Skeletal & Muscular Systems: Movement and Support: Seventh Grade Science
The skeletal and muscular systems function together as an integrated musculoskeletal biomechanical framework that provides structural support, shields vital organs, and generates bodily movement.
The Human Skeletal System
The adult human skeleton consists of 206 bones composed of dynamic, living mineralized connective tissue. Rather than dry, inert scaffolding, bones are active organs supplied with blood vessels, nerves, and metabolic cells continuously engaged in mineral remodeling, hematopoiesis, and structural repair.
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| AXIAL VERSUS APPENDICULAR SKELETON |
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| AXIAL SKELETON (80 Bones) APPENDICULAR SKELETON (126 B.) |
| - Central longitudinal axis - Appendages and girdles |
| - Cranium & facial bones - Pectoral girdle (clavicle, |
| - Vertebral column (33 vertebrae) scapula) & upper limbs |
| - Thoracic cage (sternum & 12 (humerus, radius, ulna, |
| pairs of ribs) carpals, metacarpals) |
| - Primary role: Protect central - Pelvic girdle (coxal bones) |
| nervous system and vital organs & lower limbs (femur, patella|
| tibia, fibula, tarsals) |
| - Primary role: Locomotion |
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Microscopic and Macroscopic Bone Anatomy
A long bone, such as the human femur or humerus, features distinct anatomical regions:
- Epiphyses: The expanded ends of the bone, capped with smooth articular cartilage that reduces friction at synovial joints. The interior consists of spongy (trabecular) bone containing red bone marrow, the primary site of hematopoiesis (blood cell formation).
- Diaphysis: The central tubular shaft composed of dense compact bone engineered to resist bending and compressive loads. Inside lies the medullary cavity filled with yellow bone marrow, which stores triglycerides (fats).
- Periosteum: A tough fibrous vascular membrane enveloping the external surface of the bone, serving as the attachment anchor for tendons and ligaments while supplying osteoblasts for bone widening.
- The Osteon (Haversian System): The microscopic structural unit of compact bone. Concentric rings of calcified collagen matrix, called lamellae, surround a central Haversian canal carrying nerves and capillaries. Osteocytes (mature bone cells) reside inside tiny chambers called lacunae, communicating through microscopic fluid-filled canals called canaliculi.
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| THE OSTEON MICROSCOPIC ARCHITECTURE |
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| Central Haversian Canal (Nerve & Blood Vessel) |
| | |
| +--------------v--------------+ |
| / (.) (.) (.) \ |
| | (.) [Concentric] (.) (.)| |
| | (.) [ Lamellae ] (.) (.)| |
| | (.) (.) (.) (.) (.) | |
| \ (.) (.) (.) / |
| +-----------------------------+ |
| (.) = Osteocytes resting inside Lacunae |
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Joint Classification and Articulations
Bones articulate with one another at joints, categorized structurally by the material binding them:
- Fibrous Joints: Immovable sutures held by dense collagen, such as the cranial bones of the skull.
- Cartilaginous Joints: Slightly movable articulations joined by cartilage, such as the pubic symphysis and intervertebral discs.
- Synovial Joints: Freely movable joints enclosed in a fluid-filled synovial capsule. Examples include ball-and-socket joints (shoulder, hip) providing multi-directional rotation, and hinge joints (elbow, knee) providing planar flexion and extension. Ligaments are dense bands of fibrous connective tissue that connect bone to bone, stabilizing synovial joints against dislocation.
The Human Muscular System
The human muscular system contains over 600 skeletal muscles that comprise approximately forty percent of human body mass. Skeletal muscles are attached to bones by tendons, which are dense cords of collagen fibers that transmit tensile forces generated by contracting muscle fibers directly to bone levers.
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| THE THREE TYPES OF MUSCLE TISSUE |
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| Muscle Type | Microscopic View | Control & Physiological Role |
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| Skeletal Muscle | Striated, multinuclear| Voluntary; somatic motor |
| | cylindrical fibers | control; moves skeletal levers |
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| Cardiac Muscle | Striated, branched, | Involuntary; autonomic and |
| | intercalated discs | nodal control; pumps blood |
+--------------------+-----------------------+--------------------------------+
| Smooth Muscle | Non-striated, spindle | Involuntary; autonomic control;|
| | shaped, single nucleus| regulates visceral diameter |
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The Sliding Filament Mechanism of Muscle Contraction
At the microscopic level, each muscle fiber contains bundles of cylindrical myofibrils divided into repeating contractile segments called sarcomeres. The boundaries of each sarcomere are marked by dark protein bands termed Z-lines.
