Introduction to Behavioral Neuroscience · Motor Control
The Physiological Actions Implementing Movement – Contraction of Muscles
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In 30 seconds
Every movement you make — typing, walking, smiling — ends in the same place: skeletal muscle contraction. Skeletal muscles are the only organs that produce voluntary movement, and they do it by shortening. The whole motor system — from the motor cortex to the spinal cord — exists to control a single final step: making muscle fibers contract, with the right timing, force, and coordination.
Muscle contraction works at the molecular level through the sliding filament mechanism. Muscle fibers contain two types of protein filaments: thin filaments made mostly of actin and thick filaments made of myosin. When a muscle contracts, myosin heads attach to actin, pivot, and pull the thin filaments toward the center of the sarcomere The repeating contractile unit of a muscle fiber, from Z-disc to Z-disc Full entry → (the repeating contractile unit), shortening the muscle without the filaments themselves shrinking. This process is driven by ATP and switched on by calcium.
The chain of events from nerve to contraction is called excitation–contraction coupling: an action potential arrives at the neuromuscular junction, releases acetylcholine, triggers an action potential in the muscle fiber membrane, which travels down T-tubules into the fiber, releases calcium from the sarcoplasmic reticulum, and calcium finally turns on the molecular machinery. Understanding this chain explains muscle twitches, tetanus (sustained contraction), fatigue, rigor mortis Post-mortem stiffening from ATP depletion and locked cross-bridges Full entry →, and a range of neuromuscular disorders.
Why this matters
- It is the output stage of the entire nervous system. Every motor command, reflex, and posture adjustment ultimately does one thing: change muscle contraction. The brain's plans are meaningless until they produce muscle force.
- Explains everyday physiology. Why do muscles fatigue? Why does a muscle twitch when a nerve fires once? Why does a dead body stiffen (rigor mortis)? All of these follow directly from the sliding filament and excitation–contraction coupling machinery.
- Foundation for motor control topics. This chapter's later topics (lower motoneurons, reflex arcs, and upper motor systems) all assume you know what a muscle does when it receives input. The language of motor units, twitch, and graded contraction is used throughout.
- Clinical relevance. Myasthenia gravis (autoimmune attack on acetylcholine receptors), muscular dystrophy (dystrophin defects), malignant hyperthermia (uncontrolled calcium release), and even muscle cramps are all disorders of this machinery.
- Exam favorite. Expect questions on sarcomere structure, the roles of ATP and calcium, the sequence of excitation–contraction coupling, and the difference between twitch, summation, and tetanus.
The college version
Core Concepts
Skeletal muscle architecture: from whole muscle to filaments
A skeletal muscle is a bundle of muscle fibers (muscle cells — long, multinucleated, and striated). Each fiber is packed with myofibrils, chains of repeating units called sarcomeres — the smallest contractile unit. A sarcomere runs from one Z-disc to the next and contains:
- Thin filaments (actin plus the regulatory proteins troponin Calcium-sensitive protein on the thin filament Full entry → and tropomyosin Protein that blocks myosin-binding sites on actin Full entry →) anchored at the Z-discs.
- Thick filaments (myosin) centered in the sarcomere, with heads that can bind actin.
In cross-section the filaments overlap in a characteristic pattern (the A-band is the myosin region, the I-band the actin-only region), and the overlap pattern changes with contraction — the I-band shortens while the A-band stays constant. That observation was key evidence for the sliding filament model.
The sliding filament mechanism
Contraction is a ratchet-like cycle:
- cross-bridge Myosin head bound to actin formation — myosin heads bind to actin.
- Power stroke — the myosin head pivots, pulling the thin filament Actin filament with troponin and tropomyosin Full entry → toward the sarcomere center; ADP is released.
- Detachment — a new ATP molecule binds myosin, causing it to release actin.
- Recocking — ATP hydrolysis re-energizes the head into its "cocked" position, ready to bind the next actin site.
Each cycle shortens the sarcomere a tiny amount; many cycles per second across thousands of sarcomeres produce visible movement. Without ATP, myosin cannot detach from actin — which is why rigor mortis stiffens the body after death: ATP is exhausted and cross-bridges are locked in place.
