Difficulty: Intermediate | Prerequisites: Basic cell biology, protein structure
This chapter sits at the intersection of cell biology and organismal physiology. You need to understand how individual protein interactions (actin and myosin) scale up to produce whole-body movement, and how different muscle types serve different physiological roles. If you are comfortable with protein structure and ATP hydrolysis from earlier chapters, you are ready. The material here connects directly to topics on the nervous system (which triggers contraction) and to animal form and function more broadly.
Muscles contract when myosin heads on thick filaments pull actin thin filaments inward, shortening sarcomeres. Three muscle types (skeletal, cardiac, smooth) handle different jobs. Animals support and move their bodies using one of three skeleton types: hydrostatic, exo-, or endoskeletons.
Thin filaments
Filaments composed of the globular protein actin. They are the tracks along which myosin heads walk during contraction, and they also play roles in general cell motility. Think of them as the rails in the sliding-filament system.
Thick filaments
Staggered arrays of myosin molecules. Each myosin has a head that binds actin and hydrolyses ATP to generate force. Think of them as the motors.
Myofibrils
Cylindrical bundles inside muscle cells (fibres) that contain the organised thick and thin filaments. A single muscle fibre holds many myofibrils running in parallel.
Sarcomere
The basic contractile unit of skeletal muscle, defined as the region between two Z-discs (Z-lines). When a muscle contracts, every sarcomere shortens simultaneously. In simple terms, it is one repeating "unit" of the striped pattern you see in striated muscle.
Sliding-filament model
The model describing how contraction works: thin and thick filaments do not themselves shorten. Instead, myosin heads ratchet the thin filaments inward past the thick filaments, pulling the Z-discs closer together and shortening the sarcomere.
Transverse tubules (T-tubules)
Infoldings of the muscle cell's plasma membrane that carry the action potential deep into the interior of the fibre, ensuring the signal reaches every myofibril at once.
Sarcoplasmic reticulum (SR)
A specialised form of the endoplasmic reticulum found in muscle cells. It stores and releases calcium ions (Ca²⁺), which are the immediate trigger for contraction.
Tropomyosin
A regulatory protein that winds along the actin filament. At rest, it blocks the myosin-binding sites on actin. Think of it as a safety latch.
Troponin complex
A set of regulatory proteins bound to the actin strands of thin filaments. When Ca²⁺ binds troponin, it shifts tropomyosin out of the way, exposing the binding sites so contraction can proceed.
Myoglobin
An oxygen-storing protein found in muscle tissue. It holds a reserve of O₂ for use during sustained contraction, particularly in slow-twitch fibres. Think of it as the muscle's personal oxygen tank.
Fast-twitch fibres (Type II)
Muscle fibres that enable brief, rapid, powerful contractions. They fatigue quickly, rely more on anaerobic metabolism, and are prominent in sprinters and animals that need explosive movement.
Slow-twitch fibres (Type I)
Muscle fibres that sustain long, steady contractions. They are rich in mitochondria and myoglobin, rely on aerobic metabolism, and dominate in endurance activities like distance running or maintaining posture.
Muscle organ → bundles of muscle fibres (cells)
Each fibre contains many myofibrils running lengthwise
Each myofibril is a chain of sarcomeres, the repeating contractile units
Each sarcomere contains interleaved thick filaments (myosin) and thin filaments (actin)
An action potential travels along the motor neuron to the neuromuscular junction.
The signal propagates along the muscle fibre membrane and dives into the T-tubules.
T-tubule depolarisation triggers the sarcoplasmic reticulum to release Ca²⁺ into the cytoplasm.
Ca²⁺ binds the troponin complex, which shifts tropomyosin off the myosin-binding sites on actin.
Myosin heads (energised by ATP hydrolysis) bind exposed actin sites, forming cross-bridges.
The myosin head pivots (the "power stroke"), pulling the thin filament toward the centre of the sarcomere.
A fresh ATP molecule binds myosin, releasing it from actin. ATP is hydrolysed, re-cocking the head for the next cycle.
