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Today, the neural mechanism of consciousness still remains a mystery. According to William James, an American philosopher and psychologist, consciousness is an awareness of oneself and the environment. A person is in the conscious state only 5% or less versus the rest of the time being in un-conscious activity. So how does consciousness work and what are the mechanisms involved? The brain is an incredible complex organ, consisting of billions of neurons that create a large number of synaptic connections with other neurons. Each neuron may be connected up to ten thousand to a million other neurons while using neurotransmitters to form as many as a trillion synaptic connections. Unlike many other systems in the body, consciousness is completely subjective. It integrates emotions, memories, attention, and perceptions of an individuals surroundings and experiences to form ones unique consciousness. Memories, which involve the amygdala, are crucial because the mind will trick an individual into believing that nothing has changed in an environment or situation. It does this by resurfacing the same images and recollections. It is noteworthy that a persons consciousness does this automatically and smooth enough without an individual even realizing or actually having to think about doing it.
Apart of the reason that the mechanism of consciousness is such an enigma is due to the fact that there is not one particular area in the brain that applies to consciousness. Unlike the sensory and motor systems which go to defined areas in the brain through extensive reciprocating connectivity, consciousness is integrated with numerous diverse cells, pathways, and regions of the brain. It remains unclear what specific area produces consciousness; however, it can be said that there are many connections and parts involved, and a proper timing mechanism. This phenomenon is called functional connectivity and the areas/pathways involved include: the brainstem reticular formation (RF), thalamus, both the sensory and motor system, amygdala, and the prefrontal cortex (PFC).
The RF is a highly organized and complex structure that is imperative for controlling autonomic functions, as well as playing a crucial role in arousal, consciousness, and pain modulation. The RF communicates information to the thalamus and cerebral cortex which in turn controls sensory signals. These sensory signals will then exclusively be brought to an individuals attention when necessary. For instance, according to Anthony Hudetz and his study regarding consciousness and the mechanism of general anesthesia, somatosensory signals may be blocked peripherally. It can be inferred that this is a result of the RF receiving information from the peripheries and not conveying the pain signals to the thalamus, therefore modulating the signals. The RF is also the location where neuromodulators are produced and sent throughout the CNS via the reticular activating system (RAS). These neuromodulators are vital for anesthesia because they can alter how GABA, glutamate, and other receptors work by enhancing their excitatory or inhibitory state. As a result, by altering this connectivity, ones state of consciousness will be affected. Injury, such as lesions to the RF, can impair consciousness and may even lead to an irreversible coma.
One of the main attributes of the thalamus is that it receives and integrates different sensory inputs from the body. Once this information is received, it relays it to the appropriate area in the sensory system. The sensory system is located in the posterior portion of the brain and is composed of the occipital, temporal, and parietal lobe. This area is commonly referred to as the hot zone and is essential to forming ones consciousness. Images, sounds, and all other sensations that are perceived by an individual is generated at these particular regions of the brain. It is evident that the thalamus is more than just a relay station for somatosensory information. The thalamus also regulates motor signals, levels of consciousness, sleep and alertness. Hudetz believes that when an individual receives anesthesia, the brain is partially still active, particularly at the sensory areas. Anesthesia does not fully block the transmission of somatosensory information to the primary sensory cortex, therefore auditory and visual material is still processed. Implicit memory is also not fully blocked because it does not require conscious thought.
The thalamocortical loop is another reciprocating system that plays a necessary role in levels of consciousness and sleep. The loop is not just a gate into the brain, but rather a reciprocating pathway to and from the rest of the cortex. The thalamic reticular nucleus (TRN) is a modulating system that contains only GABAergic neurons. As thalamocortical neurons project through to the cortex, the signals are modulated and become inhibitory. This is particularly important for sleep, alertness, and levels of consciousness. Hudetz questions whether or not the thalamus is the key to providing anesthesia. For instance, it is noted that when GABA is injected into the intralaminar thalamus, an induced sleep state will result. However, not all anesthetics work on reducing thalamic activity, nor on GABA receptors. Therefore, it becomes clear that even though the thalamus plays a significant role, there are other areas that are just as crucial.
