Final answer:
An increase in ventilation during the onset of exercise is caused by psychological stimuli, activation of motor neurons, and stimulation of proprioceptors in muscles and tendons.
Step-by-step explanation:
The three neural factors that cause an increase in ventilation as exercise begins are the psychological stimulus related to the conscious decision to exercise, the stimulation of motor neurons that innervate the muscle groups involved in the physical activity, and the activation of proprioceptors found within the muscles, joints, and tendons which sense movement and stretching. These proprioceptors create a stimulus that triggers the respiratory centers of the brain. This response aligns with the observed sudden increase in ventilation when exercise is initiated. It is also notable that ventilation decreases rapidly after exercise concludes, which underscores the neural control of these processes.