Final answer:
Type A fibers have the largest diameter, which are typically thick, myelinated axons enabling fast action potential propagation. Fast fibers have large glycogen reserves and many mitochondria. The largest neurons with the longest axons can be found in the spinal cord and are crucial for muscle contraction.
Step-by-step explanation:
Type type A fibers have the largest diameter axons. These fibers are typically thick, myelinated axons, which allows them to propagate action potentials at a higher speed compared to thinner and unmyelinated axons.
The true statement about fast fibers is that they have large glycogen reserves. Fast fibers are designed to enable rapid contractions and therefore have a higher demand for energy, which glycogen provides. Additionally, fast fibers have many mitochondria to support their energy needs during rapid, intense movements.
Regarding the neurons that control the muscles, some of the largest neurons in the spinal cord have very long axons. For example, the motor neuron that causes contraction of the big toe is one of the largest cells in the body, with its axon reaching lengths of up to a meter.
To propagate an action potential fastest, a thick, myelinated axon is ideal. The combination of having a larger diameter and myelination drastically increases the speed of nerve impulse conduction.