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like plasma membrane polarized; their is a potential voltage difference across the membrane and the inside is negative relative to the outer membrane face

Action Potential

electrical charge which occurs along the entire surface of the sarcolemma; 3 steps involved for this to occur

Generation of an Action Potential Across the Sarcolemma (1)

(local depolarization and generation of an end plate potential); Binding of ACh molecules to ACh receptors at neuromuscular junction opens (ligand) gated ion channels that allow Na+ and K+ to pass; More Na+ diffuses in then K+ diffuses out and interior of sarcolemma becomes less negative (depolarization [local electrical event called, end plate potential)


-70 (resting plasma potential)


30 (for action potential to occur)

Generation of an Action Potential Across the Sarcolemma (2)

(generation and propagation of the action potential); [neuron send AP through axon] end plate potential ignites AP that spreads in all directions from neuromuscular junction across sarcolemma; this depolarization (end plate potential) spreads to adjacent membrane areas and opens voltage gated sodium channels; Na+ enters and reaches voltage threshold (+30), and an action potential is generated. AP is propegated along length of sarcolemma as depolarization wave spreads to adjacent areas of sarcolemma and opens voltage gated sodium channels there; again, Na+ difusses into cell

Generation of an Action Potential Across the Sarcolemma (3)

(repolarization); sarcolemma is restored to intitial polarized state; Na+ channels close and voltage-gated K+ channels open; K+ efflux rapidly restores the resting polarity; Fiber cannot be stimulated and is in a refractory period until repolarization is complete; Ionic conditions of the resting state are restored by the Na+-K+ pump

Excitation-Contraction Coupling

Sequence of events that convert s action potentials in a muscle fiber to a contraction; Action potential travels across entire sarcolemma; occur during hidden (laten) period, between AP initiation and the beginning of mechanical activity (contraction); electrical signal does not act directly on myofilaments (it causes rise in intracellular calcium ion concentration that allows filaments to slide)

Excitation-Contraction Coupling: Step 1

action potential travels across entire sarcolemma and are rapidly conducted to interior of muscle fibers by transverse tubules

T Tubules

Regularly spaced infoldings of sarcolemma that branch extensively throughout the muscle fiber; At numerous junctions, make contact with calcium storing membranous network known as sarcoplasmic reticulum

Terminal Cisternae

formed by SR (On portion of t tubule and adjacent terminal cisternae); saclike bulges where it abuts t tubules

Excitation-Contraction Coupling (2)

traveling down of action potential causes t tubule voltage sensitive protein to change shape; opens a calcium release channel in SR allowing calcium ions to flee sarcplasm; this rapid influx of calcium triggers contraction of muscle fibers

Excitation-Contraction Coupling (3)

calcium binds to tropinin and removes the blocking action of tropomyosin; when Ca2+ binds, troponin changes shape, exposing binding sites for myosin on the thin filaments

Excitation-Contraction Coupling (4)

contraction begins; myosin binding to actin forms cross bridges and contraction begins; at this point E-C coupling is over

Role of Calcium (Ca2+) in Contraction

At low intracellular Ca2+ concentration:
(Tropomyosin blocks the active sites on actin) (Myosin heads cannot attach to actin) (Muscle fiber relaxes)
At higher intracellular Ca2+ concentrations:
[Ca2+ binds to troponin ] [Troponin changes shape and moves tropomyosin away from active sites] [Events of the cross bridge cycle occur] [When nervous stimulation ceases, Ca2+ is pumped back into the SR and contraction ends]


Functional unit of contraction in skeletal muscle fiber; Shorten when myosin heads in thick myofilaments form cross bridges with actin molecules in thin myofilaments

Formation of Cross Bridge

Initaited when calcium ions released from SR bind to troponin (This causes tropinin to change shape); Tropomyosin moves away from myosin binding sites on actin allowing myosin head to bind actin and form a cross bridge; Myosin head has to be activated before a cross bridge cycle can begin; ATP combines with myosin head and is hydrolized to ADP and inorganic phosphate (Energy from hydrolozied Atp activates myosin head forcing It to be in cocked position)

Cross Bridge Cycle: Step 1

cross bridge formation:Activated myosin head binds to actin forming a cross bridge; Inorganic phospahte released; Bond between myosin and actin becomes stronger

Cross Bridge Cycle: Step 2

the power stroke: ADP released and activated myosin head pivotes; Slides thin myofilament toward center of sarcomere

Cross Bridge Cycle: Step 3

cross bridge detachment: Link between mysoin head and actin weakens when another ATP ataches to myosin head; Myosin head detaches

Cross Bridge Cycle: Step 4

reactivation of myosin head: ATP hydrolized to ADP and inorganic phospahte; Energy released during hydrolizes reactivates myosin head returning it to cocked postion

Muscle Tension

the force exerted by a containing muscle on an object


the opposing force exerted on the muscle by the weight of the object to be moved


if muscle tension develops but the load is not moved; increasing muscle tension is measured


if the muscle tension developed overcomes the load and muscle shortening occurs; amount of muscle shortening is measured

