During a Muscle Contraction The muscle fiber is in a resting state. The resting state is slightly negative inside and slightly positive outside. It has to be in this state in order to receive a message. First the brain sends a message to the muscle using a neuron then reaches the axon terminal of the neuron. Calcium gates then open on the axon terminal allowing calcium to rush in. Ach release from axon terminals, diffuses across the synaptic clef and binds to (receptors) on motor end plate.
Then Ach receptor channel opens and increases permeability of Na+ into karyoplasms. This allows sodium gates to open. Na+ enters muscle fiber, rapid depolarization of sarcolemma occurs. Voltage changes to a less negative charge. An increase of Na+ rushing into cell allows for more gates to open (depolarization wave). Acetylcholinesterase is an enzyme that resets and closes gates once Ach is removed from its receptor. Once sodium gates close it initiates K+ gates to open.
K+ rushes out of the cell this and once repolarized the gates closes by the use of a pump and return to a resting state. ATP powers the Ca++ active transport pumps. The action potential spreads away from the end plate in all directions and depolarizes the T-tubules and dips down into the SR and depolarizes it. Calcium gates on the SR return open and allows calcium to rush out into the sarcoplasm. Calcium gates close, when a gate opens it’s a one way movement.
Ca++ combines with protien troponin and changes shape exposing the myosin binding sites on actin. This is the latent period, the lag time between stimulation and contraction. Myosin heads or cross bridges attach to actin binding sites on thin filaments. The sliding filament theory of a muscle contraction begins. When myosin binds to actin it pulls toward the m-line this is the “power stroke”. Once myosin head if flexed, ATP binding site is exposed and ATP binds to the head. Every single myosin head that attaches to actin has to have ATP.
Now the myosin head detaches from actin binding sites in result of ATP binding. Energy from ATP returns the myosin head to cocked forward position and now attaches to a new binding site on actin. Ca++ is then removed from troponin and changes shape allowing tropomyosin to cover myosin binding sites on actin. ATP is required to assist with the active transport of Ca++ to the SR. This process is considered the contraction and is repeated over and over as long as ATP and Ca++ are available.
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