The cycle repeats as long as Ca²⁺ and ATP are available. When the signal stops, Ca²⁺ is pumped back into the SR, tropomyosin re-covers the binding sites, and the muscle relaxes.
The key insight: the filaments do not shorten. They slide past each other, pulling the Z-discs closer together. This is why it is called the sliding-filament model.
Attached to bones via tendons; responsible for voluntary movement and posture
Striated (sarcomeres produce a banded appearance under the microscope)
Multinucleated, because each fibre forms from the fusion of many precursor cells
Under voluntary (somatic) nervous system control
Contains both fast-twitch and slow-twitch fibres in varying proportions depending on the muscle's role
Found only in the heart
Striated, like skeletal muscle, but cells are shorter and typically have a single nucleus
Cells are connected by intercalated discs, which contain gap junctions that allow direct electrical coupling between cells. This is why the heart can beat as a coordinated unit.
Involuntary; the heart's own pacemaker cells set the rhythm, modulated by the autonomic nervous system
Found in the walls of hollow organs: blood vessels, the digestive tract, the bladder, the uterus
Not striated (lacks the ordered sarcomere arrangement)
Involuntary; controlled by the autonomic nervous system, hormones, and local chemical signals
Responsible for peristalsis, the rhythmic wave-like contractions that move food through the digestive tract
Contracts more slowly than skeletal muscle but can sustain contraction for much longer periods
Muscles can only pull (contract); they cannot push. Every movement therefore requires something for the muscle to pull against. That something is the skeleton.
Found in soft-bodied invertebrates such as earthworms, cnidarians, and flatworms
Consists of fluid held under pressure in a closed body compartment
Muscles arranged in circular and longitudinal layers contract against the incompressible fluid, changing the body's shape to produce movement
No rigid elements, so the animal can squeeze through tight spaces
A hard external encasement deposited on the animal's outer surface
Found in arthropods (insects, crustaceans, spiders); composed largely of chitin, a polysaccharide similar to cellulose
Muscles attach to the inner surface of the exoskeleton and pull across joints
Must be moulted (shed) for the animal to grow, which is a vulnerable period
A hardened internal skeleton buried within the animal's soft tissues
Found in vertebrates (bone and cartilage) and some invertebrates (echinoderms, sponge spicules)
Grows with the animal, so moulting is unnecessary
Muscles attach to bones via tendons and move the skeleton at joints
Locomotion is the active travel of an animal from place to place. It requires the coordination of muscles and skeleton and is shaped by the medium the animal moves through: water, land, or air.
The sliding-filament model underpins how we understand rigor mortis: after death, ATP is depleted, so myosin heads lock onto actin and cannot release, leaving muscles stiff.
Fast-twitch vs. slow-twitch fibre composition explains why some people excel at sprinting while others are better suited to marathon running. Training can shift the metabolic profile of fibres but not convert one type to the other wholesale.
Smooth muscle pharmacology is central to medicine: drugs that relax smooth muscle in blood vessel walls lower blood pressure, while drugs that stimulate uterine smooth muscle induce labour.
Understanding chitin-based exoskeletons has informed the development of biodegradable packaging materials and wound dressings.
Students often think the filaments themselves shorten during contraction. They do not. The thin filaments slide inward along the thick filaments; the filament lengths stay the same.
Students frequently confuse the sarcoplasmic reticulum with the endoplasmic reticulum in other cell types. The SR is a specialised version of the ER, adapted specifically for rapid Ca²⁺ storage and release in muscle cells.
It is common to assume that cardiac muscle is under voluntary control because you can "feel" your heartbeat speed up. Cardiac muscle is involuntary. What you feel is modulation by the autonomic nervous system, not conscious control.
Students sometimes think exoskeletons are heavier than endoskeletons. For small animals, exoskeletons are proportionally lighter and more efficient. The scaling problem (exoskeletons become impractically heavy at large body sizes) is why you do not see insect-sized elephants in exoskeletal armour.
⚠️ Be able to trace the full sequence from action potential → T-tubule → SR Ca²⁺ release → troponin/tropomyosin shift → cross-bridge cycling → relaxation. This is a classic multi-step exam question.
