Unit 19: The Skeletal and Muscular Systems
How bones, joints, and muscles turn a chemical signal into the physical motion of a body moving through space.
5 topics · 15 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
The Structure and Function of Bone
- How it worksHow does a honeycomb-like internal structure make bone both lightweight and strong?
- Common mistakeWhy isn't bone a dead, inert material once it stops growing?
- Going furtherWhat happens to bone density over years when a person doesn't get enough calcium or weight-bearing exercise?
Your turn1 of 2
A human femur weighs only about half a kilogram, light enough to lift easily with one hand, yet it can support several times a person's body weight without breaking under normal daily activity. Why can a bone be this light yet this strong at the same time?Joints and Range of Motion
- How it worksHow does the shape of a joint's bone surfaces determine what kind of motion it allows?
- How it worksHow do ligaments and cartilage at a joint keep bones aligned while still allowing movement?
- Common mistakeWhy doesn't cartilage at a joint work the same way a ligament does?
Your turn1 of 2
A shoulder joint can rotate a person's arm in almost any direction, while a knee joint mostly just bends and straightens like a door hinge. Why can your shoulder rotate in almost every direction while your knee only bends like a hinge?How Skeletal Muscles Move Bones
- How it worksHow do a bicep and tricep work as an antagonistic pair to bend and straighten your arm?
- Common mistakeWhy can't a single muscle both bend and straighten a joint on its own?
- Going furtherWhat happens to arm movement if one muscle in an antagonistic pair is damaged?
Your turn1 of 2
Bending your elbow to bring your hand toward your shoulder relies on your bicep contracting, while straightening that same elbow back out relies on your tricep contracting instead. Why does bending your elbow require two separate muscles instead of just one?The Sliding Filament Model of Muscle Contraction
- How it worksWalk me through how actin and myosin filaments slide past each other to contract a muscle fiber.
- How it worksHow does calcium released into a muscle cell trigger the myosin heads to start pulling on actin?
- Common mistakeWhy doesn't the length of the actin and myosin filaments themselves change during contraction?
Your turn1 of 2
A contracting muscle fiber measurably shortens overall, yet under a microscope the individual actin and myosin protein filaments inside it stay exactly the same length throughout that contraction. Why does a muscle fiber get shorter even though its individual filaments don't shrink?Types of Muscle Tissue
- How it worksHow do skeletal, cardiac, and smooth muscle differ in structure and in whether you control them voluntarily?
- How it worksHow does cardiac muscle's structure let it contract rhythmically without becoming fatigued like skeletal muscle?
- Common mistakeWhy isn't smooth muscle found only in one organ instead of throughout many internal organs?
Your turn1 of 2
Your heart contracts rhythmically all day and night without you ever having to consciously think about it, while flexing your bicep requires a deliberate, conscious decision to move. Why does your heart keep beating without you ever consciously controlling it?