BIO 004 · Human Anatomy
Muscle Structure & Sarcomeres
Muscle Structure and Sarcomeres
A reference for the muscle structure video and lab. This page covers the structure of a skeletal muscle from its connective tissue coverings, down through the muscle fiber, to the sarcomere and its myofilaments. The contraction mechanism belongs to physiology and is not covered here.
How to use this sheet
Notes prints the full reference for review.
Study prints as a fill-in-the-blank worksheet to complete while you watch the video or read.
Quiz me is on-screen practice: type the term, or click Reveal to check yourself.
The tables and sequences respond to Study and Quiz too, with a Reveal button on each row.
- Name the connective tissue coverings of a skeletal muscle and trace how they connect to the tendon.
- Order the levels of muscle organization, from the whole muscle down to the myofilament.
- Identify the parts of a muscle fiber, including the T tubules, sarcoplasmic reticulum, and triad.
- Diagram a sarcomere, name its bands, lines, and zones, and state which filaments lie in each.
Your pre-work
Work through these the evening before class. None of it is turned in. It is how you learn the material and build your spaced recall.
This is more than a checklist. Ticking these boxes is the start, not the finish. Committing this material to memory and being able to apply it takes considerable time and repeated effort. You are not done when the boxes are checked. Put in the real hours, and keep coming back for frequent recall and review until the material is genuinely yours.
Skeletal Muscle, an Overview
A skeletal muscle is an organ. It is built of muscle tissue plus connective tissue, blood vessels, and nerves, and it is wrapped, bundled, and anchored at every level of its structure.
- Skeletal musclea voluntary, striated muscle organ, usually attached to bone, that produces movement when it contracts
- Muscle fibera single skeletal muscle cell, long and cylindrical, with many nuclei
- Originthe attachment of a muscle to the more stationary bone
- Insertionthe attachment of a muscle to the more movable bone, the bone that is pulled toward the origin
- Bellythe thick, fleshy middle region of the muscle between its attachments
Attachments and Action
A muscle crosses at least one joint and attaches to the articulating bones. When it contracts, one bone stays put and the other moves.
- Originthe tendon's attachment to the stationary bone, usually the proximal one
- Insertionthe tendon's attachment to the movable bone, usually the distal one, pulled toward the origin
- Actionthe movement the muscle produces when it contracts
How Muscles Work Together
Skeletal muscles are arranged in opposing pairs across a joint, and each contraction has muscles playing different roles.
- Agonist (prime mover)the muscle that contracts to produce the action
- Antagonistthe opposing muscle that stretches and yields to the prime mover
- Synergista muscle that aids the action or prevents unwanted movement
- Fixatora muscle that stabilizes the origin so the prime mover acts efficiently
- Compartmenta group of muscles, vessels, and nerves with a common function, such as the flexor and extensor compartments
How Skeletal Muscles Are Named
A muscle's name usually encodes one or more of these features. Learn the clues and the name tells you the muscle.
| Naming clue | Examples in the name |
|---|---|
| Direction of fibers | rectus, transverse, oblique |
| Size | maximus, minimus, longus, brevis, vastus |
| Shape | deltoid, trapezius, serratus, rhomboid, orbicularis |
| Action | flexor, extensor, abductor, adductor, levator, pronator, supinator |
| Number of origins | biceps, triceps, quadriceps |
| Location | temporalis, over the temporal bone |
| Origin and insertion | sternocleidomastoid, sternum and clavicle to the mastoid process |
The Three Muscle Tissue Types
All three are muscle, but their structure differs. Compare them by location, appearance, control, and cell features.
| Type | Location | Striations and control | Cell features |
|---|---|---|---|
| Skeletal | attached to the skeleton | striated, voluntary | long cylindrical fibers, many nuclei pushed to the edge |
| Cardiac | the heart wall | striated, involuntary | branching fibers, one central nucleus, joined by intercalated discs (desmosomes for adhesion, gap junctions for electrical coupling) |
| Smooth | walls of hollow organs and vessels | non-striated, involuntary | spindle-shaped cells, one central nucleus |
Connective Tissue Coverings
Three connective tissue sheaths wrap the muscle at three scales. They are continuous with each other and with the tendon, so a fiber's pull is carried all the way to the bone. Compare the three sheaths.
| Sheath | What it wraps | Tissue |
|---|---|---|
| Epimysium | the entire muscle, the outermost layer | dense irregular connective tissue |
| Perimysium | each fascicle, a bundle of muscle fibers | dense irregular connective tissue |
| Endomysium | each individual muscle fiber | a fine sheath of areolar connective tissue with reticular fibers |
- Superficial fasciathe subcutaneous layer of areolar and adipose tissue between the skin and the muscles
- Deep fasciaa sheet of dense connective tissue that wraps muscles, separates them, and binds those with a common action into functional groups
- Tendona cord of dense regular connective tissue, the merged continuation of all three sheaths, that attaches a muscle to bone
- Aponeurosisa broad, flat sheet of tendon-like connective tissue, an attachment used where a muscle is wide
Levels of Organization
A skeletal muscle is a set of structures nested inside one another. Follow them in order, from the whole organ down to the protein strands.