⚠️ Know the differences among skeletal, cardiac, and smooth muscle (striation, nuclei count, voluntary vs. involuntary, location). Comparison tables are common on exams.
⚠️ Understand why ATP is needed both for the power stroke (myosin head pivot) and for releasing myosin from actin. The role of ATP in detachment, not just attachment, trips students up.
⚠️ Be ready to compare the three skeleton types (hydrostatic, exo-, endo-) and give an example organism for each.
⚠️ The distinction between fast-twitch and slow-twitch fibres, including their metabolic profiles (aerobic vs. anaerobic) and where each is advantageous, is frequently tested.
True or False: During contraction, the thick and thin filaments become shorter.
False. The filaments slide past each other; their lengths remain the same.
Fill in the blank: The sarcoplasmic reticulum releases ______ ions to initiate contraction.
Calcium (Ca²⁺)
True or False: Cardiac muscle cells are connected by intercalated discs containing gap junctions.
True.
Fill in the blank: An exoskeleton is made primarily of ______, a polysaccharide similar to cellulose.
Chitin
True or False: Slow-twitch fibres rely primarily on anaerobic metabolism.
False. Slow-twitch fibres are aerobic; fast-twitch fibres rely more on anaerobic pathways.
Q: Describe the sequence of events from the arrival of an action potential at a muscle fibre to the initiation of cross-bridge cycling.
A: The action potential propagates along the sarcolemma and into the T-tubules. T-tubule depolarisation causes the sarcoplasmic reticulum to release Ca²⁺. Ca²⁺ binds to the troponin complex on the thin filament, shifting tropomyosin away from the myosin-binding sites on actin. Myosin heads, already energised by ATP hydrolysis, bind to the exposed sites, and cross-bridge cycling begins.
Q: Why does ATP need to be present for a muscle to relax, not just to contract?
A: ATP is required for myosin to detach from actin after the power stroke. Without ATP, myosin heads remain locked to actin (this is the molecular basis of rigor mortis). ATP binding causes the myosin head to release, and subsequent hydrolysis re-cocks the head for the next cycle.
Q: Compare the three types of skeletal support in animals. For each, name a representative organism and one advantage of that skeleton type.
A: Hydrostatic skeleton (earthworm): allows the animal to change shape and squeeze through narrow spaces. Exoskeleton (crab or insect): provides external protection and a waterproof barrier. Endoskeleton (human or fish): grows with the animal, eliminating the need to moult.
Q: A researcher observes a muscle that is striated, involuntary, and connected by intercalated discs. What type of muscle is it?
A: Cardiac muscle. Skeletal muscle is striated but voluntary. Smooth muscle is involuntary but not striated. Only cardiac muscle is both striated and involuntary, and intercalated discs are unique to cardiac tissue.
Q: Explain why an arthropod must moult in order to grow, and identify one risk associated with moulting.
A: The exoskeleton is a rigid external casing that does not expand. The animal must shed it periodically and secrete a new, larger one. During and immediately after moulting, the new exoskeleton is soft, leaving the animal vulnerable to predation and physical damage.
This material connects directly to the nervous system (how action potentials trigger contraction), to cellular respiration (ATP supply for cross-bridge cycling and the aerobic vs. anaerobic distinction between fibre types), and to animal form and function (how body plans and skeleton types constrain movement). The calcium signalling pathway here is a specific instance of the broader second-messenger signalling you encounter in cell biology.
Actin, myosin, sarcomere, sliding-filament model, cross-bridge cycle, T-tubules, sarcoplasmic reticulum, calcium signalling, tropomyosin, troponin, myoglobin, fast-twitch, slow-twitch, Type I fibres, Type II fibres, skeletal muscle, cardiac muscle, smooth muscle, intercalated discs, peristalsis, exoskeleton, endoskeleton, hydrostatic skeleton, chitin, locomotion, muscle contraction, striated muscle, voluntary muscle, involuntary muscle, neuromuscular junction, rigor mortis, Z-disc, Z-line, motor unit