How many sensory neurons are in the human body?

Question

While I understand that there is a wide disparity of precision when it comes to specifying what constitutes "sensory neurons/receptors", I'm trying to find an authoritative source to provide at least a rough order of magnitude estimate on the number of receptors in the human peripheral nervous system (including skin, tongue, eyes, ears; but also mechanoreceptors and proprioceptors as well). The only sources I have found so far (here and here) only provide hints at the answer. I have found estimates as high as 15 billion, and as low as 3,000,000, but none of these guesses cite authoritative sources.

As I'm sure you can tell, I am not a student of anatomy (my interests lie in cognitive science and artificial neural networks), so forgive me if my question is too imprecise. But to put it simplistically: what I want to know is, roughly how many externally-produced sensory signals (channels) does the human brain have the capacity to process? Again, I am not looking for a precise answer, but I'm hoping to at least reduce the uncertainty to less than four orders of magnitude.

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CONTEXT: My purpose in this investigation is to try to quantify the magnitude of the computational capability of the brain with regard to processing external signals. If one were to imagine an "artificial nervous system" (as distinct from an artificial neural network), how many input sensors would it be able to simultaneously process if it were on par with a human brain?

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UPDATE: I recognize from the comments the inherent ambiguity of my question, so let me attempt to clarify. Assuming that the number of incoming signals to the brain at any one time is limited by the number of neural connections between the brain stem and the spinal column*, can someone provide an indication of what that number might be.

* _{(if that assumption is overly simplistic, feel free to enlighten me)}

Answer 1

This is a really interesting question. However, I'm going to try to address the context and update sections of your post rather then attempt to quantify all afferent signals in the nervous system. This is because quantifying those signals would be an exceedingly difficult task. It's not only logistically difficult to measure on an individual level, but to say anything definitive regarding the entire species would be superfluous. The number of signals at any one time is constantly in flux, as not every sensory receptor generates an action potential at the same time. Even in instances where similar or "competing" receptor types are simultaneously stimulated, those transmissions will be modulated and/or pathways suppressed. A good example can be found in this editorial article[1] on pain, where inducing painful stimuli can alleviate an itch sensation. I reference this to illustrate that sensory signals are not entering the brain unchecked. In the most simple of terms, they are heavily regulated.

Based on the context on your question, it seems what you really want to know is what is the maximum threshold. In an artificial nervous system, how many signals could we pump into it before it becomes inefficient or overwhelmed? However, what makes the nervous system unique is that it's a living system that adapts to stimuli. When there's more signals then it can handle or process, it regulates them. You can read more about the process and its importance in this article on sensory gating[2]. For this reason, a healthy brain that isn't being artificially stimulated shouldn't become overwhelmed. In fact, this process makes it far more efficient then any super computer. A fun example of this was the six chess games between Kasporov and Deep Blue in 1996. Think of the sheer number of moves Deep Blue could calculate compared to Kasporov: hundreds of millions of moves per second. It lost that match. Computational capability isn't conditional on the number of inputs the system can receive, it's how that information is processed, utilized, and applied once it's in the system.

I know I didn't answer your question in regards to a hard number, but hopefully it helped give a different perspective. Please let me know if you'd like anything further clarified.

[1]: Focus on pain. Nature Neuroscience 17, 145 (2014). doi.org/10.1038/nn.3644

[2]: Sensory gating, inhibition control and gamma oscillations in the human somatosensory cortex. Scientific Reports 6, Article number: 20437 (2016). doi.org/10.1038/srep20437

Answer 2

Estimates of the number of sensory cells range from a low in the first decade of millions to as many as double digit billions. The number of neurons going to the brain is not an indicator of how many signals are in the body. Think of it like one would the internet. Billions of people are connected, but they all go to nodes from individual devices to local data centers, to a relatively few major cables to which everyone is connected.

The way this happens on the web is that every individual user has an IP address, and even though the signals pass through cables that carry millions of messages only those directed to that IP address will be received on that device.

This all happens on a digital basis, but the human neural system is analog, which gives a much broader range of possible addresses and data combined than can pass in a digital signal. So what you are asking is far more complex than your question. You need a thorough understanding of analog information transfer as well as the digital domain from which I believe you are operating.