Muscle Contraction Tournament
Two onboarding diagrams orient you in the muscle contraction network. Then eight MCAT-DoK quiz rounds: Ca2+ binding troponin C to unblock actin, why rigor mortis proves ATP is needed to relax, the cross-bridge cycle in order, the DHPR-RyR-SERCA calcium loop, SERCA stoichiometry, which sarcomere bands change during shortening, smooth muscle's calmodulin/MLCK alternative, and the length-tension relationship.
Where the Striated muscle contraction fits in Muscle contraction
Reactome splits muscle contraction into three sub-programs: striated muscle contraction (skeletal and cardiac, built on repeating sarcomeres), cardiac conduction (the electrical system that paces the heart), and smooth muscle contraction (no troponin, no sarcomeres). The highlighted striated panel is the sarcomere machinery the MCAT tests - thin and thick filaments, troponin and tropomyosin, and the ATP-driven cross-bridge cycle. Click the highlighted Striated muscle contraction panel to enter the tournament.
Click the highlighted Striated muscle contraction box to continue.
What this tournament tests
Each task maps to a distinct MCAT cognitive demand. The first two orient you in the broader topology; the next eight test the high-yield mechanism, regulation, sequence and quantitative reasoning that consistently appear on test day.
The Bigger Picture
Anchor striated muscle contraction inside the whole muscle contraction pathway on the live Reactome map.
Whole-Pathway Overview
Pan and zoom the curated WikiPathways striated muscle figure before you start answering.
Fill in the Blank
Recall that Ca2+ binds troponin C, which shifts tropomyosin off actin's myosin-binding sites.
Rigor Mortis Disruptor
Reason from ATP depletion to locked cross-bridges - ATP is required for detachment AND for SERCA-driven relaxation.
Sequence Ordering
Trace action potential -> T tubule -> DHPR -> RyR1 -> Ca2+ -> troponin C -> cross-bridge binding -> power stroke -> ATP-dependent detachment.
Match the Pairs
Pair each component (Z line, thick filament, tropomyosin, DHPR, RyR, SERCA, creatine phosphate, titin) with its exact job.
Numeric Input
Recall SERCA's P-type ATPase stoichiometry of 2 Ca2+ pumped into the SR per ATP.
Select All That Apply
Identify what happens to the I band, H zone, A band, and Z lines when a sarcomere shortens.
Odd One Out
Separate skeletal features (troponin C, striations, T tubule DHPR-RyR) from smooth muscle's myosin light chain kinase.
Length-Tension Disruptor
Explain why overstretching lowers active tension - fewer possible cross-bridges, the descending limb, and Frank-Starling.
Public leaderboard
Your score posts to a global, persistent leaderboard scored by points first, time as tiebreaker.
Muscle contraction in 60 seconds
A skeletal muscle fiber is a bundle of myofibrils built from repeating sarcomeres, the Z-line-to-Z-line contractile unit. Thin filaments (actin plus tropomyosin and troponin) anchor at the Z lines; thick filaments (myosin) center on the M line. In the sliding filament model the filaments never shorten - they slide past each other, so overlap grows. The I band (thin only) and H zone (thick only) narrow, while the A band stays constant because thick filament length is fixed.
Contraction is switched on by calcium. An action potential runs down the sarcolemma into the T tubules, where the DHPR voltage sensor mechanically opens the RyR1 release channel of the sarcoplasmic reticulum. Cytosolic Ca2+ binds troponin C, troponin I lets go, and tropomyosin rolls off the myosin-binding sites on actin. Cardiac muscle differs in one key way: its DHPR actually conducts Ca2+ inward, and that trigger Ca2+ opens RyR2 - calcium-induced calcium release - so the heart depends on extracellular Ca2+ while skeletal muscle does not.
The cross-bridge cycle then runs: cocked myosin (ADP + Pi) binds actin -> Pi release drives the power stroke -> ADP leaves, giving the tightly bound rigor state -> a new ATP binds and detaches the head, and hydrolysis re-cocks it. ATP has a second, equally testable job: SERCA pumps Ca2+ back into the SR to cause relaxation. Both jobs fail after death, which is why rigor mortis stiffens the body. In the first seconds of exercise ATP is regenerated from creatine phosphate by creatine kinase before glycolysis and oxidative phosphorylation take over.
Force is graded, not all-or-none, at the whole-muscle level: motor unit recruitment adds more fibers (smallest, slow oxidative type I units first), and higher firing frequency produces summation and finally tetanus as Ca2+ never fully clears. Type I fibers are mitochondria-rich, myoglobin-rich, and fatigue-resistant; type II fibers are glycolytic, fast, and fatigable. Within a single fiber, tension tracks the number of available cross-bridges, giving the length-tension relationship - the muscle-level basis of Frank-Starling in the heart. Smooth muscle solves the same problem differently, using Ca2+-calmodulin and myosin light chain kinase to phosphorylate myosin itself.
FAQ
Why does muscle need ATP to relax, not just to contract?
Two reasons. Myosin can only let go of actin when a fresh ATP binds its head, so without ATP the cross-bridge stays locked in the rigor state. And SERCA, an ATP-driven pump, is what clears cytosolic Ca2+ back into the sarcoplasmic reticulum so that tropomyosin can re-cover actin. Rigor mortis is the experiment nature runs: no ATP, high cytosolic Ca2+, permanently attached heads.
How is skeletal excitation-contraction coupling different from cardiac?
Skeletal coupling is mechanical - the T tubule DHPR physically pushes RyR1 open, so no extracellular Ca2+ needs to enter and a single action potential releases plenty of SR Ca2+. Cardiac coupling is chemical - L-type Ca2+ channels conduct a small inward Ca2+ current that opens RyR2 (calcium-induced calcium release). That dependence on trigger Ca2+ is why calcium channel blockers reduce cardiac contractility but do not paralyze skeletal muscle.
What is malignant hyperthermia and how does it fit here?
Malignant hyperthermia is caused by inherited RyR1 (or DHPR) mutations that make the channel hypersensitive. Volatile anesthetics or succinylcholine trigger massive uncontrolled SR Ca2+ release, producing rigidity, hypercarbia, hyperthermia, rhabdomyolysis, and acidosis as ATP is burned by continuous cross-bridge cycling and SERCA pumping. Dantrolene treats it by blocking RyR1 and stopping the Ca2+ leak.
How does the body grade the strength of a contraction?
Two mechanisms. Motor unit recruitment adds motor units in order of increasing size (small slow oxidative units first, per Henneman's size principle), and frequency summation stacks twitches on top of each other because SERCA has not finished clearing Ca2+ before the next stimulus. Enough frequency fuses the twitches into a smooth tetanic contraction at maximal force.
Do I need an account to play?
No. The tournament is fully public. You get a randomized handle and your score posts to the public leaderboard at the bottom of this page.
Keep going
The electrical trigger for excitation-contraction coupling - how the impulse reaches the T tubule in the first place.
Where the ATP that powers cross-bridges and SERCA actually comes from in a working fiber.
Overview diagram: Reactome Pathway R-HSA-397014, licensed CC BY 4.0.