muscle spindle respond to both changes in muscle length and velocity and transmit this activity to the
spinal cord in the form of changes in the rate of
action potentials. Likewise, secondary type II sensory fibers respond to muscle length changes (but with a smaller velocity-sensitive component) and transmit this signal to the spinal cord. The Ia afferent signals are transmitted
monosynaptically to many
alpha motor neurons of the receptor-bearing muscle. The reflexly evoked activity in the alpha motor neurons is then transmitted via their efferent axons to the extrafusal fibers of the muscle, which generate force and thereby resist the stretch. The Ia afferent signal is also transmitted polysynaptically through
interneurons (Ia inhibitory interneurons), which inhibit alpha motorneurons of antagonist muscles, causing them to relax.
Sensitivity modification The function of the gamma motor neurons is not to supplement the force of muscle contraction provided by the extrafusal fibers, but to modify the sensitivity of the muscle spindle sensory afferents to stretch. Upon release of
acetylcholine by the active gamma motor neuron, the end portions of the intrafusal muscle fibers contract, thus elongating the non-contractile central portions (see "fusimotor action" schematic below). This opens stretch-sensitive
ion channels of the sensory endings, leading to an influx of
sodium ions. This raises the
resting potential of the endings, thereby increasing the probability of
action potential firing, thus increasing the stretch-sensitivity of the muscle spindle afferents. Recent transcriptomic and proteomic studies have identified unique gene expression profiles specific to muscle spindle regions. Distinct macrophage populations, known as muscle spindle macrophages (MSMPs), have been observed, suggesting an immunological component in muscle spindle maintenance and function. Immunostaining and sequencing have enabled tissue-level identification of novel markers, contributing to an advanced cellular atlas of the muscle spindle. Regarding the structural-functional correlation; muscle spindle density is not uniform across the musculoskeletal system. Recent biomechanical modeling suggests that spindle abundance correlates with muscle fascicle length and fiber velocity during dynamic movement, emphasizing the relationship between muscle structure and proprioceptive requirements. How does the central nervous system control gamma fusimotor neurons? It has been difficult to record from gamma motor neurons during normal movement because they have very small axons. Several theories have been proposed, based on recordings from spindle afferents. • 1)
Alpha-gamma coactivation. Here it is posited that gamma motor neurons are activated in parallel with alpha motor neurons to maintain the firing of spindle afferents when the extrafusal muscles shorten. • 2)
Fusimotor set: Gamma motor neurons are activated according to the novelty or difficulty of a task. Whereas static gamma motor neurons are continuously active during routine movements such as locomotion, dynamic gamma motorneurons tend to be activated more during difficult tasks, increasing Ia stretch-sensitivity. • 3)
Fusimotor template of intended movement. Static gamma activity is a "temporal template" of the expected shortening and lengthening of the receptor-bearing muscle. Dynamic gamma activity turns on and off abruptly, sensitizing spindle afferents to the onset of muscle lengthening and departures from the intended movement trajectory. • 4)
Goal-directed preparatory control. Dynamic gamma activity is adjusted proactively during movement preparation in order to facilitate execution of the planned action. For example, if the intended movement direction is associated with stretch of the spindle-bearing muscle, Ia afferent and stretch reflex sensitivity from this muscle is reduced. Gamma fusimotor control therefore allows for the independent preparatory tuning of muscle stiffness according to task goals.
Development Genetic pathways critical for spindle formation include neuregulin-1 signaling via ErbB receptors, which induce intrafusal fiber differentiation upon sensory innervation. Disruption of these pathways impairs proprioception, as seen in gene knockout models. It is also believed that muscle spindles play a critical role in
sensorimotor development. Additionally, gain-of-function mutations in HRAS (e.g.: G12S) observed in Costello syndrome are associated with increased spindle number, providing insight into genetic regulation of spindle density. ==Clinical significance==