Skeletal muscle structure
Muscles make up about 40% of body mass. Their layered structure, from whole muscle down to protein filaments, explains how they generate force.
Functions of muscle
Muscles move the body, maintain posture, stabilise joints, and generate heat, which is why you shiver when cold. Smooth muscle moves food and regulates blood vessel diameter; cardiac muscle pumps blood. This unit focuses on skeletal muscle.
From muscle to fibre
A whole muscle is wrapped in connective tissue (epimysium) and divided into bundles called fascicles, each wrapped in perimysium. Each fascicle holds many muscle fibres, individual long cells, wrapped in endomysium. These connective layers merge into tendons that attach muscle to bone.
Inside a muscle fibre
A muscle fibre is packed with myofibrils, long rods made of repeating units called sarcomeres. The fibre's membrane is the sarcolemma, and its specialised endoplasmic reticulum, the sarcoplasmic reticulum, stores calcium ions. T-tubules carry electrical signals deep into the fibre.
The sarcomere
The sarcomere is the basic contractile unit. Thick filaments are made of myosin; thin filaments are made of actin, anchored to Z discs at each end. The overlapping arrangement creates the light and dark bands that make skeletal muscle look striated. When a muscle contracts, every sarcomere shortens.
Sliding filament theory
During contraction, filaments do not shorten; they slide past each other. Myosin heads bind to actin, forming cross-bridges, pivot to pull the thin filaments toward the centre of the sarcomere, release, and rebind further along, like a team pulling a rope hand over hand. This pulls the Z discs closer together.
Origin, insertion, and actions
A muscle's origin is its attachment to the more stationary bone; its insertion is on the bone that moves. The prime mover (agonist) does most of the work; the antagonist opposes it; synergists assist. Muscles can only pull, never push, so movements are reversed by opposing muscles.
Naming muscles
Muscle names often describe them: by location (tibialis anterior, near the tibia), shape (deltoid, triangular), size (gluteus maximus, largest), number of origins (biceps has two heads, triceps three), direction of fibres (rectus means straight), or action (flexor, extensor).
Worked example
When you curl a dumbbell, the biceps brachii is the agonist, flexing the elbow. Its origin is on the scapula, its insertion on the radius. The triceps brachii is the antagonist and relaxes. To lower the weight under control, the biceps lengthens while still active.
Key takeaways
Muscle moves, stabilises, and generates heat. Skeletal muscle is organised as muscle, fascicles, fibres, myofibrils, and sarcomeres, wrapped in connective tissue. Sarcomeres contain myosin thick and actin thin filaments. Contraction happens as filaments slide via cross-bridges. Muscles pull from origin toward insertion, working in agonist-antagonist pairs.
Flashcard
Single skeletal muscle cell
Check your understanding
During muscle contraction, what happens to the length of the actin and myosin filaments?