Muscles attach to the skeleton via two types of connective tissue: tendons, which are cord-like masses, or aponeuroses, which are sheet-like.
Clinical Note: Compartment Syndrome
Muscles are often organized into "compartments"—spaces enclosed by fascia containing muscles, blood vessels, and nerves. If fluid accumulates within one of these spaces, it can lead to compartment syndrome, where increased pressure restricts oxygen and nutrient delivery, causing severe pain and potential tissue damage.
The Microscopic Muscle Fiber
If we zoom in on a single skeletal muscle fiber, we see a highly specialized, multinucleated cell. The cell membrane is known as the sarcolemma, and the cytoplasm is called the sarcoplasm.
The interior of the fiber is packed with myofibrils, long parallel structures that run the length of the cell. These myofibrils are the actual contractile elements, composed of repeating functional units called sarcomeres.
The Sarcomere: The Unit of Contraction
The sarcomere is defined by a specific arrangement of thick and thin filaments that create a striated (striped) appearance under a microscope:
- Thick Filaments: Composed of the protein myosin.
- Thin Filaments: Composed of the protein actin, which is associated with regulatory proteins troponin and tropomyosin.
- Z lines (Z discs): The boundaries of the sarcomere that anchor the filaments.
- I band: The light band containing only thin actin filaments.
- A band: The dark band containing the full length of the myosin filaments, including the areas where they overlap with actin.
- H zone: The center of the A band, consisting only of thick myosin filaments.
- M line: The very center of the sarcomere that anchors the thick filaments.
To ensure the electrical signal reaches the center of these large cells, the sarcolemma contains T tubules (transverse tubules) that relay impulses deep into the cell. These tubules are closely associated with the sarcoplasmic reticulum (SR), a specialized endoplasmic reticulum that stores calcium ions (Ca2+). A single T tubule and two adjacent SR cisternae form a structure called a triad.
Excitation-Contraction Coupling
How does a nerve impulse turn into a physical contraction? This process is known as excitation-contraction coupling.
It begins at the neuromuscular junction (NMJ), a synapse where a motor neuron meets the motor end plate of a muscle fiber. When a nerve impulse arrives, the neuron releases the neurotransmitter acetylcholine (ACh) into the synaptic cleft. ACh binds to receptors on the motor end plate, triggering an action potential (muscle impulse) that travels along the sarcolemma and down the T tubules.
This electrical impulse signals the sarcoplasmic reticulum to release its stored Ca2+ into the sarcoplasm. Once released, the calcium binds to troponin, causing it to change shape. Because troponin is attached to tropomyosin, this shape change pulls the tropomyosin away from the binding sites on the actin filaments, exposing them to the myosin heads.
The Sliding Filament Model and Cross-Bridge Cycling
Once the actin binding sites are exposed, the Sliding Filament Model takes effect. In this model, the thick and thin filaments do not actually change their own length; instead, they slide past one another, increasing their overlap and shortening the overall sarcomere.
The mechanical movement is driven by cross-bridge cycling:
- Binding: The myosin head attaches to the exposed binding site on actin, forming a cross-bridge.
- The Power Stroke: The myosin head pulls the thin filament toward the center of the sarcomere (the M line).
- Release: ADP and phosphate are released from the myosin head.
- Detachment: A new molecule of ATP binds to the myosin head, causing the cross-bridge to break.
- Reset: The ATP is split, providing the energy to cock the myosin head back into its original position, ready for the next cycle.
Energy, Metabolism, and Fatigue
Muscle contraction is an energetically expensive process that requires a constant supply of ATP.
ATP Production
- Immediate Source: Small amounts of stored ATP are used first.
- Short-term Source: Creatine phosphate can rapidly regenerate ATP from ADP, fueling about 10 seconds of intense activity.
- Long-term Source: Cellular respiration breaks down glucose to produce large amounts of ATP. This occurs in two phases: glycolysis (anaerobic phase in the cytoplasm) and the aerobic phase (in the mitochondria). Muscles also use myoglobin to store extra oxygen for aerobic metabolism.
Oxygen Debt and Fatigue
During strenuous activity, the body may reach the anaerobic threshold, where oxygen supply cannot meet demand. At this point, metabolism shifts to anaerobic pathways, producing lactic acid.
After exercise, you may experience oxygen debt—the extra oxygen required by the liver to convert lactic acid back into glucose and to restore ATP and creatine phosphate levels.
If the muscle cannot maintain its energy or ion balance, it experiences muscle fatigue (an inability to contract). Additionally, changes in electrolyte concentrations can lead to a muscle cramp, which is a sustained, involuntary contraction.
Exam Tip: ATP's Dual Role
Remember that ATP is required for both contraction and relaxation. It is needed for the myosin head to pull the actin (contraction), but a new ATP molecule must also bind to the myosin head to make it detach from the actin (relaxation).
