380 THE BIOLOGY OF MARINE ANIMALS 



thelial cell prolonged basally into muscle fibres. A common type of poly- 

 chaete muscle fibre has a U-shaped trough of myofibrillae enclosing a 

 mass of sarcoplasm. The latter, emerging between the two limbs of the 

 myofibrillae, contains the single nucleus. The large muscle cells of nematodes 



Fig. 9.7. Electron Photograph of a Fragment of Crab Leg Muscle 

 {Paragrapsus quadriden ta tus) 



Formalin fixed, digested briefly with trypsin and shadowed with uranium. The 

 photograph shows myofilaments overlaid by A and Z bands. (From Farrant and 

 Mercer (37).) 



likewise consist of an outer fibrillar portion, enclosing sarcoplasm and 

 nucleus, with long protoplasmic strands extruding to mid-dorsal and 

 mid-ventral lines (52, 55«, 100, 107a). 



Neuromuscular Transmission 



The terminals of motor-nerve fibres form intimate associations with 

 muscle fibres at neuromuscular junctions. These have varied forms: in 

 skeletal muscles of vertebrates the junction is a motor end-plate, a com- 

 plex structure made up of terminal branches of the axon and sarcoplasm; 

 in vertebrate smooth muscles the motor axons branch and ramify among 

 the muscle cells; motor fibres of crustaceans penetrate the sarcolemma and 

 cortical sarcoplasm and terminate on end-plates close to columns of 

 fibrils ; in echinoderms large ribbon axons form extensive synaptic contacts 

 on muscle fibres ; motor axons in anemones terminate on branching end- 

 plates, etc. (54,93, 116). 



On reaching the axon terminals, the nerve action potential excites the 

 muscle fibre. At the vertebrate end-plate the nervous impulse produces a 

 local depolarization which can be detected as an end-plate potential 

 (e.p.p.). This spreads electrotonically for a short distance around the end- 

 plate and, when of sufficient magnitude, initiates an action potential or 

 impulse in the muscle fibre. The e.p.p. is localized and graded in size 



