The complexity of the SROs, which brought Korr
4 to infer their role in somatic dysfunction, is quite evident on investigation (
Figure). The SROs, which encompass the intrafusal fibers and their neural attachments, are 4 to 10 mm in length and 80 to 250 µm in diameter, with the wide range in sizes due to the varying number of fibers in each spindle.
3,29,30 The entire SRO is enveloped in an external capsule, similar to a perimysium that attaches into the perimysium of extrafusal fibers.
31 While there are numerous intrafusal fibers in each SRO, much of the volume is due to the specialized intracapsular fluid. This fluid has been compared to lymph, but in addition it includes a high concentration of hyaluronan, the polysaccharide found in synovial fluid.
32 While hyaluronan may aid cellular homeostasis and the exchange of nutrients and wastes,
33 it also has the capacity to greatly enhance the fluid viscosity and help generate a low friction environment in which intrafusal fibers can sense and respond to changes in muscle length. The fluid also allows clear separation among the intrafusal fibers and reduces the problem of the intrafusal fibers being compressed by the contraction of extrafusal fibers squeezing the SRO.
The SROs lie parallel among extrafusal muscle fibers in the central areas of whole muscles.
3 Due to this parallel arrangement and the low-friction intrafusal environment, SROs are exquisitely sensitive to changes in muscle length. However, while all muscles contain SROs, these receptors are not equally distributed in muscles, with the highest SRO density found in muscles of fine motion with over 100 SROs per gram of muscle (eg, extraocular and lumbricals) and those helping maintain posture (eg, soleus). The lowest density of SROs is found in muscles initiating gross movement or responsible for great force generation (eg, gastrocnemius).
34,35 Embryonically, intrafusal fibers have the same mesodermal origins as extrafusal fibers, being multinucleated and arising from fusion of myotubes.
31 Being skeletal muscle fibers, the intrafusal fibers share characteristics with extrafusal fibers. Each fiber is covered by an internal capsule similar to the endomysium of extrafusal fibers but are generally thinner, averaging 10 µm in diameter compared with the typical extrafusal fiber diameter of 60 to 80 µm and are much shorter at 4 to 7 mm.
36,37
The fibers within the SRO are of 2 general anatomic types as illustrated in the
Figure and first described by Boyd
36: slender, cylindrical cells referred to as
nuclear chain fibers, which have their nuclei arranged in a straight “chain” in their equatorial region; and
nuclear bag fibers, out pouched in their equatorial regions, thus appearing as a bags with their numerous nuclei in a dense equatorial grouping. Being sensory in these equatorial regions, intrafusal fibers also have contractile capacity in their polar regions, allowing them to shorten in tandem with extrafusal fibers so that they do not go slack when extrafusal fibers are shortened. The fiber type of the polar regions of intrafusal fibers is much more diverse than the fiber types (slow-, intermediate-, and fast-twitch) of extrafusal fibers,
30,37 and their sarcoplasmic reticular calcium-ATPase isoforms are also diverse.
38
While there is some variability, afferent innervation of each SRO generally includes a single “primary” group Ia neuron that coils around and innervates the equatorial regions of all intrafusal fibers, forming the annulospiral endings and multiple smaller secondary group II neurons innervating the nuclear chain fibers in “flower-spray” receptor endings.
39,40 Both afferents have a baseline signal frequency that increases as they are stretched. Such increased firing is due to the increase in the dual sodium-calcium activation current of the mechanosensitive terminals.
41 However, this afferent signaling is complex and has both tonic and dynamic components.
3 The nuclear chain fibers are classified as tonic sensors, with their afferent signal frequency related to their absolute length. On the other hand, nuclear bag fibers, thought for many years to function solely as dynamic sensors, are now recognized to be of the following functional types: bag1 fibers, which signal dynamic changes in length which is then signaled to the CNS by the primary group Ia afferents, and bag2 fibers which, like nuclear chain fibers, signal absolute length.
42-46 Likewise, as the muscle shortens, the firing rate of both afferents decrease as the fibers return to their original length.
36 However, the more that is known, the more complexities are revealed, and it is recognized that some SROs lack bag1 fibers and are still capable of generating a dynamic response.
47 Although this area requires further investigation for clarification, it implies that the bag1 fibers are not essential to the dynamic response of primary endings and adds complication to the understanding of the signaling process.
The afferent response of SROs to muscle stretch is well recognized as playing an essential feedback role in regulating the state of extrafusal fibers. At the most basic level, stretching of intrafusal fibers initiates the monosynaptic myotatic reflex, generating the afferent component of the reflex, which will then cause coactivation of the efferent alpha- and gamma-motoneurons to activate the extrafusal and intrafusal fibers of the same, ie, homonomous, muscle. This homeostatic reflex helps extrafusal muscle fibers resist rapid, strong, and damaging stretch, but because of the complexity of the reflex, there are points within the reflex at which malfunctions could happen. The alpha-gamma coactivation ensures that intrafusal fibers shorten in tandem with the extrafusal fibers of the homonomous muscle, shortening so that the SROs do not go slack and are thus able to retain their sensitivities and respond to consequent stretches.
45 The tonic intrafusal fibers and dynamic bag1 fibers are specifically activated by gamma-static and gamma-dynamic motoneurons, respectively.
48