4.5 The Musculoskeletal System

The musculoskeletal system combines two organ systems that work as a unit to give the body shape, shield its organs, and produce movement. It includes bones, muscles, cartilage, tendons, ligaments, and joints. On the TEAS you should be ready to list the jobs of the skeleton, classify bones and joints, distinguish the three muscle types, and outline how a muscle actually contracts. A recurring exam theme is that bone is not the dead, static material it appears to be — it is living tissue that constantly rebuilds itself and stores critical minerals.

Functions of the Skeletal System

The skeleton does far more than hold you upright. The TEAS expects all five of its functions.

The mineral-storage role explains why the skeleton is metabolically active: when blood calcium drops, the body draws calcium out of bone, and when it is plentiful, it deposits calcium back.

Bone Classification and the Two Skeletons

The adult body has 206 bones, grouped by shape and by region.

The 206 bones split into two skeletons. The axial skeleton (80 bones) forms the central axis — skull, vertebral column, ribs, and sternum. The appendicular skeleton (126 bones) forms the appendages — the arms, legs, shoulder girdle, and pelvic girdle. A quick check: 80 + 126 = 206.

Bone Cells: Build, Maintain, Break Down

Three cell types continuously remodel bone, and the TEAS often asks you to tell them apart by their similar names.

Remembering “osteoblasts build, osteoclasts chew/cut” reliably separates the two most-confused cells.

Joints: Where Bones Meet

A joint (articulation) is any place two bones meet. Joints are classified by how much they move.

Synovial joints are the freely movable joints that matter most clinically. They are enclosed in a capsule, cushioned by articular cartilage, and lubricated by synovial fluid secreted by the synovial membrane. Synovial joints come in several subtypes worth knowing:

• Ball-and-socket (shoulder, hip): greatest range of motion, moves in all directions
• Hinge (elbow, knee): bends in one plane like a door
• Pivot (between the first two neck vertebrae): allows rotation
• Gliding (wrist, ankle): bones slide past one another

The Three Muscle Types

Muscle tissue comes in three forms the TEAS contrasts directly. The two questions to ask are: is it voluntary or involuntary, and is it striated (striped) or smooth?

Note that cardiac muscle is the unusual one: it is striated like skeletal muscle but involuntary like smooth muscle, and it is found only in the heart.

How Muscles Contract

Muscle contraction is explained by the sliding filament theory, which involves two proteins: thin actin filaments and thick myosin filaments. The simplified steps are:

1. A nerve impulse reaches the muscle and triggers release of calcium inside the fiber.
2. Calcium exposes binding sites on the actin filaments.
3. Myosin heads grab onto actin, forming cross-bridges.
4. The myosin heads pull, sliding the actin filaments inward.
5. The contractile unit (the sarcomere) shortens, so the whole muscle contracts.

The filaments themselves do not shorten — they slide past each other, which is the key insight in the name.

Connective Tissues: Tendons vs. Ligaments

A single high-yield distinction closes out this system. Tendons connect muscle to bone, transmitting the muscle's pull to move the skeleton. Ligaments connect bone to bone, stabilizing joints. A useful memory aid: “tendons tie muscle to bone.”

Worked Example: A runner sprains an ankle and the doctor says a ligament was overstretched, not a tendon. What is the difference? A ligament connects one bone to another and holds the joint together; an ankle sprain stretches or tears these bone-to-bone connectors. A tendon, by contrast, connects a muscle to a bone (an injury there would be a strain). Because the ankle is a synovial joint, the surrounding ligaments are what stabilize it during sudden twists — making them the structures most often injured in a sprain.