Which of the following is an example of spatial summation? a) More acetylcholine (ACh) is released into the neuromuscular junction of one muscle cell of your quadriceps. b) Calcium ATPase is required to transport calcium against its concentration gradient towards the sarcomeres. c) Several motor neurons simultaneously generate action potentials, and each of them depolarizes between 10-100 muscle fibers in your quadriceps. d) The elasticity of the sarcomeres allows for myosin and actin to be positioned in a way that facilitates the formation of crossbridges. e) The frequency of action potentials in one muscle fiber is so high that all possible crossbridges are active at the same time.