Muscles & Muscle Tissue
Skeletal muscle structure, excitation–contraction coupling, energy and smooth muscle
By the end of this unit you can…
- ✓Compare skeletal, cardiac and smooth muscle and list four functional properties of muscle
- ✓Describe the connective tissue wrappings and the microscopic anatomy of a skeletal muscle fiber
- ✓Explain the sliding filament model and the cross-bridge cycle
- ✓Describe events at the neuromuscular junction and excitation–contraction coupling
- ✓Explain motor units, twitches, summation, tetanus and isotonic vs isometric contraction
- ✓Describe ATP sources, fatigue and fiber types; compare smooth muscle
Key terms
Practice →1Overview of muscle tissue
Muscle makes up nearly half of body mass. Its cells, called muscle fibers, transform the chemical energy of ATP into mechanical work. Four functions: movement, posture, joint stabilization and heat generation (skeletal muscle produces ~85% of body heat). It also protects organs and forms valves (sphincters).
- Excitability
- Responds to stimuli (a neurotransmitter).
- Contractility
- Shortens forcibly when stimulated.
- Extensibility
- Can be stretched.
- Elasticity
- Recoils to resting length after stretch.
2Skeletal muscle anatomy
| Wrapping | Surrounds |
|---|---|
| Epimysium | The whole muscle (dense irregular CT) |
| Perimysium | A fascicle (bundle of fibers) |
| Endomysium | Each individual muscle fiber (areolar CT) |
Skeletal muscles attach to bones at an origin (less movable) and an insertion (more movable), usually via tendons (cords) or aponeuroses (sheets). Each muscle has a rich nerve and blood supply — every fiber is served by a nerve ending.
3The muscle fiber & sarcomere
A skeletal muscle fiber is a long cylindrical cell with many peripheral nuclei (formed by fusion of embryonic cells). Its sarcoplasm is packed with myofibrils — rods of contractile proteins — plus mitochondria, glycogen and myoglobin (an O₂-binding red pigment).
Each myofibril is a chain of sarcomeres, the smallest contractile unit, running from one Z disc to the next. The repeating light and dark bands are what make the muscle striated:
| Region | What it contains |
|---|---|
| A band (dArk) | The full length of the thick (myosin) filaments — overlaps with thin filaments at the ends |
| I band (lIght) | Thin (actin) filaments only; bisected by the Z disc |
| H zone | Center of the A band with thick filaments only (no overlap) |
| M line | Middle of the H zone; proteins holding thick filaments together |
| Z disc | Anchors thin filaments; boundary of the sarcomere |
- Thick filaments
- Myosin molecules: a tail plus two globular heads that bind actin and split ATP (ATPase).
- Thin filaments
- Actin strands with myosin-binding sites, plus regulatory proteins tropomyosin (covers the binding sites at rest) and troponin (binds Ca²⁺, then moves tropomyosin away).
- Sarcoplasmic reticulum (SR)
- Smooth ER around each myofibril that stores and releases Ca²⁺. Its enlarged ends are terminal cisterns.
- T tubules
- Inward extensions of the sarcolemma that carry the action potential deep into the fiber. A T tubule + two terminal cisterns = a triad.
- Titin
- Elastic protein that holds thick filaments in place and helps the sarcomere spring back.
4The sliding filament model
During contraction the thin filaments slide past the thick filaments toward the M line, so the sarcomere shortens — but neither filament changes length. Z discs move closer, I bands and H zones shrink or disappear, and the A band stays the same length.
5Neuromuscular junction & excitation
- 1A nerve impulse (action potential) arrives at the axon terminal of the somatic motor neuron.
- 2Voltage-gated Ca²⁺ channels open and Ca²⁺ enters the axon terminal.
- 3Ca²⁺ triggers exocytosis of synaptic vesicles: acetylcholine (ACh) is released into the synaptic cleft.
- 4ACh diffuses across and binds ACh receptors on the motor end plate of the sarcolemma.
- 5The receptors are ligand-gated channels: Na⁺ rushes in (more than K⁺ leaves) → end plate potential → an action potential spreads along the sarcolemma.
- 6Acetylcholinesterase in the cleft rapidly breaks ACh down, ending its effect.
6Excitation–contraction coupling & cross-bridge cycle
- 1The action potential travels along the sarcolemma and down the T tubules.
- 2Voltage-sensitive proteins in the T tubule change shape, opening Ca²⁺ release channels in the SR terminal cisterns.
- 3Ca²⁺ floods the cytosol and binds troponin.
- 4Troponin changes shape and pulls tropomyosin off the myosin-binding sites on actin.
- 5Myosin heads bind actin → cross-bridge cycling → contraction. When stimulation stops, Ca²⁺ is pumped back into the SR (needs ATP) and tropomyosin re-covers actin.