Turning contraction on: the role of calcium and regulatory proteins
Actin's myosin-binding sites are normally blocked by tropomyosin, with troponin acting as the calcium-sensitive switch. When cytosolic calcium rises, calcium binds troponin, tropomyosin shifts aside, and myosin can bind actin. When calcium is pumped back into the sarcoplasmic reticulum (SR), tropomyosin returns, and relaxation occurs. So calcium concentration is the master switch for contraction — and calcium pumps consume ATP, which is why relaxation also requires energy.
Excitation–contraction coupling: from nerve impulse to shortening
The link between the muscle fiber's electrical signal and its contraction:
- A motor neuron action potential arrives at the neuromuscular junction and releases acetylcholine (ACh).
- ACh binds nicotinic receptors on the muscle membrane, producing an end-plate potential Depolarization of muscle membrane caused by ACh binding Full entry → large enough to trigger a muscle action potential.
- The action potential sweeps along the muscle membrane and down T-tubules (invaginations of the membrane that reach deep into the fiber).
- T-tubule depolarization triggers ryanodine receptors on the SR to open, releasing calcium into the cytosol.
- Calcium binds troponin → tropomyosin shifts → cross-bridge cycling runs → contraction.
- Relaxation follows as calcium is pumped back into the SR.
Because a single action potential normally releases enough calcium to trigger a twitch, and the twitch outlasts the action potential, a second nerve impulse arriving before relaxation adds force — temporal summation. Rapid, repeated stimulation produces a smooth, sustained contraction called tetanus (not to be confused with the bacterial disease). Normal voluntary contractions are essentially tetanic: the nervous system drives motoneurons fast enough to fuse individual twitches.
Motor units and graded force
A motor unit One motoneuron plus the muscle fibers it innervates Full entry → is one motoneuron plus all the muscle fibers it innervates. Force is graded in two ways:
- Recruitment — activating more motor units (the nervous system's main way to increase force; small units are recruited before large ones — the size principle, covered in the next topic).
- Rate coding — increasing the firing rate of already-active motoneurons, which raises force by summation toward tetanus.
Muscle fibers within a motor unit are of one type: slow-oxidative fibers (endurance, fatigue-resistant, smaller force), fast-oxidative fibers (moderate speed and endurance), and fast-glycolytic fibers (fast and powerful but quick to fatigue). Muscle fiber type is largely determined by the motoneuron that innervates it — an example of neural influence on muscle properties.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Muscle contraction and filament shortening | Filaments getting shorter | Filaments slide past each other; the sarcomere shortens while the filaments keep their length |
| Calcium's role and ATP's role | Both are "needed for contraction" | Calcium is the on/off switch; ATP is the fuel that powers the stroke, detaches myosin, and drives relaxation |
| Temporal summation and recruitment | Two ways to increase force | Summation = one motor unit fires faster (twitches fuse); recruitment = more motor units turn on |
| Tetanus (physiology) and tetanus (disease) | Same word, different meanings | Physiological tetanus is normal fused contraction; the disease is caused by a bacterial toxin that blocks inhibitory transmission, causing uncontrolled spasms |
| Acetylcholine and the end-plate potential | The signal vs. its effect | ACh is the neurotransmitter; the end-plate potential is the depolarization it produces in the muscle membrane |
| Slow-twitch and fast-twitch fiber "speed" | Only about contraction speed | Fiber types also differ in force, fatigue resistance, and ATP source (oxidative vs. glycolytic) |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Your muscles work like a team of tiny rope-pullers. Inside each muscle, small "thin" ropes and "thick" ropes slide past each other to make the muscle shorter — like pulling the two ends of a telescope together. A message from your brain tells the muscle to release a special key (calcium) that lets the rope-pullers grab and pull. When the key is put away, the muscle relaxes.
Worked example
Picture lifting a heavy suitcase. Your motor cortex plans the lift and sends commands down to the spinal cord; lower motoneurons fire rapid bursts of action potentials at the biceps and forearm muscles. At each neuromuscular junction, acetylcholine opens nicotinic receptors, and within milliseconds the muscle membrane fires action potentials that race down the T-tubules. Calcium floods out of the sarcoplasmic reticulum, troponin catches it, tropomyosin steps aside, and myosin heads begin their pull-release-pull cycle — hundreds of cross-bridges per filament, thousands of sarcomeres shortening together. The heavier the suitcase, the more motor units the nervous system recruits and the faster it fires them, driving force toward full tetanus.
