The action potentials of individual neurons can be recorded with microelectrodes. David Hubel (1926-2013) and Torsten Wiesel (1924-) began such recordings in the early 1950s (Hubel and Wiesel 2005). They recorded from cells in the primary visual cortex of unconscious anesthetized cats whose eyes were propped open, and they drew conclusions about how those types of cells responded to elements in the visual field. They found cells in the visual cortex that responded to specific visual stimuli within their receptive fields.
The receptive field comprises the range of stimuli within which sensory neurons respond. Hubel and Wiesel found that cells in the primary visual cortex responded to simple geometric figures, such as lines, their orientations, and their movements. Their work on receptive fields also led to theories of how vision could be understood as the construction of complex representations from simple ones.
When a muscle is tensed, motor neurons fire proportionally to the tension. In a firm handshake, the relevant motor neurons fire faster than in a loose one. Neuroscientists think of this as rate coding. It works in the same way for somatosensory (touchsignaling) neurons. There are several variations on rate coding. Dynamic rate coding neurons activate briefly, and static rate coding ones do so for extended periods. Dynamic rate coding neurons detect change, while static rate coding neurons monitor ongoing stimuli. The static rate coding neurons quiet down with time, explaining why a constant uniform noise, such as that from an office fan, can be tolerable.
-  Neurons responding to a wide array of bodily sensations: touch, pressure, pain, temperature, jointposition, muscle position.