- 1Cross-bridge formation — an energized myosin head (holding ADP + Pᵢ) binds actin.
- 2Power stroke — ADP and Pᵢ are released; the head pivots and pulls the thin filament toward the M line.
- 3Cross-bridge detachment — a new ATP binds myosin, and the head lets go of actin.
- 4Cocking of the myosin head — ATP is hydrolyzed to ADP + Pᵢ, re-energizing (“cocking”) the head.
7Contraction of a whole muscle
A motor unit is one motor neuron plus all the muscle fibers it supplies. Muscles needing fine control (fingers, eyes) have small motor units (a few fibers); large weight-bearing muscles (thigh) have huge ones (hundreds to thousands). Fibers of one motor unit are spread throughout the muscle.
A twitch is the response to a single stimulus: latent period (excitation–contraction coupling, no tension yet) → period of contraction → period of relaxation.
- Wave (temporal) summation
- A second stimulus before the muscle fully relaxes produces a stronger contraction.
- Unfused (incomplete) tetanus
- Rapid stimuli → sustained but quivering contraction.
- Fused (complete) tetanus
- Very rapid stimuli → smooth, sustained maximal contraction (no relaxation).
- Recruitment
- Activating more and larger motor units to increase force (the size principle: small units first, largest last).
- Muscle tone
- Constant slight contraction from spinal reflexes, keeping muscles firm and ready.
| Contraction | Muscle length | Example |
|---|---|---|
| Isotonic concentric | Shortens while generating force | Lifting a book (biceps) |
| Isotonic eccentric | Lengthens while generating force | Lowering the book slowly; walking downhill |
| Isometric | No change — tension doesn't exceed load | Pushing against a wall; holding a plank |
8Muscle metabolism & fatigue
| Pathway | Speed / ATP yield | Duration it supports |
|---|---|---|
| Creatine phosphate (direct phosphorylation): CP + ADP → creatine + ATP | Fastest; 1 ATP per CP | ~15 seconds |
| Anaerobic glycolysis: glucose → pyruvate → lactic acid | Fast; 2 ATP per glucose; no O₂ needed | ~30–40 seconds of strenuous activity |
| Aerobic respiration (in mitochondria): glucose, fatty acids, amino acids + O₂ → CO₂ + H₂O | Slow; ~32 ATP per glucose | Hours — prolonged activity |
Muscle fatigue is physiological inability to contract despite stimulation — due to ionic imbalances (K⁺, Ca²⁺, Pᵢ), not simply lack of ATP. After exercise, excess postexercise oxygen consumption (EPOC) replenishes O₂ reserves, glycogen and creatine phosphate and converts lactic acid back to glucose — why you keep breathing hard.
| Fiber type | Features | Best for |
|---|---|---|
| Slow oxidative (red) | Slow, fatigue-resistant, lots of myoglobin, mitochondria, capillaries | Posture, endurance (marathon) |
| Fast oxidative | Fast, moderately fatigue-resistant | Walking, sprinting (middle-distance) |
| Fast glycolytic (white) | Fast, powerful, fatigue quickly; little myoglobin; large diameter | Short bursts — lifting heavy objects |
Aerobic (endurance) exercise increases capillaries, mitochondria and myoglobin (more endurance); resistance exercise causes hypertrophy — more myofibrils, bigger fibers. Disuse causes atrophy.
9Smooth & cardiac muscle
Smooth muscle forms the walls of hollow organs (except the heart), usually in two layers: longitudinal and circular — alternating contraction produces peristalsis. Cells are spindle-shaped with one central nucleus; no sarcomeres or striations, no T tubules, a sparse SR. Thin filaments are anchored to dense bodies.
| Feature | Skeletal | Smooth |
|---|---|---|
| Ca²⁺ source | SR | SR and extracellular fluid |
| Ca²⁺ binds | Troponin | Calmodulin → activates myosin light chain kinase, which phosphorylates myosin |
| Speed / energy | Fast; fatigues | Slow, sustained, very energy-efficient; resistant to fatigue |
| Control | Somatic motor neurons (voluntary) | Autonomic nerves, hormones, stretch, local chemicals |
| Other | — | Stress-relaxation response (bladder fills without pressure spike); can divide (hyperplasia) |
Single-unit (visceral) smooth muscle — the common type — has cells linked by gap junctions that contract as a unit, often with self-excitatory pacemaker cells. Multi-unit smooth muscle (large airways, arteries, arrector pili, iris) has independent fibers each with its own nerve supply.
Cardiac muscle is striated like skeletal muscle but involuntary, with branched cells joined by intercalated discs (gap junctions + desmosomes). Covered fully in Unit 18.