10.3 Muscle Fiber Contraction and Relaxation

Muscle Fiber Contraction and Relaxation

Muscle contraction is the result of proteins, ions, and energy molecules working together inside individual muscle fibers. Understanding this process at the molecular level is central to the rest of the course, since it connects nervous system signaling to the physical force your muscles produce.

This section covers the key structural components of a muscle fiber, the sliding filament theory, and the full sequence from neural stimulation through relaxation.

Components of Muscle Fiber Contraction

The sarcomere is the basic contractile unit of a muscle fiber. It's made up of overlapping thick and thin filaments that slide past each other during contraction.

• Thick filaments are primarily composed of myosin, a motor protein. The globular portions of myosin, called myosin heads, are the parts that physically bind to actin and generate force. This binding is called cross-bridge formation.
• Thin filaments are built on a backbone of actin, which provides the binding sites that myosin heads attach to. Two regulatory proteins sit on the thin filament:
  • Tropomyosin wraps around actin and physically blocks the myosin-binding sites when the muscle is at rest.
  • Troponin is a three-part complex that controls tropomyosin's position:
    • Troponin C binds calcium ions ($Ca^{2+}$), which is the trigger that starts contraction.
    • Troponin I inhibits the actin-myosin interaction when $Ca^{2+}$ is absent.
    • Troponin T anchors the troponin complex to tropomyosin.

Several other structures play supporting roles:

• The sarcoplasmic reticulum (SR) is a specialized smooth ER that stores and releases $Ca^{2+}$. The terminal cisternae are enlarged regions of the SR that sit close to the T-tubules and hold the highest concentration of $Ca^{2+}$.
• T-tubules (transverse tubules) are inward folds of the sarcolemma (the muscle cell membrane). They carry action potentials deep into the interior of the fiber so that the signal reaches the SR quickly and uniformly.
• Myoglobin is an oxygen-binding protein found in muscle fibers. It acts as a local oxygen reserve, facilitating oxygen delivery to mitochondria during contraction.

Sliding Filament Theory

The sliding filament theory explains how sarcomeres shorten to produce force. The filaments themselves don't get shorter. Instead, the thin filaments slide inward along the thick filaments, pulling the Z-discs closer together and reducing sarcomere length.

During relaxation, the thin filaments slide back outward, and the sarcomere returns to its resting length.

The molecular engine behind this sliding is the cross-bridge cycle, which repeats as long as $Ca^{2+}$ and ATP are available:

1. Cross-bridge formation: A myosin head, energized from prior ATP hydrolysis, binds to an exposed binding site on actin.
2. Power stroke: The myosin head pivots, pulling the thin filament toward the center of the sarcomere (the M-line). ADP and inorganic phosphate ($P_i$) are released during this step.
3. Cross-bridge detachment: A new molecule of ATP binds to the myosin head, causing it to release from actin. Without ATP, the myosin head stays locked to actin, which is exactly what happens in rigor mortis.
4. Recovery stroke: The myosin head hydrolyzes the bound ATP ($ATP \rightarrow ADP + P_i$), and the released energy re-cocks the head back to its high-energy position, ready to bind actin again.

ATP's role in this cycle is worth emphasizing: ATP is needed both for detachment (step 3) and for re-energizing the myosin head (step 4). A common misconception is that ATP only powers the power stroke, but the power stroke actually uses energy that was stored from previous ATP hydrolysis.

During intense, short-duration activity, creatine phosphate donates its phosphate group to ADP to rapidly regenerate ATP, keeping the cycle going before slower metabolic pathways catch up.

Neural Stimulation Through Relaxation: The Full Sequence

Here is the complete sequence from nerve signal to muscle relaxation, step by step:

Stimulation and Signal Transmission

1. An action potential travels down a motor neuron and arrives at the neuromuscular junction (NMJ).
2. The motor neuron terminal releases acetylcholine (ACh) into the synaptic cleft.
3. ACh binds to nicotinic acetylcholine receptors on the sarcolemma.
4. This binding depolarizes the sarcolemma, generating an action potential that spreads along the surface and dives into the T-tubules.

Excitation-Contraction Coupling

1. Depolarization of the T-tubules activates voltage-sensitive dihydropyridine receptors (DHPRs).
2. Activated DHPRs mechanically trigger the opening of ryanodine receptors (RyRs) on the SR membrane.
3. $Ca^{2+}$ floods out of the SR into the sarcoplasm (the cytoplasm of the muscle cell).

Contraction

1. $Ca^{2+}$ binds to troponin C, causing a shape change in the troponin complex.
2. This shift moves tropomyosin away from the myosin-binding sites on actin.
3. Myosin heads bind to the now-exposed actin sites, and cross-bridge cycling begins.
4. The cycle repeats as long as $Ca^{2+}$ remains bound to troponin C and ATP is available.

Relaxation

1. SERCA pumps (sarco/endoplasmic reticulum $Ca^{2+}$-ATPase) actively transport $Ca^{2+}$ back into the SR. This is an ATP-dependent process.
2. As sarcoplasmic $Ca^{2+}$ concentration drops, $Ca^{2+}$ dissociates from troponin C.
3. Tropomyosin slides back over the actin-binding sites, blocking myosin attachment.
4. Cross-bridge cycling stops, and the muscle fiber returns to its resting state.

Types of Muscle Contractions

• Isotonic contraction: The muscle changes length while maintaining relatively constant tension. Think of the bicep curl as you raise the weight: the muscle shortens (concentric) or lengthens under load (eccentric).
• Isometric contraction: The muscle generates force but does not change length. Holding a heavy box in front of you with your arms steady is a good example: your muscles are working hard, but nothing is moving.
• Rigor mortis: After death, ATP production stops. Without ATP, myosin heads cannot detach from actin (step 3 of the cross-bridge cycle is blocked). The result is widespread muscle stiffness that persists until the muscle proteins begin to degrade.