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** Muscle Contraction Cycle

Cuesta College-Human Anatomy (BIO 205)-Fall 2010-Dr. S. Sachs, Instructor
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Muscle Contraction Cycle
Muscle fiber generates tension through the action of actin and myosin cross-bridge cycling; while under tension, the muscle may lengthen, shorten or remain the same
Events in Muscle Contraction [1]
1. A nerve impulse causes ACh (acetylcholine) release at a neuromuscular junction. ACh binds receptors on the motor end plate, initiating a muscle impulse
Events in Muscle Contraction [2]
2. The muscle impulse spreads quickly along the sarcolemma and into the muscle fiber along T-tubule membranes, causing calcium ions to be released into the sarcoplasm
Events in Muscle Contraction [3]
3. Calcium ions bind to troponin, causing tropomyosin to move and expose active sites on actin. Myosin heads attaché to the actin and form crossbridges
Events in Muscle Contraction [4]
4. Myosin heads go through cyclic "attach, pivot, detach, return" events as the thin filaments are pulled past the thick filaments. ATP is required to detach the myosin heads and complete the sequence of cyclic events. The sarcomere shortens, and the muscle contracts. The cyclic events continue as long as calcium ions remain bound to the troponin
Events in Muscle Contraction [5]
5. Calcium ions are moved back into the sarcoplasmic reticulum by ATP-driven ion pumps to reduce calcium concentration in the sarcoplasm, leading to relaxation. Termination of the muscle impulse results in the passive sliding of myofilaments back to the original state
sarcomeres
protein filaments inside a myofibril are organized into repeating functional units
T-tubules
extends into sarcoplasm, filled with extracellular fluid, electrical impulses conducted into cells
sarcoplasmic reticulum
stores and releases calcium [Ca++] into sarcoplasm after nervous stimulation
myosin
protein that makes up the thick filaments in striations in skeletal muscle cells
actin
protein that mainly makes up the thin filaments in striations in skeletal muscle cells
Myosin binding sites are found on the thin or thick filaments?
thin
tropomyosin
covers myosin binding sites on the actin molecules
troponin
moves tropomyosin aside & exposes myosin binding sites when Ca++ is released
Ca++ [Calcium]
When an action potential reaches a muscle cell, Ca++ flows through through the cell to trigger a contraction; muscle contraction remains as long as Ca++ is abundant in sarcoplasm
ATP
Adenosine triphosphate (ATP) is the biochemical way to store and use energy; binds myosin, allowing it to release actin and be in the weak binding state (a lack of ATP makes this step impossible, resulting in the rigor state characteristic of rigor mortis); the myosin then hydrolyzes the ATP and uses the energy to move into the "cocked back" conformation