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PART 2 Support and Movement

contraction. The point of contact of motor neuron axon brancheswith the muscle fiber is called the neuromuscular junction, or synapse (figure 9.6).The neuromuscular junction consists of a group of enlargedaxon terminals that rests in an invagination of the sarcolemma.Therefore, a neuromuscular junction consists of the axon termi-nals and the area of the muscle fiber sarcolemma they innervate.Each axon terminal is the presynaptic (pr ē ′ si-nap ′ tik) terminal.The space between the presynaptic terminal and the muscle fiberis the synaptic (si-nap ′ tik) cleft, and the muscle plasma membranein the area of the junction is the postsynaptic (p ō st-si-nap ′ tik)membrane, or motor end-plate (figure 9.6). Each presynaptic ter-minal contains numerous mitochondria and many small, sphericalsacs approximately 45 μm in diameter, called synaptic vesicles.The vesicles contain acetylcholine (as-e-til-k ō ′ 1 ē n; ACh ), anorganic molecule composed of acetic acid and choline. Acetylcho-line is a neurotransmitter (noor ′ ō -transmit ′ er), a substancereleased from a presynaptic membrane that diffuses across thesynaptic cleft and alters the activity of the postsynaptic cell. Neu-rotransmitters can stimulate (or inhibit) the production of an

action potential in the postsynaptic membrane (the  sarcolemma)by binding to ligand-gated ion channels.

Sliding Filament Model

The primary function of skeletal muscle cells is to generate forceby contracting, or shortening. The parallel arrangement of myo-filaments in a sarcomere allows them to interact, which causesmuscle contraction. This interaction is described by the slidingfilament model (figure 9.7).When a muscle contracts, the actin and myosin myofila-ments in the sarcomere slide past one another but remain thesame length as when the muscle is at rest. When the myo-filaments slide past each other and the sarcomeres shorten,the myofibrils also shorten because the myofibrils consist ofsarcomeres joined end to end. The myofibrils extend the lengthof the muscle fibers, and when they shorten the muscle fibersshorten. Groups of muscle fibers make up a muscle fasciculus,and several muscle fascicles make up a whole muscle. Therefore,when sarcomeres shorten, myofibrils, muscle fibers, musclefascicles, and muscles shorten to produce muscle contraction.

(sarcolemma)

Axonbranch

Neuromuscularjunction

Skeletalmusclefiber

(c)

FIGURE 9.6 Neuromuscular Junction

( a ) Several branches of an axon form the neuromuscular junction with a single muscle fiber. ( b ) The presynaptic terminal containing synaptic vesicles is sepa-rated from the postsynaptic membrane (sarcolemma) by the synaptic cleft. This region of the postsynaptic membrane is specialized and is shown in blue .( c ) Photomicrograph of neuromuscular junctions.