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| consist mainly of a twisted chain of actin molecules, plus some other proteins. |
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| are bundles of many myosin molecules. |
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| Skeletal muscle (cardiac also) is also called this because of its striped appearance. |
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| These are skeletal muscle cells. They are large and have many nuclei because they are a fusion of many individual cells. Each muscle fiber is packed with bundles of myofibrils, each made up of thin actin units surrounding thick myosin units. |
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| Myofibrils consist of repeating units |
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| Each sarcomere is bounded by these, which anchor the thin actin filaments |
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| The bundles of myosin filaments are held in position within the sarcomere, and centered in a group of actin filaments by this protein. , titin molecules are very elastic, accounting for the resistance to stretch in relaxed skeletal muscle |
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| sliding filament theory of muscle contraction |
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Definition
| When a muscle contracts, the sarcomere shortens, the H zone and the I band become much narrower, and the Z lines move toward the A band as if the actin filaments were sliding into the region occupied by the myosin filaments.Actin and myosin slide past each other as the muscle contracts |
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Each myosin molecule consists of two long polypeptide chains coiled together, each ending in a large globular head.
A myosin filament is made of many such molecules arranged in parallel. Myosin heads also have ATPase activity. |
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| blocks the myosin binding sites on the actin filament |
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| When Ca2+ is released to the sarcoplasm, it binds to troponin causing troponin to change shape which in turn causes tropomyosin to change conformation, thereby exposing the binding sites for myosin heads on the actin. |
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Muscle contractions are initiated by action potentials from motor neurons.
Each motor neuron branches and synapses with up to a hundred muscle fibers. These fibers constitute a |
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| transverse tubules (T tubules) |
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Definition
| They branch through cytoplasm. this is how actions potentials travel deep into muscle cells. Run close to sarcoplasmic reticulum. |
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| Intracellular membranes that surround every myofibril. At rest, there is a high concentration of Ca2+ in the sarcoplasmic reticulum and a low concentration in the sarcoplasm |
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| Mechanisms that Control the Strength of a Muscle Contraction |
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Definition
1. Increase APF in each motor neuron
- Electrical summation of epsps (and some additional factors, such as facilitation).
- Mechanical summation of tension. The mechanical event is much slower than the electrical.
2. Recruitment of motor units |
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| Slow-twitch fibers (red muscle) |
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Definition
have many mitochondria and a lot of the oxygen-binding molecule myoglobin to provide steady, prolonged aerobic ATP production.
Red muscle is also well supplied with blood vessels and fuel reserves (glycogen and fat).
Long-term aerobic work such as distance running and swimming depends on this type of fiber. |
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| Fast-twitch fibers (white muscle) |
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Definition
have fewer mitochondria, more SR and very little myoglobin. They develop maximum tension more rapidly and produce greater tension, but fatigue rapidly (go anaerobic).
The myosin of fast-twitch fibers has a high ATPase activity, but they cannot replenish ATP fast enough to sustain long-time contraction. |
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| induces formation of new actin and myosin filaments, hence, bigger (diameter) muscles. The muscle cells get larger, but don’t change much in number. |
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enhances the oxidative capacity of muscles by increasing the number of mitochondria, enzymes, and the density of capillaries that deliver oxygen.
There is also an increase in myoglobin, which facilitates the diffusion of oxygen throughout the muscle fibers and provides a store of oxygen for use when the blood supply is insufficient. |
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preformed ATP and creatine phosphate used. CP stores energy in a phosphate bond and transfers it to ADP when ATP levels are low.
The total energy available from the immediate system is small. |
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metabolizing carbohydrates to lactate and pyruvate and producing ATP. glycolysis rapidly leads to the accumulation of lactic acid, which slows the process.
The glycolytic and immediate systems can provide energy for less than one minute. |
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| : metabolizing carbohydrates or fats to H20 and C02, producing ATP thru the electron transport system andit requires many reactions and takes place in the mitochondria, so the rate at which oxidative metabolism can make ATP available to the myosin filaments is slower than that of the other two systems. ATP synthase in mitochondria. Depends on blood flow. |
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| are branched and appear striated (banded) because of the regular arrangement of their actin and myosin filaments. |
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| provide strong mechanical adhesions between adjacent cells. |
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provides contraction for internal organs, which are under the control of the autonomic nervous system, moves food through the digestive tract, controls the flow of blood, empties the urinary bladder, produces contractions of the uterus during childbirth, diaphram, etc.
Smooth muscle lacks striations, however, with some differences, the same basic contractile mechanisms are operative.
Smooth muscle cells are the simplest muscle cells structurally; they have a single nucleus and are usually long and spindle-shaped. However, smooth muscle from different sites is quite varied. |
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