Stretch reflex When a<unk>  is stretched, primary type Ia sensory fibers of the<unk>  respond to both changes in<unk>  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<unk>  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<unk> . The reflexly evoked activity in the alpha motor neurons is then transmitted via their efferent axons to the extrafusal fibers of the<unk> , 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<unk> s, causing them to relax. Sensitivity modification The function of the gamma motor neurons is not to supplement the force of<unk>  contraction provided by the extrafusal fibers, but to modify the sensitivity of the<unk>  sensory afferents to stretch. Upon release of acetylcholine by the active gamma motor neuron, the end portions of the intrafusal<unk>  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<unk>  afferents. 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<unk>  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<unk>  afferents when the extrafusal<unk> s 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<unk> . Dynamic gamma activity turns on and off abruptly, sensitizing<unk>  afferents to the onset of<unk>  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<unk> -bearing<unk> , Ia afferent and stretch reflex sensitivity from this<unk>  is reduced. Gamma fusimotor control therefore allows for the independent preparatory tuning of<unk>  stiffness according to task goals. Development It is also believed that<unk> s play a critical role in sensorimotor development.
Muscle spindle
Efferent nerve fiber
Postganglionic nerve fibers
Nerve plexus
Nerve fascicle
Muscle tissue
Cerebrospinal fibers
Sensory nerve