Source: UF APK2105 Elite Workbook v2 (Final Edition), Chapters 7 + 8 + 12
Tags: skeletal muscle, excitation-contraction coupling, crossbridge cycle, sliding filament theory, sarcomere, actin, myosin, troponin, tropomyosin, calcium, sarcoplasmic reticulum, SR, DHP receptor, ryanodine receptor, RyR, SERCA, T-tubule, power stroke, rigor mortis, ATP, muscle fiber types, Type I, Type IIa, Type IIx, motor unit, recruitment, tetanus, wave summation, temporal summation, length-tension relationship, muscle fatigue, Golgi tendon organ, muscle spindle, gamma motor neuron, calmodulin, MLCK, smooth muscle, cardiac muscle, UF APK2105
Difficulty: Intermediate to Advanced | Prerequisites: Neuronal Physiology notes, Synaptic Transmission and NMJ notes.
This topic covers what happens after the muscle action potential fires: how an electrical signal is converted into mechanical force. Excitation-contraction (E-C) coupling links the muscle action potential to calcium release from the sarcoplasmic reticulum. The crossbridge cycle then uses that calcium and ATP to produce force. Understanding how the body regulates the amount of force a whole muscle produces, through motor unit recruitment, stimulation frequency, and the length-tension relationship, completes the picture from nerve impulse to movement. This is also where you compare skeletal, cardiac, and smooth muscle at the molecular level.
A muscle action potential travels down T-tubules, activating DHP receptors that open ryanodine receptors on the SR, releasing Ca2+. Calcium binds troponin, shifting tropomyosin to expose actin binding sites, and crossbridge cycling begins. Force is graded by recruiting more motor units and increasing stimulation frequency (not by changing AP size). Muscle relaxation requires ATP-powered Ca2+ reuptake by SERCA.
Excitation-contraction coupling (E-C coupling)
The process that links a muscle action potential (the electrical event) to crossbridge cycling (the mechanical event). The key steps are T-tubule depolarization, DHP receptor activation, ryanodine receptor opening, and Ca2+ release from the SR.
In simple terms, E-C coupling is the translation from "the muscle received a signal" to "the muscle is pulling."
T-tubule (transverse tubule)
An infolding of the sarcolemma that carries the action potential deep into the muscle fibre, ensuring that the signal reaches the sarcoplasmic reticulum throughout the entire cell, not just at the surface.
DHP receptor (dihydropyridine receptor)
A voltage sensor located in the T-tubule membrane. When the muscle action potential depolarises the T-tubule, the DHP receptor changes shape and mechanically opens the ryanodine receptor on the adjacent SR membrane. In skeletal muscle, the DHP receptor does not need to conduct Ca2+ itself; it acts as a voltage sensor.
In simple terms, the DHP receptor "feels" the electrical signal and physically pulls open the calcium release channel.
Ryanodine receptor (RyR)
A Ca2+ release channel on the sarcoplasmic reticulum membrane. In skeletal muscle, it is mechanically opened by the DHP receptor. When it opens, Ca2+ floods from the SR into the cytoplasm (sarcoplasm).
SERCA pump (sarco/endoplasmic reticulum Ca2+-ATPase)
An ATP-powered pump that actively transports Ca2+ from the cytoplasm back into the SR, lowering cytoplasmic calcium and allowing the muscle to relax. Impaired SERCA activity means Ca2+ stays elevated in the cytoplasm, delaying relaxation.
Troponin
A regulatory protein complex on the thin filament (actin). When Ca2+ binds to the troponin C subunit, troponin changes shape and pulls tropomyosin away from the myosin-binding sites on actin, allowing crossbridge formation.
In simple terms, troponin is the calcium-sensitive "switch" that either permits or blocks contraction.
Tropomyosin
A rod-shaped protein that wraps around actin filaments and blocks myosin-binding sites at rest. It is moved out of the way when troponin binds calcium.
Crossbridge cycle
The four-step cycle of force production: (1) myosin head binds actin (crossbridge formation), (2) power stroke (myosin pulls actin toward the M-line, releasing ADP and Pi), (3) ATP binds myosin, causing detachment from actin, (4) ATP is hydrolysed, re-cocking the myosin head for the next cycle.