Consciousness involves the prefrontal cortex (PFC), aka the Executive part of the brain. This particular area is essential for complex behaviors, emotions, planning, and linking together meaning. Since the PFC is highly interconnected with much of the brain, it requires all areas to continuously be interacting with each other. For example, it constantly requires the amygdala, particularly how you remember a certain experience and how that effects your current emotions and behaviors. Hudetz discusses and applies an Integrative Theory of Unconsciousness which involves the PFC and the posterior parietal association area (which was previously discussed earlier). The only way to effectively diminish consciousness is to disrupt the entire feedback connection. Recurrent processing is important for consciousness, therefore if there is even the slightest deviation in the timing mechanism (for example with anesthesia or due to a brain defect), the entire functional connectivity of the brain will be distorted. The connection between the PFC and the posterior parietal association area is the exact location where ESPSs and ISPSs must be altered to modify the timing. Anesthesia plays a key role in disconnecting the PFC and the posterior area which will result in the patient becoming unconscious and entering a sleep-like state.
When an individual is asleep, he or she will have a diminished conscious awareness, yet the brain will remain active. According to a research study completed at the University of Wisconsin entitled, Breakdown of Cortical Effective Connectivity During Sleep, it was found that consciousness fades due to lack of high frequency oscillations and effective connectivity between sensory inputs to the cortex. Again, this involves the thalamocortical feedback loop. The loop is comprised of a negative feedback mechanism that has two distinct modes: tonic and oscillatory. Oscillatory mode is in the delta frequency range and is seen in NREM sleep stages 3 and 4. Delta sleep has the greatest oscillation synchrony, which binds together all critical sensory information and creates a disconnection. It is important to note that as previously stated, the RF projects ACH to modulate other areas in the CNS; the loop is one of those areas. Once again this demonstrates that consciousness (and sleep) involve many areas of the brain. When a person is awake, the RF will project a large amount of ACH, which will in turn inhibit the thalamocortical relay neurons. On the other hand, when a person is in NREM sleep ACH is inhibited. This allows the relay neurons to provide long lasting inhibition and cause a disruption in sensory connectivity. This can be characterized by observing slow-waves on an EEG.
Anesthesia and sleep both depress an individuals level of consciousness. Therefore, similar areas of the brain are affected, however precise mechanisms and outcomes differ. It has been proposed that certain anesthesia medications, like propofol, reduce brain waves similarly to slow-waves seen on an EEG during natural sleep. On the other hand, there are many differences between sleep and anesthesia. As previously stated, anesthesia does not fully block sensory connectivity, versus NREM sleep which does. However, in sleep there is also a heightened threshold to sensory input. A Certified Nurse Anesthetist (CRNA) can administer a medication that can actually decrease the sensation of pain. Anesthesia also provides temporary amnesia by creating disconnections between reciprocating pathways and the amygdala. However, as previously stated implicit memory is not fully blocked because it does not require conscious thought. On the other hand, while a person is asleep, memories are consolidated. NREM sleep particularly plays a strong role in declarative memories versus REM sleep which involves more procedural memory. Lastly, when an individual is given anesthesia there is complete paralysis of skeletal muscles. When a person is asleep and is in the NREM state, he or she has occasional, involuntary movements. Instead, REM state causes muscle paralysis.
Neuroscientists have been trying to solve the riddle behind consciousness for hundreds of years. Unfortunately, it is not an easy task. The brain is a complex organ that involves millions of synaptic connections. Consciousness does not involve one particular area, but many reciprocating and overlapping areas in the brain. Hopefully as technology and medical research improves, we will be able to uncover the mysteries behind unconsciousness.
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