Further morphological and electrophysiological studies on snake muscle spindles

1. Electron microscope studies using ruthenium red, horseradish peroxidase, and colloidal lanthanum disclosed a feature characteristic of the snake spindle capsule: it consists of a single layer of flattened cells connected in series, each of which forms a cylinder enclosing the intrafusal fiber. 2. In intact spindles none of the markers penetrated the capsule. The diffusion barrier appeared to be located along the intercellular space near the inner surface of the capsule layer. Through the openings at the ends of the capsule the marker gained access to the periaxial space, but only for a limited distance. 3. Through a punctured hole in the capsule, ruthenium red had access to the periaxial space, disclosing a dense network consisting of fine filaments and granular structures. Since this space was also shown to contain alcian blue-positive substrate, the network may represent acid mucopolysaccharide (probably hyaluronic acid + protein molecules), which may account for the observed limited longitudinal diffusion of this dye in the periaxial space. The intercellular space between the intrafusal fiber, the sensory ending, and the satellite cell was filled with ruthenium red, and no special junction was found between these cells. 4. Using glass microelectrodes, potential profiles across the sensory region of muscle spindles of the snake were examined. 5. When the electrode was advanced through the capsular region, two steps of negative potential were usually observed. The first negative step of 5-10 mV was confirmed by fluorescent dye injection to be due to penetration of the capsule. The second larger step of 40-60 mV negative to the bathing solution was demonstrated by the dye injection to be due to penetration of the intrafusal fiber. Occasionally, a negative potential of 20-30 mV relative to the bathing solution was also recorded. This potential, characterized by small spikelike potentials occurring in synchrony with nerve impulses, was considered to be due to penetration of the sensory ending. 6. both the capsule and the intrafusal fiber showed rectification to injected currents. From input resistance, input capacitance and surface area specific resistance and capacitance were estimated with some assumptions to be 4.1-4.8 k omega . cm2 and 5.1-14.3 micro F/cm2 for the capsule, and 1.1-2.8 k omega . cm2 and 11-23 micro F/cm2 for the intrafusal fiber. The intrafusal fiber of short-capsule spindles lacked an action potential, whereas that of long-capsule spindles responded with an all-or-none action potential to depolarizing current. 7. In about half of the short-capsule spindles examined (8/15), current injected through an intracellular electrode in the intrafusal fiber neither initiated impulses in the afferent axon nor affected background nerve activity. The rest of the spindles responded to depolarizing current with initiation of nerve impulses and concomitant maintained contraction of the intrafusal fiber during applied current...