Power stroke
The force-generating step of the crossbridge cycle, where the myosin head pivots and pulls the actin filament toward the centre of the sarcomere.
Rigor mortis
The stiffness that develops in muscles after death. Without ATP, myosin heads cannot detach from actin, locking the muscle in a contracted state.
Sarcomere
The basic contractile unit of a muscle fibre, defined as the region between two Z-discs. During contraction, the sarcomere shortens as thin filaments slide past thick filaments.
I band
The light band in a sarcomere that contains only thin filaments (actin). It shrinks during contraction as actin slides inward.
H zone
The central region of the A band that contains only thick filaments (myosin). It shrinks and can disappear during contraction as thin filaments overlap it.
Thick filament (A band length)
Composed of myosin. The thick filament itself does not change length during contraction. The A band (which spans the entire thick filament) stays the same width.
In simple terms, myosin molecules do not get shorter; the thin filaments slide past them.
Motor unit
One motor neuron and all the muscle fibres it innervates. All fibres in a motor unit contract simultaneously when the motor neuron fires.
Recruitment (multiple motor unit summation)
The process of activating additional motor units to increase whole-muscle force. Motor units are recruited in order from smallest (Type I) to largest (Type IIx), following the size principle.
Wave (temporal) summation
An increase in force that occurs when successive stimuli arrive before the muscle has fully relaxed from the previous contraction. Residual Ca2+ in the cytoplasm adds to the Ca2+ released by the new stimulus.
Tetanus (fused and unfused)
Unfused tetanus occurs when stimulation frequency is high enough to produce summation but the muscle still partially relaxes between stimuli. Fused tetanus occurs at even higher frequencies where the muscle reaches a sustained maximum force with no relaxation between stimuli.
Length-tension relationship
The relationship between sarcomere length and the force a muscle can generate. At optimal length, there is maximum overlap between actin and myosin for crossbridge formation. If the muscle is stretched too far, overlap decreases and force drops. If the muscle is too short, excessive filament overlap interferes with effective crossbridge cycling.
Type I (slow oxidative) fibres
Slow-twitch fibres with high mitochondrial density, rich myoglobin content, and high fatigue resistance. They rely primarily on aerobic (oxidative) metabolism and are recruited first during most activities.
Type IIx (fast glycolytic) fibres
Fast-twitch fibres with lower mitochondrial density, lower myoglobin, and high fatigability. They produce the greatest peak power but rely heavily on anaerobic glycolysis, so they fatigue quickly. Recruited last, during high-intensity efforts.
Type IIa (fast oxidative-glycolytic) fibres
Intermediate fibres with moderate fatigue resistance and moderate force output, using a mix of aerobic and anaerobic metabolism.
Golgi tendon organ (GTO)
A proprioceptor located at the muscle-tendon junction that monitors muscle tension (force). It provides feedback to the nervous system about how hard the muscle is pulling.
Muscle spindle
A proprioceptor embedded within the muscle that monitors muscle length and rate of length change (stretch). Gamma motor neurons control the sensitivity of muscle spindles.
Gamma motor neuron
A motor neuron that innervates intrafusal (spindle) muscle fibres. Its role is to maintain muscle spindle sensitivity across different muscle lengths. Damage to gamma motor neurons impairs the spindle's ability to detect stretch, not voluntary contraction itself.
Calmodulin and MLCK
In smooth muscle, calcium binds calmodulin (instead of troponin). The Ca2+-calmodulin complex activates myosin light-chain kinase (MLCK), which phosphorylates the myosin head to allow crossbridge formation. This is a fundamentally different regulatory mechanism from the troponin-tropomyosin system used in skeletal and cardiac muscle.
The correct sequence for skeletal muscle E-C coupling (Q60: answer A):
ACh release at the NMJ
End-plate potential develops on the motor end plate
Muscle action potential propagates along the sarcolemma and into T-tubules
DHP receptors in the T-tubule membrane detect the depolarization and change shape
DHP receptors mechanically open ryanodine receptors (RyR) on the SR
Ca2+ floods out of the SR into the cytoplasm
Ca2+ binds troponin C, causing tropomyosin to shift and expose actin binding sites
Crossbridge cycling begins (force production)
Blocking DHP receptors interrupts the sequence between the muscle action potential and SR calcium release (Q8: answer B). The action potential still propagates, but it cannot trigger Ca2+ release.
