Muscle Physiology- Pt 3 Quiz

Muscles: General Information

• Muscles are contractile tissue with functions of locomotion, respiration, digestion, swallowing, parturitions, blood and lymphatic circulation, and generating body heat.
• Muscles have properties of contractility, excitability, extensibility, and elasticity.
• There are three main types of muscles: skeletal muscle, cardiac muscle, and smooth muscle.

Muscle Types

• Skeletal muscle makes up 70% of body mass and produces fast contractions under voluntary control.
• Skeletal muscle can be found in the trunk of the body, the head, neck, and extremities.
• Skeletal muscle has striations.
• Smooth muscle produces slow contractions under involuntary control and can be found in viscera and blood vessels.
• Smooth muscle lacks striations.
• Cardiac muscle produces fast contractions under involuntary control and can be found in the heart.
• Cardiac muscle has striations.

Skeletal Muscle: General Info

• Skeletal muscle is crucial for body movement as it is attached to bones via tendon, where it moves and supports the skeleton.
• Skeletal muscle is considered to be under voluntary response because it is stimulated by motor nerves.
• The thin filament composition consists of 2 intertwined helical strands of actin protein and 2 strands of tropomyosin protein.

Troponin

• Troponin is located intermittently along the tropomyosin-actin strand.
• Troponin has an affinity for calcium ions.

Thick Filaments

• The thick filament is myosin.
• The myosin filament contains a tail of intertwined helices and 2 globular heads, where 1 head binds to ATP and the other binds to actin.
• Myosin heads can flex and relax.
• Myosin acts as an ATPase enzyme by using ATP as the source of energy for contractions.
• Approximately 500 myosin heads form cross-bridges that interact with actin to shorten the sarcomere.

Titin Filamentous Molecules

• Titin filamentous molecules keep myosin and actin filaments in place.
• Titin filamentous molecules are very elastic filamentous molecules with 1 end attached to the end of the Z disk and another part attached to myosin.

Myofilament: Organization

• Myosin and actin filaments partially interdigitate, resulting in the myofibrils to have alternating light and dark bands, also known as striations.
• I bands are the light bands (thin filaments) which are isotropic to polarized light.
• A bands are the darker bands (thick filaments) which are anisotropic to polarized light.
• The end of actin filaments are attached to Z disks (AKA: Z lines), forming the periphery of the sarcomere and transversely bisecting the I bands.
• The H-zone is the light line in the center of the sarcomere from the thick filaments.
• The M-line is composed of thick filaments and accessory proteins and can be found inside the H-zone, forming the center of the sarcomere.

Skeletal Muscle: Fiber Types

• Type 1 (slow) muscle fibers are red fibers.
• Type 1 fibers are slow twitch oxidative muscle fibers, meaning they highly rely on oxidative metabolism.
• Type 1 fibers are slow contracting and fatigue-resistant.
• Type 1 fibers are rich in mitochondria, myoglobin, and capillaries, which cause this fiber type to have a red appearance.
• Type 2 (fast) muscle fibers are white fibers.
• Type 2 muscle fibers are fast twitch glycolytic muscle fibers.
• Type 2 fibers are fast contracting and are more fatigable fibers, in comparison to Type 1 fibers.
• Type 2 fibers rely on glycolytic metabolism and have less mitochondria than Type 1 fibers.
• Type 2 fibers lack myoglobin, causing it to have a white pigmentation.
• Type 2 muscle fibers can be further classified as "Type 2A" and "Type 2B".
• Type 2A fibers use mixed oxidative-glycolytic metabolism and are fast contracting with fatigue resistance.
• Type 2B fibers use glycolytic metabolism and are fast contracting, fast-fatiguing fibers.

Muscle Cell: Organelles

• Sarcoplasmic Reticulum is a specialized endoplasmic reticulum which manages calcium storage, release, and reuptake, aiding in muscle contraction.

• The sarcoplasmic reticulum is larger in cells with faster contracting fibers, such as the Type 2 (white) fibers.

• Mitochondria is present in large numbers within muscle cells and lay parallel to myofibrils.

• Mitochondria is more prevalent within slow fibers, such as Type 1 (red) fibers.

• Mitochondria supplies the contracting myofibrils with large supplies of ATP to be used for energy.

• T Tubules are tubules that contain periodic invagination within the sarcolemma and are arranged transversely to myofibrils.

• T Tubules contain ECF (extra cellular fluid).

• T Tubules allow the plasma membrane of the muscle fiber to carry depolarization of action potentials to the interior of the muscle fiber.

• In mammals, T Tubules are located at junctions between A bands and I bands of the sarcomere (2 per sarcomere in longitudinal view).

• Each T Tubule is flanked by 2 terminal cisternae of the sarcoplasmic reticulum, resulting in the formation of triad structures.

Myoglobin

• Myoglobin is a protein composed of a single polypeptide chain with only 1 HEME group (also known as an oxygen binding site).
• Myoglobin is located primarily in the striped muscles of vertebrates and is found exclusively in skeletal and cardiac muscle.
• Myoglobin constitutes up to 5-10% of all cytoplasmic proteins.
• Myoglobin serves as a local oxygen reservoir where it can temporarily provide oxygen whenever blood oxygen supply is low during periods of intense muscular activity.
• Myoglobin can also function in the homeostasis of nitric oxide and the detoxification of reactive oxygen species.