Morphological and physiological measures of trophic dependence in a crustacean muscle

Degeneration, Trophic Interactions, and Repair of Severed Axons: A Reconsideration of Some Common Assumptions

We suggest that several interrelated properties of severed axons (degeneration, trophic dependencies, initial repair, and eventual repair) differ in important ways from commonly held assumptions about those properties. Specifically, (1) axotomy does not necessarily produce rapid degeneration of distal axonal segments because (2) the trophic maintenance of nerve axons does not necessarily depend entirely on proteins transported from the perikaryon—but instead axonal proteins can be trophically maintained by slowing their degradation and/or by acquiring new proteins via axonal synthesis or transfer from adjacent cells (e.g., glia). (3) The initial repair of severed distal or proximal segments occurs by barriers (seals) formed amid accumulations of vesicles and/or myelin delaminations induced by calcium influx at cut axonal ends—rather than by collapse and fusion of cut axolemmal leaflets. (4) The eventual repair of severed mammalian CNS axons does not necessarily have to occur by neuritic outgrowths, which slowly extend from cut proximal ends to possibly reestablish lost functions weeks to years after axotomy—but instead complete repair can be induced within minutes by polyethylene glycol to rejoin (fuse) the cut ends of surviving proximal and distal stumps. Strategies to repair CNS lesions based on fusion techniques combined with rehabilitative training and induced axonal outgrowth may soon provide therapies that can at least partially restore lost CNS functions.

Crustaceans as a model for microgravity-induced muscle atrophy

Atrophy of skeletal muscles is a serious problem in a microgravity environment. It is hypothesized that the unloading of postural muscles, which no longer must resist gravity force, causes an accelerated breakdown of contractile proteins, resulting in a reduction in muscle mass and strength. A crustacean model using the land crab, Gecarcinus lateralis, to assess the effects of spaceflight on protein metabolism is presented. The model is compared to a developmentally-regulated atrophy in which a premolt reduction in muscle mass allows the withdrawal of the large claws at molt. The biochemical mechanisms underlying protein breakdown involves both Ca(2+)-dependent and multicatalytic proteolytic enzymes. Crustacean claw muscle can be used to determine the interactions between shortening and unloading at the molecular level.

Decrease in transmitter output and synaptic ultrastructure at lobster neuromuscular terminals with decentralization

The effects of decentralization on the physiology and ultrastructure of neuromuscular terminals were examined by transecting the single excitor axon to the distal accessory flexor muscle in the walking legs of lobsters (Homarus americanus). Decentralization caused a reduction in the amplitude of the excitatory junctional potential without altering the resting potential or input resistance of the muscle fiber thereby suggesting a reduction in transmitter release. Confirmation was obtained by recording of synaptic currents at focal sites which showed failure of transmission and a reduced amplitude on decentralized fibers compared to their intact counterparts on the contralateral leg. The mean quantal content of synaptic transmission decreased approximately 2-7-fold at these decentralized sites compared to their intact counterparts. The ultrastructure of these identified sites was examined with serial section electron microscopy. There are few if any qualitative changes in synaptic ultrastructure between decentralized and control terminals. However, quantitatively there were changes in synaptic ultrastructure which were progressive in nature depending on the severity of the reaction to decentralization. Thus terminals showing a moderate decline in quantal content were characterized by a reduction in the number of presynaptic dense bars and synapses. Terminals showing a severe drop in transmitter release showed in addition to the above changes, a reduction in the size of synapses and terminals. These results show a progression in the loss of the structural parameters controlling transmitter release. Finally synaptic vesicles and mitochondria did not reveal any consistent or marked change with decentralization.