A mutation preventing DHP receptors from activating ryanodine receptors still allows normal muscle membrane depolarization (Q67: answer C). Everything upstream of Ca2+ release is unaffected.
DHP receptors detect membrane depolarization, making this the correct statement about E-C coupling (Q49: answer B). SERCA pumps Ca2+ back into the SR (it does not release it), and RyR releases Ca2+ (it does not pump it back).
ATP hydrolysis by myosin ATPase cocks the myosin head into the high-energy position. This occurs before the crossbridge attaches to actin.
Once attached, the myosin head performs the power stroke (pivots), pulling actin toward the M-line and releasing ADP and Pi.
A new ATP molecule then binds, causing myosin to detach from actin.
ATP is hydrolysed again, re-cocking the head, and the cycle repeats as long as Ca2+ keeps the binding sites exposed.
The event immediately before calcium binds troponin is calcium release through ryanodine receptors (Q55: answer B).
If myosin ATPase cannot hydrolyse ATP, crossbridge cycling is most directly impaired (Q73: answer A), because the myosin head cannot be re-cocked for the next cycle.
After death, ATP is depleted. Without ATP, myosin cannot detach from actin, leaving muscles locked in a rigid state (Q37: answer B).
Injecting ATP into a rigor muscle: the first event is myosin detaching from actin, because ATP binding to the myosin head is what breaks the actin-myosin bond (Q50: answer A).
If troponin cannot bind calcium, tropomyosin never moves off the actin binding sites, so no crossbridge formation, no power stroke, and no force. However, calcium release from the SR still occurs normally because it depends on DHP-RyR coupling, not on troponin (Q19: answer C).
Defective troponin explains reduced muscle force when calcium enters the cytoplasm normally but the muscle still cannot contract effectively (Q2: answer A).
SERCA actively pumps Ca2+ from the cytoplasm back into the SR. If SERCA is impaired, cytoplasmic Ca2+ remains elevated after contraction and the muscle has difficulty relaxing. This manifests as delayed skeletal muscle relaxation (Q7: answer B).
A muscle biopsy showing normal contractile proteins but elevated cytoplasmic Ca2+ after contraction points to defective SERCA (Q77: answer A).
During shortening, the sarcomere gets shorter, the I band narrows, and the H zone narrows or disappears.
The thick filament (and therefore the A band) does not change length (Q12: answer C). Myosin molecules stay the same size; actin slides past them.
At optimal sarcomere length, maximum crossbridge overlap produces maximum force.
Stretching well beyond optimal length reduces force because there is less overlap between actin and myosin, meaning fewer crossbridges can form (Q34: answer A).
At extremely shortened lengths, excessive filament overlap interferes with effective crossbridge cycling (thin filaments from one side collide with those from the other, or with the opposite Z-disc), reducing force (Q48: answer A; Q69: answer B).
Individual muscle fibres obey all-or-none action potentials, so the body cannot grade force by changing the size of each fibre's action potential. Instead, force is regulated through motor unit recruitment and stimulation frequency (Q79: answer B).
Recruiting additional motor units increases whole-muscle force by activating more fibres simultaneously (Q53: answer C).
Increasing stimulation frequency produces temporal (wave) summation: Ca2+ from successive stimuli accumulates because SERCA cannot pump it all back before the next release. More Ca2+ means more exposed binding sites and more crossbridges (Q16: answer B; Q65: answer B).
When a researcher increases stimulation frequency to the same motor neuron, the first variable to change is the frequency of muscle action potentials (Q57: answer B).
A muscle that contracts normally after the first stimulus but cannot maintain force during repeated stimulation most likely suffers from reduced ATP availability (Q33: answer A). Without adequate ATP, crossbridge cycling slows, SERCA cannot maintain Ca2+ reuptake, and force drops.
Type I (slow oxidative): highest mitochondrial density (Q51: answer C), highest fatigue resistance, lowest peak force, relies on aerobic metabolism.
Type IIx (fast glycolytic): greatest power output, fastest fatigue, relies on anaerobic metabolism (Q38: answer C). Has faster myosin ATPase activity than Type I.
