Treppe Contraction

The treppe contraction is the staircase effect. Graded contractions acummulate because calcium hasn't been absorbed , this causes a graded response . A single stimulus that produces a twitch and the stimulus gets greater ; muscles are warmer and enzymes are more efficient ;  calcium remains in the sarcoplasm and is not all taken up into the sacoplasmic recticulum.

Events That Occur During Each Phase of a Muscle Twitch

Lag Phase : 1] An action potential is progated to the presynatic terminal of the motor neuron.. 2] The action potention causes the permeability of the presynaptic terminal to increase. 3] Calcium ions diffuse into the presynaptic teminal , causing acetylcholine contained within several synaptic vesicles to be released by exocytosis intso the synaptic cleft. 4]Acetylcholine released from the presynaptic terminal diffuses across the synaptic sleft and binds to acetylcholine receptor molecules in the postsynaptic membrame  of the sarcolemma . 5] The binding of acetylcholine to its receptor site causes ligand-gated Na+ channels to open ,and the postsynaptic membrane becomes more permeable to Na+. 6 ] Sodium ions difuse into muscle fiber ,causing a local depolarization that exceeds threshold and produces an action potential .7] Acetycholine is rapidly degraded in the synaptic cleft to acetic acid and choline by acetylcholinesterase , thus limiting the length of  time acetylcholine is bound to its reptor site .The result is that one presynpatic action potential produces one postsynaptic action potential in each muscle fiber . 8] The action potentia produced in a muscle fiber is progated from the postsynaptic membrane near the middle of both ends and into the T tububles .9] The depolarization that occurs in the the T tubule in response to the actio potential causes Ca+ channels of the membrane of the membrane of the sarcoplasmic recticulum to open ,and the membrane of the sarcoplasmic recticulum becomes very permeable to Ca2+. 10] Calcium ions diffuse from the sacroplasplasmic recticulum into the sacrcoplasm. 11] Calcium ions bind troponin ; The troponin -tropomyosin complex changes its postionand exposes the active site on the on the acttin myofilaments.
Contraction Phase
12 ]Cross- bridges  between actin molecules and myosin molecules form, move , release ,and reform many  times , causing the sarcomeres to shorten . Energy stored in the head of the myosin molecule allows cross-bridge formation and movements. After cross-bridge movement has occurred  ,ATP must bind to myosin head . The ATP is broke down to ADP , and some of the energy is used to released the cross-bridge  and cause the head of the myosin molecule to move back to it's resting postion , where its ready to form another cross-bridge . Some of the energy from the ATP is stored in the myosin head and is used for the next cross-bridge formationand monement.
Relaxation Phase
13] Calcium ions are actively transported into the sacoplasmic reticulum .
14] The trponin-tropomyosin complexes inhibit cross-bridge formation.
15] The muscle fibers lengthen passively .