Exam 3 Review:  Chapter 09:  Skeletal Muscle Cell = Fiber Histology

myofiber - An alternate term for a muscle cell or muscle fiber; these cells are specialized for contraction using the sliding filament mechanism.

myofibril - The non-membrane-bound cell organelle of skeletal and cardiac (striated) muscle cells which is a long cylindrical mass of contractile protein fibers (actin + myosin), organized as a series of repeating sarcomeres, these contractile organelles are triggered to contract in an all-or-none fashion during excitation-contraction coupling when Ca²⁺ ions to diffuse into the general sarcoplasm where they serve as the second messenger, triggering the sliding filament mechanism of contraction.

cross-striations - The series of parallel lines or light and dark bands observed in the cytoplasm of striated muscle cells, the bands are perpendicular to the long axis of the muscle cell; the banding pattern is due to the orderly arrangement of contractile myofilaments organized into repeating sarcomeres within each myofibril.

myofilament - Any of the ultramicroscopic protein filaments, comprised of actin and myosin molecules, which are the structural components of a myofibril; they form the physical elements acting during the sliding filament mechanism of contraction.

sarcolemma - The outer lipid bilayer cell membrane or plasmalemma of a skeletal muscle cell; muscle contraction is initiated when the sarcolemma is depolarized in response to a nerve impulse from a motor neuron at the motor end plate.

sarcoplasm - The general cytoplasm of a striated or skeletal muscle fiber containing all of the organelles and resources required for the sliding filament mechanism of contraction.

sarcoplasmic reticulum - The smooth endoplasmic reticulum found in striated muscle fibers which stores calcium phosphate salts in a gelatinous state until the muscle is stimulated at which point the Ca²⁺ release channels open, permitting Ca²⁺ ions to diffuse into the general sarcoplasm where they serve as the second messenger, triggering the sliding filament mechanism of contraction.

Ca²⁺ release channels - The integral membrane proteins which have the ability, when regulated to be open,  to permit calcium ions to diffuse from one side of the membrane to the other; e.g., the calcium release channels in the sarcoplasmic reticulum membranes of skeletal muscle cells which release calcium ions into the sarcoplasm when muscle contraction is to be initiated.  [nickname:  "foot proteins"]

Ca⁺⁺ ATPase - An integral membrane protein which has the ability to use direct ATP hydrolysis to transport calcium ions from one side of the membrane to the other; e.g., the calcium pumps in the sarcoplasmic reticulum membranes of skeletal muscle cells which transport calcium ions back into the interior of the cisternae when muscle contraction is ending.

calmodulin - A calcium-binding second messenger regulatory protein found in all nucleated cells which affects the activity of many calcium-sensitive enzymes, including those involved in muscular contraction; structurally similar to troponin C component.

calsequestrin - A calcium-binding storage protein found within the cisternae of the sarcoplasmic reticulum of striated muscle cells.

transverse tubules = T tubules - The irregular network of tiny tunnel-like extensions of the sarcolemma which penetrate into the interior of striated muscle cells and make intimate contact with the membranes of the terminal cisterns of the sarcoplasmic reticulum; when the striated muscle cell is stimulated by neurotransmitter and depolarization occurs, the wave of depolarization is propagated along these membrane passageways and the depolarization is transferred to the membrane of the sarcoplasmic reticulum causing its depolarization and the opening of the Ca²⁺ release channels.

terminal cisterns - The slightly expanded edges of the sarcoplasmic reticulum which are found adjacent to the T = transverse tubules in the interior of striated muscle cells.

triad - The group of three membranous structures seen in the interior of striated muscle cells, consisting of one T = transverse tube and the two adjacent terminal cisterns in electron microscopy.

Describe: (or you may sketch and label)

5. The differences between muscle fiber, myofibril, and myofilament.
 

7. The role of calcium in muscle contraction.  Include the specific molecular interactions.

Ca2+ ions are located in the smooth endoplasmic reticulum = sarcoplasmic reticulum found in striated muscle fibers.  They are stored as calcium phosphate salts in a gelatinous state until the muscle is stimulated at which point the Ca2+ release channels open, permitting the Ca2+ ions to diffuse into the general sarcoplasm where they serve as the second messenger, triggering the sliding filament mechanism of contraction.  Ca2+ ions play two roles as second messengers triggering the sliding filament mechanism of contraction.  (1)  Ca2+ ions interact with the troponin ":switch" molecules, causing a conformational change in the troponin-tropomyosin complexes which exposes the myosin binding sites on individual G-actin monomers of the thin filaments.  (2)  Ca2+ ions interact with the myosin heads, triggering their temporary covalent bonding to the myosin binding sites on individual G-actin monomers of the thin filaments so that the power stroke can be made.  In order for contraction to end, Ca2+ ions must be removed from the general sarcoplasm by the action of Ca2+ ion pumps which restore the Ca2+ ions to the interior of the sarcoplasmic reticulum.

Sketch and Label:

3. The microscopic anatomy of muscle fibers (cells).

Skeletal Muscle Cells cut in cross-section.