Motor Unit

the nerve-muscle functional unit; consists of a motor neuron and all the muscle fibers it supplies; small motor units (more precise movement) [ex: fingers], larger motor units (less precise movement) [ex: hip muscles, bone]

Motor Nerve

served each muscle; each motor nerve contains axons of up to hundreds of motor neurons


a graphic recording of contractile activity; line recording activity is called tracing

Muscle Twitch

response of a motor neuron to a single action potential of its motor neuron

Latent Period

the first few milliseconds following stimulation when excitation-contraction coupling is occuring; during this period, muscle tension is beggining to increase

Period of Contraction

cross bridges are active, from the onset to the peak of tension development, and the myogram tracing rises to a peak

Period of Relaxation

final phase, lasting 10-100ms, is initiated by reentry of Ca2+ into the SR; muscle tension decreases to zero and tracing returns to baseline

Graded Muscle Responses

muscle contraction can be graded in two ways: (1) by changing the frequency of stimulation and (2) by changing the strength of stimulation

Temporal/Wave Summation

if two identical stimuli (electrical shocks or nerve impulses) are delivered to muscle in rapid succession, the second twitch will be stronger then the first; temoral or wave summation is this second twitch; this occurs because second contraction occurs before the muscle has completely relaxed; primary function is to produce smooth continous muscle cells

Unfused or Incomplete Tetanus

if the stimulus strength is held canstant and the muscle is stimulated at an increasingly faster rate, the relaxation time between the twitches become shorter and shorter, the concentration of Ca2+ in the cytosol higher and higher, and the degree of wave sumation greater and greater, progressing to a sustained but quivering contraction

Fused or Complete Tetanus

as the stimulation frequency continues to increase, muscle tension increases until a maximal tension is reached; at this point all evidence of muscle relaxation dissapears and the contractions fuse into a smooth, sustained contraction plateau

Recruitment (Multiple Motor Unit Summation)

controls force of contraction; achieved by delivering shocks of increasing voltage to the muscle, calling more and more muscle fibers to play

Subthreshold Stimulus

stimuli that produce no observable contractions

Threshold Stimulus

the stimulus at which the first observable contraction occurs

Maximal Stimulus

the strongest stimulus that produces increased contractile force; represent the point at which all the muscles motor units are recruited

Muscle Tone

relaxed muscles that are almost slightly contracted; its due to spinal reflexes that activate first one group of motor units then another in response to activation of stretch receptors in muscles

Isotonic Contractions

muscle length changes and moves the load, the tension remains relatively constant through the rest of the contractile period; come in two flavors concentric and eccentric

Concentric Contractions

those in which the muscle shortens and does work, such as picking up a book or kicking a ball;

Eccentric Contractions

the muscle generates force as it lengthens; are important for coordination and purposeful movements; occur in calf muscle for example; 50% more forceful then concentric

Isometric Contractions

tension may build to the muscles peak tension producing capacity, but the muscle neither shortens nor lengthens; occur when a muscle attempts to move a load that is greater then the force (tension) the muscle is able to develop

Direct Phosphorylation

ATP producing way; Metabolic Pathway; chemical reaction where CP (creatine phosphate) and ADP are used. Phosphate from CP is taken and put into ADP and turns to ATP; 1 to 1 ration; energy last for less then 10 seconds; does not require Oxygen;first stage body goes to to make energy; occurs in cytoplasm of cell

Anearobic Pathway (Glycolosis)

metabolic pathway without oxygen; produced 4 ATP; 10 step process; every step is a chemical reaction and changes everytime (10 times); 1st 5 steps are known as energy investment phase (they require energy) [2 ATPS to produce these 5 steps]; 2nd Stages known as Reinvestment stage; makes only 2 ATPs; this process occurs in cytoplasm of cell; 2 energy sources glucose and glycogen; makes 2 or 3 ATP + 2 Pyruvic acids

Glucose Anearobic Pathway

yields 2 ATPs; does not require oxygen; also yeilds 2 pyruvic acids

Glycogen Anearobic Pathway

used when their is not enought glucose; stored fat; Yields 3 ATPs because it has allready been partially hydrated; also yields 2 pyruvic acids

Lactic Acid

made if their is no oxygen present or work is more then you can breathe; pyruvic acid is transformed to lactic acid; made either way by glucose or glycogen; liver can use or send back to be formed to pyruvic

Aerobic Pathway

yields the most ATP with Oxygen; needs pyruvic acid to be shuttled from mitochondria (it is shuttled from Anearobic Pathway when 2 ATP and 2 Pyruvic acids); Pyruvic acid is transformed to Acetylcoa (happens in cytoplasm of cell); Krebs cycle produces 32 ATP

Aerobic Endurance

the length of time a muscle can continue to contract using aerobic pathways

Anaerobic Threshold

the point at which muscle metabolism converts to anaerobic glycolysis

Muscle Fatigue

the state of physiological inability to contract even though the muscle still may be receiving stimuli; due to a problem in excitation-contraction coupling or, in rare cases, problems at the neuromuscular junction


lack in ATP; states of continuous contraction because the cross bridges are unable to detach

Oxygen Deficit

the extra amount of oxygen that the body must take in for these restorative processes

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