Muscle Contractions

Muscles pull on tendon fibers Pull=Tension Tension is an active force Energy must be applied to produce an active force Applied tension must overcome an objects resistance

Resistance= a passive force that opposes movement Compression= the push applied to an object to force the object away Muscles can use energy to shorten fibers (thick/thin) to create tension Muscles CANNOT use energy to lengthen fibers so they cannot create compression

Contractions are activated by neurons in the sarcolemma This stimulation is called excitation-contraction coupling In excitation-contraction coupling calcium ions are released from the cisternae in the sarcoplasm reticulum Calcium ions trigger reactions between thick/thin filaments This trigger causes muscle fiber contractions and the consumption of energy (ATP) Net result Active Tension Overview of a muscle contraction

Vocabulary Terms Neuromuscular Junction- specialized intercellular connection where neurons communicate with muscle fibers Synaptic Terminal- branch ends of the axon on the neuron that branch within the perimysium Acetylcholine- neurotransmitter released by a neuron to change the permeability of the cell membrane Synaptic Cleft- a space between the synaptic terminal from the motor end plate Motor End Plate- contain deep creases called junctional folds which increase the surface area and thus the number of ACh sites (receptors) Acetylcholinesterase (AChE)- enzyme that breaks down ACh

Neuron Stimulation An electricla impulse called an action potential stimulates the release of Ach (Acetycholine) at the synaptic terminal The action potential causes exocytosis of a vesicle filled with ACh and the vesicle is released into the synaptic cleft ACh is released and binds to ACh receptors on the motor end plate. This binding causes a change in the permeability of the membrane(sarcolemma) at the motor end plate to sodium ions. This influx continues until AChE (Acetylcholinesterase) removes all the ACh from the receptors. Influx causes an action potential in the sarcolemma As the action potential spreads across the entire sarcolemma, AChE breaks down ACh and it is reabsorbed by the synaptic terminal for resynthesis of ACh so the process can be repeated

Excitation-Contraction Coupling Neuron stimulation creates the release of Calcium ions from the cisternae of the sarcoplasm reticulum Although it only lasts about.03 seconds, the levels of Calcium reaches 100 times the resting levels Cisternae are situated in the zone of overlap The zone of overlap is where thick/thin filaments interact Troponin is the lock and calcium is the key As calcium binds to troponin and its shape changes, tropomysium position changes which allows for myosin fibers to interact with actin filament.

Contraction Cycle Exposure of Active Sites- During excitation- contraction coupling calcium binds with troponin and tropomysium so that the interaction of myosin and actin can be completed. Formation of Cross-Bridge- Myosin heads binds to Actin sites to form cross bridges. Pivoting Of Myosin Heads- Myosin heads are usually positioned away from the M-line, but now they are “cocked” like a mouse trap. This step also requires the breakdown of ATP to provide the energy for the power stroke (myosin head moves toward the M-line)

Contraction Cycle Continued Detachment of Cross-Bridge- As another ATP molecule binds to the the myosin head, the bond to the Actin site is broken. The active site is now exposed and another cross-bridge can be formed. Reactivation of Myosin- As the new ATP molecule is broken down, the energy released can “recock” the myosin head. If calcium levels remain at elevated levels and ATP reserves are sufficient power strokes will continue. This rate will shorten sarcomeres by 1 percent and the shortening will continue based upon the cycling rate. As the sarcomere shortens, so does the muscle fibers within the whole muscle.

Cycling rates depend on resistance The higher the resistance, the slower the contraction rate The lower the resistance, the faster the rate. Think about it... Heavy weight= less reps and slower accomplishment, Lighter weight= more reps and faster accomplishments. In other words, light weight allows for fast reps and more of them, heavy weights allow for slow reps and less of them