The neuromuscular spindles are stretching receptors located inside the striated-voluntary muscles; with their activity they are able to capture the state of stretching of the muscles and send the collected information to the spinal cord and brain. The activity of the neuromuscular spindles is therefore very important both to prevent injuries related to excessive stretching, both to maintain normal muscle tone, and to perform fluid movements in a harmonious and controlled manner.
All skeletal muscles, with the exception of one muscle of the jaw, contain within them several neuromuscular spindles, which are particularly concentrated at the level of the muscles of chewing, spine, eyes, limbs and hands. Here, the neuromuscular spindles, about 5-10 mm long, are arranged in parallel to the ordinary muscle fibers and thanks to this particular arrangement "side by side" they are able to capture the degree of elongation.
Anatomy
The neuromuscular spindle consists of a connective tissue capsule that surrounds a small group of muscle fibers (from 4 to 10), equipped with a "special" cytological structure; these fibers are often called intrafusal, to distinguish them from the ordinary ones, which, for a level playing field, are awarded the adjective "extrafusal".
The physiology of intrafusal fibers is explained, first of all, by examining their anatomical structure in detail. At their ends they are quite similar to ordinary fibers and therefore contain contractile striated fibrils. The real difference lies in the equatorial portion, which is enlarged, devoid of myofibrils and rich in sensory endings sensitive to stretching, immersed in a gelatinous substance.
For this reason, it is said that the fibers of the neuromuscular spindles are effector at the two poles (they contract in response to a nervous stimulus) and emitters at the center (from which they send information on the state of elongation).
Anatomically, intrafusal muscle fibers are divided into nuclear sac fibers (also called bag or pouch fibers) and nuclear chain fibers. The former have an enlarged central area, rich in nuclei. Nuclear chain fibers, on the other hand, have an elongated nuclear distribution, always concentrated in the equatorial region, but also extended in the periphery; they are also shorter and thinner than the previous ones.
From the anatomical point of view, the sensory terminations of the neuromuscular spindle are arranged, partly by rolling up to the median region (annulus-spiral or primary terminations) and partly forming a sapling branch in the neighboring regions (flowered or secondary terminations).
The primary terminations are thicker, have a high conduction speed, belong to the class of Ia fibers, and branch off from both the sack and nuclear chain fibers; the secondary terminations, belonging to the class of type II fibers, instead they are thinner, less fast in the propagation of impulses and mainly innervate the nuclei chain fibers.
From the physiological point of view, however, we distinguish fast-conducting sensitive fibers (type Ia) and slower-conducting sensitive fibers (type II). The former, although having terminations on both types of fiber, are annulus-spiral terminations characteristic of the pouch fibers of dynamic nuclei (see below).The slower fibers II, on the other hand, have annulus-spiral terminations that wrap the fibers in a bag of static nuclei and the chain fibers; the flowered terminations also belong to this category.
Unlike extrafusal muscle fibers, which receive input from alpha motor neurons, spindle fibers contract under the action of gamma motor neurons (nerve fibers originating from the anterior horn of the spinal cord characterized by a reduced caliber).
MORE: Physiology of neuromuscular spindles "
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