Introduction
The major functions of muscles include movement, toning muscle, propelling foods and bodily fluids, making the heart beat, and distributing heat.
Structure of a Skeletal Muscle
Connective Tissue Coverings
Fascia
Fascia is dense connective tissue that separates a skeletal muscle from adjacent muscles and holds it in position.
Epimysium
This is a layer of connective tissue that surrounds each skeletal muscle closely.
Perimysium
Layers of connective tissue that extend inward from the epimysium and separate muscle tissue into small sections.
Fascicles
Sections created by perimysium, which are bundles of skeletal muscle fibers.
Endomysium
The muscles' fibers within fascicles lie within a layer of connective tissue called the endomysium.
Structures Beyond the Muscle
The two structures that may extend beyond the muscle are tendons and aponeuroses. Tendons may attach the muscle to bone, while aponeuroses may attach to bone, skin, or connective tissue of nearby muscles.
Parts of a Skeletal Muscle
- Sarcolemma: Cell membrane of a muscle fiber.
- Sarcoplasm: Cytoplasm of a muscle fiber.
- Myofibrils: Threadlike parts of muscle fibers that help with muscle contraction. Thick myofilaments are composed of myosin, and thin myofilaments are composed of actin.
Striations of Skeletal Muscle
The striations of skeletal muscle are caused by the arrangements of myofilaments in myofibrils. The two parts of the striation pattern are light bands made of thin filaments attached to Z lines, and dark bands that extend the length of thick filaments.
Sarcoplasmic Reticulum and Transverse Tubules
The sarcoplasmic reticulum stores calcium, and the transverse tubules work together with the sarcoplasmic reticulum to help activate muscle contraction when the fiber is stimulated.
Nerve Cells and Synapses
Motor neurons control skeletal muscle and communicate through synapses by releasing neurotransmitters.
Neuromuscular Junction and Synaptic Cleft
A neuromuscular junction is a synapse between a neuron and the muscle fiber it controls. The muscle fiber membrane forms a motor end plate. A synaptic cleft is a gap that separates the membrane of a neuron and a membrane of a muscle fiber.
Skeletal Muscle Contraction
Myosin and Actin Molecules
A myosin molecule has twisted protein mon strands with globular heads projecting from one end. Actin molecules are globular and can bind to myosin molecules. Tropomyosin and troponin are associated with the actin filaments.
Muscle Contraction and ATPase
The force for skeletal muscle contraction comes from myosin heads pulling on the thin filaments, forming cross-bridges. The sliding filament model describes how thick and thin filaments slide past each other, and the role of ATPase is to catalyze the breakdown of ATP to ADP and phosphate to provide energy for straightening myosin heads into "cocked" positions to help muscles prepare for contraction.
Neurotransmitter Stimulating Muscle Contraction
The neurotransmitter that stimulates skeletal muscle contraction is stored in the synaptic vesicles and released into the synaptic cleft, which starts the process of muscle contraction.
