Reversal of the static component of spindle potential by imposed depolarizing current in the frog muscle spindle

Intramembrane structure of the sensory axon terminals in bullfrog muscle spindles

Much physiologic and morphologic research has been done into the sensory mechanism of the frog muscle spindle. However, no freeze-fracture study has described in detail the shape and intramembrane structure of the nonmyelinated sensory axon terminals of the frog muscle spindle. In this study, muscle spindles were isolated from the red part of bullfrog semitendinous muscles. Chemically fixed spindles were subjected to freeze fracturing. The sensory axon endings were reconstructed, and the size and density of intramembrane particles (IMPs) were measured along the sensory nerve endings. The axon terminals had four distinctive parts: parent trunks (>0.5 microm in diameter), primary branches (0.15-0.5 microm), terminal branches (<0.1 pm), and varicosities (0.02-0.5 microm). IMPs ranged from 5 nm to 21 nm in diameter and were present in the intramembrane space of the plasma membrane all throughout the nonmyelinated 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 in 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 and largest in the varicosity. Mean IMP densities (numbers of IMPs per microm2) 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 distributed evenly in the PF and the EF. The characteristic intramembrane structure of sensory nerve endings is discussed.

Intramembrane particles and responses of sensory axon terminals during reinnervation of the frog muscle spindle

Changes in the density of intramembrane particles (IMPs) of sensory nerve terminals in the bullfrog muscle spindle were correlated with recovery in the response of the spindle to stretch during postcrush reinnervation. A few IMPs on the protoplasmic (P) face in summer experiments (June to October) reappeared by the 3rd week after the nerve crush, then rapidly increased to 110% and 120% of control values 2 and 2.5 months after the crush. Afferent responses to stretch could be recorded after the mean IMP density on the P-face in terminal branches had recovered to more than 25% of the control value. The discharge rate showed a plateau pattern during the period of the excessive IMPs. This was supplanted by a normal pattern after a myelinated branch of the sensory axon was cut. The IMPs in winter experiments (November to April) reappeared by day 90 after nerve crush, and then slowly increased. The sustained responses to stretch reappeared after 5 months, when the mean IMP density on the P-face was restored to 25% or more of the control. Neither excessive density of the IMPs nor plateau pattern of the discharge rate were observed in winter experiments. The relation between the regenerated IMP densities and the functional recovery is discussed.

Effects of cyclic nucleotides and calcium on transduction and encoding processes in frog muscle spindle

In decapsulated muscle spindles, application of dibutyryl cyclic AMP (d-cAMP, 0.1-10 mM) and forskolin (10-100 microM) increased the rate of spontaneous discharges and decreased the responsiveness to stretch. Addition of 2-5 mM CaCl2 or 0.5-2 mM BaCl2 to the above drugs prevented the deterioration of the responsiveness to stretch and the increase in the rate of spontaneous discharges. Similar changes in the afferent discharges were observed with application of isobutylmethyl-xanthine (IBMX, 0.5-2 mM) or carbonyl cyanide chlorophenylhydrazone (CCCP, 1-10 microM). Application of 0.1-1 mM quercetin resulted in a prolonged increase in the discharge rate, lasting 15-20 s after the release of stretch. These results suggest that cAMP is involved in the regulation of the sensory processes through a modification of intracellular calcium activity.

Intramembrane particles and terminal responses following denervation of frog muscle spindles

The sizes and population density of intramembrane particles in the non-myelinated sensory nerve terminals of frog muscle spindles were studied in comparison with changes in the responses to spindles to stretch 1-28 days after denervation. The sciatic nerve in the right leg was cut at the dorsal part of the hip joint. Spindle receptors in the left leg were used as controls. Muscle spindles were isolated from semitendinosus and iliofibularis muscles. The mean density of particles, which occurred mainly on the protoplasmic (P) face, declined rapidly to 60% of the control values 2 days after denervation, 30% after 4 days and to less than 10% after 7 days. The mean diameter of particles changed from 8.0 +/- 1.5 nm in the control to 12.8 +/- 2.5 nm 7 days after denervation. No significant difference between the results for the trunks (greater than 2 micron in diameter) and the branches (less than 0.5 micron) of the non-myelinated axons was observed. The peak rate of afferent discharges during a ramp-and-hold stretch survived up to 4 days after denervation, whereas the static component disappeared after 3 days. It is suggested that the decline to less than 20% of the control density of particles removes the mechanosensitivity of the sensory endings.