Identified muscle fibers in a crab

Summary

• 1. A set of 10 superficial fibers is described for the closer muscle of the crabEriphia spinifrons. The fibers are found in identical positions from one preparation to the next.

• 2. According to innervation pattern and neuromuscular responses, the identified fibers can be classified in 4 major groups (I to IV). They are representative for the remaining fibers comprising the closer muscle.

• 3. All muscle fibers of the closer are innervated by the fast excitor. Only the fibers also innervated by the slow excitor are further innervated by the common inhibitor (CI). CI innervation seems to be linked to the distribution of the slow axon.

• 4. CI is most effective in fibers 2, 3 and 4 (group I fibers) through post- and mainly presynaptic mechanisms. Presynaptic inhibition drastically reduces excitatory transmission at the terminals of both the slow and the fast axon. Fiber 5 (group II fibers) receive(s) only postsynaptic inhibition.

• 5. The strength of presynaptic inhibition varies at different junctions along a given muscle fiber from about 20 to almost 100%.

• 6. Excitatory and inhibitory nerve terminal potentials were recorded allowing determination of the time relationship for optimal presynaptic inhibition. Inhibition reaches its optimum when the action potential in the inhibitory ending precedes that in the excitatory terminal by 5 to 8 ms. Presynaptic inhibition shows a long persistance. Half maximal inhibition is present when action potentials in the inhibitory endings occur 18 to 30 ms before those in the excitatory endings.

• 7. CI plays an important role in the division of labor among muscle fibers receiving identical excitatory input. Fibers of group I (tonic fibers) are selectively inhibited and prevented from participation in tension generation during walking of the crab. During locomotion mainly group II fibers are active. A very large proportion of muscle fibers (group III and IV) appears not to be activated during slow walking.