The function of an afferent neuron, of course, is to respond to a sensory stimulus applied to its sensory dendrites. It then sends action potentials into the central nervous system encoding information about the stimulus.
The very first step in the process is for the sensory stimulus to open gated ion channels. The type of gated ion channel determines the type of sensory stimulus that is the afferent neuron detects. For our example here, we are going to use a sensor such as a Meissner's corpuscle, which responds to fine touch. Thus the ion channels in the sensory dendrites in a Meissner's corpuscle are mechanically gated ion channels. But the entire sequence would be the same for other types of sensors except the factor that gates the ion channels would be different.
When the gated ion channels open, ions flow through channel causing a change in the membrane potential. This type of change of membrane potential, which occurs only in sensory dendrites, is called a receptor potential.
Below is an interactive figure that shows conceptually what is happening in the sensory dendrites. First, apply a very light stimulus to the sensory dendrites and observe the ion channels that open. (The stimulus shown here is extremely brief.) Then apply the stimulus a second time and watch what happens to the charges separated by the membrane. Next, apply the second, stronger stimulus and watch what happens. And finally, apply an even larger stimulus.
Assume this afferent neuron is in a Meissner's corpuscle in a fingertip. What might the subject feel for the first two stimuli? Answer
What does the kapow! indicate in the figure? Answer
Why do you suppose I am using kapow! to indicate an action potential occurs? Answer
Let's now see what a recording of the membrane potential in the sensory dendrites would look like during the above three stimuli. As you follow along the recording, be sure you are clear what ion channels are responsible for each change in the membrane potential. Remember, the above figure shows what happens for extremely brief stimuli. (Also note that the figure stops at 0 mV. The peak of the action potential is higher than shown. Typically it would be somewhere between +20 to +40 mV.)
Now let's move the electrode to a node of Ranvier well away from the sensory dendrites. Notice the type of ion channels found there. Hopefully it is clear why here you see only one aspect of the response you analyzed above.
The above recordings are in response to an extremely brief touch. Now let's apply a slightly longer touch (say, 0.01 second long). We have moved the electrode back to the sensory dendrites and are using the third intensity of stimulation. Notice that the central nervous system now is receiving enough information to potentially discern that more stimulus was applied.
QUESTION: Let's say I didn't tell you the electrode was moved back to the sensory dendrites for the final recording. But you would have known that anyway. Why?
QUESTION: Notice the receptor potential declines with time and thus the frequency of the action potential decreases. What is this phenomenon called?
QUESTION: Suppose an even more intense stimulus was applied, but of the same duration. What would you expect to see?