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| 61. | sarcolemma | plasma membrane of a skeletal muscle fiber |
| 62. | sarcomere | longitudinally, repeating functional unit of skeletal muscle, with all of the contractile and associated proteins involved in contraction |
| 63. | sarcopenia | age-related muscle atrophy |
| 64. | sarcoplasmic reticulum (SR) | specialized smooth endoplasmic reticulum, which stores, releases, and retrieves Ca++ |
| 65. | sarcoplasm | cytoplasm of a muscle cell |
| 66. | satellite cell | stem cell that helps to repair muscle cells |
| 67. | skeletal muscle | striated, multinucleated muscle that requires signaling from the nervous system to trigger contraction; most skeletal muscles are referred to as voluntary muscles that move bones and produce movement |
| 68. | slow oxidative (SO) | muscle fiber that primarily uses aerobic respiration |
| 69. | smooth muscle | nonstriated, mononucleated muscle in the skin that is associated with hair follicles; assists in moving materials in the walls of internal organs, blood vessels, and internal passageways |
| 70. | somites | blocks of paraxial mesoderm cells |
| 71. | stress-relaxation response | relaxation of smooth muscle tissue after being stretched |
| 72. | synaptic cleft | space between a nerve (axon) terminal and a motor end-plate |
| 73. | T-tubule | projection of the sarcolemma into the interior of the cell |
| 74. | tetanus | a continuous fused contraction |
| 75. | thick filament | the thick myosin strands and their multiple heads projecting from the center of the sarcomere toward, but not all to way to, the Z-discs |
| 76. | thin filament | thin strands of actin and its troponin-tropomyosin complex projecting from the Z-discs toward the center of the sarcomere |
| 77. | treppe | stepwise increase in contraction tension |
| 78. | triad | the grouping of one T-tubule and two terminal cisternae |
| 79. | tropomyosin | regulatory protein that covers myosin-binding sites to prevent actin from binding to myosin |
| 80. | troponin | regulatory protein that binds to actin, tropomyosin, and calcium |
| 81. | twitch | single contraction produced by one action potential |
| 82. | varicosity | enlargement of neurons that release neurotransmitters into synaptic clefts |
| 83. | visceral muscle | smooth muscle found in the walls of visceral organs |
| 84. | voltage-gated sodium channels | membrane proteins that open sodium channels in response to a sufficient voltage change, and initiate and transmit the action potential as Na+ enters through the channel |
| 85. | wave summation | addition of successive neural stimuli to produce greater contraction |
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