Raising neurotransmitter levels is only one means by which antidepressants act. The activity of modern drugs appears to readjust the balance between serotonin and noradrenaline by more fundamental means. Likely mechanisms include correction of the diminished sensitivity and numbers of specific serotonin and noradrenaline receptors found in depression. Also responsible may be changes in the normal pattern of movements of these transmitters across the synaptic membranes within the diffuse modulatory systems. It is possible that by increasing the serotonin levels these drugs may increase neurogenesis, suggesting that neuronal losses may be a fundamental cause of the symptoms.