Figure 1. The basic function of sensory receptors is shown for a photoreceptor. The flow of information is given by the black arrow. Photic energy (shown as a lightning bolt) is absorbed by the membrane in the transduction segment of the cell (A). Energy absorption produces membrane depolarization, which extends beyond the cell body (B) to the synaptic segment (C), where neurotransmitter is released.

Figure 2. Pacinian receptor. Within the onion-like lamella of connective tissue resides a nerve terminal (A) whose membrane is depolarized by mechanical energy. The sensory cell is ‘protected’ from slow skin indentation but rendered exquisitely sensitive to quickly changing indentations. The receptor potential is generated whenever the membrane is stretched and becomes permeable to ion flux. Action potentials are generated just outside the corpuscle (B) and are transmitted along a myelinated axon (C) towards the spinal cord.

Figure 3. The arrangement of sensory receptors in the skin (whisker hair follicles, shown as points indicated by the red arrow) is replicated across the cerebral cortex by a matrix of cortical columns (shown as hollow cylinders, indicated by the blue arrow). The dotted line is the border of the entire skin representation.