| A | B |
|---|
| Skeletal muscle | Cigar shaped cells that are multinucleated, and the largest of the muscle types |
| Smooth muscle | No striations, involuntary, found in the walls of visceral organs |
| Cardiac muscle | Found only in the heart, striated and voluntary |
| Sarcolemma | Muscle cell membrane |
| Myofibrils | Long ribbon-like organelles, which nearly fill up the cytoplasm |
| Sarcomere | Tiny contractile units aligned end to end |
| Myofilaments | Arrangement of smaller structures within the sarcomere |
| Thick filaments | The type of filament that extends the entire length of the A band |
| Myosin | Thick filaments are made mostly of bundled molecules of the protein myosin |
| Cross bridges | Also called myosin heads, they link the thick and thin filaments during contraction |
| Thin filaments | Composed of actin and some regulatory proteins that are anchored to the Z line |
| Sarcoplasmic reticulum | Specialized smooth endoplasmic reticulum |
| Irritability | The ability to receive and respond to a stimulus |
| Contractility | The ability to shorten when an adequate stimulus is received |
| Motor unit | Composed of one neuron and all the skeletal muscle cells it stimulates |
| Neuromuscular junctions | A long threadlike extension of the neuron called an axon, reaches the muscle, it branches into a number of axonal terminals, each of which forms junctions with the sarcolemma of a different muscle cell |
| Synaptic cleft | The gap between the nerve endings and the muscle cell membranes |
| Neurotransmitter | When the nerve impulse reaches the axonal terminals |
| Acetylcholine | The specific neurotransmitter that stimulates skeletal muscle cells |
| Action potential | Electrical current |
| Muscle twitch | Single, brief jerky contractions |
| Tetanus | Muscle is stimulated so rapidly that no evidence of relaxation is seen |
| Muscle fatigue | A muscle is unable to contract even though it is still being stimulated |
| Oxygen debt | Not able to take in oxygen fast enought to keep the muscles supplied with all the oxygen they need when they are working |
| Isotonic contraction | Myofilaments are successful in their sliding movements, the muscle shortens and movement occurs |
| Isometric contraction | Muscles do not shorten, the myofilaments are trying to slide |
