Understanding Muscle Types and Contraction Mechanisms
Striated Muscle Characteristics
Light microscopy reveals that both skeletal and cardiac muscle fibers exhibit alternating light and dark bands, or striations. These bands change relative size during contraction. Striated muscle fibers contract rapidly but fatigue easily; they require periods of rest before contracting again.
Cardiac Muscle Details
Located in the walls of the heart, cardiac muscle is involuntary, controlled by the autonomic nervous system. Its fibers are joined end-to-end, branching and reuniting to form complex networks. They are cylindrical, elongated, and striated, typically containing one or two nuclei per fiber. The contraction speed and the ability to sustain contraction are intermediate. A key characteristic of heart muscle tissue is the presence of intercalated disks, specialized junctions between fibers. This muscle contracts rhythmically, driving blood circulation with each beat.
Skeletal Muscle Details
Skeletal muscle is attached to the skeleton. It is voluntary, controlled by the central nervous system. Its fibers are elongated (up to 2 or 3 cm), cylindrical, and have blunt ends. Like cardiac muscle, it displays striations. Each skeletal muscle fiber is multinucleated, an exception to the typical single-nucleus cell structure. Contraction is faster compared to other muscle tissues, but its ability to sustain contraction is lower. Skeletal muscles form the large muscle masses attached to the body’s bones. When stimulated by a single, brief impulse (typically only in laboratory settings), it undergoes a rapid twitch called a simple spasm.
Muscle Contraction: Neurotransmitters and Cycle
The process of muscle contraction involves neurotransmitters and a specific cycle:
- In a motor unit, a single motor neuron connects functionally to approximately 150 muscle fibers. When stimulated, the motor neuron releases the neurotransmitter acetylcholine into the synaptic cleft (the space between the neuron and muscle fiber).
- Acetylcholine binds to receptors on the muscle fiber, causing an electrical change called depolarization along the sarcolemma (the muscle cell membrane).
- This depolarization generates an electrical impulse, or action potential.
- The action potential spreads along the sarcolemma, through the membranes of T-tubules (invaginations of the plasma membrane), and passes through the sarcoplasmic reticulum.
- The action potential changes the permeability of the sarcoplasmic reticulum, opening protein channels and allowing the release of calcium ions (Ca²⁺) into the cytoplasm (sarcoplasm).
- These released calcium ions become available to interact with the contractile proteins: actin and myosin.
- Calcium ions bind to troponin molecules located on the actin filaments, modifying their structure. This change allows myosin filaments to attach to actin, forming cross-bridges.
- Segments of myosin, known as heads, extend from the myosin filament. These heads can break down ATP (adenosine triphosphate) in the presence of calcium, using the released energy for contraction. When the muscle fiber is activated, calcium ions trigger the breakdown of ATP to ADP (adenosine diphosphate) and phosphate, releasing energy. With this energy, the myosin heads bind to actin, flex (in a movement often called the power stroke), pulling the actin filament toward the center of the sarcomere. This cycle of binding, flexing, and detaching repeats as long as calcium ions and ATP are available, resulting in the shortening of the muscle as actin filaments slide past myosin filaments.