Anatomy and Physiology: Unity of Form and Function, by Saladin. Chapter 15, section III, sub-sections B and C; section IV, all sub-sections.
Edit: those are the sections covering innervation by the somatic nervous system, including reflexes.
Edit: nerve branches are divided functionally. There is overlap between different nerve branches. Interneurons may recruit more of these branches, depending on the motor program. This is achieved by activating more afferent fibers. It is not, however as was proposed: that the training of one fiber interferes with the activation of another. Having done anatomy and physiology courses yourself, I hope you will agree -- the nervous system's training process is entirely within the central nervous system, not the peripheral nervous system.
I agree with this post. I don't know the book you're quoting, but it sounds like a direct quote and is in line with what I've learned.
Your previous post sounded to me like you were saying fast twitch and slow twitch impulses are controlled by separate nerves altogether, rather than separate branches of the same nerves, which is what your source is basically saying.
In this context, yes, the training process is within the CNS, not the PNS. However, the PNS will grow more branches and innervation points for finer motor control and better fiber recruitment, so one could make the argument that you're training both. I'm really splitting hairs with that one though
You mentioned reflexes in your previous post also. Reflexes are not controlled by the brain; they're controlled by interneurons in the spinal cord. For example, if you put your hand on a hot stove, your brain doesn't tell your hand to move. The pain impulse gets to the spinal cord, an interneuron in the spinal cord* then sends an impulse to the arm and hand to move. The pain impulse is simultaneously sent to the brain, but by this point the reflex action has already been initiated. This is why there's a very slight delay in moving away and actually feeling the pain. The affarent nerves aren't myelinated, making by them slower than efferent nerves which are myelinated. An example I use is unexpectedly putting your hands under scalding water; you pull your hands away then feel the pain afterward.
*I can't remember the name of the structure in the spinal cord that controls reflexes but I think it's the s.g. I also can't remember what s.g. stands for.
Thanks for the trip down neurology lane. I think I broke my brain
Edit: Actually, I think the s.g. determines if the impulse will be passed onto the brain or block it. I don't remember, it was a while ago.