Muscle Figs and Smooth Muscle Figs

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Muscle Figs and Smooth Muscle Figs

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University of Southern California
Bisc 307l - General Physiology
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BISC 307L 2nd Edition Lecture 13 Current Lecture Muscle Fiber Types There are 3 categories of muscle fibers in well- studied 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 slow- twitch 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 ...

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