Type IIx fatigues faster than Type I because it relies more heavily on anaerobic metabolism, which depletes substrates quickly and produces metabolic byproducts (Q58: answer B).
Motor units are recruited from smallest to largest. During prolonged endurance exercise, small Type I units are recruited first. Large Type IIx motor units are recruited last, only when high force is demanded (Q74: answer B).
The first adaptation contributing to increased strength in the first few weeks of resistance training is improved neural recruitment efficiency, not muscle hypertrophy (Q40: answer B). The nervous system learns to recruit motor units more effectively before structural changes in the muscle occur.
Gamma motor neurons innervate intrafusal fibres within muscle spindles. Their job is to maintain spindle sensitivity as the muscle changes length.
Selective damage to gamma motor neurons impairs the maintenance of muscle spindle sensitivity (Q52: answer B). Voluntary contraction (alpha motor neurons), neuromuscular transmission, and cardiac function are unaffected.
Golgi tendon organs monitor muscle tension (force), not length (Q39: answer B). Muscle spindles monitor length.
Smooth muscle uses calmodulin and myosin light-chain kinase (MLCK) instead of the troponin-tropomyosin system (Q36: answer C). Calcium still initiates contraction, but it acts through a different signalling pathway.
Cardiac muscle is less likely to develop tetanus because it has a prolonged refractory period (Q9: answer B). The long absolute refractory period of the cardiac action potential (due to the plateau phase) prevents a new action potential from arriving before the muscle has relaxed, making summation and tetanus functionally impossible. This protects the heart from sustained contraction, which would stop it from filling with blood.
Crossbridge cycle (simplified steps):
ATP hydrolysis (cocking) → Crossbridge attachment → Power stroke (ADP + Pi release) → ATP binding (detachment) → repeat
E-C coupling chain:
Muscle AP → T-tubule depolarization → DHP activation → RyR opening → Ca2+ release → Troponin-Ca2+ binding → Tropomyosin shift → Crossbridge cycling
The length-tension relationship is why physiotherapists position joints at specific angles for strength testing: they want the muscle at or near its optimal length. Rigor mortis is directly explained by ATP depletion preventing myosin detachment. Malignant hyperthermia, a life-threatening reaction to certain anaesthetics, involves uncontrolled Ca2+ release from the SR through defective ryanodine receptors, causing sustained muscle contraction and dangerous heat production.
Students often think the thick filament (myosin) shortens during contraction. It does not. The filaments slide past each other; their individual lengths stay the same.
Students confuse "too short" and "too long" on the length-tension curve. Both reduce force, but for different reasons: too long means insufficient overlap, too short means excessive overlap interfering with crossbridge cycling.
Students sometimes think ATP is needed for contraction (attachment and power stroke) but not for relaxation. ATP is also essential for detachment (breaking the actin-myosin bond) and for SERCA to pump Ca2+ back into the SR.
Students assume that the first adaptation to resistance training is muscle growth. Early strength gains are primarily neural (better recruitment and firing synchrony).
⚠️ The full E-C coupling sequence (ACh release → EPP → muscle AP → DHP → RyR → Ca2+ → troponin → crossbridge cycling) is heavily tested. Memorise the order.
⚠️ Know what each protein does: DHP senses voltage, RyR releases Ca2+, SERCA pumps Ca2+ back, troponin binds Ca2+, tropomyosin blocks/exposes binding sites.
⚠️ The crossbridge cycle and the role of ATP at each step (hydrolysis for cocking, binding for detachment) appear frequently. Remember that ATP is needed for both contraction and relaxation.
⚠️ Length-tension questions test both ends of the curve. Be ready to explain reduced force at both excessively long and excessively short sarcomere lengths.
⚠️ Fibre type comparisons (Type I vs. IIx) focus on mitochondria, fatigue resistance, metabolism type, and force output. Know the trade-offs.
⚠️ Cardiac muscle's prolonged refractory period preventing tetanus is a classic exam comparison question.
True or False: The thick filament shortens during muscle contraction.
Fill in the blank: The DHP receptor acts as a ______ sensor in the T-tubule membrane.
True or False: SERCA releases calcium from the sarcoplasmic reticulum.
Fill in the blank: Rigor mortis occurs because ______ is unavailable to detach myosin from actin.
True or False: Type IIx fibres have the highest mitochondrial density of any skeletal muscle fibre type.
