Introductory Skeletal Muscle – Histology Flash Cards

Structural Organization in Skeletal Muscle

Part of a Multinucleate Skeletal Muscle Fiber (Cell) Sarcolemma Striations Nuclei Myofibrils Mitochondria b A diagrammatic view and a micrograph part of one skeletal muscle fiber (cell).

Structure and Internal Organization of a Skeletal Muscle Fiber Myofibril Muscle fiber Sarcolemma Nuclei Sarcoplasm Mitochondria Terminal cisterna Sarcolemma Sarcolemma Sarcoplasm Myofibril Myofibrils Thin filament Thick filament Triad Sarcoplasmic reticulum T tubules

Levels of Functional Organization in a Skeletal Muscle The Sarcomere I band A band Contains: Thick filaments Thin filaments Z line M line Titin Z line H band

The Sarcomere

The contraction cycle Z line distance: reduced I band H band Z line distance: reduced I band distance: smaller H band distance: smaller

Neuro-Muscluar Junction

The Steps of Skeletal Muscle Contraction Neuromuscular Junction Skeletal muscle activity is controlled at the neuromuscular junction (NMJ) This is a special intercellular connection between the nervous system and the skeletal myofiber It controls calcium ion release into the sarcoplasm

Arriving action potential Junctional fold of motor end plate The cytoplasm of the synaptic terminal contains vesicles filled with molecules of acetylcholine, or ACh. Acetylcholine is a neurotransmitter, a chemical released by a neuron to change the permeability or other properties of another cell’s plasma membrane. The synaptic cleft and the motor end plate contain molecules of the enzyme acetylcholinesterase (AChE), which breaks down ACh. The stimulus for ACh release is the arrival of an electrical impulse, or action potential, at the synaptic terminal. An action potential is a sudden change in the transmembrane potential that travels along the length of the axon. Arriving action potential Vesicles ACh The synaptic cleft, a narrow space, separates the synaptic terminal of the neuron from the opposing motor end plate. Junctional fold of motor end plate AChE 11

Excitation–Contraction Coupling In EC Coupling, an action potential reaches a triad, releasing Ca2+, triggering contraction This process requires the myosin heads to be in the “cocked” position (loaded by ATP)

The Exposure of Active Sites SARCOPLASMIC RETICULUM Calcium channels open Myosin tail (thick filament) Tropomyosin strand Troponin G-actin (thin filament) Active site Nebulin In a resting sarcomere, the tropomyosin strands cover the active sites on the thin filaments, preventing cross-bridge formation. When calcium ions enter the sarcomere, they bind to troponin, which rotates and swings the tropomyosin away from the active sites. Cross-bridge formation then occurs, and the contraction cycle begins. 14

The Contraction Cycle Tropomyosin shifts Active-site is exposed Cross-Bridge forms Myosin head pivots (advances) Cross-bridge detaches Myosin reactivates

Contraction Cycle Begins Active-Site Exposure The contraction cycle, which involves a series of interrelated steps, begins with the arrival of calcium ions within the zone of overlap. Calcium ions bind to troponin, weakening the bond between actin and the troponin– tropomyosin complex. The troponin molecule then changes position, rolling the tropomyosin molecule away from the active sites on actin and allowing interaction with the energized myosin heads. Sarcoplasm Myosin head Troponin Active site Tropomyosin Actin 16

Cross-Bridge Formation Myosin Head Pivoting Once the active sites are exposed, the energized myosin heads bind to them, forming cross-bridges. After cross-bridge formation, the energy that was stored in the resting state is released as the myosin head pivots toward the M line. This action is called the power stroke; when it occurs, the bound ADP and phosphate group are released. 17

Cross-Bridge Detachment Myosin Reactivation When another ATP binds to the myosin head, the link between the myosin head and the active site on the actin molecule is broken. The active site is now exposed and able to form another cross-bridge. Myosin reactivation occurs when the free myosin head splits ATP into ADP and P. The energy released is used to recock the myosin head. 18

Steps Involved in Skeletal Muscle Contraction and Relaxation Steps in Initiating Muscle Contraction (ACTIVE) Steps in Muscle Relaxation (PASSIVE) Synaptic terminal Motor end plate T tubule Sarcolemma Action potential reaches T tubule ACh released, binding to receptors ACh broken down by AChE Sarcoplasmic reticulum releases Ca2 Sarcoplasmic reticulum recaptures Ca2 Ca2 Actin Active site exposure, cross-bridge formation Active sites covered, no cross-bridge interaction Myosin Contraction ends Contraction begins Relaxation occurs, passive return to resting length 19