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๐Ÿ’ช Physiology ยท Muscle

Memory tricks for muscle physiology

Sliding filament theory, excitation-contraction coupling, and motor units.

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๐Ÿ’ช Muscle Physiology

Memory Tricks

Proven Mnemonics & Acronyms โ€” fast to learn, hard to forget.

Sliding Filament Theory
Thin slides in ยท Thick stays ยท Sarcomere shortens ยท Z lines close
Actin (thin) slides over myosin (thick) โ€” myosin heads pull actin inward
How muscles shorten โ€” actin slides over myosin, neither filament actually shortens
During contraction, actin (thin filaments) slide over myosin (thick filaments) โ€” neither filament actually changes length. The sarcomere shortens because the Z lines at each end are pulled inward. Myosin heads (cross-bridges) bind actin โ†’ power stroke pulls actin toward center (M line) โ†’ myosin releases โ†’ cocks back โ†’ rebinds โ†’ repeats. This cycle requires ATP. I band and H zone shorten during contraction. A band stays the same. Z lines move closer together. The whole muscle shortens as millions of sarcomeres shorten in series and parallel.
A band
Stays SAME โ€” contains myosin. Full length of myosin filament.
I band
Shortens โ€” actin only zone between A bands. Z line is center of I band.
H zone
Shortens โ€” myosin only zone (no actin overlap). Disappears at full contraction.
Z lines
Move CLOSER together โ€” mark sarcomere boundaries. Attach actin filaments.
Difficulty: Intermediate
Cross-Bridge Cycle
ARPC โ€” Attach ยท Release ยท Power stroke ยท Cock back
Four steps of the myosin cross-bridge cycle โ€” requires ATP at two steps
The cross-bridge cycle โ€” how myosin heads generate force step by step
The cross-bridge cycle generates the force of muscle contraction. Attach: myosin head (in low-energy state after power stroke) binds actin โ€” rigor state. Release: ATP binds myosin โ†’ myosin releases actin (rigor mortis = no ATP, cross-bridges stuck). Power stroke: ATP hydrolysis (ADP + Pi) cocks myosin head to high-energy position โ†’ myosin rebinds actin โ†’ Pi released โ†’ power stroke โ€” actin pulled ~10 nm toward M line. Cock back: ADP released โ†’ myosin returns to low-energy position. Cycle repeats as long as Ca2+ present and ATP available.
Attach
Myosin head binds actin in post-power stroke (low energy) position.
Release
ATP binds myosin โ†’ releases from actin. No ATP = rigor mortis (stuck).
Power stroke
ATP hydrolysis cocks head โ†’ rebinds actin โ†’ Pi released โ†’ stroke โ†’ ADP released.
Rigor mortis
No ATP after death โ†’ cross-bridges stuck in attached state โ†’ muscle rigid 12-72 hrs.
Difficulty: Advanced
Excitation-Contraction Coupling
AP โ†’ T-tubule โ†’ SR โ†’ Ca2+ โ†’ Troponin โ†’ Tropomyosin moves โ†’ Actin exposed โ†’ Contract
Action potential triggers calcium release which switches on actin binding sites
How a nerve signal becomes a muscle contraction โ€” step by step
Motor neuron action potential โ†’ ACh released at NMJ โ†’ muscle action potential โ†’ travels along T-tubules deep into muscle โ†’ activates dihydropyridine receptors (voltage sensors) in T-tubule โ†’ open ryanodine receptors (RyR) on sarcoplasmic reticulum โ†’ Ca2+ floods out of SR into cytoplasm. Ca2+ binds troponin C โ†’ conformational change in troponin-tropomyosin complex โ†’ tropomyosin shifts โ†’ exposes myosin-binding sites on actin โ†’ cross-bridge cycle begins โ†’ contraction. Relaxation: Ca2+ pumped back into SR by SERCA pump (requires ATP) โ†’ tropomyosin covers actin sites โ†’ relaxation.
T-tubules
Extensions of plasma membrane deep into muscle โ€” carry AP to SR junction.
SR (sarcoplasmic reticulum)
Ca2+ storage organelle of muscle. Releases via RyR, reuptakes via SERCA.
Troponin C
Ca2+ sensor โ€” binds Ca2+ โ†’ moves tropomyosin off actin binding sites.
SERCA pump
Pumps Ca2+ back into SR โ€” requires ATP. Blocked by caffeine (delays relaxation).
