Some of the fundamental behaviours involved in survival are escape reactions from harm. If we ask, why do we withdraw the finger a flame, most people will answer, “because it hurts” or ”because I feel pain”. These would be wrong explanations. Nociceptive (‘harm sensing‘, from Latin ‘nocere’, to harm)1 sensory neurons convey information via their nerve fibres to spinal neural circuits, with a consequent reflex motor response resulting in retraction of the finger. Thus, the removal of the finger from the flame is due to activation of neural circuits entirely contained within the spinal cord, with no involvement of the brain (nociceptive reflex). This is one of the primordial sensory-motor loops described above and underlies the defence reaction to harmful stimuli. Effective stimuli for these pathways are mainly tissue lesions, which could be caused by severe heat or cold, trauma, infection, toxic substances or the growth of a tumour, for example. Chemicals released locally by lesioned tissue or from irritants introduced from outside are major stimulants of nociceptors2.
Where does the ‘hurting’, the ‘feeling of pain’ experienced from the burning of the finger occur then? The nociceptive neurons not only connect with interneurons and motor neurons within the spinal cord to trigger the defence reaction, but these same neurons branch in the spinal cord. These axon branches project further up to higher levels of the brain (ascending pathways) including the brain stem and, via further synaptic connections, to the sensory cortex. From there descending pathways project down the spinal cord to modulate motor activity, forming entirely internal neural loops. The subjective unpleasant ‘feeling of pain’ takes place at these higher neural levels, in parallel with threat detection via other sensory inputs, such as vision and hearing, and is modulated by previous experience and context. Amongst other areas, the amygdala has a key role in coordinated visceral and somatic responses to the perceived threat.
I will not review here the extensive literature covering multiple levels of integration of pain perception. Suffice to know that the feelings of pain occur with some delay compared to the rapid withdrawal of the burned finger mediated by the primordial spinal sensory-motor loop. This is because the ascending neural signals travel longer distances, requiring more time to reach the upper loops in the brain that generate the conscious feeling of pain. From these higher levels, descending pathways to lower sensory-motor loops can modulate nociceptive sensory inputs, including inhibition of their activity. A dramatic example of suppression of pain by inhibitory descending neural pathways is the proverbial bravery of a Roman soldier Muzio Scevola (Mucius Scaevola) who, by power of his will, held his hand steadily over a flame to demonstrate his Roman courage to the Etruscans.
Image: Mucius Scaevola in the Presence of Lars Porsenna by Matthias Stom, (early 1640s), Art Gallery of New South Wales. From: https://en.wikipedia.org/wiki/Muzio_Scevola
In summary, while the spinal sensory-motor pathways are sufficient to generate the escape behaviour, pain is felt at higher levels. This explains why it is possible to feel pain even in the absence of a harmful external stimuli when loops further up in the defence circuits become abnormally active (‘central pain’).
What applies to the pain circuits applies to all sensory pathways. Every sensory signal entering the spinal cord via sensory neurons connects with local spinal circuits, but, in parallel, also connects via ascending pathways with levels further up, which, in turn, connect via descending pathways to lower neural loops, thereby forming internal neural loops superimposed to the primordial sensory-motor loops.
- For a review of different types of nociceptors in the animal kingdom, see E StJ Smith & GR Lewin (2009): Nociceptors: a phylogenetic view. Journal of Comparative Physiology A 195, 1089-1106 ↩︎
- For a review of how nociceptors respond to different stimuli, see AE Dubin & A Patapoutian (2010): Nociceptors: the sensors of the pain pathway. Journal of Clinical Investigation 120, 3760- 3772. ↩︎