The name Horace Wells (1815–1848) is probably not familiar to you, although many of us at some point benefited from his discovery: anesthesia, a word derived from the Greek meaning “without sensation.”
As for sensations, there is a part of our body that understands them: the nervous system. It is not for nothing that it receives sensory information 24 hours a day from outside our body (somatic information) and from internal organs (visceral). It specializes in detecting stimuli, processing them, and, most importantly for our survival, responding.
This explains why our bodies have a large number of nociceptors – sensory receptors – that can differentiate between harmless and harmful stimuli, i.e. those that damage tissue or may do so. This is where pain comes into play.
Complex alarm system
Pain is a protective response that alerts the body to imminent damage or danger, allowing us to make a decision to avoid injury. The International Association for the Study of Pain defines it as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage.” This means that it is not only a sensory experience, but also has emotional and subjective components. Moreover, it can occur without a somatic reason to justify it.
Thus, pain occurs when nociceptors in the skin, tissue, muscles, and internal organs detect a threat. It is a response to different types of stimuli: thermal, such as extreme temperatures (hot or cold); mechanical, such as strong pressure or physical injury; and chemical, such as substances released by damaged or inflamed cells.
As could not be otherwise, these receptors are located at the end of the axon of the sensory neuron. In other words, they have a direct connection with the nervous system. Thus, they transform the harmful stimulus into a signal that the neurons understand: an electrical current that is transmitted through the axons to the spinal cord.
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Once there, the signals begin to be processed and finally reach the brain, where they are interpreted and perceived as pain. Many areas of the brain are involved in this processing, such as the thalamus (analyzes and integrates sensory and motor functions), the somatosensory cortex (mainly receives and processes tactile information), the limbic system (associated with emotions), and the prefrontal cortex (an area of great importance for our survival and our coexistence in society).
In short, a stream of information is transmitted from the receptor that detects possible damage (for example, touching a burning iron) to various areas of our brain involved in processing this signal, which allows us to issue a survival response (remove our hand).
This is how anesthesia works
It is clear that pain is necessary because it tells us that our body is damaged or in danger. Although, of course, it doesn’t seem so useful to us when we are undergoing surgery. Don’t worry, that’s what we have anesthesia for.
Its purpose is to cause a temporary loss of sensation or consciousness so that patients can undergo surgery and other medical procedures without experiencing pain. In short, it involves “switching off” our brains for a period of time. Although the exact mechanisms are not fully understood, they are known to involve interfering with the flow of information (transmission of nerve signals) from nociceptors to the nervous system.
Interestingly, a recent study conducted on flies of the genus Drosophila shows that anesthetics do not affect all types of synapses (connection points between two neurons) equally. For example, many agents, such as barbiturates and propofol, enhance the action of gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter in the central nervous system. These compounds thus enhance the “switching off” of neurons, leading to sedation, hypnosis, and amnesia.
Other anesthetics, such as ketamine, block NMDA (N-methyl-D-aspartate) receptors, which are involved in the transmission of excitatory nerve signals. This reduces the excitability of neurons and causes a state of unconsciousness and analgesia.
Finally, inhalational anesthetics such as sevoflurane and isoflurane act on potassium and calcium channels, altering membrane potential and hence synaptic transmission (communication between neurons).
Consciousness goes into mode switched off
There are three components associated with pain: sensation, emotion, and cognition. Therefore, a good anesthetic should produce at least the following effects: amnesia (inability to remember what happened), analgesia (suspension of sensitivity to pain), hypnosis (unconsciousness), and immobility.
Of all of them, perhaps one of the most interesting and – still – mysterious aspects has to do with consciousness. Or rather, with the loss of it.
Read more: Coma or reality?: measuring consciousness is not easy
In this sense, a new study conducted at the Massachusetts Institute of Technology (MIT) on animals confirms that consciousness depends on an unusually synchronized communication through the cerebral cortex and that some anesthetics, such as propofol, interrupt the flow of sensory information from nociceptors to various parts of the cortex involved in pain processing.
That is, the anesthetized brain will receive all kinds of stimuli – auditory, tactile, olfactory, etc. – but this information will not reach the higher cortical centers responsible for processing them. The stimuli are left half-depleted, suggesting that consciousness requires coordination between all the cortical areas involved in their processing.
Studies of this type suggest that the state of unconsciousness that occurs during anesthesia is not due to a complete “numbness” of the brain, but to a cessation of the flow of information between different areas.
This fact may explain what happens in what is called “intraoperative awakening,” also known as intraoperative consciousness, when a patient regains consciousness during surgery and can remember the event. During these unusual but serious episodes, a person may experience varying degrees of consciousness, from feeling awake but unable to move (due to the effects of muscle relaxants) to feeling pain or discomfort.
A window into the mysteries of the mind
Despite the enormous benefits it brings, research into anesthesia still has many unknowns. Beyond its medical applications, its research could open new doors in our knowledge of the human brain, perhaps allowing us to explore the deepest mysteries of the mind and perception. Can we understand processes like sleep, coma, or anesthesia itself with complete clarity? Or even more philosophical concepts like mind-body dualism.
A field designed to dull the mind may, paradoxically, be the key to unlocking the secrets of our consciousness.