BISC 307L 1st Edition Lecture 12 Current Lecture Motor Units o Every one of these qualities vary continuously across fiber types o 3 types of muscle fibers o type 2b fast fatigue able o type 2a fast fatigue resistant o type 1 slow twitch fibers o muscle tend to be predominant in one type or the other although these fibers do intermix o 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 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 capacitance Same excitation will depolarize this small neuron more 1a afferent initiates stretch reflex if you give a tap to the tendon the units that respond tend to be the slow units not the fast ones o Muscle units Fast twitch ones produce more force Should show that there are more muscle fibers in the fast twitch ones Big powerful motor units in fast twitch ON the right slow force is much weaker fewer and smaller fibers the twitch is longer in duration build up and decay o Fatigue Stimulate continuously type 2b can maintain initial level of force for shorter time than type 1 Slow can maintain for an hour Type 2a is in between these two o IF you record form a living animal you see that the type 2B fibers fire at much higher frequencies o Type one would be lower frequencies action potential has longer refractory period o Thresholds relatively the same though takes less excitation for the type 1 to get to that level o Type 1 dark meat o Type one has more storage of oxygen myoglobin capillaries and has more mitochondria o Type 2B light meat How the brain controls the motor pool o Size principle Describes order of recruitment o All these properties together how the brain controls motor pool Motor pool All of the given neurons that go to a given muscle Motor neurons are scattered in central horn of spinal cord sort of clustered but more scattered This is a mixed muscle Type 1 red and type 2B blue If the muscle consists of many motor units the size principle will determine order If 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 force How 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 first A lot of force will stimulate type 2 b Decision is built into the intrinsic properties of the motor neurons simplifies task of brain as 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 up also 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 inactive Type 1 motor units are used for the postural tasks Type 2 b are used for strong or ballistic movements don t need to be oxidative glycolysis will supply their needs Asynchronous Activation Smoothes Contractions o How are muscle fibers activated Most motor units are activated at a frequency that allows the contractions to fuse together Get single twitch spontaneous firing of one motor unit This is strange motor units are usually activated at much higher frequencies and more than one is activated at the same time Stimulate once and then again right after you would get fused twitches If you continuously did this you would get a condition called tetanus Movements are smooth because there is more than one motor unit firing at one time not synchronized but still smooth as an average Smooth muscle Structure and force generation o Uni nucleated muscle cells o Embedded in the walls of hollow organs GI uterus blood vessel o Made up of several layers but the middle layer is muscular o Cells can form a band sphincter or randomly oriented and when contract do not seal off opening Relaxed Small in diameter as compared to skeletal muscle fiber But tend to be very long Same mechanism of contraction actin in thin filaments and myosin in thick filaments Contain tropomyosin but NOT TROPONIN for thin filaments There is sarcoplasmic reticulum also but not as organized s skeletal muscle Overall there is much less of everything in the smooth muscle Proteins arranged in oblique arrays just less protein in general Not striated smooth No z disks but there are dense bodies purple which are structures that play the same role as Z disks thin fllaments attached When the muscle fiber is activated it tends to ball up on itself shorter and fatter Arrangement of myosin molecules in the thick filament Shafts 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 muscle Can 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 muscle Calcium 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 membrane Does not have a t tubule system way excitation spreads is different than skeletal fiber Smooth muscle attaches to each other and connective tissue by desmosomes Contracted Contractions are slow Relative 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