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USC BISC 307L - Smooth Muscle
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Motor UnitsEvery one of these qualities vary continuously across fiber types3 types of muscle fiberstype 2b – fast fatigue abletype 2a- fast fatigue resistanttype 1- slow twitch fibersmuscle tend to be predominant in one type or the other although these fibers do intermixSizeMotor neurons larger for type 2b>2a>1Everything about them bigger not just somaMeans that a given amount of excitation delivered to both types of neurons will excite the small ones first because the smaller neuron has less total membrane so they have a higher total resistance between inside and outside (input resistance higher for small neuron). Since there’s less membrane it also has less capacitanceSame excitation will depolarize this small neuron more1a afferent: initiates stretch reflexif you give a tap to the tendon, the units that respond tend to be the slow units not the fast onesMuscle unitsFast twitch ones produce more forceShould show that there are more muscle fibers in the fast twitch onesBig powerful motor units in fast twitchON the right : slow – force is much weaker, fewer and smaller fibers, the twitch is longer in duration (build up and decay)FatigueStimulate continuously, type 2b can maintain initial level of force for shorter time than type 1Slow can maintain for an hourType 2a is in between these twoIF you record form a living animal, you see that the type 2B fibers fire at much higher frequencies.Type one would be lower frequencies (action potential has longer refractory period)Thresholds: relatively the same though takes less excitation for the type 1 to get to that levelType 1= dark meatType one has more storage of oxygen (myoglobin), capillaries and has more mitochondriaType 2B= light meatHow the brain controls the motor poolSize principleDescribes order of recruitmentAll these properties together- how the brain controls motor poolMotor pool- All of the given neurons that go to a given muscleMotor neurons are scattered in central horn of spinal cord- sort of clustered but more scatteredThis is a mixed muscle (Type 1-red and type 2B-blue)If the muscle consists of many motor units, the size principle will determine orderIf you ask subject to increase force on the muscle the order of recruitment says that the small weak slow ones will be recruited first (type 1) and as you continue to increase force you are recruiting the faster and stronger units (Type 2b)- last ones to kick in. This is how the muscle grades the amount of forceHow does brain control the motor pool?When the brain wants to recruit motor units it slowly increases the level of excitation to the motor pool bringing it to threshold or increasing inhibition. As you increase excitation, higher resistance and lower capacitance of type 1 neurons will cause them to be recruited firstA lot of force will stimulate type 2 b“Decision” is built into the intrinsic properties of the motor neurons- simplifies task of brainas you recruit more and more units, each additional force is more or less a constant fraction of the baseline force – this smoothes the grading of contractions, force doesn’t need a lot of big jumps to go upalso, all of these properties together – if low levels of excitation stimulate the type 1 motor units first, then the type 1 units will be active the whole time.Type 2 b units spend most of the time inactiveType 1 motor units are used for the postural tasksType 2 b are used for strong or ballistic movements (don’t need to be oxidative- glycolysis will supply their needs)Asynchronous Activation Smoothes ContractionsHow are muscle fibers activated?Most motor units are activated at a frequency that allows the contractions to fuse togetherGet single twitch – spontaneous firing of one motor unitThis is strange- motor units are usually activated at much higher frequencies and more than one is activated at the same timeStimulate once and then again right after, you would get fused twitchesIf you continuously did this you would get a condition called tetanusMovements are smooth because there is more than one motor unit firing at one time- not synchronized but still smooth as an averageSmooth muscleStructure and force generationUni-nucleated muscle cellsEmbedded in the walls of hollow organs (GI, uterus, blood vessel)Made up of several layers but the middle layer is muscularCells can form a band (sphincter) or randomly oriented and when contract do not seal off openingRelaxed:Small in diameter as compared to skeletal muscle fiberBut tend to be very longSame mechanism of contraction (actin in thin filaments and myosin in thick filaments)Contain tropomyosin but NOT TROPONIN for thin filamentsThere is sarcoplasmic reticulum also but not as organized s skeletal muscleOverall, there is much less of everything in the smooth muscle. Proteins arranged in oblique arrays – just less protein in generalNot striated-smoothNo z disks but there are dense bodies (purple) which are structures that play the same role as Z disks- thin fllaments attachedWhen the muscle fiber is activated it tends to ball up on itself (shorter and fatter)Arrangement of myosin molecules in the thick filamentShafts are located at 45 degree angles, which makes it possible for the thick filament to slide all along the length of the thin filament unlike skeletal muscleCan extend these and get a lot of overlap of myosin and actin (important for stretching of organs when contents change – bladder)- this is unlike skeletal muscleCalcium required for contractions but not in the same way. Doesn’t just come from the SR but can come from the calcium channels in the plasma membraneDoes not have a t tubule system- way excitation spreads is different than skeletal fiberSmooth muscle attaches to each other and connective tissue by desmosomesContracted:Contractions are slowRelative amount of force and duration of force in a “twitch” is much smaller and way slower than a slow twitch in skeletal muscle and lasts long.BISC 307L 1st Edition Lecture 12 Current Lecture- Motor Unitso Every one of these qualities vary continuously across fiber typeso 3 types of muscle fiberso type 2b – fast fatigue ableo type 2a- fast fatigue resistanto type 1- slow twitch fiberso muscle tend to be predominant in one type or the other although these fibers do intermixo Size Motor neurons larger for type 2b>2a>1 Everything about them bigger not just soma Means that a given amount of excitation delivered to both types of neurons will excite the small ones


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