Expert: John Locke - 7/23/2009

Question
We are going over how nerve cells receive and transmit signals and the subject was brought up of why some nerve signals are stronger than others.  It was explained to us that there are only a certain amount of receptors, and all neurotransmitters in excess of the number of those receptors would be lost.  So it was not the number of neuroreceptors, but the speed at which the signal travelled to where it is processed.  At the end of class we felt a bit incredulous of this answer.  Is it the speed?  Why does the speed make a difference?  Does the number of nerve sensors stimulated make a difference when you are poked with a blunt object verses being poked with a brand or a blade?  Or is the type of receptor, free ends of nerve cells vs Pacinian corpuscles for example, what elicits these different sensations?

Answer
Thanks for using AllExperts. There are various questions within your larger question, and I will try to address the major one first--why are some nerve signals stronger than others--and the address your specific questions as to blunt/sharp objects. I will be referencing cutaneous sensation a lot here, as that is the clearest example of what I'll be talking about.

The strength of a sensory nerve signal is indeed linked to the number of receptors that are stimulated up the saturation point, at which all sensory elements are being stimulated. It is also linked to the sensory modality (vibration, pain, cold, mechanical pressure, etc.), intensity of the stimulus, stimulus frequency, and the location of the stimulus. Let's consider each in turn.

The modality of the stimulus is the readily identified class of sensation, some of which are noted above. Different regions of skin contain varying amounts of receptors for given modalities, meaning that a stimulus equal in all other aspects but differing in modality will produce a distinct response on different regions of the body. To take a concrete example, cold and hot sensory neurons are found in the greatest concentration in the distal fingertips; they will respond to temperature changes more quickly and to a greater extent than some other area of exposed skin.

The stimulus location is signaled by the activation of a particular population of sensory neurons that are affected by the stimulus. Not every location on the body has a similar population of neurons, hence the effect will be distinct at different locations.

The stimulus frequency will affect the perceived sensation depending upon how quickly the sensory neurons are capable of responding to repeated stimuli. Slowly-reacting neurons may only fire a single action potential in response to a series of very fast stimuli, for instance, which will affect the perceived sensation.

Neurons react differently in terms of what response they will produce for a given stimulus. For most sensory neurons, the relationship is roughly exponential:

Response=(Constant value)*(Stimulus^n)

where the exponent n is a number greater than zero. This means that, broadly, most sensory neurons show a greater response as a stimulus increases in intensity, which is what you might expect intuitively.

Beyond this, sensory neurons function as a series of interconnected relay circuits. First order neurons, which actually receive the stimulus, synapse with second order neurons in the spinal cord. Some processing may take place at that point. The signal then travels into the thalamus, where it synapses with third order fibers that may syanapse in turn with fourth-order fibers in the cerebral cortex.

To answer your question about speed, all the mechanical receptors within the skin have myelinated axons, so they transmit quickly and at a similar rate. I'm not sure that speed is the relevant factor here. Sensory modalities are divided up between different types of neurons--as described above--and each of those modalities of neurons functions in a unified way. In other words, all Meissner's corpuscles respond to low-frequncy vibrational stimuli, and they all respond in about the same way.

To discuss pain for a second: pain sensation is handled by two major classes of receptors, A-delta mechanical nociceptors and C polymodal nociceptors. A-delta receptors react quickly to noxious mechanical stimuli (such as pricking the skin or crushing it with forceps), while C polymodal receptors respond slowly via unmyelinated nerve fibers to all varieties of painful stimuli--mechanical, chemical, thermal. They are responsible for the longer, duller sensation of pain that follows the sharp pain of initial injury. If you poke the skin with a sharp object, pain receptors would most likely be stimulated. When the skin is prodded with a blunt object, in contrast, Merkel cells or Ruffini's corpuscles would be stimulated instead. In that case, the type of recpetor that is responding would be most important.

What does this all mean? The answer is somewhat more complicated than just how many receptors are present or the speed at which they transmit. The number, type, location, and characteristics of the recpetors all contribute to the initial neuron response; sensory processing that takes place in higher-order neurons affects the perceived sensation even further. The type of receptor is highly important for distinguishing different types of sensations, as you suspected. The speed, so far as I'm aware, is less important than what was explained to you.

If you have further questions, I'd be happy to answer them. Good luck.