Now imagine the same scenario one hour later: the suitcase feels heavier, and your arm trembles. The muscle fibers are fatiguing — ATP resynthesis can't keep pace with demand, calcium handling becomes less efficient, and the power stroke weakens. If the weight were extreme and ATP ran out entirely, the cross-bridges would lock: the muscle would stiffen in place until ATP was restored. The same chemistry explains rigor mortis after death — when ATP is gone for good, the muscles cannot relax.
Finally, consider a patient with myasthenia gravis: their immune system attacks acetylcholine receptors at the NMJ. The end-plate potential becomes too small to reliably trigger muscle action potentials, so commands that should produce strong contraction produce weakness and fatigue instead — treatable in part by drugs that slow ACh breakdown, giving each ACh molecule more time to act. One disease, and suddenly every step of the coupling chain matters clinically.
Key takeaways
- Sliding filament model: myosin heads pull actin filaments toward the sarcomere center; filaments slide, they don't shrink.
- Sarcomere = repeating unit from Z-disc to Z-disc; I-band shortens during contraction, A-band does not.
- Calcium is the on-switch: Ca²⁺ binds troponin, tropomyosin moves, myosin binds actin. No calcium, no contraction.
- ATP plays three roles: powers the power stroke, detaches myosin from actin, and drives the calcium pumps needed for relaxation. No ATP → rigor.
- Excitation–contraction coupling order: ACh at NMJ → end-plate potential → muscle action potential → T-tubules → SR calcium release → troponin → contraction.
- A single nerve impulse → twitch; rapid firing → summation → tetanus. Voluntary contractions are tetanic.
- Force is graded by recruitment (size principle) and rate coding.
- Motor units contain one fiber type — slow-oxidative (endurance), fast-oxidative, or fast-glycolytic (power, fatigable).
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
List, in order, the events of excitation–contraction coupling from the motoneuron action potential to contraction.
Show answer
Motoneuron AP → ACh release at NMJ → ACh binds nicotinic receptors → end-plate potential → muscle AP → AP travels down T-tubules → SR releases Ca²⁺ → Ca²⁺ binds troponin → tropomyosin shifts → cross-bridge cycling → contraction; Ca²⁺ pumps restore relaxation.
Why does rigor mortis happen, and what does it tell you about ATP's role?
Show answer
After death, ATP is depleted. Myosin cannot detach from actin without ATP, so cross-bridges stay locked and muscles stiffen. This shows ATP is required for cross-bridge detachment (and for relaxation).
A single nerve impulse produces a twitch. How does the nervous system produce a strong, smooth contraction instead?
Show answer
By firing the motoneuron rapidly so twitches sum (temporal summation) into a fused, sustained tetanic contraction, and by recruiting additional motor units for more force.
What changes visibly in a sarcomere during contraction — the I-band, the A-band, or both? Why?
Show answer
The I-band shortens (actin-only zone narrows as filaments slide inward); the A-band (myosin zone) stays constant. That pattern is classic evidence for sliding filaments.
Name the three skeletal muscle fiber types and one property of each.
Show answer
Slow-oxidative (slow, fatigue-resistant, endurance), fast-oxidative (fast, moderate fatigue resistance), fast-glycolytic (fast, powerful, quick to fatigue).
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- sarcomere
- The repeating contractile unit of a muscle fiber, from Z-disc to Z-disc
- thin filament
- Actin filament with troponin and tropomyosin
- thick filament
- Myosin filament with cross-bridge heads
- cross-bridge
- Myosin head bound to actin
- troponin
- Calcium-sensitive protein on the thin filament
- tropomyosin
- Protein that blocks myosin-binding sites on actin
- neuromuscular junction (NMJ)
- The synapse between a motoneuron and a muscle fiber
- end-plate potential
- Depolarization of muscle membrane caused by ACh binding
- sarcoplasmic reticulum (SR)
- Calcium-storing organelle inside muscle fibers
- motor unit
- One motoneuron plus the muscle fibers it innervates
- tetanus (physiological)
- Sustained contraction from rapid, summed stimulation
- rigor mortis
- Post-mortem stiffening from ATP depletion and locked cross-bridges
Sources & references
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