Human Anatomy & Physiology I Supplemental · Exam Block 4
Week 11 · Ch. 10 · Muscle Tissue
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Week 11 · Chapter 10
Muscle Tissue
This week begins Exam Block 4 and moves from bones and joints into the tissue that actually moves them. Muscle tissue converts chemical energy into force — you'll cover the three muscle types, skeletal muscle's internal organization, and the sliding filament mechanism that powers every contraction.
4Explain the sliding filament theory of contraction
5Describe the role of calcium and ATP in muscle contraction
6Connect muscle fatigue to its underlying physiological causes
Big Picture
Three muscle tissue types
This is the fourth primary tissue type from Chapter 4, now covered in depth. Skeletal muscle is voluntary and striated (visibly striped under a microscope) — this is the type covered in most depth this week. Cardiac muscle is also striated but involuntary, found only in the heart. Smooth muscle is involuntary and non-striated, found in the walls of organs and blood vessels.
Vocabulary & Word Parts
Muscle tissue roots
my/o-muscle
Myofibril = a contractile strand inside a muscle fiber. Myosin = a key muscle contraction protein.
sarc/o-flesh, muscle
Sarcomere = the basic contractile unit of muscle. Sarcolemma = a muscle fiber's cell membrane.
-lemmahusk, sheath
Sarcolemma = the membrane surrounding a muscle fiber.
fasc/o-bundle, band
Fascicle = a bundle of muscle fibers grouped together.
-troph/o-nourishment, growth
Hypertrophy = muscle growth from increased use. Atrophy = muscle shrinkage from disuse.
tetan/o-rigid, tense
Tetany = a sustained, maximal muscle contraction from rapid, repeated stimulation.
Structure & Function
Skeletal muscle's hierarchical organization
A whole muscle is built from progressively smaller units, each nested inside the last: the muscle organ contains bundles called fascicles, each fascicle contains individual muscle fibers (cells), and each fiber contains many myofibrils — the actual contractile strands. This nesting pattern is worth memorizing in order, since exam questions often test the sequence directly.
Structure & Function
The sarcomere: muscle's basic contractile unit
A sarcomere runs from one Z-disc to the next and contains two types of protein filaments: thin filaments (actin) anchored to the Z-discs, and thick filaments (myosin) in the center. This repeating Z-disc-to-Z-disc unit, stacked thousands of times along a myofibril, is what gives skeletal muscle its striped (striated) appearance under a microscope.
Structure & Function
Sliding filament theory
This is the central mechanism of the whole chapter. Muscle contraction does NOT happen because the filaments themselves shrink — they stay the same length. Instead, myosin heads on the thick filaments repeatedly grab, pull, and release the thin filaments, ratcheting them further into the center of the sarcomere. As every sarcomere in a muscle fiber shortens this way, the whole muscle contracts.
Structure & Function
What triggers the slide: calcium and ATP
Calcium (Ca²⁺) — released from storage inside the muscle fiber when a nerve signal arrives; it exposes the myosin-binding sites on actin, allowing the contraction cycle to begin.
ATP — provides the energy for each myosin head to release its grip on actin and "re-cock" for another pull. Without ATP, muscle fibers can't relax — this is part of why rigor mortis occurs after death, when ATP production stops entirely.
The cycle repeats — as long as calcium and ATP are both available, myosin heads keep pulling, actin keeps sliding, and the sarcomere keeps shortening.
Your Turn — Guided Practice
Apply it before the quiz
After an intense workout, a runner's leg muscles become fatigued and can no longer contract with the same force.
Question 1 — At the molecular level, which two resources becoming depleted best explains this fatigue?
Sustained contraction depends on adequate ATP and calcium to keep the sliding filament cycle running. Bone density and joint cartilage are skeletal structures unrelated to the muscle contraction cycle itself.
Question 2 — Which structural level of muscle organization directly contains the sarcomeres responsible for this contraction?
Sarcomeres are found within myofibrils — the smallest structural level in the hierarchy that directly contains the contractile units. A fascicle is a much larger bundle of whole muscle fibers, several organizational levels above the myofibril.
Clinical Correlation
Rigor mortis and the ATP connection
This is a striking, memorable application of the ATP requirement covered this week.
🩺 In practice
Rigor mortis — the stiffening of muscles after death — occurs because ATP production stops when circulation and cellular metabolism cease. Without ATP, myosin heads that are gripping actin at the moment of death can't release their grip, leaving the muscle locked in a contracted state until the muscle tissue itself begins to break down. This is a direct, unforgettable illustration of ATP's role in muscle RELAXATION, not just contraction.
Clinical Correlation
Muscular dystrophy and structural muscle proteins
This connects the sarcomere's structural proteins to a well-known group of genetic diseases.
🩺 In practice
Muscular dystrophy refers to a group of genetic disorders that disrupt structural proteins supporting the muscle fiber membrane (the sarcolemma) and its connection to the contractile machinery inside. Over time, this leads to progressive muscle weakness and degeneration — a direct consequence of a breakdown at the structural level this week covers, even though the disease itself isn't part of the standard exam content.
Mini Knowledge Test
Show what you know — 7 questions
Question 1
Which muscle type is voluntary and striated?
Skeletal muscle is voluntary and striated. Cardiac muscle is also striated but involuntary, and smooth muscle is both involuntary and non-striated — neither matches skeletal muscle's combination of traits.
Question 2
Which structure directly contains a bundle of muscle fibers?
A fascicle is a bundle of muscle fibers. A myofibril is a contractile strand inside a single fiber, and a sarcomere is an even smaller repeating unit within a myofibril — both are much smaller structures than a fascicle.
Question 3
A sarcomere is defined as the region between two:
A sarcomere runs from one Z-disc to the next. Fascicles and muscle fibers are much larger structural units — a sarcomere is a microscopic component found within a single myofibril.
Question 4
Which protein makes up the thin filaments?
Actin makes up the thin filaments. Myosin makes up the thick filaments instead, and collagen is a structural connective tissue protein unrelated to the sarcomere's contractile filaments.
Question 5
During contraction, what happens to the length of the actin and myosin filaments themselves?
The filaments stay the same length — contraction happens because they slide past each other, not because they shrink. This is the key insight of sliding filament theory, distinguishing it from a simple 'shrinking' model of contraction.
Question 6
Which ion must be released for a muscle contraction to begin?
Calcium release exposes the myosin-binding sites on actin, allowing contraction to begin. Sodium and potassium play roles in nerve signaling more broadly, but calcium is the specific trigger for the contraction cycle itself.
Question 7
Rigor mortis occurs because, after death:
Rigor mortis happens because ATP production stops, leaving myosin heads unable to release their grip on actin. Muscle tissue does not convert to bone, and while calcium changes occur after death, the defining mechanism tested here is the loss of ATP.
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Debrief
Week 11 Quick-Reference Recap
One page to glance at before Exam 4 — everything from this week, condensed.
Next — Week 12: The Muscular System + Exam 4 Synthesis. You'll build on this week's contraction mechanism by naming and locating the body's major skeletal muscles, then pull Chapters 10–12 together for Exam 4, including a review of Chapter 12, Neural Tissue.