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Anatomy and Physiology Unit 3: Skeletal and muscular systems

Bones support and protect the body, and muscles pull on bones to move them.

Covers: Bone structure and functions, joints, axial vs appendicular skeleton, muscle contraction, sliding filament model

Study guide

The skeletal system supports the body, protects organs, allows movement, stores calcium and other minerals, and makes blood cells in red bone marrow. An adult has 206 bones, classified by shape as long, short, flat or irregular. A long bone has a shaft (diaphysis), ends (epiphyses), a hard compact bone outer layer and spongy bone inside, with marrow in its cavity. Bones are living tissue, constantly remodeled by osteoblasts (which build bone) and osteoclasts (which break it down).

The skeleton is divided into the axial skeleton (skull, vertebral column and rib cage), which protects vital organs, and the appendicular skeleton (limbs plus the shoulder and pelvic girdles), which allows movement.

Joints connect bones. They're classified by movement: immovable (skull sutures), slightly movable (vertebrae) and freely movable synovial joints, such as hinge (elbow, knee), ball-and-socket (shoulder, hip) and pivot joints. Ligaments connect bone to bone, and tendons connect muscle to bone.

Skeletal muscles move bones by pulling. Muscles work in antagonistic pairs, like the biceps and triceps. A muscle is made of fibers (cells) containing myofibrils divided into repeating units called sarcomeres.

Contraction follows the sliding filament model. A nerve signal at the neuromuscular junction releases acetylcholine, which triggers calcium release inside the muscle fiber. Calcium lets myosin heads bind to actin filaments and pull them toward the center of the sarcomere, using ATP. The filaments slide past each other and shorten the muscle without changing length themselves. When the signal stops, calcium is pumped away and the muscle relaxes.

The calcium that triggers contraction is stored in the sarcoplasmic reticulum, the muscle fiber's specialized ER. When calcium is released, it binds troponin, which shifts tropomyosin off the binding sites on actin so myosin heads can attach. ATP is needed both for the power stroke and for myosin to let go of actin. A single muscle fiber contracts fully or not at all, so the body controls force by recruiting more motor units (a motor neuron plus the fibers it controls). In an isotonic contraction the muscle changes length: it shortens in a concentric contraction and lengthens under load in an eccentric one. In an isometric contraction, tension rises but the muscle's length stays the same.

Flexion decreases the angle at a joint, and extension increases it. Abduction moves a limb away from the midline, and adduction moves it toward the midline. Rotation turns a bone around its own axis, and circumduction moves a limb in a cone shape. Long bones grow in length at the epiphyseal (growth) plate, a layer of cartilage that is replaced by bone and closes after puberty. Compact bone is built from osteons, which are rings of bone matrix around a central canal carrying blood vessels and nerves. Hormones such as parathyroid hormone and calcitonin adjust remodeling to control blood calcium.

Slow-twitch fibers are rich in mitochondria and myoglobin and resist fatigue, which suits endurance and posture. Fast-twitch fibers contract quickly and powerfully but tire fast. Commonly tested muscles include the deltoid, pectoralis major, biceps and triceps brachii, quadriceps, hamstrings, gastrocnemius and masseter.

Key ideas

• Bones support, protect, store minerals and make blood cells (red marrow).
• The adult skeleton has 206 bones: axial (skull, spine, ribs) and appendicular (limbs, girdles).
• Ligaments connect bone to bone; tendons connect muscle to bone.
• Muscles can only pull; they work in opposing pairs.
• Sliding filament model: actin and myosin slide past each other, using ATP and calcium.

Formulas and rules

Common mistakes

• Mixing up tendons and ligaments.
• Thinking muscles push. They only pull.

Words to know

Adult skeleton

206 bones.

Tendon

Connects muscle to bone.

Ligament

Connects bone to bone.

Red bone marrow

Where blood cells are made.

Actin and myosin

The protein filaments that slide during muscle contraction.

Compact bone

The dense outer layer of bone.

Spongy bone

Lightweight bone tissue inside bones.

Diaphysis / epiphysis

A long bone's shaft / its ends.

Osteoblast / osteoclast

Builds bone / breaks bone down.

Axial skeleton

Skull, vertebral column and rib cage.

Appendicular skeleton

Limbs plus shoulder and pelvic girdles.

Synovial joint

A freely movable joint, like hinge or ball-and-socket.

Sarcomere

The repeating contractile unit of a muscle fiber.

Sliding filament model

Myosin pulls actin filaments inward, shortening the sarcomere.

Neuromuscular junction

Where a motor neuron releases acetylcholine onto a muscle fiber.

Antagonistic muscles

Pairs that move a joint in opposite directions, like biceps and triceps.

Functions of bones

Support, protection, movement, mineral storage and blood cell formation.

Sample practice questions

5 more questions like these, from level 1 to 5, are in the practice set.