1. Vertebrate skeletal muscle is comprised of long, parallel cells, called fibers, which run from tendon (attached to bone) to tendon (Eckert, Fig. 10-1). Within these cells, bundles of thick and thin fibers, the contractile protein machinery of muscle, are wrapped in sarcoplasmic reticulum. These bundles are called myofibrils.

2. Lengthwise, each myofibril consists of a repeating structure called the sarcomere (Eckert, Fig. 10-2) (Eckert, Fig. 10-3). Sarcomeres contain the actin thin filaments, attached to the Z-lines or Z disks, as they may be more accurately called. (Z for zig-zag, although the surface of the Z-disk is more like a waffle pattern.) The interior waffle-like structure of the Z-disks contains a lot of the protein alpha-actinin, which is also used to connect microfilaments to proteins on the plasma membrane, whereas the periphery of the Z-disks has desmin attached. Desmin is one of the molecules used to "glue" various types of cells together to keep tissues intact. (So far you should know about sarcomeres, Z-disk, actin, thin filaments, alpha-actinin, and desmin.)

3. Microfilaments are filaments consisting of a double strand of globular Actin units.

• In eukaryotic cells, myosin interacts (Eckert, Fig. 10-6c) with Actin microfilaments to cause movement or contraction (look!). Myosin I has just one motor domain but Myosin II has two motor domains, or heads. Most forms of myosin "walk" from the pointed (-) end of microfilaments towards the slightly barbed (+) end.

• Another form of myosin, called Myosin V, walks along microfilaments in nonmuscle cells, just as a person would walk across a river on stepping stones, carrying cargo with them. Myosin V is very similar to Myosin II, having two motor domains. There are even other types of Myosin, including Myosin VI and Myosin VII. At least one of these forms appears to walk along microfilaments in the opposite direction, from + to -, from the others. (At this point you should know something about the overall structure of Myosins, which consist of a globular head or heads, which contain the catalytic site or sites, the substrate for which is ATP, and a structural tail, usually helical in shape)

4. Contraction of skeletal (and heart) muscle occurs when the myosin thick filaments slide relative to the actin thin filaments, thus shortening length of the sarcomere.

5. Myosin ATPase produces contractile force from the energy released from ATP hydrolysis.

• ATP binding to myosin causes release of the myosin "head" from actin (Eckert, Fig. 10-11a). After release, the energy from ATP hydrolysis is stored in the myosin (Eckert, Fig. 10-11b) and released to cause contraction when myosin again comes in contact with actin.

• Myosin kinetics and the previous diagram help us to understand the cycle of active contraction, the relaxed state, and the development of rigor: (A = Actin, M = Myosin, P = Phosphate. AM = Actomyosin, Myosin with its head region attached to Actin. AM*ATP = Myosin with ATP attached, bound to Actin.) Notice that the presence of a long-lived enzyme-product complex (myosin-ADP-phosphate) shows us that the kinetics can be more complicated than Michaelis-Menten theory predicts.

• Make sure you know what chemical conditions produce rigor, active contraction, or the relaxed state of muscle.

6. Low ATP concentrations within myofibrils cause rigor (RYE-GORE). Therefore, vertebrate skeletal muscle has several mechanisms which result in keeping ATP at suitable concentrations, even under adverse circumstances.

• Myoglobin in the muscle cells provides a temporary source of oxygen and releases it when blood oxygen pressures become low.

• Fermentation, producing Lactic Acid, permits muscle cells to make a little ATP even if there is no oxygen available in the muscle cells.

• The Lohman reaction provides a buffer against heavy demand for ATP lowering ATP concentrations in the myofibrils. ATP produced by the mitochondria, usually located in the periphery of the cell, is used to phosphorylate creatine to make creatine phosphate. The creatine phosphate diffuses into the myofibrils, where it is used to phosphorylate ADP to ATP, as needed. (Eckert, Fig. 10-29)

7. The coupling of electrical excitation and contraction is the result of Ca⁺⁺ release from the lumen of the sarcoplasmic reticulum when a depolarization passes into the muscle cell interior via the T-tubules (Eckert, Fig. 10-21a) (Eckert, Fig. 10-21b). An action potential traveling along the external cell membrane follows the cell membrane into the T-tubules and deep into the muscle cell interior.

• The T-tubules end in sacs which lie next to the membranes at the ends of sacs of sarcoplasmic reticulum. Ca⁺⁺ channels are opened in response to depolarization and Ca⁺⁺ flows into the cytosol, where the actin and myosin are located (Eckert, Fig. 10-24d) (Eckert, Fig. 10-24c). Ca⁺⁺ concentrations in the cytosol rise from 10^-8 or 10^-7M to about 10^-6M. Active transport pumps transport Ca⁺⁺ back into the lumen of the sarcoplasmic reticulum so that when Ca⁺⁺ is no longer being released, cytosol concentrations of Ca⁺⁺ drop back to resting levels. Another diagram makes clearer the relationships between the T-tubules and the sarcoplasmic reticulum:
  

• Ca⁺⁺ activates human and frog muscle by 2 Ca⁺⁺ ions binding to each Troponin-C (TN-C) unit. A conformational change in troponin causes the TN-I subuit to release from the actin filament, resulting in tropomyosin sliding deeper into the grooves in the actin helix and uncovering binding sites for myosin on actin. Calicum control for human and frog muscle is on the actin filament but in other animals, calcium may control contraction through binding to small proteins attached to myosin and in yet other animals, control may be through binding to proteins attached to actin and proteins attached to myosin. Look here!

• At this point you should know that action potentials travel down the T-tubules, causing Ca++ release from the sarcoplasmic reticulum - from what concentrations at rest to what concentrations during contraction? You should understand the placement of troponin and tropomyosin on the actin thin filaments and know which protein binds Ca++ and which physically blocks actin-myosin binding at rest.

All text and images, not attributed to others, including course examinations and sample questions, are Copyright, 2006, Thomas J. Herbert and may not be used for any commercial purpose without the express written permission of Thomas J. Herbert.