Muscular System
Big picture
Muscle force starts in the nervous system, is switched on by calcium, is powered by ATP, and is shaped by fiber type. Hypertrophy adds contractile tissue, atrophy removes it, and fatigue and cramps are what happen when that chain breaks down.
Five ideas to know cold:
Hierarchy: muscle > fascicle > fiber (one cell) > myofibril > sarcomere (actin + myosin).
Hypertrophy means more sarcomeres in parallel inside existing fibers, supported by satellite cells adding nuclei. You do not gain fibers.
Force production is entirely neural: nerve signal → Ca²⁺ release → troponin unblocks actin → cross-bridges form (using ATP).
Fiber types: Type I is slow and fatigue resistant; IIa is in between; IIx is fastest, strongest and tires first.
Fatigue is central (brain/nerve drive), peripheral (muscle itself) and metabolic (substrate depletion). Cramps have two proposed mechanisms: altered neuromuscular control and electrolyte/dehydration imbalance.
Muscle structure
Connective tissue wraps muscle in layers, from the outside in:
Layer | What it surrounds |
|---|---|
Epimysium | The whole muscle and all fascicles (outermost fascia, often called aponeurosis) |
Perimysium | One fascicle |
Endomysium | One muscle fiber |
Fascicle: a bundle of multiple muscle cells.
Fiber: a single, multinucleated cell containing sarcoplasm, sarcolemma and satellite cells.
Satellite cells: stem cells that repair and monitor muscle cell health.
Sarcomeres: each cell has many, arranged both in series (end to end) and in parallel (side by side).
Sliding filament theory: thick myosin filaments attach to and pull on thin actin filaments. The muscle can shorten, hold its length, or hold tension while lengthening.
Sarcomere landmarks: Z lines mark the ends, the A band is the myosin length, the I band is actin only, the H zone is the center of the A band, and the M line is the very middle.
Growth, repair, hypertrophy and atrophy
Satellite cells respond to exercise by dividing and fusing into the fiber. They can raise the number of nuclei per fiber, which may be a precursor to hypertrophy. More myofibrillar tissue (actin, myosin, tropomyosin) needs more nuclei to manage it.
Where muscle grows
More sarcomeres in parallel inside a fiber is what is generally considered hypertrophy.
More sarcomeres in series is an adaptation to eccentric exercise.
Individual sarcomeres can also lengthen, again an eccentric adaptation.
Humans show almost no evidence of gaining or losing whole fibers, only sarcomeres within fibers.
Three stimuli for hypertrophy
Time under tension: tension is sensed and triggers protein synthesis (new or replacement myofibrils).
Myofibrillar damage: stimulates repair and satellite cell activity, but is not necessary for growth.
Metabolic stress: cell swelling or metabolite buildup can also signal growth.
Hypertrophy implications: more contractile machinery means more force (strength). The same force needs less neural input, which lowers metabolic cost.
Atrophy implications (e.g., sarcopenia in aging):
Lower myofibril density means less net tension (weakness).
Fewer nuclei means a worse adaptive response to exercise.
Less contractile tissue means more neural drive (rate coding and motor unit number) is needed for the same force, at a higher metabolic cost.
Contraction, relaxation and ATP
Neural and calcium control. Depolarization of the final neuron at the muscle triggers Ca²⁺ release. Calcium "unlocks" troponin, which frees the binding sites on actin. No calcium means no force, so force production depends entirely on neural drive.
Cross-bridge cycle (ATP):
Resting: myosin is not attached to actin.
Myosin binds actin and a cross-bridge forms.
Pi is released and myosin changes shape.
Power stroke: myosin pulls actin and ADP is released.
A new ATP binds and myosin releases actin.
ATP is broken down and myosin resets.
ATP is needed both to detach myosin (step 5) and to reset it (step 6), so a muscle with no ATP cannot release.
Relaxation (to end a contraction):
The neural signal stops (no more neurotransmitter at the synapse).
A Ca²⁺ pump removes Ca²⁺ from the sarcoplasm back into the sarcoplasmic reticulum.
Troponin covers the binding sites again, blocking myosin binding and tension.
Twitch and tetanus: a single stimulus gives a twitch (latent period, contraction, relaxation). Rapid repeated stimuli cause force summation and then tetanus, a sustained maximal contraction. All high force outputs are tetanus.
Fiber types
Type I fibers are slow and fatigue resistant, and Type IIx fibers are the fastest, strongest and least fatigue resistant. Type IIa sits between them.
Feature | Type I (slow twitch) | Type IIa (fast) | Type IIx (fastest) |
|---|---|---|---|
Contraction speed | Slower (myosin type) | Fast | Fastest |
Actin + myosin per cross-sectional area | Fewest | More | Most |
Metabolism | Highly oxidative (high mitochondrial density) | Mixed oxidative and glycolytic | More glycolytic |
Fatigue resistance | High | Moderate | Lowest |
Hybrid fibers (IIa/IIx) exist and express both myosin types.
Highest Type I %: soleus (~70–90%), then multifidi, adductor longus, vastus intermedius, gastrocnemius.
Lowest Type I % (more Type II): triceps brachii, biceps femoris, biceps brachii (~30–50%).
The pattern fits function: postural and endurance muscles are slow-fiber heavy, while muscles used for power are faster.
Fatigue and cramps
Fatigue is a decline in force or performance, and it has three kinds of cause:
Central: the brain and spinal cord send less drive to the muscle.
Peripheral: the muscle itself produces less force even though the drive is intact.
Metabolic: substrate depletion in the bioenergetic systems (acute).
How it shows up: force declines, EMG declines (magnitude or mean frequency), and task performance drops (fewer reps or less weight).
Central vs peripheral clue: if electrical stimulation (estim) of the muscle still produces less force, the problem is peripheral. If estim restores force, the drive from the nervous system was the limit.
Exercise-associated muscle cramps tend to follow extended and/or high-intensity exercise, like fatigue. Two mechanisms are proposed:
Altered neuromuscular control: with fatigue or injury, errors in length and tension sensory input may increase excitation and decrease inhibition of motor neurons. Both lower the action potential (AP) threshold.
Electrolyte (Na⁺, Ca²⁺, K⁺) deficiency or imbalance: dehydration concentrates electrolytes in the interstitial space, creating a constant depolarization stimulus.
Why stretching ends a cramp: if cramps were purely chemical, stretching should not help. Stretching raises the AP threshold by increasing the inhibiting motor neuron pool, which supports the neuromuscular explanation.