Lecture 17: Muscle & Skeletons

What is vertebrate skeletal muscle?

Skeletal muscle is an evolutionary innovation that gave vertebrates speed, endurance, and fine motor control.

Its structure – from actin and myosin filaments to whole muscle groups – determines how a species moves through its environment.

What is the structure and function of vertebrate skeletal muscle?

Vertebrates skeletal muscles are attached to bones of the skeleton by cords of connective tissues (tendons) and are controlled voluntarily by the somatic nervous system.

Depending on its points of attachment, contraction of a single skeletal muscle may extend or bend body parts, or may rotate one body part with respect to another.

A skeletal muscle consists of bundles of elongated, multinucleate cells (muscle fibers), which run the entire length of the muscle.

Muscle fibers are held in parallel bundles by sheaths of connective tissue, which merge with the tendons that connect muscles to bones or other structures – supplied with nutrients and oxygen by an extensive network of blood vessels that penetrates the muscle tissue.

What are myofibrils?

Muscle fibers are packed with cylindrical contractile elements (myofibrils) that run lengthwise inside the cells.

Each myofibril consists of thick filaments (13-18 nm in diamater) and thin filaments (5-8 nm in diameter).

The arrangement of thick and thin filaments forms a pattern of alternating dark bands and light bands, giving skeletal muscle a striated appearance.

What are thick filaments?

Thick filaments are parallel bundles of myosin molecules.

• Myosin has heads that act like oars → pull on actin.

• Myosin heads bind to actin to form cross-bridges.

• ATP binds to myosin → makes the head prime + power stroke.

What are thin filaments?

Thin filaments contain two linear chains of actin molecules twisted into a double helix. Regulatory proteins:

• Tropomyosin blocks binding sites on actin.

• Troponin binds Ca2+ to move tropomyosin out of the way.

What are striation bands?

Alternating light and dark bands in muscle tissue that give it a striped appearance. These bands are formed by the organized arrangement of protein filaments within the sarcomeres, the basic contractile units of muscle fibers.

What are the different types of striation bands?

A bands = dark bands at both ends of stacked thick filaments where thin filaments overlap.

H zone = lighter middle region of an A band, which contains only thick filaments.

M line = a disc of proteins in the center of the H zone, which holds the stack of thick filaments together.

I bands = light bands consisting of the parts of the thin filaments not in the A band.

Z line = a thin disc in the center of each I band to which the thin filaments are anchored.

The region between 2 adjacent lines is a sarcomere – the basic unit of contraction in a myofibril.

What do striations do?

Striations allow for extremely fast, powerful, and controlled contractions.

Because sarcomeres are arranged end-to-end like repeating units, when calcium triggers contraction, all sarcomeres shorten together.

→ This produces forceul movement – ideal for locomotion.

How did muscle evolve?

Smooth muscle came first evolutionarily (only contractile protein present = actin/myosin, but in a disorganized lattice → slow and constant contractions).

Striated muscle evolved later around 550 million years ago, in early bilaterians (animals with bilateral symmetry).

It allowed rapid, directional movement – critical during the Cambrian explosion, when predation and active locomotion appeared.

When do muscles contract?

Muscles contract when calcium is present and relax when it’s removed.

Skeletal muscles are voluntary and connect to bone and have multiple nuclei.

What is flexion?

When the biceps muscle (the flexor muscle) contracts and raises the forearm, its antagonistic partner, the triceps muscle (the extensor muscle), relaxes.

What is extension?

When the triceps muscle contracts and extends the forearm, the biceps muscle relaxes.

How does muscle contraction work?

NMJ (signal), T-tubules (delivery), and SR (calcium release) work together to turn a neuron’s electrical message into mechanical force.

What is sarcoplasmic reticulum (SR)?

Specialized smooth ER in muscle fibers that stores calcium (Ca2+).

Ca2+ releases from SR → initiates muscle contraction.

The cell membrane folds inward to form T-tubules, which carry the action potential deep into the muscle fiber so every sarcomere contracts at the same time.

What is the neuromuscular junction (NMJ)?

Synapse between a motor neuron and a muscle fiber.

Motor neuron releases acetylcholine (ACh) → ACh binds receptors on the muscle → triggers an action potential in the muscle cell membrane.

What is a muscle twitch?

A single action potential arriving at a neuromuscular junction causes a single, weak contraction called a muscle twitch.

What is tetanus?

When action potentials arrive so rapidly (about 25 msec apart) that the fiber can’t relax at all between stimuli, and twitch summation produces a peak level of continuous contraction.

What is eye twitching?

Eyelid twitching = normal muscle fasciculation.

A small group of muscle fibers are firing involuntarily due to spontaneous action potentials.

Common triggers = stress, fatigue/lack of sleep, too much caffeine.

What is the evolution of muscle contraction?

Sliding filament mechanism (actin + myosin cross-bridges) is ancient and conserved across almost all animals.

Actin and myosin predate animals – they first evolved in unicellular eukaryotes for movements and intracellular transport.

Over evolutionary time, this basic contractile mechanism was repurposed for:

• Flagella/cilia motion (protists)

• Digestion and vessel constriction (smooth muscle)

• Fast locomotion (striated skeletal muscle)

• Endurance rhythmic contraction (cardiac muscle)

What is the ecology of muscle contraction?

Predator species → need extreme speed & power to capture prey → more fast-twitch fibers (rapid Ca2+ release and tetanus ability).

Prey species → need endurance and sustained contraction to escape → more slow-twitch fibers (mitochondria, high myoglobin).

Burrowers → pressurized, long-lasting contractions → more smooth muscle and slow ATP cycling.

Aquatic animals → efficient contraction for swimming waves → highly organized sarcomeres along myotomes.

What are exoskeletons?

Rigid external body covering that supports and protects internal tissues.

Muscle contraction force is applied against the exoskeleton.

Many molluscs (clams, oysters) have an exoskeleton consisting of a hard calcium carbonate shell secreted by glands in the mantle.

Arthropods (insects, spiders, crustaceans) have a chitinous cuticle secreted by underlying tissue – joints are moved by muscles that extend across the inside surfaces.

What are the exoskeletons of arthropods like?

Arthropods (insects, spiders, crustaceans) have an external skeleton made of chitin.

Because the skeleton is external the muscles attach to the inside at inward-pointing ridges called apodemes.

Apodemes function like internal anchor points.

When the muscles contracts it pulls the skeleton from the inside, causing movement.

How did skeletons evolve in different species?

Molluscs (squid, cuttlefish)

• Internal cartilaginous structures reinforce the mantle, gills, and siphon.

• Protects the brain and helps maintain body shape during jet propulsion.

Echinoderms (sea stars, sand dollars, sea urchins)

• Endoskeletons made of ossicles (plates of calcium carbonate).

• Sand dollars and sea urchins have fused ossicles forming a rigid internal “shell.”

Vertebrates (fish, mammals)

• Endoskeleton made of bone and cartilage.

• Major functions: supports body and anchors muscles; protect organs (skull, ribs, vertebrae); stores calcium and phosphate; houses bone marrow → cell formation.

How is the endoskeleton organized?

Bones composed of = several types of bone cells; blood vessels; nerves; stores of adipose tissue in some bones.

Bone tissue distributed between compact bone, which is dense and generally forms the outer surfaces; spongy bone, which is more open and found in interior regions.

The interior of some flat bones are filled with red marrow (which is the primary source of new red blood cells in mammals and birds).