<u>Answer</u>:
<em>Part A</em>
<u>6</u> actin filament
<u>3</u> muscle fiber = muscle cell
<u>1</u> muscle
<u>2</u> bundle of muscle fibers
<u>4</u> myofibril
<u>5</u> myosin filament
<em>Part B</em>
<em>1</em> Action potential in motor neuron
<em>11</em> Myosin head releases ADP
<em>6</em> Troponin shifts tropomyosin, exposing myosin binding sites on actin
<em>7</em> Myosin head extends
<em>8</em> Myosin’s ATP is hydrolyzed to form ADP-P
<em>15</em> Troponin loses calcium, so tropomyosin shifts and covers myosin binding site on actin
<em>5</em> Calcium binds troponin
<em>9</em> Myosin head with ADP-P binds to actin
<em>2</em> Action potential in neuromuscular junction
<em>13</em> Muscle excitation ends so calcium release from sarcoplasmic reticulum ends
<em>10</em> Myosin head releases P, causing myosin head to flex, pulling filaments past each other = the power stroke
<em>4</em> Action potential in muscle fiber cell membrane and T-tubules
<em>12</em> Myosin head binds ATP and realeases actin
<em>14</em> Calcium pumps of sarcoplasmic reticulum take up calcium and reduce its concentration in sarcoplasm
<em>3</em> Release of calcium from sarcoplasmic reticulum into sarcoplasm surrounding myofibril
