What problems does a neuron need to overcome to maintain the health and growth of the axon? I thought it has a problem with axon's length? Another question is: How would you estimate the muscle mass of a person? I know that muscle does not float, but adipose tissue does. Would I just determine it by how tall the person is? and.. An athletic injury can result in a tear to a muscle or the tendon that attaches the muscle to bone. What would be torn ( specific materials/ cells/ etc). What do you imagine the repair process for each type of tear to be like?