Skeletal Muscle Contraction and Fiber Types

Sarcomere Contraction

• During contraction, the sarcomere decreases in length.
• Z lines come closer to the A band.
• The length of the A band does not change, but the I band shortens in length.
• Thin filaments are pushed towards the center of the sarcomere by myosin heads, pulling Z lines together.
• The H band gradually decreases in length, and thin filaments penetrate deeper into the sarcomere.

Skeletal Muscle Fiber Types

• Three types of skeletal muscle fibers: slow oxidative, fast oxidative, and fast glycolytic.
• Slow oxidative fibers are dark red due to abundant myoglobin, have many mitochondria, and contract slowly but are resistant to fatigue.
• Fast oxidative fibers stain less darkly than slow oxidative fibers, produce ATP via aerobic metabolism, contract faster and more powerfully than slow oxidative fibers, and are abundant in lower limbs.
• Fast glycolytic fibers stain pale due to little myoglobin, are the largest of the three fiber types, depend on anaerobic glycogenolysis for ATP, contract rapidly but fatigue easily, and are found in extraocular muscles and finger muscles.

Neuromuscular Control

• Skeletal muscle contraction is controlled by a nerve impulse transmitted by a motor nerve from the brain or spinal cord.
• A motor unit consists of all the muscle fibers controlled by a single motor neuron.
• Fine control muscles (e.g., eyelid muscles) have fewer muscle fibers per nerve (2:1).
• A contraction is initiated by an action potential (nerve impulse), followed by the release of a chemical neurotransmitter at the neuromuscular junction (NMJ).
• The neurotransmitter for skeletal muscle is acetylcholine.
• Every muscle fiber is innervated by a single motor neuron.
• Contractions may be graded or full depending on the number of muscle fibers that respond to the stimulus: the more fibers, the greater the muscle contraction.
• Synapse: functional connection between a nerve fiber and its target cell.
• Neuromuscular junction: synapse between a motor nerve and a muscle fiber.

Neuromuscular Junction

• Synaptic knob, terminal, or bouton: bulbous swelling at the end of a motor nerve above the motor end plate on the muscle fiber.
• Synaptic cleft: gap between the synaptic knob and the motor end plate.
• Synaptic vesicles: small packets of neurotransmitter chemical (e.g., acetylcholine, norepinephrine).

Cardiac Muscle

• Basic morphologic unit: cardiac muscle cell (cardiomyocyte) with cross striations and often branched.
• Nuclei (1-2) are situated in the center of the cell.
• Intercalated discs (ID) are connections between cardiac muscle cells.
• On the transverse site of IDs are: fascia adherentes (α - actinin) for actin filament attachment and desmosomes for strong connection between cells.
• On the lateral site of IDs are: gap junctions (nexus) for ion transport, impulse spreading, and metabolism.

Functions of Cardiac Muscle Tissue

• Plays the most important role in the contraction of the atria and ventricles of the heart.
• Causes the rhythmic beating of the heart, circulating blood throughout the body.
• Influenced involuntarily by endocrine (hormones) and autonomic nervous systems.

Smooth Muscle

• Generally occurs as bundles or sheets of elongated fusiform cells, with a centrally located single nucleus and finely tapered ends.
• Controls slow, involuntary movements of the walls of hollow organs (e.g., stomach, gastrointestinal tract, uterus, glands, blood vessels).
• Does not exhibit cross striations because myofilaments are not arranged in the same ordered way as in striated muscle.
• Found in the walls of the digestive system (middle esophagus to anus), arteries, veins, large lymphatics, urinary and genital duct walls, respiratory tract, arrector pili muscle, dartos muscle, and iris and corpus ciliare (eye).

Smooth Muscle Tissue

• Contains both thick and thin filaments and a cytoskeleton of desmin and vimentin intermediate filaments.
• Not arranged in orderly sarcomeres.
• Gap junctions interconnect smooth muscle cells.
• Caveolae in the sarcolemma instead of T tubules.
• Dense bodies provide an attachment site for thin filaments and intermediate filaments, serving as intracellular analogs of the striated muscle Z lines.
• Dense bodies are important for transmitting contractile forces generated inside the cell to the cell surface, altering cell shape.
• During contraction, filaments pull on the dense bodies, causing shortening of the muscle fiber.
• Dense bodies are distributed throughout the sarcoplasm in a network of intermediate filaments containing the proteins desmin and vimentin.

Proteins Essential for Contraction

• Myosin light chain kinase (MLCK) is important in smooth muscle contraction.