Skeletal Muscle Cells Overview

Skeletal Muscle Cells

• Skeletal muscle is striated and voluntary, controlled by the somatic nervous system.
• These cells are long, cylindrical, and multinucleated, formed from the fusion of myoblasts during development.
• Individual muscle fibers are composed of numerous myofibrils, the fundamental contractile units.
• Myofibrils contain repeating sarcomeres, the functional units of contraction.
• Sarcomeres are composed of overlapping thick (myosin) and thin (actin) filaments.
• The sliding filament model describes how myosin heads bind to actin filaments, leading to the shortening of the sarcomere and muscle contraction.
• The neuromuscular junction is the synapse between a motor neuron and a muscle fiber.
• Acetylcholine is the neurotransmitter released at the neuromuscular junction, triggering muscle action potentials.
• The action potential travels along the sarcolemma and down the T-tubules, initiating calcium release from the sarcoplasmic reticulum.
• Calcium ions bind to troponin, causing tropomyosin to move, exposing myosin-binding sites on actin.
• Myosin heads bind to actin, forming cross-bridges and pulling the actin filaments towards the center of the sarcomere.
• ATP is required for the detachment of myosin heads from actin and for the continued cycle of contraction.
• Muscle relaxation occurs when calcium levels decrease, troponin returns to its original conformation, tropomyosin blocks myosin-binding sites on actin, and the muscle fiber returns to its resting length.
• Different types of skeletal muscle fibers (e.g., fast-twitch and slow-twitch) exhibit varying properties like speed of contraction, fatigue resistance, and metabolic pathways.

Smooth Muscle Cells

• Smooth muscle is non-striated and involuntary, found in the walls of internal organs and blood vessels.
• These cells are spindle-shaped and have a single nucleus.
• Smooth muscle contraction is regulated by various stimuli, including hormones, neurotransmitters, and local factors.
• The contraction mechanism differs from skeletal muscle, involving a different set of regulatory proteins and intracellular signaling pathways.
• Calcium ions are also essential for smooth muscle contraction, but the pathway for calcium entry and its interaction with the contractile proteins differ.
• Smooth muscle contractions are generally slower and more sustained than skeletal muscle contractions.
• The organization of the contractile proteins within the smooth muscle cell is less ordered than that of skeletal muscle cells.
• The interaction between actin and myosin is different in smooth muscle, involving calmodulin and myosin light-chain kinase.

Cardiac Muscle Cells

• Cardiac muscle is striated and involuntary, found only in the heart.
• These cells are branched and connected by intercalated discs, specialized junctions that allow for rapid communication and coordination of contraction.
• Each cardiac muscle cell has a single centrally located nucleus.
• The cardiac muscle contraction is initiated by the heart's pacemaker cells, which are specialized cardiac muscle cells.
• Cardiac muscle contraction is sustained and rhythmic, driven by electrical signals generated within the heart itself.
• Cardiac muscle cells have a longer refractory period compared to skeletal muscle, preventing tetanus (sustained contraction).
• The contraction mechanism is similar to skeletal muscle, but the regulation differs significantly due to the unique properties of cardiac muscle tissue, like the specialized calcium handling.
• The heart's electrical conduction system ensures coordinated contraction of the atria and ventricles to pump blood throughout the body.