Muscular System
Structure & Contraction
Master the sliding filament model, sarcomere anatomy, and muscle contraction mechanisms for the MCAT.
MCAT Biological Foundations
Three Types of Muscle Tissue
Skeletal
Voluntary, striated, multinucleated. Attached to bones for movement.
Cardiac
Involuntary, striated, branched with intercalated discs. Found only in the heart.
Smooth
Involuntary, non-striated, spindle-shaped. Lines organs and blood vessels.
Skeletal Muscle Hierarchy
The Sarcomere: Functional Unit
Key Contractile Proteins
Myosin
Thick filament with globular heads that have ATPase activity and bind actin during cross-bridge cycling.
Actin
Thin filament: G-actin monomers polymerize into F-actin. Contains myosin-binding sites.
Tropomyosin
Wraps around actin, blocking myosin-binding sites in resting muscle.
Troponin
Regulatory complex that binds \( \text{Ca}^{2+} \), shifting tropomyosin to expose binding sites.
The Sliding Filament Model
During contraction, thin filaments slide over thick filaments, pulling Z-lines closer together. The filaments themselves do not shorten: the sarcomere shortens.
The Cross-Bridge Cycle
Excitation-Contraction Coupling
1
Action potential travels along the motor neuron to the neuromuscular junction (NMJ).
2
Acetylcholine (ACh) is released into the synaptic cleft and binds nicotinic receptors on the sarcolemma.
3
Depolarization spreads along the sarcolemma and into T-tubules.
4
T-tubule signal triggers sarcoplasmic reticulum (SR) to release \( \text{Ca}^{2+} \).
5
\( \text{Ca}^{2+} \) binds troponin C, shifting tropomyosin and exposing myosin-binding sites on actin.
6
Cross-bridge cycling begins: contraction occurs as long as \( \text{Ca}^{2+} \) and ATP are available.
Muscle Relaxation
Relaxation is not passive: it requires active transport of \( \text{Ca}^{2+} \) back into the sarcoplasmic reticulum.
1
ACh is degraded by acetylcholinesterase (AChE) in the synaptic cleft.
2
SERCA pump (a \( \text{Ca}^{2+} \)-ATPase) actively transports \( \text{Ca}^{2+} \) back into the SR, using ATP.
3
Tropomyosin re-covers the myosin-binding sites. Cross-bridge cycling stops.
Comparing Muscle Types
| Feature | Skeletal | Cardiac | Smooth |
|---|---|---|---|
| Control | Voluntary | Involuntary | Involuntary |
| Striations | Yes | Yes | No |
| Nuclei | Multinucleated | 1-2 per cell | 1 per cell |
| T-tubules | Yes (at A-I junction) | Yes (at Z-lines) | None (caveolae) |
| Pacemaker | No | Yes (SA node) | Some (slow waves) |
| Gap junctions | No | Yes (intercalated discs) | Yes |
| Regulation | Troponin-tropomyosin | Troponin-tropomyosin | Calmodulin-MLCK |
Smooth Muscle: Calmodulin Pathway
Smooth muscle lacks troponin. Instead, contraction is regulated by the calmodulin-MLCK pathway:
1
\( \text{Ca}^{2+} \) enters the cytoplasm (from SR and extracellular sources).
2
\( \text{Ca}^{2+} \) binds calmodulin, forming a \( \text{Ca}^{2+} \)-calmodulin complex.
3
The complex activates myosin light-chain kinase (MLCK).
4
MLCK phosphorylates the myosin head, enabling it to bind actin and contract.
Motor Units and Recruitment
Motor Unit
A single motor neuron plus all the muscle fibers it innervates. The smallest functional unit of motor control.
Recruitment
Activating more motor units increases force. Small units fire first (size principle), then larger ones.
Muscle Fiber Types
Type I (Slow Oxidative)
Red fibers, rich in myoglobin and mitochondria. Fatigue-resistant: marathons, posture.
Type IIa (Fast Oxidative)
Intermediate: moderate mitochondria, moderate fatigue resistance. Middle-distance running.
Type IIb (Fast Glycolytic)
White fibers, few mitochondria, rapid fatigue. High force: sprinting, weightlifting.
Example: Band Changes During Contraction
Question: A muscle fiber contracts. Which bands/zones change in width?
Example: Summation and Tetanus
Common Mistakes
✕ A band shortens
The A band remains constant during contraction. Only the I band and H zone shorten.
✕ Filaments contract
Filaments slide past each other: they do not shorten or compress. The sarcomere shortens, not the filaments.
✕ Relaxation is passive
Relaxation requires ATP for the SERCA pump to resequester \( \text{Ca}^{2+} \) into the SR. It is an active process.
✕ Smooth = troponin
Smooth muscle uses calmodulin-MLCK, not troponin-tropomyosin. This is a frequent MCAT trap.
✕ ATP for power stroke
ATP is used for detachment and re-cocking the myosin head. The power stroke is driven by release of \( \text{ADP} + \text{P}_i \).
Question 1 of 5
During skeletal muscle contraction, which of the following remains constant in width?
The A band spans the full length of the myosin thick filament, which does not change during contraction. The I band, H zone, and sarcomere all shorten as actin slides over myosin.
Question 2 of 5
What is the immediate trigger for tropomyosin to shift and expose myosin-binding sites on actin?
\( \text{Ca}^{2+} \) released from the sarcoplasmic reticulum binds troponin C, causing a conformational change that shifts tropomyosin away from the myosin-binding sites on actin. The other choices are upstream events in the signaling cascade.
Question 3 of 5
In smooth muscle, contraction is initiated when \( \text{Ca}^{2+} \) binds to which molecule?
Smooth muscle lacks troponin. \( \text{Ca}^{2+} \) binds calmodulin, forming a complex that activates MLCK. MLCK then phosphorylates the myosin light chain, enabling cross-bridge cycling. This is thick-filament regulation (vs. thin-filament in skeletal).
Question 4 of 5
A patient is exposed to a toxin that blocks the SERCA pump. What is the most likely immediate effect on skeletal muscle?
The SERCA pump resequesters \( \text{Ca}^{2+} \) back into the sarcoplasmic reticulum. If blocked, \( \text{Ca}^{2+} \) remains elevated in the cytoplasm, keeping troponin in the active conformation and causing sustained contraction (similar to the mechanism of rigor).
Question 5 of 5
Which of the following correctly describes the role of ATP in the cross-bridge cycle?
ATP binding to myosin causes it to release actin (detachment). ATP is then hydrolyzed to \( \text{ADP} + \text{P}_i \), which re-cocks the myosin head into the high-energy position. The power stroke occurs when \( \text{ADP} \) and \( \text{P}_i \) are released after the cross-bridge forms. ATP does not directly power the stroke itself.