160-The Motor Unit And Muscle Action-short

The Motor Unit and Muscle ActionLu Chen, Ph.D.MCB, UC Berkeley1Three types of musclesSmooth muscle: internal actions such as peristalsis and blood flow.Cardiac muscle: pumping blood.Skeletal muscle: moving bones.A motor unit consists of a motor neuron and themuscle fibers that it innervates.Skeletal musclefibers are innervatedby α-motor neurons.21

The Skeletal Muscle - OrganizationMuscleÆ Fascicles Æ Muscle Fibers Æ Myofibrils50-100 um2-6 cmThick filament(myosin)Thin filament (f-actin,tropomyosin, troponin)3The Skeletal Muscle - OrganizationMuscle UnitÆ Ensemble of muscle fibers innnervated by the same motorneuronMotor UnitÆ A muscle unit together with the motor neuron that innervates it.Except during development, each muscle fiber is only innervated by one motorneuron in one place (in mammals).The Sarcomere is the contractile Unit of the Striated Muscle42

The Thick FilamentThick filaments are composed of aprotein called myosin. Each myosinmolecule has a tail which forms the coreof the thick filament plus a head thatprojects out from the core of thefilament. These myosin heads are alsocommonly referred to as cross-bridges.The myosin head has several important characteristics:1)2)3)it has ATP-binding sites into which fit molecules of ATP. ATP represents potential energy.it has actin-binding sites into which fit molecules of actin.it has a "hinge"at the point where it leaves the core of the thick filament. This allows thehead to swivel back and forth, and the "swiveling" is, as will be described shortly, whatactually causes muscle contraction.5The Thin FilamentThe actin molecules (or G-actin as above) arespherical and form long chains. Each thin filamentcontains two such chains that coil around eachother.Tropomyosin molecules are long, thin moleculesthat wrap around the chain of actin.At the end of each tropomyosin is an troponinmolecule. The tropomyosin and troponinmolecules are connected to each other.Each of these 3 proteins plays a key role in muscle contraction:ACTIN - when actin combines with myosin head, the ATP associated with the head breaks down into ADP.This reaction releases energy that causes the myosin head to swivel.TROPOMYOSIN - In a relaxed muscle, the myosin heads of the thick filament lie against tropomyosinmolecules of the thin filament. As long as the myosin heads remain in contact with tropomyosin nothinghappens (i.e., a muscle remains relaxed).TROPONIN - Troponin molecules have binding sites for calcium ions. When a calcium ion fills this site itcauses a change in the shape and position of troponin. And, when troponin shifts, it pulls the tropomyosin towhich it is attached. When tropomyosin is moved, the myosin head that was touching the tropomyosin nowcomes in contact with an underlying actin molecule.63

Excitation – Contraction Coupling I: The t-tubules and the SR784

Excitation – Contraction Coupling II: The cross-bridge cycleAttached (rest)ATP-bindingCross bridge formationATP hydrolysis and Recock of myosinheadHead rotation and force generation9Excitation – Contraction Coupling II: The cross-bridge cycle105

Type of contractions1 - isotonic - tension or force generated by the muscle is greater than the load & themuscle shortens (successfully lift up a bucket or water)2 - isometric - load is greater than the tension or force generated by the muscle & themuscle does not shorten (tried to lift up a bucket of water but failed)Each single AP of the nerveelicit a single muscle twitch,which contains:Latent periodContraction periodRelaxation period11The force of the contraction of a muscle depends on:1- The frequency of the action potentialsfired2- The muscle fibers’ lengths (whichdepend on the initial overlap between thickand thin fibers)Active force: dependent on the degree of overlap of thickand thin filament.10 gPassive force: generated when lengthening the connectinfilaments that tether the thick filaments between the z disks126

Three types of muscle unitsType 1Type 2AType 2B25 pps20 pps13 pps40 pps for 0.33 s,repeated everysecond13Types of Striated Muscle/ Types of Muscle Fibers:RedWhiteFiber typeSlow –twitch fibers(type I)fast-fatigable (type IIB)fast-fatigue resistant (typeIIA)Fiber Sizesmalllargelarge (but maybe not aslarge as type wfastfastMitochondriamanyvery littleintermediateOxidativecapacityhighlowmoderate to highFatigueFatigue resistanteasy fatigablefatigue resistant (thoughless than type I)‘typical usage’posturegallop, jumprunThe MN that innervates fast-twitch muscle fibers usually innervates many large fibers. These MNhave relatively large cell bodies and large axon diameters, so they can conduce action potentialsfast. (the opposite is true for those MN that innervate the small fibers)147

The Motor Units are Recruited in a fixed orderExample of recruitment oftwo motor unitsThe size principle of MN recruitment V I x R15The Motor Units are Recruited in a fixed order168

The Motor Unit and Muscle Action Lu Chen, Ph.D. MCB, UC Berkeley 2 Three types of muscles Smooth muscle: internal actions such as peristalsis and blood flow. Cardiac muscle: pumping blood. Skeletal muscle: moving bones. A motor unit consists of a motor neuron and the muscle fibers