Intrafusal fibers

Gamma motor neurons innervate intrafusal fibers of the muscle spindle, which monitors muscle length. 

It has been suggested that botulinum toxin might have other effects on neuronal excitability. This toxin, for example, might also inhibit contraction of intrafusal muscle fibers that are located within skeletal muscle, and help control sensitivity of the stretch reflex.33 Inhibiting these intrafusal fibers would diminish activity in the afferent limb of the stretch reflex, thereby contributing to the antispasticity effects of this intervention.33... 

Skeletal muscles are organized into extrafusal and intrafusal fibers. Extrafusal fibers are the strong, outer layers of muscle. This type of muscle fiber is the most common. Intrafusal fibers, which make up the central region of the muscle, are weaker than extrafusal fibers. Skeletal muscles fibers are additionally characterized as fast or slow based on their activity patterns. Fast, also called white, muscle fibers contract rapidly, have poor blood supply, operate anaerobically, and fatigue rapidly. Slow, also called red, muscle fibers contract more slowly, have better blood supplies, operate aerobically, and do not fatigue as easily. Slow muscle fibers are used in movements that are sustained, such as maintaining posture. 

The extrafusal fibers EF of the muscle M and the intrafusal fibers IF of the muscle spindles MS contract simultaneously when commanded by the central nervous system G. 

The sensory afferents of the muscle spindle comprise group la fibers (primary endings), which send branches to every intrafusal fiber in the muscle spindle, and group n fibers (secondary endings), which innervate the nuclear chain endings only. The larger primary endings (la) surround the center of the muscle spindle much like a coil, whereas the smaller secondary end-... 

Servo-assist function of the muscle spindle reflex. If the extrafusal fibers contract less than the intrafusal fibers, then the muscle spindles will maintain a stretch reflex to further excite the extrafusal fibers. This technique is used in active resistive myofascial therapy. 

Diagram of the structures involved in the stretch reflex arc. I is an inhibitory interneuron E indicates an excitatory presynaptic terminal la is a primary intrafusal afferent fiber Ca2+ denotes activator calcium stored in the sarcoplasmic reticulum of skeletal muscle RyR channels indicates the Ca2+ release channels. 

Muscle spindles are intrafusal mechanoreceptors that are widely distributed within skeletal muscle fibers in the belly of the muscle. They exist in parallel with the much larger extrafusal skeletal muscle fibers, and the cotmective tissue around the muscle spindles is continuous with the connective tissue around the other muscle fibers. The muscle spindles mediate a response to a load placed on the muscle this is known as the load reflex. Muscle spindles have a dampening function, as well. They prevent some types of oscillation and jerkiness in body movement. In fact, tremors noted, especially during times of extreme anxiety or stimulation, represent a failure to dampen this response smoothly. In addition to serving at a subconscious level, the muscle spindle reflex is invoked in voluntary motor activity. 

Muscle spindles are composed of two types of intrafusal muscle fibers nuclear bag fibers and nuclear chain fibers. The nuclear bag fibers have nuclei that appear to be bunched in the center of the cell, and the nuclear chain fibers have then-nuclei aligned single-file in the center as in a series. There are two to five nuclear bag fibers and six to ten nuclear chain fibers in a typical muscle spindle. The central portions of either type of fiber have very poor contractile abihty, whereas the ends that are attached have greater contractility. The components of an intrafusal muscle spindle are diagrarmned in Figure 6-3. 

FIG. 6-4 (A) Neutral position. (B) Stretch of extrafusal and intrafusal alpha motor neuron stimulated to reflexively initiate extrafusal muscle contraction. (C) Gamma motor neuron innervation of muscle spindle. Contraction of ends stretches spindle, activating lb fibers, lb sensory fiber activates alpha motor neuron to cause extrafusal muscle contraction. (D) Compression of whole muscle or contraction of extrafusal muscle results in shortening of muscle spindle and deactivation of sensory fiber firing. 

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