Nociception: how nerves transmit pain

Nociception is the process by which nerves transmit information about stimuli that damage or threaten damage to normal tissue (“ IASP Terminology – IASP ,” 2017).  We can’t properly call it pain, because you’ll remember that pain is always a psychological state, even though it usually involves nociceptive elements.

One of the first serious attempts to describe the nociceptive pathways was made by Rene Descartes in his Treatise on Man, first published posthumously in 1664. He described the system as:

Next, in order to understand how the external objects that strike the sense organs can instigate the machine to move its members in a thousand different ways, note that the tiny fibres (which, as I have already told you, come from the innermost part of its brain and make up the marrow of the nerves) are arranged in every part serving as the organ of some sense in such a way that they are easily moved by the objects of that sense. And when they are moved, with however little force, they simultaneously pull on the parts of the brain from which they come and thereby open the entrances to certain pores in the internal surface of the brain. The animal spirits in the cavities of the brain immediately begin to make their way through these pores into the nerves and so into the muscles, which act so as to cause movements in the machine very like those we are naturally instigated to make when our senses are similarly affected.

Descartes' depiction of pain pathways

Thus, for example, if fire [A] is near foot [B], the tiny parts of this fire – which as you know move very rapidly – have sufficient force to move with them the area of skin that they touch, and in this way they pull the tiny fibre [cc] which you see attached to it, and simultaneously open the entrance to the pore [de], located opposite the point where this fibre terminates: just as when you pull on one end of a cord you cause a bell hanging at the other end to ring at the same time.
Now when the entrance to the pore or small tube [de] is opened this way, the animal spirits from cavity [F] enter and are carried through it, some to the muscles that serve to pull the foot away from the fire, and some to the muscles that make the hands move and the whole body turn in order to protect itself.

(Descartes, 1998)

Nociceptive Pathways - a modern understanding
When fire burns the skin, the nociceptive signal travels along the first-order neuron, shown in green, in the peripheral nervous system. It synapses to the second-order neuron, shown in red, and continues up the spino-thalamic tract until it synapses with the third-order neuron, shown in blue, in the thalamus. The third-order neuron takes the signal to the cortex for processing. The results of this processing determine whether the nociception is actually pain.

An updated understanding can add more details to this basic system. We split the nervous system conceptually into the central nervous system, consisting of the brain and the spinal cord, and the peripheral nervous system. We know that there are different types of nerve fibres, and that the nerves that carry information about touch are different than the ones that carry information about nociception. We describe the journey of the nociception as going through three stages. The first-order neuron, in the peripheral nervous system, carries a signal from the site of injury or insult to the spinal cord. The signal then passes into the central nervous system, where a second-order neuron takes the message up the spine to the brain. In the midbrain (which is part of the brain stem), the signal is transferred to a third-order neuron, which takes it to the cortex for processing (S. Wallace & Staats, 2004). So far, the system is not very dissimilar from that described by Rene Descartes, hundreds of years ago.

Of course, when we look more closely at what actually happens, things become more complex.

Nerves do not exist in isolation, as Descartes described them. They respond to outside stimuli, like the damaging heat the burns the foot in the diagram, but they also get signals from other nerves and respond to molecules released by other cells around them. Within a neuron, or nerve cell, signals are transmitted electrically, but the electrical activity within the cell is affected by neurotransmitters and neuropeptides that bind to specialised receptor sites, including opioid and cannabinoid receptors. These neurotransmitters and neuropeptides from other cells in the body work to excite (turn up) or inhibit (turn down) the intensity of the signal. If adding up all the exciting factors and subtracting all the inhibiting factors takes the electrical charge over a certain threshold, then an action potential is triggered: the electrical charge of the cell spikes, and it releases its load of neurotransmitters to pass the message to the next cell in the relay. If there are enough inhibiting forces that it doesn’t pass that threshold, however, the nociceptive message does not get passed forward (Cheng & Rosenquist, 2018).

IASP Terminology – IASP . (2017, December 14). Retrieved November 19, 2018, from http://www.iasp-pain.org/terminology?navItemNumber=576#Nociception
Cheng, J., & Rosenquist, R. W. (Eds.). (2018). Fundamentals of Pain Medicine. Springer International Publishing. doi:10.1007/978-3-319-64922-1
Descartes, R. (1998). The World and Other Writings. (S. Gaukroger, Trans.). Cambridge: Cambridge University Press.
S. Wallace, M., & Staats, P. (2004). Pain Medicine and Management. Toronto: McGraw Hill Professional.

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