🧵 Muscle Structure
The connective tissue wrapping around each individual muscle fiber.
Endomysium
The protein filament anchored to the Z discs.
Actin
The neurotransmitter released by the motor neuron at the neuromuscular junction.
Acetylcholine (ACh)
The baseline low-level contraction that keeps muscles firm and ready to contract.
Muscle tone
These fibers contract slowly and primarily produce ATP through oxidative phosphorylation.
Slow oxidative
From largest to smallest, put these in order: epimysium, perimysium, endomysium.
Epimysium → Perimysium → Endomysium
The protein filament anchored at the M line.
Myosin
Calcium binds to this regulatory protein, causing tropomyosin to move.
Troponin
A contraction produces force while the muscle shortens.
Concentric
These fibers have large diameters, high glycogen reserves, and relatively few mitochondria.
Fast glycolytic
A muscle structure contains many fascicles, which are each surrounded by their own connective tissue layer. What surrounds the entire muscle?
Epimysium
During contraction, the distance between these structures decreases as actin is pulled toward the M line.
Z discs
What must happen to tropomyosin before myosin can attach to actin?
It unblocks the binding sites
A muscle generates force but its length does not change enough to produce movement.
Isometric
A fiber contracts quickly and primarily uses the electron transport chain to make ATP. Which fiber type is it?
Fast oxidative
This muscle-fiber structure stores calcium and is a specialized form of smooth ER.
Sarcoplasmic reticulum
A sarcomere region containing only thin filaments becomes smaller during contraction.
I band
In the cross-bridge cycle, myosin pulls actin toward the M line during this stage.
Power stroke
A second stimulus arrives while the previous twitch is still relaxing, causing the responses to add together.
Summation
An endurance athlete needs fibers with many mitochondria, many capillaries, and high fatigue resistance. Which type fits?
Slow oxidative
An action potential must reach all parts of the muscle fiber. Which structures carry it deep into the sarcoplasm?
T-tubules
During contraction, the filaments themselves do not shorten. Instead, this mechanism pulls one filament toward the M line.
Sliding filament mechanism
Put these events in order: calcium binds troponin, ACh release, calcium release from SR, tropomyosin moves.
ACh release → Ca²⁺ release → Ca²⁺ binds troponin → tropomyosin moves
Stimuli arrive so rapidly that the relaxation phase disappears completely, producing continuous contraction.
Tetanus
Resistance training increases fiber size and stored glycogen, producing this change in skeletal muscle.
Hypertrophy