Isolation of viable human muscle spindles for electron microscopic and physiologic study

Staining of human thyroarytenoid muscle with myosin antibodies reveals some unique extrafusal fibers, but no muscle spindles

This study describes the myosin composition of extrafusal and intrafusal muscle fibers found in the human thyroarytenoid (TA) and sternohyoid (control) muscles. We sought to determine the presence of muscle spindles in the TA muscle, and to identify unusual extrafusal fiber types, using the commonly accepted approach of tissue staining with myosin isoform specific antibodies. Extrafusal fibers are organized into motor units, which subsequently produce muscle movement, whereas intrafusal fibers compose muscle spindles, the primary stretch receptor that provides afferent (feed back) information to the nervous system for regulation of motor unit length and tonicity. Immunohistochemical identification of muscle spindles was confirmed in sternohyoid, but not in TA samples; however, some extrafusal fibers contained tonic myosin. These results indicate that human TA muscle functions similar to some mammalian extraocular muscle, performing unloaded (non-weight bearing) contractions without afferent information from native muscle spindles.

Scanning electron microscopy of teased intrafusal muscle fibers from rat muscle spindles

Scanning electron microscopic (SEM) studies on intrafusal muscle fibers and their various sensory and motor endings are not available thus far although SEM greatly increases the resolution power for analyzing surface structures in comparison to light microscopic preparations. From a total of 68 isolated muscle spindles in lumbrical muscles of Wistar rats, only one primary sensory ending could be isolated. Secondary sensory endings were of an annulospiral, a flat, cuff-shaped, or a more complicated branched type; they were always completely surrounding intrafusal muscle fibers. Motor terminals were more focally arranged not surrounding the muscle fibers. Three types could be distinguished. Collagen fibrils were seen to insert on capsular elements as well as on the surface of intrafusal muscle fibers. Thus SEM helps understanding the most complicated electromechanical system for motor control.

[Fine structure of intrafusal and extrafusal nerve segments of the rat muscle spindle after sciatic nerve transsection]

Intrafusal and extrafusal nerve segments in muscle spindles from lumbricalis muscles of the hindpaw of the rat were studied by electron microscopy from 10 h to 5 days after severance of the sciatic nerve. In several spindles examined, nerve fascicles piercing the spindle capsule in the equatorial region contained a large myelinated, a smaller myelinated, and an unmyelinated fiber. Unmyelinated fibers were not present in small fascicles leading to the polar region. The changes in the extrafusal nerve segments followed the pattern of Wallerian degeneration. Intra-axonal glycogen deposits were prominent in sensory fibers. The unmyelinated fibers were the earliest to degenerate, the large myelinated ones the latest. Differences between motor and sensory fiber degeneration emerged in their preterminal intrafusal segments and were analogous to those of the nerve endings. Terminal nerve fibers in the spindle equator succumbed to attack of mesenchymal cells, leaving extensive basement membrane reduplications around myelin debris-laden Schwann cells, while polar fibers were engulfed by Schwann cell processes, leaving regular bands of Büngner.

Human muscle spindles: fine structure of the primary sensory ending

The primary sensory innervation of muscle spindles obtained by muscle biopsy of normal human volunteers was studied with the light and electron microscopes. The parent IA sensory fibre branched 4-6 times, became unmyelinated for 25-30 mum, then formed sensory terminals on each nuclear bag and chain intrafusal muscle fibre. The first 4--5 mum of the unmyelinated segment is believed to be an encoder zone because the plasmalemma was undercoated by a dense granular layer similar to that under other membranes where action potentials originate. A reconstruction from micrographs of serial longitudinal sections showed that the primary sensory ending on a nuclear bag fibre is an irregular coil with branches and varicose swellings. The terminals contain central aggregates of microfilaments often surrounded by mitochondria, small numbers of vesicles, cisterns and tubular profiles. The latter merge with the plasma membrane. Junctional complexes between the plasma membranes of the terminals and intrafusal muscle resemble fascia adherns and are postulated to act as attachment plaques. These could contribute to the transduction process by incresing the degree of distortion of the terminal's membrane when stretch is applied to the spindle. A mechanism is described which could account for some of the differences in sensitivity of the primary and secondary sensory endings.