Correlated electrophysiological and ultrastructural studies of a crustacean motor unit

Structural and functional interrelationships between the pre- and postsynaptic elements of a singly motor innervated crab muscle (stretcher of Hyas araneus L.) were examined using electrophysiological and electron microscopic techniques. Excitatory postsynaptic potential (EPSP) amplitude at 1 Hz was found to be inversely related to the extent of facilitation, and directly related both to the amount of transmitter released at 1 Hz and the muscle fiber input resistance (R(in)). The extent of facilitation (F(e)), taken as the ratio of the EPSP amplitude at 10 Hz to that 1 Hz, was inversely related to muscle fiber R(in), tau(m), and sarcomere length. Sarcomere length was directly related to R(in) and tau(m). The excitatory nerve terminals of low F(e) muscle fibers had larger neuromuscular synapses than did those of high F(e) fibers. Inhibitory axo-axonal synapses were more often found in low F(e) muscle fibers. These structural features may account for the greater release of transmitter at low frequencies from the low F(e) nerve terminals as well as provide for a greater amount of presynaptic inhibition of low F(e) muscle fibers. The implications of these findings for the development and physiological performance of the crustacean motor unit are discussed. It is proposed that both nerve and muscle fiber properties may be determined by the developmental pattern of nerve growth.

Fine structure of rat intrafusal muscle fibers. The equatorial region

An ultrastructural study has been undertaken on the equatorial (sensory) region of the rat muscle spindle. Two kinds of intrafusal muscle fibers, a nuclear bag fiber and a nuclear chain fiber, have been identified in this region on the basis of fiber diameter, nuclear disposition, and M-band appearance. The large-diameter nuclear bag fiber contains an aggregation of tightly packed vesicular nuclei, while the small-diameter nuclear chain fiber contains a single row of elongated, well-separated nuclei. Both muscle fibers contain an attenuated peripheral cylinder of myofilaments surrounding a central core of sarcoplasm. Elements of the sarcotubular system, dilatations of the sarcoplasmic reticulum, and the presence of other sarcoplasmic organelles and inclusions are considerably more abundant in the nuclear chain fiber than in the nuclear bag fiber. Leptomeric organelles and membrane-bounded sarcoplasmic granules are present in both intrafusal fiber types and may be situated between the myofibrils or in intimate association with the sarcolemma. The functional significance of some of these structural findings is discussed.