Frog Skeletal Muscle Structure

Muscle Structure

• A muscle is composed of bundles of muscle fibers, each surrounded by a layer of connective tissue called endomysium.
• Muscle fibers are grouped into bundles called fascicles, which are surrounded by perimysium.
• An entire muscle is surrounded by epimysium.

Muscle Fiber

• A muscle fiber is a long, striated cell that can be up to 30cm (12 inches) in length.
• The striations in skeletal muscle are a result of many repeating units called sarcomeres.
• Sarcomeres are the functional units of the muscle and are found in myofibrils.

Motor Unit

• A motor unit is a single motor neuron and all the muscle fibers it innervates.
• Motor units vary in size, from just a few muscle fibers to thousands.
• The smaller the motor unit, the finer the control of movement in that muscle.
• Muscles controlling fine movements, such as those in the fingers and eyes, have small motor units.
• Muscles controlling larger movements, such as those in the limbs, may have larger motor units.

Muscle Contraction

• Skeletal muscle contraction is an all-or-none response, meaning it responds to a stimulus in an all-or-none fashion.
• The strength of a muscle contraction can be increased in two ways: by increasing the number of active motor units (recruitment) or by stimulating motor units more frequently.
• The absolute force a muscle can generate is dependent on the total number of muscle fibers.
• Muscles with larger cross-sectional areas can generate larger forces than those with smaller areas.

Neuromuscular Junction

• Motor neurons release the neurotransmitter acetylcholine from their terminals, which binds to ACh receptors on the muscle membrane.
• The binding of acetylcholine to ACh receptors opens cation-selective ion channels, depolarizing the sarcolemma and triggering action potentials.
• Depolarization leads to the release of intracellular calcium from the sarcoplasmic reticulum, which triggers muscle contraction.
• Acetylcholine is broken down by acetylcholinesterase on the skeletal muscle membrane, preventing accumulation in the junctional cleft.

Learn about the structure of skeletal muscle fibers in frogs, including the arrangement of contractile proteins like actin and myosin. Understand how muscle fibers function independently without gap junctions found in cardiac muscle cells.