Although a huge body of research has investigated how 'touch' sensations work in mammals (especially humans), the 'mechanotransduction' channels are still not fully explained. In particular, mechanisms providing the 'pleasant' aspects of touch have not yet been adequately described.
And a subset of Aδ nerve fibers - also known as mechanonociceptor (AM) fibers - that respond to noxious mechanical and thermal stimuli, have end-organs that are as yet are poorly understood.
“Currently, we understand the detailed classification of mechanoreceptive cells and the response characteristics of associated sensory neurons. One of the next major questions is how somatosensory information is integrated and utilized in the spinal cord and the central nervous system for feeding, rearing, avoiding or emotionally bonding with others.”