Have you ever wondered how sensory receptors transfer the energy in a stimulus into neural output? This process is known as sensory transduction.
To understand how sensory transduction works I will use skin receptors to illustrate the process. Deep within the skin there are mechanical sensory receptor organs known as the pacinian corpuscles. They respond to pressure stimuli. The pacinian corpuscles are wrapped in a fibrous connective tissue capsule and are at the end of the sensory fiber. The nerve ending has the same structural and functional properties of neuronal dendrites and acts as a true transducer in this system.
The nerve endings, emerging from the capsule are myelinated containing nodes of Ranvier having the same functional property of axons. The connective tissue capsule helps spread the mechanical stimulus and contributing to receptor adaptation. The pacinian corpuscles activate when there is a strong enough pressure initiated on the skin. Since the pacinian corpuscles lie deep within the skin layers the pressure must be strong to initiate them. Once the pressure is exerted the well cushioned capsule of the connective tissue fibers send an action potential down the nerve to the nerve ending in the corpuscles core. The negatively charged sodium ions then diffuse into the cell becoming positively charged ions which depolarize the membrane, this is called the graded potential. There is then an established charged current along the nerve ending and the node of Ranvier in the axon. When the stimulation provokes a greater threshold response an action potential will initiate in the node which will then signal the central nervous system. This sequence will continue as long as the pressure is being presented on the skin. If the pressure increases or continues to be sustained, the current graded potential will continue. When the pressure overrides the threshold of the sensory receptors for long periods and the nerves are already firing off, adjacent corpuscles are recruited.
When the pressure is sustained for long periods and appears not to be causing any damage or pain to the tissue this is known as receptor adaptation.Though the receptors are still receiving impulses, the action potentials are below threshold transferring the information into the autonomic nervous system turning off conscious awareness. An example of this would be wearing clothes or wearing a wrist watch. When you first put your clothes on the receptors acknowledge the stimulus but within a few minutes the stimulation is weakened and adaptation occurs.
Daryl Conant, M.Ed