Organisation
Skeletal muscle is organised from whole muscle to fascicles, fibres, myofibrils and sarcomeres.
The sarcomere extends from one Z line to the next and contains:
- thick filaments: myosin
- thin filaments: actin with troponin and tropomyosin
The A band corresponds to the length of the thick filament. The I band contains thin filaments without thick-filament overlap. The H zone contains thick filaments without thin-filament overlap.
Skeletal muscle is organised from whole muscle to fascicles, fibres, myofibrils and sarcomeres. Individual fibres are multinucleated cells surrounded by endomysium; fascicles are surrounded by perimysium and the whole muscle by epimysium. These connective-tissue layers transmit force to tendon and help define intramuscular pathways for vessels and nerves.
Sliding filament mechanism
During contraction:
- a motor nerve action potential reaches the neuromuscular junction
- acetylcholine depolarises the muscle membrane
- the action potential travels along sarcolemma and T tubules
- calcium is released from sarcoplasmic reticulum
- calcium binds troponin C
- tropomyosin moves, exposing actin binding sites
- myosin cross-bridge cycling shortens the sarcomere
The filaments do not shorten; their overlap changes.
During sarcomere shortening:
- Z lines move closer
- I band shortens
- H zone shortens
- A band remains essentially constant
Motor unit
A motor unit is a motor neuron and all muscle fibres it innervates. Small motor units permit fine control; larger units generate greater force.
Fibre types
Type I
- slow twitch
- fatigue resistant
- high oxidative capacity
- abundant mitochondria and capillaries
Type II
Faster fibres with greater power but lower fatigue resistance. Type IIa fibres have more oxidative capacity than the fastest glycolytic fibres.
Type I fibres are oxidative, fatigue-resistant and suited to sustained activity. Type II fibres generate greater power but fatigue more rapidly, with subtypes showing different oxidative and glycolytic characteristics.
Training, disuse, ageing and neurological disease can alter fibre size and functional characteristics.
Types of contraction
- isometric: muscle develops tension without appreciable change in length
- concentric: muscle shortens while producing force
- eccentric: muscle lengthens while producing force
Eccentric contraction can generate high force with relatively low metabolic cost but may produce greater muscle microdamage.
Length-tension relationship
Active force depends on actin-myosin overlap. Maximum active force occurs near an optimal sarcomere length. At very short or very long lengths, effective cross-bridge formation decreases.
Active force is greatest over a mid-range of sarcomere length where actin-myosin overlap is optimal. Excessive shortening reduces useful cross-bridge interaction; excessive lengthening reduces overlap. Passive tension rises as elastic structures are stretched.
This relationship matters in tendon transfer: the transferred muscle must be tensioned so that its operating range allows useful force generation.
Force-velocity relationship
During concentric contraction, shortening velocity falls as load rises. Eccentric contractions can generate force greater than maximal isometric force.
Pennation
Pennate muscles pack more fibres into a given volume and can generate greater force, although their fibre shortening translates into less longitudinal excursion than parallel-fibre muscles.
Clinical links
- denervation causes rapid weakness and later atrophy
- immobilisation causes muscle wasting and reduced force
- rehabilitation should consider strength, endurance, contraction type and functional loading
Sarcomere and contraction
The sarcomere extends from Z line to Z line. Thin filaments contain actin, while thick filaments contain myosin. Titin contributes elastic behaviour and alignment of the thick filament.
Depolarisation reaches the muscle fibre through the neuromuscular junction and travels along the sarcolemma and T-tubules. Calcium is released from the sarcoplasmic reticulum. Calcium binds troponin C, moves tropomyosin away from actin-binding sites and permits cross-bridge cycling. ATP is required both for cross-bridge detachment and restoration of ion gradients.
During contraction:
- the A band length remains essentially constant
- the I band shortens
- the H zone shortens
- Z lines move closer together
The filaments slide; they do not themselves shorten.
Motor units
A motor unit consists of one alpha motor neuron and all the muscle fibres it supplies. Small motor units provide fine control; large motor units provide greater force. Force is increased by recruitment of additional motor units and by increasing firing frequency.
Recruitment generally follows the size principle, with smaller, fatigue-resistant units recruited before larger, more powerful units.
Contraction types
- Concentric: muscle shortens while producing force
- Eccentric: muscle lengthens while producing force
- Isometric: force is generated with little change in muscle length
Eccentric contraction can generate high force efficiently but is also associated with greater microscopic muscle damage during unfamiliar loading.
Denervation and reinnervation
After denervation, muscle fibres lose neural input and progressively atrophy. Reinnervation may occur by axonal regeneration or collateral sprouting from adjacent motor units. Chronic reinnervation can produce motor-unit grouping.
The rate of peripheral nerve regeneration and distance to the target muscle therefore matter clinically because prolonged denervation reduces the quality of eventual functional recovery.
Muscle-tendon unit in surgery
A tendon transfer succeeds only if:
- donor muscle has adequate power
- excursion is sufficient
- the line of pull is appropriate
- joints remain supple
- antagonist imbalance is considered
- the donor function is expendable
- the transfer is appropriately tensioned
FRCS synthesis
For viva questions, link microscopic physiology to clinical application: sarcomere mechanics explain tensioning, motor-unit recruitment explains graded power, and denervation biology explains why delayed nerve recovery can become functionally irreversible.