INTRAMEMBRANE STRUCTURE OF THE SENSORY AXON TERMINALS IN BULLFROG MUSCLE-SPINDLES

Much physiologic and morphologic research has been done into the senso ry mechanism of the frog muscle spindle. However, no freeze-fracture s tudy has described in detail the shape and intramembrane structure of the nonmyelinated sensory axon terminals of the frog muscle spindle. I n this study, muscle spindles were isolated from the red part of bullf rog semitendinous muscles. Chemically fixed spindles were subjected to freeze fracturing. The sensory axon endings were reconstructed, and t he size and density of intramembrane particles (IMPs) were measured al ong the sensory nerve endings. The axon terminals had four distinctive parts: parent trunks (>0.5 Fun in diameter), primary branches (0.15-0 .5 mu m), terminal branches (<0.1 mu m), and varicosities (0.02-0.5 mu m) IMPs ranged from 5 nm to 21 nm in diameter and were present in the intramembrane space of the plasma membrane all throughout the nonmyel inated sensory nerve endings. Mean IMP sizes in the protoplasmic face (PF) and the external face (EF), respectively were 8.1 nm and 8.4 nm i n the parent trunks, 8.8 nm and 8.8 nm in the primary branches, 9.4 nm and 9.0 nm in the varicosities, and 8.7 nm and 8.7 nm in the terminal branches. Mean IMP size in the PF was smallest in the parent trunk an d largest in the varicosity. Mean IMP densities (numbers of IMPs per m u m(2)) in the PF and the EF, respectively, were 2,500 and 700 in the parent trunks, 2,200 and 500 in the primary branches, 1,700 and 400 in the varicosities, and 1,000 and 300 in the terminal branches. Density decreased with the tapering of the axon terminal, with IMPs distribut ed evenly in the PF and the EF. The characteristic intramembrane struc ture of sensory nerve endings is discussed. Anat. Rec. 252:340-354, 19 98. (C) 1998 Wiley-Liss, Inc.