Difficulty: Advanced
Neuromuscular Junction
NMJ โ€” ACh released ยท nAChR on muscle ยท EPP โ†’ AP โ†’ contraction
Nicotinic ACh receptors generate end-plate potential โ†’ muscle action potential
The neuromuscular junction โ€” how nerve signals cross to muscle
The NMJ is the synapse between a motor neuron and skeletal muscle fiber. Motor neuron AP โ†’ Ca2+ enters terminal โ†’ ACh vesicles fuse โ†’ ACh released into synaptic cleft โ†’ binds nicotinic ACh receptors (nAChR) on motor end plate โ†’ Na+ enters muscle โ†’ end-plate potential (EPP) โ†’ if large enough, triggers muscle AP โ†’ E-C coupling โ†’ contraction. ACh degraded by acetylcholinesterase (AChE) in cleft. Myasthenia gravis: autoimmune attack on nAChR โ†’ fewer receptors โ†’ weak EPP โ†’ muscle weakness (worse with use, improved by AChE inhibitors). Botulinum toxin blocks ACh release โ†’ flaccid paralysis.
nAChR
Nicotinic ACh receptor โ€” ligand-gated Na+ channel. One per end plate.
AChE
Acetylcholinesterase โ€” degrades ACh in cleft. Neostigmine/pyridostigmine inhibit it โ†’ more ACh โ†’ treats MG.
Myasthenia gravis
Anti-nAChR antibodies โ†’ fatigable weakness. Worse afternoon/evening. Ptosis common.
Botulinum toxin
Cleaves SNARE proteins โ†’ blocks ACh vesicle fusion โ†’ flaccid paralysis.
Difficulty: Intermediate
Muscle Fiber Types
Type I = Slow Steady Red ยท Type II = Fast Furious White
Type I: slow-twitch oxidative ยท Type IIa: fast oxidative ยท Type IIx: fast glycolytic
Three muscle fiber types โ€” and which athletes have more of each
Type I (slow-twitch, red): slow contraction, highly fatigue-resistant, aerobic metabolism, many mitochondria, high myoglobin (red color). Used for endurance โ€” marathon runners, postural muscles. Type IIa (fast-twitch oxidative): fast, moderate fatigue resistance, both aerobic and anaerobic. Middle-distance athletes. Type IIx (fast-twitch glycolytic, white): fastest and most powerful, fatigue rapidly, anaerobic glycolysis, few mitochondria, low myoglobin (white). Used for sprinting, weightlifting. Fiber type determined by motor neuron โ€” the neuron dictates fiber type. Training can shift IIx toward IIa but rarely I.
Type I
Slow, red, fatigue-resistant. Aerobic. Endurance. Heart and postural muscles.
Type IIa
Fast, intermediate โ€” can be trained toward either I or IIx. Moderate endurance.
Type IIx
Fastest, white, fatigues fast. Anaerobic glycolysis. Power and sprint athletes.
Motor neuron
Determines fiber type. Cross-reinnervation experiment proves this.
Difficulty: Intermediate
Motor Unit Recruitment
Size principle โ€” small first ยท large last ยท smooth force grading
Henneman's size principle: small motor units (Type I) recruited first, large (Type II) last
How the nervous system grades force โ€” Henneman's size principle
A motor unit = one motor neuron + all the muscle fibers it innervates. Force is graded by recruiting more motor units (spatial summation) and firing them faster (temporal summation โ€” unfused tetanus โ†’ fused tetanus). Henneman's size principle: small motor neurons have lowest threshold โ†’ recruited first. As force needed increases, progressively larger motor neurons recruited. Small neurons = Type I fibers (fatigue-resistant). Large neurons = Type IIx (fast, powerful, fatigue quickly). This ordering means low-force tasks use only fatigue-resistant fibers, while max effort recruits all types. Explains why prolonged exertion eventually calls in fast-fatigue fibers.
Motor unit
1 motor neuron + its muscle fibers. All same fiber type. Fires all-or-none.
Size principle
Small neurons โ†’ Type I first. Large neurons โ†’ Type IIx last. Orderly recruitment.
Temporal summation
Faster firing โ†’ more force. Twitch โ†’ unfused tetanus โ†’ fused tetanus (max force).
Tetanus
Fused tetanus = maximum sustained force โ€” Ca2+ accumulates, no relaxation between APs.
Difficulty: Intermediate
Smooth vs Cardiac Muscle
Skeletal = voluntary striated ยท Cardiac = involuntary striated ยท Smooth = involuntary unstriated
Three muscle types โ€” different Ca2+ sources, speed, and control
How the three muscle types differ โ€” key physiological distinctions
Skeletal muscle: voluntary, striated, Ca2+ from SR only, no spontaneous activity, somatic motor control, fatiguable. Cardiac muscle: involuntary, striated, Ca2+ from SR AND extracellular (CICR โ€” Ca2+-induced Ca2+ release), connected by intercalated discs with gap junctions (syncytium), pacemaker activity (SA node), cannot be tetanized (long refractory period). Smooth muscle: involuntary, non-striated (actin and myosin but no sarcomeres), Ca2+ from SR and extracellular, calmodulin activates myosin light chain kinase (not troponin), slow sustained contractions, can maintain tone indefinitely without fatigue.
