Skeletal Muscle Fibers, Contraction Mechanics, and Metabolism
Sarcomere Structure & Terminology
A skeletal muscle fibre = a single skeletal muscle cell.
Sarcomere
Functional contractile unit; defined as distance between two Z-discs (drawn as black squiggly lines in the video).
Components
Thick filament → myosin molecules; each myosin has projecting heads (cross-bridges).
Thin filament → actin molecules (video shows ONE strand of F\text{-actin} for simplicity; physiologically there are TWO intertwined strands).
H-zone
Region between opposing thin filaments (space where only thick filaments lie).
Width changes with contraction → diminishes as thin filaments slide inward.
Length–Tension Relationship
Optimal length–tension relationship
Achieved when every available myosin head can bind actin ⇒ maximal number of cross-bridges.
Shown in mid-panel illustration: all 4 heads (per side, per layer) attached.
Optimal H-zone width correlates with maximal force.
Excessive stretch (top diagram)
Large H-zone, few cross-bridges engaged ⇒ reduced force even though potential shortening distance is large.
Excessive shortening (bottom diagram)
Thin filaments overlap, H-zone already minimal ⇒ no further shortening possible ⇒ low additional force.
Key principle
Force ∝ number of cross-bridges
Optimal sarcomere length just before contraction starts gives greatest tension.
Cellular Energy Sources for Contraction
Muscles need ATP for every cross-bridge cycle & for Ca^{2+} re-uptake.
Four sequential/overlapping sources
Residual (stored) ATP in fibre
Suffices for < 1 s of activity.
Substrate-level phosphorylation
Phosphate transferred from creatine phosphate (CP) to ADP.
\text{Creatine~P} + ADP \rightarrow Creatine + ATPExtremely fast, anaerobic, supports a few additional seconds.
Glycolysis
First stage of cellular respiration; occurs in cytosol; O$_2$ not required.
Glucose → 2 pyruvate + 2~ATP quickly.
Without O$_2$, pyruvate → lactic acid (lactic fermentation) → contributes to fatigue.
Together, 1–3 sustain ~60 s of maximal activity.
Oxidative phosphorylation (OXPHOS)
Occurs in mitochondria; needs O$_2$.
Utilises electron transport chain, proton motive force, ATP synthase (discussed in earlier lectures).
Slow to ramp up but yields ( \approx 30!\text{–}!32~ATP\/glucose).
Twitch, Summation, and Tetanus
Twitch = one contraction–relaxation cycle (mechanical event)
Not an action potential (electrical event).
Duration: 7 – 70 ms depending on fibre type.
Latent period: brief delay between action potential (AP) and tension rise (time for E-C coupling).
Action potential in muscle fibre
Duration ≈ 1–2 ms; triggers Ca^{2+} release via DHP–ryanodine receptor pathway.
Summation (temporal summation)
Repetitive APs arrive before fibre fully relaxes.
Keeps sarcoplasmic reticulum (SR) channels open or repeatedly reopened → sustained cytosolic Ca^{2+}.
Each new stimulus adds to residual tension → higher peak force.
Tetanus (mechanical)
Incomplete/unfused: slight relaxation visible; force oscillates.
Complete/fused: no relaxation; maximal, sustained tension.
Requires high-frequency stimulation; muscle cannot generate additional force beyond this plateau.
Motor Unit Recruitment
Motor unit = 1 somatic motor neuron + all skeletal muscle fibres it innervates (can be ~3 to thousands of fibres).
Recruitment = activating additional motor units to increase total muscle force.
Small/light task (e.g. lifting an Expo marker) → few motor units suffice.
Heavy task (e.g. 30 lb dumbbell curl) → CNS progressively recruits more units.
Example illustration
Two neuron branches, each innervating three fibres (blue & green); simultaneous firing brings six fibres into action.
Mechanisms to Increase Muscle Force
Optimal length–tension (sarcomere starting length).
Recruitment of additional motor units (spatial summation).
Temporal summation leading to tetanus (maintained Ca^{2+} availability).
Skeletal Muscle Fiber Types (Slow vs Fast Twitch)
Nomenclature
Slow = Type I; Fast = Type II (plus an intermediate Type IIa not detailed here).
Contraction kinetics
Fast: twitch as short as 7 ms; Slow: twitch up to 70 ms.
Enzymatic differences
Fast fibres have higher activities of
Myosin ATPase (on myosin heads)
Ca^{2+}-ATPase (SR pump)
These ATPases hydrolyse ATP rapidly ⇒ quicker cross-bridge cycling & Ca^{2+} re-sequestration.
Metabolic profile
Fast (Type II) → glycolytic, anaerobic; favour glycolysis; fatigue quickly.
Slow (Type I) → oxidative; rely on OXPHOS; abundant mitochondria; fatigue-resistant.
Structural/visual traits
Diameter: Fast larger (thicker; think sprinters/body-builders). Slow smaller (lean endurance athletes).
Colour: Slow = dark ("dark meat") owing to high myoglobin, dense capillaries, many mitochondria. Fast = pale ("white meat").
Functional correlates
Fast suited for explosive, brief activities (sprints, power lifts).
Slow suited for posture, endurance (distance running, maintaining tone).
Integrative & Practical Insights
Fatigue
Rapid ATP use via glycolysis leads to lactic acid accumulation → contributes to fatigue in Type II fibres.
Adequate O$_2$ delivery (capillaries, myoglobin) permits sustained ATP generation in Type I fibres.
Training adaptations
Endurance training ↑ capillary density, mitochondrial biogenesis, myoglobin → shifts some fibres toward oxidative characteristics.
Strength/anaerobic training ↑ fibre diameter (hypertrophy) and glycolytic enzyme content.
Clinical/ethical note
Tetanus (clinical disease) involves bacterial toxin causing uncontrolled summation due to inhibitory neuron blockade; shares term but distinct from physiological tetanus described here.
Real-world applications
Athletic coaching leverages length–tension knowledge (e.g. correct joint angles) & recruitment strategies (warm-up sets recruit additional motor units).
Rehabilitation uses electrical stimulation at controlled frequencies to evoke incomplete vs complete tetanus for muscle re-education.