BISC 307L 2nd Edition Lecture 13 Current Lecture Muscle Fiber Types There are 3 categories of muscle fibers in wellstudied mammals determined by the 4 qualities mentioned in the last lecture While the 3 general types are widely accepted muscle physiologists have found that the qualities used to lump them into 3 categories vary continuously not discretely and there actually exists a gradient of categories But we will simply lump them into these 3 categories On the very left are the type IIb fibers and on the very right the type I or slow twitch fibers It is important to differentiate between slow and slow twitch fibers As mentioned before it is possible to have a slow muscle fiber with a sufficiently low density of VG NA channels such that they don t have an all or none action potential and instead have local graded potentials However we are talking about slowtwitch fibers Motor neurons are largest for the IIb and smallest for type I The IIa motor neurons are intermediate The differences are actually bigger than shown in the picture Everything about the IIb fibers are bigger the size and length of their dendritic branches the diameter of the axon etc What does this tell us A given amount of excitation delivered to both types of neurons will excite the smaller ones first because the smaller neuron has less total membrane which means they have a higher total resistance between the inside and the outside The technical term is input resistance the total resistance between the inside and outside which is higher for small neurons What about capacitance There is less membrane on a small neuron so it has less capacitance Therefore the same amount of synaptic current flowing out across the membrane of the small neuron will depolarize it more because there is higher resistance and less capacitance You can see in the blue box above the Ia EPSP what is that The synaptic inputs coming in from the very top are Ia afferents these are the synaptic terminals of stretch receptors This is the synapse that mediates the stretch reflex knee jerk reflex There are stretch sensitive and mechanically sensitive receptors in the muscle that when stretched like by tapping the patellar tendon which stretches the quadriceps cause an AP to go up the sensory neuron through the dorsal root into the spinal cord Branches of those axons are these synaptic terminals of big neurons whose mechanically sensitive nerve endings are in the muscle spindle At this synapse excitation between these 1a afferents afferent in the sense of a sensory nerve and the motor neuron to that same muscle will cause a large EPSP that will stimulate the motor neuron bring it to threshold and therefore the muscle contracts You will see roughly the same number of 1a afferents for each fiber type You can also see that the amplitude of the EPSP caused by the Ia afferents is smaller in the big neuron and bigger in the small neuron This is mainly a function of the fact that the neuron is smaller because inward current flowing through the channels at the postsynaptic membrane at the synapses made by those 1a afferents depolarizes the small type 1 neurons first because of their higher resistance and lower capacitance This means if you give a small tap to the tendon of a muscle and stretch it the units that respond with a reflex contraction tend to be small units rather than the fast units In the muscle the muscle fibers of the type IIb are larger Though they are the same length they have a bigger cross sectional area So these fast twitch fibers on the left produce more force because force is proportional to the cross sectional area of the muscle fiber because CSA is a measure of how much of the contractile stuff is in there This diagram should also show more fibers in the type IIb motor unit than in the type I These motor units and muscle fibers are powerful and produce big increments in force if you stimulate the motor axon it will produce 5 units of force However the time scale is only 0 100 ms On the right you see the twitch of the type I Stimulate the motor neuron and all the muscle fibers in the motor unit contract and the force you get is smaller and weaker indicating that there are fewer fibers and the fibers are smaller You can see that the twitch is longer in duration roughly twice the duration of the IIa and it takes longer to build up and longer to decay Type IIa is intermediate in every way On the bottom in the Fatigue section we are stimulating on a timetable of minutes This basically shows the force you get when you stimulate at high frequency Type IIb can maintain its initial level of force for 1 minute and a half and it falls off and fatigues Type I on the far right from 0 6 minutes maintained its force If you continue stimulating it for an hour it can maintain force for an hour Type IIa can be seen going up to 6 minutes then all the way to 60 It peters out near the end but it is still contracting So the type IIb are big fast and powerful but they fatigue quickly while type I are smaller slower and weaker but very resistant to fatigue If you record from a living animal and you tried to see the natural frequency at which their neurons fire during normal movements you find that the motor units to the type IIB fire at much higher frequencies than the motor neurons to the type I motor units In fact strong ballistic movements like running or jumping the type IIb could fire at 60 80 hertz The type 1 frequency is around 5 hertz 10 would be high They tend to be naturally activated at a much lower frequency Look at the very top motor neuron action potential was recorded intracellularly for the type II b on left and type I on the right You can see why they fire at different rates The Type IIb has a polarization with no after hyperpolarization but type I has a long after hyperpolarization We know what this is due to a delayed opening of K channels Because of the long afterhyperpolarization and the persistent opening of K channels if you gave an excitatory current to the type I motor neurons on the right they would fire at low frequencies because they have a long refractory period On the other hand the type IIb on the left without a long hyperpolarization their refractory period is short and they are ready to fire relatively quickly and at higher frequencies Their thresholds are relatively the same Type I They are fatigue resistant because they have a lot of mitochondria a higher density of capillaries and level of oxidative phosphorylation