Skeletal
Voluntary. T-tubules + SR. Troponin-tropomyosin switch. Somatic nerves.
Cardiac
Involuntary striated. CICR. Intercalated discs. Cannot tetanize. Pacemaker.
Smooth
Involuntary unstriated. Calmodulin-MLCK pathway. No troponin. Slow and sustained.
MLCK
Myosin light chain kinase โ€” smooth muscle contraction switch. Ca2+-calmodulin activates.
Difficulty: Intermediate
Muscle Fatigue
PLAK โ€” Pi accumulates ยท Lactate ยท ATP depletes ยท K+ accumulates extracellular
Four mechanisms of muscle fatigue during intense exercise
Why muscles fatigue โ€” four physiological mechanisms
Muscle fatigue is multifactorial. Inorganic phosphate (Pi) accumulates from ATP hydrolysis โ†’ directly inhibits myosin cross-bridge cycling โ†’ reduces force. Lactate (lactic acid) accumulates from anaerobic glycolysis โ†’ was believed to cause fatigue (now more nuanced โ€” H+ and Pi more important). ATP depletion: at very high intensity, ATP can't be regenerated fast enough. K+ accumulates in T-tubules during repetitive firing โ†’ disrupts membrane potential โ†’ AP generation fails. Calcium handling: SR becomes unable to release or reuptake Ca2+ rapidly โ†’ less Ca2+ available for troponin. Central fatigue: brain reduces motor neuron firing to protect the body.
Pi accumulation
Most important early fatigue mechanism โ€” directly inhibits cross-bridge force.
K+ in T-tubules
Repeated APs leak K+ out โ†’ depolarization block of T-tubule APs.
Ca2+ handling
SR releases less Ca2+ as fatigue progresses โ†’ less troponin activation.
Central fatigue
CNS reduces drive โ€” protective mechanism. Explains "mind over matter" in athletes.
Difficulty: Intermediate
Length-Tension Relationship
Optimal length = 2.0โ€“2.2 ฮผm sarcomere ยท Too short or too long = less force
Maximum force at optimal actin-myosin overlap โ€” basis of Frank-Starling in cardiac muscle
Why muscle length affects force โ€” the length-tension curve explained
Skeletal muscle generates maximum force at optimal sarcomere length (2.0โ€“2.2 ฮผm) โ€” where actin-myosin overlap is greatest, allowing maximum cross-bridge formation. Too short: actin filaments from both sides overlap each other + myosin hits Z lines โ†’ less force. Too long: actin and myosin barely overlap โ†’ fewer cross-bridges โ†’ less force. This is the physiological basis of Frank-Starling law in cardiac muscle โ€” stretch within physiological range increases overlap โ†’ stronger contraction. Clinical relevance: muscles work best at resting length โ€” joint positioning affects muscle strength (why grip strength varies with wrist angle).
Optimal (2.0โ€“2.2 ฮผm)
Maximum overlap โ†’ maximum cross-bridges โ†’ maximum force. Peak of length-tension curve.
Too short
Actin overlap + myosin hits Z lines โ†’ interference โ†’ force drops.
Too long
Minimal overlap โ†’ few cross-bridges โ†’ force drops toward zero.
Frank-Starling
Cardiac version โ€” EDV stretch โ†’ optimal length โ†’ stronger contraction โ†’ higher SV.
Difficulty: Advanced
Muscle Fiber Types
Type I = slow Ironman ยท Type II = fast Sprinter
๐Ÿ“Œ Fiber Classification
Type I fibers are slow/endurance; Type II are fast/power โ€” different fuel and fatigue
Type I (slow-twitch): lots of mitochondria, myoglobin (red), aerobic, fatigue-resistant, postural/endurance. Type IIa (fast-twitch oxidative): intermediate. Type IIx/IIb (fast-twitch glycolytic): few mitochondria, anaerobic, powerful but fatigue quickly, sprinting/lifting.
IType I = Iron(man) = endurance, slow
IIType II = Speed = power, fast fatigue
Length-Tension Relationship
Goldilocks Zone โ€” not too long, not too short, just right
๐Ÿ“Œ Muscle Mechanics
Muscle generates maximum force at optimal sarcomere length โ€” too long or short reduces force
At optimal length (~2.0-2.2 ฮผm sarcomere): maximum actin-myosin overlap โ†’ maximum cross-bridge formation โ†’ peak force. Too short: filaments overlap, less room for cross-bridges. Too long: actin filaments pulled away from myosin heads โ†’ fewer cross-bridges.
