I. Introduction  
II. Activation Functions: A Historical Perspective  
III. Biological vs Artificial Neurons  
IV. Desired Characteristics of the Activation Functions  
V. Taxonomy of Activation Functions  
VII. Complex-Valued Activation Functions  
VIII. Quantized Activation Functions  
IX. A Quest Towards an Optimal Activation Function  
X. Performance of Some Fixed and Adaptive Activation Functions for Classification Tasks  
XI. Activation Functions for Physics-Informed Machine Learning  
XII. Summary  