Skeletal Muscle Study Guide Key Points Review

Skeletal Muscle Organization

• Epimysium is the outermost layer of connective tissue, covering the entire muscle
• Perimysium surrounds each fascicle, a group of muscle fibers
• Endomysium is the layer of connective tissue between individual muscle fibers
• Myofibril is the organelle within muscle fibers responsible for contraction
• Myofilament is composed of actin and myosin proteins

Muscle Triad

• Muscle triad components include T-tubules and two terminal cisternae of the sarcoplasmic reticulum (SR)
• Function of muscle triad is to regulate excitation-contraction coupling

Sarcomere Components

• Sarcomere is the functional unit of muscle contraction, consisting of myofilaments, lines, bands, and zones
• Actin is a thin/light strand, making up the I-band
• Myosin is a thick/dark strand, making up the A-band
• Z-line denotes the length of a sarcomere
• M-line is the middle of a sarcomere
• H-zone surrounds the M-line, running from the end of one actin to the end of another actin within the same sarcomere

Actin and Myosin

• Myosin uses cross-bridge cycling to "pull" actin towards the M-line, causing contraction
• Components of actin and myosin are essential for muscle contraction

Motor Unit

• A motor unit consists of one alpha motor neuron and the muscle fibers it innervates
• Alpha motor neuron structure and function involve depolarization and the release of acetylcholine (ACh)
• Function of the motor unit is to send a signal to trigger muscle contraction

Neurotransmission

• Neuromuscular junction (NMJ) is the junction between the motor neuron and skeletal muscle
• Motor end plate is the area of the muscle fiber with a high concentration of ACh receptors
• Acetylcholine (ACh) is the neurotransmitter released to create a skeletal muscle action potential

Resting Membrane Potential

• Resting membrane potential is a negative charge, typically -70 mV in neurons and -80 mV in skeletal muscle
• Sodium-potassium pump maintains the ion gradient, pumping Na+ out and K+ in

Action Potential

• Depolarization occurs when Na+ channels open, and Na+ moves in, making the cell more positive
• Repolarization occurs when K+ moves out of the cell, making it more negative

Calcium in Muscle Contraction

• Calcium is released from the terminal cisternae of the sarcoplasmic reticulum
• Calcium attaches to tropomyosin, moving it off of troponin, making myosin heads available to bind

ATP in Muscle Contraction

• ATP binds to the cross-bridge, detaching it and hydrolyzing to energize the cross-bridge

Cross-Bridge Cycling Process

• Cross-bridge formation: actin and myosin bind
• Power stroke: myosin pulls actin towards the center/M line
• Cross-bridge detachment: ATP binding breaks the bond
• Reactivation of myosin: ATP hydrolysis causes reactivation, where myosin "cocks head" to bind to actin again

Sliding Filament Model

• The sliding filament model describes the movement of actin and myosin filaments during muscle contraction

Peripheral Nervous System

• The peripheral nervous system consists of 12 cranial nerves and 31 spinal nerves that send signals to skeletal muscles

Mechanoreceptors

• Muscle spindles are located in the muscle belly, stimulated by muscle stretch, and respond with tension
• Golgi tendon organs are found within tendons, sense the strength of contraction in the tendon, and inhibit muscle contraction if too much force is applied