OOptimal = maximum overlap = peak force
SShort = overlap interference
LLong = too few cross-bridges
Summation and Tetanus
Wave + Wave = Summation โ†’ Tetanus (no rest)
๐Ÿ“Œ Muscle Contraction
Rapid repeated stimuli produce summation; high frequency produces smooth maximal force (tetanus)
Single twitch: one AP โ†’ brief contraction. If next stimulus arrives before full relaxation โ†’ wave summation โ†’ increased force. High frequency stimulation โ†’ sustained contraction = tetanus (physiological, not the disease). Unfused tetanus = some relaxation; fused tetanus = sustained maximum force.
TTwitch โ€” single AP response
SSummation โ€” waves add up
TTetanus โ€” fused max contraction
Smooth vs Cardiac Muscle
Smooth = Slow involuntary ยท Cardiac = auto-rhythmic
๐Ÿ“Œ Muscle Types
Smooth and cardiac muscle are both involuntary but differ in structure and control
Smooth muscle: single-unit (visceral) or multi-unit, no sarcomeres/striations, slow sustained contraction, regulated by ANS/hormones/local factors. Cardiac muscle: striated, intercalated discs with gap junctions, intrinsic automaticity, cannot undergo tetanus (refractory period during contraction).
SSmooth โ€” no striations, involuntary
CCardiac โ€” striated, auto-rhythmic
IIntercalated discs โ€” cardiac's gap junctions
🎓 Common Exam Questions
Q: Walk through the steps of excitation-contraction coupling in skeletal muscle.
A: Motor neuron AP โ†’ ACh released at NMJ โ†’ binds nicotinic receptors โ†’ end-plate potential โ†’ muscle AP propagates along sarcolemma โ†’ down T-tubules โ†’ activates dihydropyridine (DHP) receptors โ†’ opens ryanodine receptors on SR โ†’ Ca2+ floods sarcoplasm. Ca2+ binds troponin C โ†’ troponin-tropomyosin complex shifts โ†’ exposes myosin binding sites on actin โ†’ myosin heads bind โ†’ power stroke (ADP + Pi released) โ†’ cross-bridge cycling โ†’ contraction. Relaxation: Ca2+ pumped back into SR by SERCA (ATP required).
Q: What is the sliding filament theory and what happens at the molecular level?
A: Sarcomere = Z line to Z line. Thin filaments (actin + troponin + tropomyosin) anchored at Z lines. Thick filaments (myosin) in center (A band). During contraction: myosin heads attach to actin โ†’ power stroke pulls thin filaments toward center โ†’ Z lines move closer โ†’ sarcomere shortens. Neither actin nor myosin filaments shorten โ€” only the overlap increases. A band stays same width; I band and H zone narrow; Z lines come closer. ATP required for cross-bridge detachment (rigor mortis = no ATP โ†’ permanent attachment).
Q: What are the differences between the three muscle types?
A: Skeletal muscle: striated, voluntary, multinucleate, fast contraction, can tetanize, controlled by somatic NS. Smooth muscle: no striations, involuntary, single nucleus, slow sustained contraction, cannot tetanize, regulated by ANS/hormones/local factors. Found in viscera, blood vessels, bladder, uterus. Cardiac muscle: striated (like skeletal), involuntary (like smooth), single nucleus with intercalated discs (gap junctions = electrical syncytium), auto-rhythmic (SA node), long refractory period PREVENTS tetanus โ€” essential to allow heart to refill between beats.
Q: What is a motor unit and how does the nervous system grade muscle force?
A: A motor unit = one motor neuron + all muscle fibers it innervates. Smaller motor units (few fibers, e.g., extraocular muscles): fine control. Larger motor units (many fibers, e.g., quadriceps): coarse power. Two ways to grade force: (1) Recruitment: activate more motor units โ†’ more force. Smaller (Type I) recruited first (size principle โ€” Henneman). (2) Rate coding: increase firing frequency โ†’ summation โ†’ tetanus โ†’ more force. Both work together. In fatigue, additional motor units are recruited to maintain force as originally active units fatigue.
Q: What is the length-tension relationship and how does it apply to cardiac physiology?
A: Skeletal muscle: maximum force at optimal sarcomere length (~2.0โ€“2.2 ฮผm) where actin-myosin overlap is greatest. Too short: filaments interfere. Too long: fewer cross-bridges possible. Applied to cardiac muscle: this IS the Frank-Starling law. Greater ventricular filling (preload โ†’ โ†‘ EDV) โ†’ greater sarcomere stretch (up to optimum) โ†’ stronger contraction โ†’ more stroke volume. The ascending limb of the Frank-Starling curve reflects this. In severe heart failure, the ventricle is overstretched (descending limb) โ†’ force actually decreases despite more filling.