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| THE SLIDING FILAMENT MECHANISM |
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| RELAXED SARCOMERE: |
| Z-Line Z-Line |
| |--------> Thin Filament (Actin) <--------| |
| | ======================== | |
| | Thick Filament (Myosin) | |
| |--------> Thin Filament (Actin) <--------| |
| |
| CONTRACTED SARCOMERE: |
| Z-Line Z-Line |
| |---> Actin Actin <---| |
| | ======================= | |
| | Myosin pulls actin in! | |
| |---> Actin Actin <---| |
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Contraction proceeds according to the sliding filament theory:
- When a somatic motor neuron releases acetylcholine at the neuromuscular junction, an action potential spreads across the muscle sarcolemma and down transverse tubules, triggering the sarcoplasmic reticulum to release stored calcium ions (Ca2+).
- Calcium binds to the regulatory protein troponin, causing tropomyosin to shift and expose active binding sites along the thin actin filaments.
- Myosin heads, energized by ATP hydrolysis into ADP and inorganic phosphate, bind to the exposed actin sites, forming cross-bridges.
- The myosin heads flex forward in a conformational movement called the power stroke, pulling the thin actin filaments toward the center of the sarcomere (M-line), shortening the distance between Z-lines.
- A new ATP molecule binds to the myosin head, causing it to detach from actin, recock, and repeat the cycle as long as calcium and ATP remain abundant.
Biomechanics: Antagonistic Muscle Pairs
Muscles can only exert mechanical force by pulling; they cannot actively push. Therefore, skeletal movement requires antagonistic muscle pairs arranged across a joint:
- The Flexor contracts to decrease the angle of the joint (e.g., the biceps brachii flexes the elbow).
- The Extensor contracts to increase the angle of the joint, returning the limb to its extended position (e.g., the triceps brachii extends the elbow while the biceps relaxes).
Exercises and Solutions
Exercise 1: Deconstructing Musculoskeletal Lever Action in Human Movement. When lifting a heavy dumbbell during an arm curl, the elbow joint functions as the fulcrum, the forearm acts as the rigid lever arm, the dumbbell in the hand provides the downward resistance load, and the biceps brachii provides the upward effort force via its insertion on the radial tuberosity. (a) Identify which class of lever (first-class, second-class, or third-class) the human elbow joint represents; (b) Explain why this anatomical lever configuration operates at a mechanical disadvantage for force but a mechanical advantage for speed and range of motion.
Solution 1: (a) The elbow joint during a biceps curl represents a third-class lever because the effort force (applied by the biceps tendon pulling on the radius) is located between the fulcrum (the elbow joint axis) and the resistance load (the dumbbell held in the hand). (b) A third-class lever operates at a mechanical disadvantage for force because the effort arm distance (from the elbow fulcrum to the biceps insertion, roughly 5 centimeters) is much shorter than the resistance arm distance (from the elbow fulcrum to the weight in the palm, roughly 35 centimeters). Consequently, the biceps muscle must generate significantly greater contractile tension than the actual mass of the dumbbell being lifted. However, this configuration provides a significant mechanical advantage in speed and distance: a small 2-centimeter contraction of the biceps muscle produces an expansive 20-centimeter sweep of the hand, allowing human limbs to move quickly and smoothly through large arcs of motion.
Exercise 2: Evaluating the Cellular Pathology of Rigor Mortis. Following clinical death, the body gradually enters a state of muscular stiffness known as rigor mortis, which begins roughly three hours post-mortem and peaks at twelve hours before gradually subsiding. Using the sliding filament mechanism and cellular ATP dynamics, explain why muscles lock into permanent contraction following death, and explain why the stiffness eventually dissipates.
Solution 2: Rigor mortis is caused by the complete cessation of aerobic respiration and cellular ATP synthesis following death. Without active metabolic generation of ATP, the calcium active transport pumps in the sarcoplasmic reticulum membrane shut down. Calcium ions leak passively out of storage into the sarcoplasm along their concentration gradient, binding to troponin and permanently shifting tropomyosin to expose actin binding sites. The myosin heads bind to actin and execute their power stroke using residual cellular ADP. However, detaching a myosin head from an actin filament requires a new molecule of fresh ATP to bind to the myosin ATPase site. Because dead cells cannot synthesize ATP, myosin heads remain permanently locked in their cross-bridge attachments, fixing all skeletal muscles into rigid contraction. Stiffness eventually subsides after twenty-four to thirty-six hours because lysosomes burst, releasing acid hydrolases that decompose and digest the actin and myosin protein filaments, causing structural muscle breakdown.