Answers: 1. False (it stays the same length; the thin filaments slide past it). 2. voltage. 3. False (SERCA pumps Ca2+ back into the SR; ryanodine receptors release it). 4. ATP. 5. False (Type I fibres have the highest mitochondrial density).
Q: What is the correct sequence for skeletal muscle excitation-contraction coupling?
A: ACh release → end-plate potential → muscle action potential → DHP receptor activation → ryanodine receptor opening → Ca2+ release from SR → crossbridge cycling.
Q: An experiment blocks DHP receptors in skeletal muscle. Which sequence is interrupted first?
A: The link between the muscle action potential and SR calcium release. The AP still propagates along the sarcolemma and into T-tubules, but the DHP receptor cannot detect it and therefore cannot open the ryanodine receptor.
Q: A patient has impaired SERCA pump activity. What is the most likely symptom?
A: Delayed skeletal muscle relaxation. Without effective SERCA, Ca2+ remains elevated in the cytoplasm, keeping troponin in its active conformation and crossbridges engaged for longer than normal.
Q: A skeletal muscle fibre shortens during contraction. Which structure remains unchanged in length?
A: The thick filament (myosin). During the sliding filament mechanism, actin slides past myosin; neither filament actually gets shorter.
Q: A skeletal muscle is stretched well beyond its optimal length before stimulation. Why does force decrease?
A: Crossbridge overlap between actin and myosin decreases. With less overlap, fewer myosin heads can bind actin, so fewer crossbridges form and less force is produced.
Q: A researcher directly injects ATP into a muscle in rigor. What is the first event?
A: Myosin detaches from actin. ATP binding to the myosin head breaks the actin-myosin bond, which is the immediate cause of rigor (the absence of ATP to allow detachment).
Q: A muscle generates greater force after additional motor units are recruited. What mechanism is this?
A: Recruitment (also called multiple motor unit summation). More motor units firing means more muscle fibres contracting simultaneously, producing greater whole-muscle force.
Q: Cardiac muscle is less likely to develop tetanus than skeletal muscle. Why?
A: Cardiac muscle has a prolonged refractory period (due to its long plateau phase). A new action potential cannot arrive before the muscle has relaxed, preventing the summation of contractions that produces tetanus.
Q: Which muscle type uses calmodulin and MLCK instead of troponin for calcium-mediated contraction regulation?
A: Smooth muscle. Calcium binds calmodulin, the complex activates MLCK, and MLCK phosphorylates the myosin light chain to permit crossbridge formation.
Q: A scientist wants to distinguish whether reduced muscle force comes from impaired NMJ transmission or defective E-C coupling. What is the best experiment?
A: Directly stimulate the muscle fibre electrically, bypassing the motor neuron. If the muscle contracts normally with direct stimulation, the NMJ is the problem. If force is still reduced, the defect is in excitation-contraction coupling or the contractile machinery itself.
Excitation-contraction coupling connects directly back to the neuronal physiology and NMJ notes, as the muscle action potential originates from neuromuscular transmission. Cardiac E-C coupling (Chapter 12) uses a similar but distinct mechanism where extracellular Ca2+ entry through L-type channels triggers Ca2+-induced Ca2+ release from the SR, rather than the mechanical coupling seen in skeletal muscle. The length-tension relationship ties into biomechanics and exercise physiology, while fibre type composition is relevant to training adaptations, ageing, and metabolic disease.
excitation-contraction coupling, E-C coupling, crossbridge cycle, sliding filament theory, sarcomere, actin, myosin, troponin, tropomyosin, calcium, sarcoplasmic reticulum, DHP receptor, dihydropyridine receptor, ryanodine receptor, RyR, SERCA, T-tubule, power stroke, rigor mortis, ATP, muscle relaxation, length-tension relationship, motor unit, recruitment, size principle, wave summation, temporal summation, tetanus, fused tetanus, unfused tetanus, muscle fatigue, Type I fibre, slow oxidative, Type IIa, fast oxidative glycolytic, Type IIx, fast glycolytic, Golgi tendon organ, muscle spindle, gamma motor neuron, intrafusal fibre, calmodulin, MLCK, smooth muscle, cardiac muscle, refractory period, APK2105, UF anatomy and physiology