Regardless of the type of exercise, the workings of these amateur athletes’ muscles is the same:   Before the muscle can contract, a/an must travel down a motor neuron from the spinal cord to the muscle in question. Near the end of the motor neuron, its axon splits into numerous branches, each of which forms a with a single muscle fiber. All of these branches together activate a group of muscle fibers that always contract at the same time; this group is called a .   Each axon terminal comes together with a muscle fiber, forming a connection called a . When the nerve signal reaches the axon terminal, a neurotransmitter called is released into a tiny space called the ___, where it binds to receptors on the . This causes an to spread along the sarcolemma and down into each ___, where it triggers the ___ to release the ion ___ into the sarcoplasm.   This ion binds to ___, causing it to move the “chaperone” ___ out of the way. Now, the ___ cycle can begin:   first, the "head" of the thick filament protein binds to the main thin filament protein, .   second, the heads of the thick filament protein rotate toward the M line, causing a small muscle contraction. This step is called the ___.   third, a molecule of ___ binds to the thick filament protein, breaking the "cross bridge" between the two filaments.   fourth, energy from the breakdown of ___ is used to "rewind" and "spring load" the thick filament protein so the cycle can begin again.   ·        acetylcholine ·        actin ·        action potential ·        ATP ·        calcium ·        cross-bridge ·        motor endplate ·        motor unit ·        myosin ·        neuromuscular junction ·        power stroke ·        sarcoplasmic reticulum ·        synaptic cleft ·        transverse tubule ·        tropomyosin ·        troponin​