. The biology of marine animals. Marine animals; Physiology, Comparative. 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. 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 myofilamen
. The biology of marine animals. Marine animals; Physiology, Comparative. 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. 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 (). 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 is localized and graded in size. Please note that these
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