Also have: SarcomeresStructures of the Sarcomere: A bands – dark thick bands made from myosin protein I bands – lighter bands made from actin protein
H zone – area between theends of the actin myofilamentswhich point to one another fromopposite ends of the sarcomere
heads actin myosin H zoneZ disk– where the actin or thinfilaments attach at one end-Z disk are made out of protein
The myosin proteins havehead-like structures on them
Sliding-Filament ModelIs the model explaining how amuscle cell or fiber is believedto contract
Sliding Filament ModelActin is surrounded by two otherproteins: 1. tropomyosin which winds around the actin blocking active sites 2. troponin which lies across the tropomyosin and keeps it in place
Sliding filament modelNerve impulse by neurotransmitteracetylcholine stimulates thesarcoplasmic reticulum to release Ca2+into the sarcoplasmCa2+ bind to the troponin and causethe position of the tropomyosin tochange exposing the active sites onthe actin filament
Sliding filament modelThe heads of the myosin filamentthen bind to the active site of the actinfilament forming a cross bridge
Sliding filament modelThe Ca2+ ions are then releasedOnce cross bridges are formed themyosin filament heads will shift calleda power stroke pulling the actinfilaments closer together;
Sliding filament modelThis happens simultaneously in allthe sacromeres and muscle fiberscausing the muscle to contract.
During sarcomere contraction:Reduced are the : H zone (shortens) I band (shortens) Overall length of sarcomere (reduced) I
During sarcomere contraction:Stays same: actin filament myosin filament A band (made by myosin)
Energy Requirements•Muscle contraction requires lotsof energy provided in the form ofATP (adenosine triphosphate).•When the first phosphate bond inATP is broken large amounts ofenergy is released.•ATP becomes ADP and Pmolecule.
ATP related to sliding filament model:•ATP bonds to head of myosinfilament•ATP ADP + P energybecomes available to the myosinhead to move.•P released from myosin, myosinthen binds to the actin strongly andpower stroke occurs
ATP related to sliding filament model:•ADP falls off and is recycled•New ATP can attach to providemore energy•New ATP attached, crossbridgeis broken
What stops the contraction?Same thing that started it – Ca ions 2+When nerve impulse stops, sarcoplasmicreticulum stops releasing Ca2+ ions whichno longer binds with troponin, and the activesites are no longer available.
Motor Units Motor unit is One motor neuron (nerve cell) and all the Muscle cells stimulated by that neuron
Parts of the motor neuron Axon – process that extends from the cell body and carries signal away Note: may branchPresynaptic terminal –very end of the axon neurotransmitter containing vesicles are found
Parts of the motor neuronNeuromuscular junction – where thenerve and muscle fiber meet (don’t touch). Parts of the neuromuscular junctionSynapse – interface between a nerve celland another cell (muscle cell)Synaptic cleft – gap between nerve andmuscle cells that is filled with interstitialfluid
Postsynaptic membrane –membrane of the muscle fiber in theregion of the synapseAcetylcholine (ACh)– Most commonneurotransmitter (chemical) released byneuron traveling across the cleftinteracting with the postsynapticmembrane of muscle cell stimulating it tocontract.
Synaptic vesicles – found inside thepresynaptic terminal containingneurotransmitters.Acetylcholinesterase – enzyme thatinactivates ACh to stop musclecontraction.
How the motor neuron stimulates muscle contraction:1.Action potential (electricalsignal) travels down axon2. Vesicles in the presynapticterminal release ACh.3. ACh travels across the synapticcleft.
How the motor neuron stimulates muscle contraction:4. ACh stimulates a new action potential in the sarcolemma through the T-tubules5. T-tubules stimulate the sarcoplasmic reticulum to release Ca+2 ions by diffusion.
How the motor neuron stimulates muscle contraction:6. We know Ca+2 ions attach to troponin changing arrangement of tropomyosin exposing sites on actin- crossbridges-powerstroke- contraction
How motor neuron is responsible for stopping contraction:1.Action potential stops2.ACh is no longer released and isbroken down by acetylcholinesterase.3. Calcium is no longer released but ispumped back into sarcoplasmicreticulum.
How motor neuron is responsible for stopping contraction:5.Troponin releases Ca ions and +2tropomyosin rearranges covering theactin active sites again.6.Crossbridges of myosin/actin arebroken7.Muscle is relaxed!!
All-or-none law of skeletal muscle contractionIndividual muscle fiberscontract with equal force inresponse to each actionpotentialEither a fiber is contracted orrelaxed no inbetween.
How do you get degrees of muscle movement? Slight, fine movement(thread a needle) compared tocoarse, extreme movements(pick up a 100 lb weight).
How do you get degrees of muscle movement?1. By the size of the motor unit oramount of axon branching A. Large motor unit – axonbranches hundreds of times stimulatinghundreds of muscle fibers at the sametime resulting in coarse movement withless control or
How do you get degrees of muscle movement?Two ways:1. The size of the motor unit or amount of axon branching A. Large motor unit – axon branches hundreds of times stimulatinghundreds of muscle fibers at the same time resulting in coarse movement with less control orB. Small motor unit – axonbranches only a few timesstimulating only a few musclefibers or just one at the timeresulting in fine movement withmore control.
How do you get degrees of muscle movement?2.Frequency of the action potentialcan affect muscle contraction Stronger the stimulus the higher the frequency of action potentials that travel down motor neuron
There are 4 levels of frequency:A.Subthreshold stimulus – toosmall to create an action potentialB.Threshold stimulus – strongenough to create an actionpotential
B. Threshold stimulus -stimulus is strong enough to create an action potentialC. Submaximal stimuli – stimuliof increasing strength that createmore action potentials alongmore neurons
C. Submaximal stimuli – stimuli of increasing strength that create moreaction potentials along more neuronsD.Maximal stimulus – a stimuluswhich is strong enough to createaction potentials in all theneurons innervating a wholenerveInnervating = controlling
Nerve stimulates several motorunits together.The more motor units (multiplemotor unit summation) activated themore muscle fibers contract thegreater the force of the musclecontraction.If motor unit is stimulated it hasbeen recruited.
•Muscle tone – the state of partialcontraction of the muscle, evenwhen not being used.•Less muscle tone when asleep,than when awake.•Motor units will take turnscontracting and relaxing to havemuscle tone.
Energy for muscle fibers comes from one of three ways:1.Aerobic respiration glucose synthesized into ATP most efficient; 36 ATP – 1Glucose slow and requires oxygen
2. Creatine phosphate• gives up a phosphate to ADP converting it to ATP• stored in the muscle cells in only small amounts• 1 ATP for each creatine phosphate stored, less efficient than aerobic
3. Anaerobic respiration• Glucose broken down to pyruvic acid to ATP• No oxygen required• Produces lactic acid toxic to muscles causes burning• Not very efficient, 1 pyruvic acid yields 2 ATP
If anaerobic is less efficient than aerobic why use it?Sometimes body cannot keep themuscle supplied with oxygen at afast enough rate.Myoglobin molecule stores oxygen inmuscle cell but sometimes notenough.