Muscle Anatomy

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Five jobs
- Movement of the body and of substances within it
- Posture, holding you upright against gravity
- Joint stability from muscle tone
- Heat production (shivering)
- Guarding openings and protecting organs (sphincters, body wall)
Definitions
- Origin:the attachment that stays relatively fixed during contraction
- Insertion:the attachment that moves; it is pulled toward the origin
- Action:the movement produced, named at the joint crossed (for example, flexion of the elbow)
Roles, with the elbow example
- Agonist (prime mover):mainly produces the movement: biceps brachii
- Antagonist:opposes and relaxes to allow it: triceps brachii
- Synergist:assists or adds force: brachialis
- Fixator:stabilizes the origin: rotator cuff steadying the shoulder
Levers
- First class:fulcrum in the middle, like a seesaw (body: nodding the head)
- Second class:load in the middle, like a wheelbarrow (body: standing on tiptoe)
- Third class:effort in the middle, like tweezers (body: a biceps curl); most common, trades force for speed and range
Naming clues
- Location: tibialis anterior
- Shape: deltoid
- Size: gluteus maximus
- Fiber direction: rectus, oblique, transversus
- Number of heads: biceps, triceps, quadriceps
- Attachments: sternocleidomastoid
- Action: flexor, extensor, adductor
Big to small (in order)
- Whole muscle wrapped in epimysium
- Fascicle wrapped in perimysium
- Muscle fiber wrapped in endomysium (membrane = sarcolemma, cytoplasm = sarcoplasm)
- Myofibril inside the fiber
- Sarcomere, the functional unit, Z-line to Z-line
Tie-in
- All three wrappers merge at the ends to form the tendon.
Bands and lines
- A band:full length of the thick filament (width never changes)
- I band:thin filaments only
- H zone:thick filaments only, no overlap
- Z-line:sarcomere boundary; thin filaments anchor here
- M line:center; thick filaments anchor here
What happens
- I band narrows
- H zone narrows
- A band stays the same, because thick filament length never changes
Filament proteins
- Thick filament:myosin
- Thin filament:actin, plus the regulatory proteins troponin and tropomyosin
- Titin:an elastic protein that anchors thick filaments to the Z-line
Excitation machinery
- T-tubules:inward folds of the sarcolemma that carry the action potential deep into the fiber
- Sarcoplasmic reticulum (SR):smooth ER that stores calcium and releases it when the signal arrives
- Triad:one T-tubule flanked by two SR terminal cisternae; where the signal and the calcium meet
Clinical reasoning
- Dystrophin anchors the contractile machinery to the sarcolemma
- In Duchenne, dystrophin is defective, so every contraction tears the fiber
- Creatine kinase (CK) rises when the sarcolemma is breached, signaling muscle damage