Lesson 1 — The Skeletal System
The skeletal system is the body's framework — 206 bones that support structure, protect organs, enable movement, produce blood cells, and store minerals like calcium and phosphorus.
Axial SkeletonSkull, vertebral column, rib cage — protects brain, spinal cord, and vital organs
Appendicular SkeletonArms, legs, shoulder and hip girdles — enables locomotion
JointsWhere bones meet; types: ball-and-socket, hinge, pivot, gliding
Bone HealthCalcium + Vitamin D + weight-bearing exercise = strong bones throughout life
Bone Growth
Bones grow at growth plates (epiphyseal plates) through childhood and adolescence. Peak bone mass is reached around age 30 — building strong bones now protects against osteoporosis later.
Bones grow at growth plates (epiphyseal plates) through childhood and adolescence. Peak bone mass is reached around age 30 — building strong bones now protects against osteoporosis later.
Quick Check: What two nutrients are most important for bone health, and how do you get them?
Lesson 2 — The Muscular System
The body has over 600 muscles of three types. Muscles work by contracting — shortening to pull bones or move substances through organs.
- Skeletal muscle — voluntary; attached to bones; enables movement; striped appearance
- Smooth muscle — involuntary; lines organs like stomach and blood vessels
- Cardiac muscle — involuntary; found only in the heart; never fatigues
Muscle Pairs
Muscles work in pairs — when one contracts (agonist), the opposing muscle relaxes (antagonist). Example: bicep contracts/tricep relaxes to flex the arm.
Muscles work in pairs — when one contracts (agonist), the opposing muscle relaxes (antagonist). Example: bicep contracts/tricep relaxes to flex the arm.
Quick Check: How do the bicep and tricep work together to move the forearm?
Lesson 3 — The Nervous System
The nervous system is the body's communication network, controlling thought, movement, sensation, and automatic functions. It has two main divisions:
Central Nervous SystemBrain + spinal cord; processes information and sends commands
Peripheral Nervous SystemAll other nerves; relays signals between CNS and body
SomaticVoluntary control of skeletal muscles
AutonomicInvoluntary control of heart, digestion, breathing (sympathetic/parasympathetic)
Neurons are the basic units of the nervous system. They transmit electrical signals across synapses using chemical messengers called neurotransmitters.
Quick Check: What is the difference between the central and peripheral nervous system?
Lesson 4 — Caring for Your Body Systems
Lifestyle choices profoundly affect the health of skeletal, muscular, and nervous systems across a lifetime.
- Weight-bearing exercise strengthens bones and builds muscle mass
- Adequate calcium (1,300 mg/day for teens) and Vitamin D protect bones
- Helmets and protective gear prevent traumatic brain injury (TBI)
- Avoid tobacco — nicotine impairs nerve transmission and muscle recovery
- Quality sleep allows the brain to consolidate memories and repair itself
- Stretching maintains muscle flexibility and joint range of motion
Quick Check: How does wearing a helmet protect the nervous system?
Chapter Vocabulary
Skeletal systemThe framework of 206 bones that supports the body, protects organs, and enables movement.
CartilageFlexible connective tissue at joints that cushions bones and prevents friction.
TendonTough connective tissue that attaches muscle to bone.
LigamentConnective tissue that connects bone to bone at joints.
Skeletal muscleVoluntary muscle attached to bones that enables body movement.
Cardiac muscleInvoluntary muscle found only in the heart that contracts continuously without fatigue.
NeuronA nerve cell that transmits electrical signals throughout the nervous system.
SynapseThe gap between neurons where chemical neurotransmitters pass signals.
ReflexAn automatic, involuntary response to a stimulus, processed in the spinal cord.
OsteoporosisA disease in which bones become weak and brittle due to loss of calcium and bone density.
