Muscle spindle

Muscle spindle is an organ that can be found inside the skeletal muscles. It involves the central muscular fibers of the muscle, two or more nerve afferents, and a fibrous capsule that surrounds it and separates it from the extrafusal skeletal muscle fibers. It is innervated by gamma motor neurons, which function is not muscle contraction, but to provide stretching-sensitivity to the muscle.

When extrafusal muscle fibers (i.e.: all muscle fibers from the muscle that are not the muscle spindle) contract, or stretch, the muscle spindle does so, which modifies the firing rates of the sensory endings of their nerve afferents, already mentioned.

The innervation of the muscle spindle consists of static motor axons (beta and gamma fibers), dynamic motor axons (beta and gamma fibers), and afferent axons, which divide in a primary ending (Ia fibers) and a secondary ending (II fibers). The capsule covers only the central part of the spindle, which is the non-contractile part innervated by motor and sensory axons. This capsule is sometimes called "bag".

Intrafusal fibers are contractile at each of their ends, but not in the middle, and it is precisely in this region where the sensory endings spiral around them. During intrafusal contractions -provoked by the release of acetylcholine from gamma motor neurons-, the contractile regions stiffen, passing the stretch "stress" to the non-contractile region, which enlarges a bit, thus provoking the opening of stretch-sensitive sodium channels, causing an influx of sodium into the muscle, thus raising the resting potential of the spindle' endings, which causes an increase in their stretch-sensitivity. Intrafusal contractions are controlled by a separate group of small spinal motoneurons, the fusimotor neurons.

In the absence of fusimotor neurons' action, spindle afferents Ia and II increase their firing rates when their parent muscle is stretched.

Dynamic fusimotor action (through gamma or beta fibers) stiffens b1 intrafusal fibers, increasing the firing rate of Ia fibers (which are the afferent fibers that wrap arond them), that becomes stronger little by little if the muscle is then progressively stretched. All components of response are scaled up more or less equally. Dynamic fusimotor neurons do not change the output of II neurons, because there are just a few of them around b1 fibers.

Static fusimotor action (gamma or beta fibers), increases Ia bias (what is this???) and reduces Ia gain. It also increases II bias and gain.

Simply: gamma and beta D axons increase Ia gain. Gamma and beta S axons increase Ia bias, and II bias and gain.

Sources:

 http://www.ualberta.ca/~aprochaz/research_interactive_receptor_model.html

 http://en.wikipedia.org/wiki/Muscle_spindle#Sensitivity_modification