Axotomy induces fusimotor-free muscle spindles in neonatal rats.
Crushing the nerve to the medial gastrocnemius (MG) muscle at birth and administering nerve growth factor to rats afterwards results in a reinnervated muscle with supernumerary muscle spindles, some of which must have formed de novo. Structure and innervation of spindles in the reinnervated MG muscles were studied in serial 1 micron transverse sections. Two types of spindle-like encapsulations were observed. The prevalent type consisted of one to three small diameter intrafusal fibers with features of nuclear chain fibers or infrequently a nuclear bag fiber. The second type of encapsulation consisted of the small-diameter fibers located in a compartment which abutted a compartment containing a large diameter extrafusal fiber. All intrafusal fibers in spindles of the experimental muscles were innervated by afferents, but most of them (85%) were devoid of efferent innervation. Thus, immature fusimotor neurons may be more susceptible than spindle afferents to cell death after axotomy at birth.
Kucera
J
J
Department of Neurology, School of Medicine, Boston University, MA 02118.
Walro
J
J
D000818:Animals; D000831:Animals, Newborn / Q000502:physiology*; D001369:Axons / Q000502:physiology*; D016923:Cell Death / Q000502:physiology; D009046:Motor Neurons / Q000502:physiology; D009132:Muscles / Q000294:innervation* / Q000502:physiology; D009475:Neurons, Afferent / Q000502:physiology; D051381:Rats; D011919:Rats, Inbred Strains