What is a sensory receptor?
A sensory receptor is a part of a sensory neuron or cell (and possibly associated cells) that receives information from the world and relates it to your nervous system. For example, you learned way back in A&P 1 lab about Pacinian corpuscles in the skin; they are the deep pressure receptors. Each Pacinian corpuscle contains the dendritic terminal of a single sensory neuron (with its cell body located in the DRG). Each Pacinian corpuscle also contains some other cells that surround and enwrap the dendritic terminal. Together, the entire structure of the dendritic terminal plus the supporting cells is a Pacinian corpuscle… this is a single sensory receptor.
A nice description of sensory receptors is given in this General Sensory Physiology course page on the Sensory Cell. You may want to read the rest of this page and then go check out that link.
Basically, some outside force has to have a way to act on dendrites (or some similar cellular region). In the case of the Pacinian corpuscle, a very forceful pressing on the skin activates it. How? That’s described below.
How does a sensory receptor work, in general?
Some sensory receptors are activated when they are bent, squished, or disturbed in some way. Others are activated by chemicals. Others by temperature. And others by light. Whatever the outside world influence is, we can call it a stimulus for the receptor. Let’s continue to use the Pacinian corpuscle as an example.
The dendrite within the Pacinian corpuscle is wrapped up by cells and connective tissue fibers to make a thick, oval-ish package, as seen if you followed the image links above. In cross-section, at rest, it can be represented by this diagram to the right; the blue areas are the layers of connective tissue, while the red area is the sensory neuron dendrite.
When the skin is pressed or tightly stretched, this sensory receptor gets deformed. This is shown in this figure. Note that both the blue and the red areas are deformed in this picture… that occurs if the pressure (the stimulus) is great enough. The deformation of the dendrite is what activates this sensory neuron. If the stimulus is weak, like a light touch, it would just bend the layers of tissue (blue areas) around the dendrite, but not the dendrite itself… One has to push really hard to actually deform the dendrite.
The images above were stolen out of the Pacinian corpuscle section of the General Sensory Physiology web page I mentioned above. This course site is a really good one, but goes much further into the material than you need.
In our skin, pressure upon the epidermis has to be hard enough to push enough into the dermis to activate these deep dermal corpuscles. This is like having another set of colored circles around the blue ones in the pictures above.
I edited the picture at the top to include the additional thicknesses for the epidermis and the dermis that the stimulus has to penetrate with pressure. My edited version is this one to the right. Can you start to see how these particular sensory receptors are only good for detecting deep pressure and not light touch?
How does the outside stimulus get transformed into a nervous system signal?
So, a mechanical deformation (or a chemical or light stimulus) can activate sensory receptors. How exactly does that happen, and how does that lead to an action potential in the sensory neuron itself (so that it can give the information to the CNS)?
Whatever the appropriate stimulus is, that will cause a depolarization to occur in the sensory receptor cell. Going back to the Pacinian corpuscle, a mechanical deformation of the sensory dendrite causes a depolarization to occur within the dendrite. This dendrite is at the tip of a unipolar neuron of the DRG; I have edited the picture from your textbook to show what this neuron is like. The sensory dendrite is embedded within the Pacinian corpuscle. The trigger zone (a.k.a. axon hillock) is just proximal to the corpuscle, still in the skin. So, any depolarization that is large enough in the dendrite itself will cause an action potential to be generated at the trigger zone. This action potential will run all the way down the axon (bypassing the cell body in the DRG) and enter the dorsal horn of the spinal cord.
The depolarization within the sensory dendrite itself (due to the stimulus) is called a receptor potential. Remember, the depolarization due to a synapse was called a postsynaptic potential. The receptor potential can occur because the dendrite has ion channels in its membrane that are stretch-gated channels. That means that they open if they are stretched. You already know about voltage-gated channels and ligand-gated channels. This is just one more type. When the dendrite is deformed, its membrane stretches, and ions can flow through the stretch-gated channels. If this occurs enough, there will be enough of a receptor potential to trigger an action potential.
How does the sensory activation stop?
After a sensory stimulus has been given, when do we stop feeling (seeing, smelling, hearing, or tasting) it?
- When the stimulus stops being applied (if it is a short stimulus).
- After a while, we stop noticing the stimulus. Our sensory receptors are said to adapt (or habituate)… that just means that they stop having a receptor potential after a while. Example: you put on your shirt in the morning and feel the material against your body… then, after a little while, you forget about your shirt because you don’t feel it anymore. The sensory receptors in your skin habituated to the shirt touch stimulus because it went on too long. We did a lab experiment on this (the one with the coins).
Adaptation is a good thing, because otherwise we would go crazy feeling our clothes all day (could you imagine) or smelling freshly-baked bread for hours (we’d have to eat a lot more of it!).