- Musclethe whole organ, wrapped in epimysium
- Fasciclea visible bundle of muscle fibers, wrapped in perimysium
- Muscle fibera single muscle cell, wrapped in endomysium
- Myofibrila long contractile rod running the length of the fiber; hundreds to thousands fill each fiber
- Myofilamentthe thick and thin protein strands within a myofibril; their ordered overlap forms the sarcomere
The Muscle Fiber
The muscle fiber is the muscle cell. Its parts carry the names of ordinary cell structures with the prefix sarco, from the Greek for flesh.
- Sarcolemmathe plasma membrane of the muscle fiber
- Sarcoplasmthe cytoplasm of the muscle fiber
- Myonucleithe many nuclei of the fiber, flattened against the inner sarcolemma; a muscle fiber is multinucleated
- Myofibrilsthe contractile rods that fill most of the sarcoplasm
- Transverse (T) tubulesnarrow tunnels of the sarcolemma that dive inward, carrying the surface signal deep into the fiber
- Sarcoplasmic reticulumthe smooth endoplasmic reticulum of the muscle fiber, a network of tubules wrapping each myofibril, it stores calcium ions
- Terminal cisternaethe enlarged end sacs of the sarcoplasmic reticulum, set on either side of a T tubule
- Triadthe structural unit of one T tubule flanked by two terminal cisternae
- Mitochondriapacked in rows between the myofibrils, they supply the ATP that contraction demands
- Myoglobina red, oxygen-binding pigment in the sarcoplasm that stores oxygen and gives muscle its color
- Glycogengranules of stored carbohydrate fuel held in the sarcoplasm
Myofilaments, Thick and Thin
Two kinds of protein filament fill each myofibril. Compare them by their main protein and structure.
| Myofilament | Main protein | Structure | Anchored at |
|---|---|---|---|
| Thick filament | myosin | a bundle of myosin molecules; each molecule has a long tail and a pair of heads, and the heads project outward as cross bridges | the M line |
| Thin filament | actin | two twisted strands of actin subunits; each subunit carries a myosin-binding site | the Z disc |
- Tropomyosina rod-shaped regulatory protein that lies end to end along the thin filament
- Troponina three-part regulatory protein bound at intervals to tropomyosin on the thin filament
The Sarcomere
The sarcomere is the contractile unit of muscle, the segment of a myofibril from one Z disc to the next. Its bands and zones are named for how the overlapping filaments look under the microscope. Compare the regions.
| Region | What it is | Filaments present |
|---|---|---|
| Z disc | the protein plate at each end of the sarcomere, its border | anchors the thin filaments |
| A band | the dark band, as wide as the thick filaments are long | thick filaments across its full width, with thin filaments overlapping at each edge |
| I band | the light band; a Z disc runs through its center, so one I band is shared by two sarcomeres | thin filaments only |
| H zone | the lighter region in the center of the A band | thick filaments only |
| M line | the protein line in the center of the H zone | anchors the thick filaments to one another |
| Zone of overlap | the region at each end of the A band | both thick and thin filaments, interleaved |
- Striationsthe stripes of skeletal muscle, the alternating dark A bands and light I bands of thousands of aligned sarcomeres
Structural Proteins of the Sarcomere
Besides the contractile proteins, a set of structural proteins holds the sarcomere in register and links it to the cell membrane. Compare them by location and role.
| Protein | Location | Role |
|---|---|---|
| Titin | spans from the Z disc to the M line | anchors the thick filament, gives the sarcomere elastic recoil, and keeps it from overstretching |
| Nebulin | runs the length of the thin filament | anchors the thin filament and sets its length |
| Alpha-actinin | the Z disc | binds the thin filaments and titin to the Z disc |
| Myomesin | the M line | binds the thick filaments together at the M line |
| Dystrophin | links the thin filaments to the sarcolemma | transfers the force of contraction from the sarcomere out to the connective tissue; faulty in muscular dystrophy |
See also: Histology: The Four Tissue Types for the muscle tissue overview, and Fascicle Arrangement and Lever Systems, the next muscle page in this block.
Study questions
Work on answering these in writing, in your own words. They are the questions to bring to class, and good practice for the reasoning the exams ask for.
- Trace the structural ladder of skeletal muscle, from the whole muscle down to the myofilaments.
- Name the parts of a sarcomere and what each one contributes.
- Compare the three muscle tissue types by striations, control, and location.
- Explain how the banding pattern of a sarcomere produces the striations you see on a slide.
Step 2 . Retrieval check
Now explain it back, in your own words.
In 60 words or more, pull together what the video just taught you. Include the key concepts. This is the point where the learning actually sticks. After you submit, your spaced-recall cards for this topic unlock.