NOTES: The Muscular System (Ch 8, part 3) Contraction of a Skeletal Muscle Fiber

Review:

Let’s Revisit Myofilaments! Each muscle fiber contains thousands of myofibrils which are made up of myofilaments (play a key role in contraction) There are 4 types of myofilaments Myosin Actin Troponin Tropomyosin Myosin is considered a thick filament, and the remaining 3 are considered thin filaments

Myosin: has a moving head that uses energy

Skeletal Muscle Fibers: **The organization of actin and myosin filaments produces STRIATIONS (bands) **The thick (myosin) and thin (actin) filaments are organized into structural units called SARCOMERES (functional units of muscle cells) border of sarcomere: Z-line

Structure of a Sarcomere

SKELETAL MUSCLE CONTRACTION Sliding Filament Mechanism of Contraction Anatomists noticed that when a muscle cell contracts, each individual sarcomere shortens, however, the myofilaments do not change length How does this occur?

SKELETAL MUSCLE CONTRACTION *Muscle fiber contraction results from a sliding movement of actin and myosin filaments. SARCOMERE

Role of MYOSIN and ACTIN:  Myosin heads of myosin filaments can form linkages with actin filaments called cross-bridges  the reaction between actin and myosin filaments generates the force of contraction

                                                                                                                                                                                                                                                                        

What causes the actin & myosin filaments to slide? When a muscle is at rest (no action potential from motor neuron), calcium levels are low within the cell. The active sites on actin are blocked by tropomyosin which is held in place by troponin

Stimulation from a nerve impulse causes calcium to flood into the cell from the sarcoplasmic reticulum The flood of calcium attaches to troponin (causes troponin to change shape) Troponin then drags tropomyosin from the active sites on actin Now myosin head can attach to actin! (= a cross-bridge)

Cool Hyperlink Animation!

STEPS of SLIDING FILAMENT MODEL Once binding sites on actin are exposed the following events occur: 1. Cross bridge attachment: myosin heads, which are strongly attracted to actin, bind to the active sites, forming a cross bridge ADP Pi Myosin head (high-energy configuration) Myosin head attaches to the actin myofilament, forming a cross bridge. 1

2. Power Stroke: as a myosin head binds, it pulls actin and slides it toward the center of the sarcomere ADP Pi Myosin head (high-energy configuration) Myosin head attaches to the actin myofilament, forming a cross bridge. Inorganic phosphate (Pi) generated in the previous contraction cycle is released, initiating the power (working) stroke. The myosin head pivots and bends as it pulls on the actin filament, sliding it toward the M line. Then ADP is released. 1 2

ATP ADP Pi Myosin head (high-energy configuration) Myosin head attaches to the actin myofilament, forming a cross bridge. (low-energy As new ATP attaches to the myosin head, the link between myosin and actin weakens, and the cross bridge detaches. 1 2 3 Inorganic phosphate (Pi) generated in the previous contraction cycle is released, initiating the power (working) stroke. The myosin head pivots and bends as it pulls on the actin filament, sliding it toward the M line. Then ADP is released. 3. Crossbridge detachment: An ATP molecule binds to the myosin head causing it to detach

4. “Cocking” of the myosin head: the myosin head is returned back to its “cocked” position by the energy from ATP At this point the cycle is back to where it started! ATP ADP hydrolysis Pi Myosin head (high-energy configuration) Myosin head attaches to the actin myofilament, forming a cross bridge. Thin filament As ATP is split into ADP and Pi, the myosin head is energized (cocked into the high-energy conformation). Inorganic phosphate (Pi) generated in the previous contraction cycle is released, initiating the power (working) stroke. The myosin head pivots and bends as it pulls on the actin filament, sliding it toward the M line. Then ADP is released. (low-energy As new ATP attaches to the myosin head, the link between myosin and actin weakens, and the cross bridge detaches. Thick filament 1 4 2 3

Each myosin cross bridge attaches and detaches many times during contraction The sliding filament theory is the most commonly accepted model of muscle contraction animation

The end of a contraction… > the muscle fiber relaxes (and the contraction ends) when cross-bridges release from actin and when calcium ions are actively transported back into the sarcoplasmic reticulum (without calcium present, the troponin-tropomyosin complex re-covers the myosin-binding sites on actin) > Acetylcholine in the neuromuscular junction is broken down by the enzyme ACETYLCHOLINESTERASE

CREEPY INFO: Rigor Mortis- stiffness that occurs in muscles after death Calcium leaks out of the sarcoplasmic reticulum causing the myofibrils to contract (Ca+2 pumps no longer working) ATP is no longer being produced so cross bridges are unable to detach leaving the muscles in a state of contraction

Energy for Contraction Two ingredients are essential for the mitochondria to provide energy for muscle fibers 1) Oxygen: necessary for aerobic respiration May also be stored on myoglobin (specialized form of hemoglobin) When the O2 supply in a cell decreases rapidly, as during exercise, it can be quickly resupplied by myoglobin Similar to hemoglobin (also contains iron, gives muscle a reddish color) With a lot of myoglobin, called red fibers White fibers have less myoglobin 2) Glucose: nutrient rich molecule that contains many chemical bonds May also be stored in muscles as glycogen

Energy Sources for Contraction ● ATP supplies the energy for muscle fiber contraction ● glucose + 02 CO2 + H20 + ATP!!! CELLULAR RESPIRATION!!