A Lecture, Higher Brain Function: Activation of the Brain and Levels of Consciousness
The nerve cell process or activity that translates into consciousness and thought remains essentially unknown... but neurophysiology has yielded some understanding of the brain regions, the nerve pathways, and the neurotransmitters which produce the various states of "consciousness." It is known that "consciousness" and thought do not emanate from a single brain structure but rather derive from an interaction among many parts of the brain. The "Holistic Theory of Thought Formation" states that thought is produced by an interaction of many brain regions. We use the term "conscious state" or "consciousness" to describe those times when we have thoughts and awareness. Our states of consciousness might be described as the "alert" or "heightened" state, "drowsiness" (when we are awake but inattentive), and finally the consciousness associated with slow wave and REM sleep.
It has been suggested that two overlapping systems underlie the various states of consciousness. One system, the reticular activating system (RAS) emanates from the brainstem and projects upward to the cerebral cortex via the thalamus. The other system is the neurohormonal (neurohumoral) system, again arising in brainstem nuclei and projecting upward to many regions of the brain, and increasing levels of certain neurohormones in various regions of the brain. The influence of these systems can be described in behavioral terms, such as the ability to think and also the kind of thoughts we have, or it can be described in terms of brain wave activity (ie. the electrical brain wave pattern observed with electroencephalography, or EEG).
The Reticular Activating System (RAS)
In anatomy class, I like to describe the RAS as the "ignition system" of the brain. It is the system, for example, that helps get your brain geared up to a higher level of activity so you can get out of bed and start your day.
The RAS, also called the bulboreticular facilitory area, arises in the reticular formation of the pons and mesencephalon. The majority of these brainstem neurons project upward to terminate on (form synapses at) nuclei in the thalamus termed "non-specific" or "diffuse" thalamic nuclei. The non-specific nuclei are interspersed among the "specific" thalamic nuclei. The specific nuclei are formed by the third order neurons of the various sensory pathways that project directly to specific areas of the sensory cortex. Examples of the "non-specific" thalamic nuclei serving the RAS include the intralaminar nuclei and the reticular nuclei at the surface of the thalamus. These "diffuse" or "non-specific" nuclei send projections to all areas of the cerebral cortex. This system of ascending fibers (ie. ascending to the cerebral cortex) is sometimes called the "non-specific" or "diffuse" thalamocortical system.
Some of the brainstem neurons of the RAS are very large and transmit impulses rapidly via cholinergic fibers that excite the cerebral cortex for brief intervals. Other brainstem neurons of the RAS arise among a network of very small brainstem neurons with slow conduction velocity which provide excitation that builds up and persists for a prolonged period of time and which contribute to the long term background excitability of the cerebral cortex. It should be noted that some of these ascending fibers bypass the thalamus and pass directly to subcortical regions of the brain. In other words, ascending fibers which pass through the thalamus then project to the cerebral cortex; those fibers which do not pass through the thalamus ascend to subcortical areas.
The same brainstem region that gives rise to the ascending fibers of the RAS also gives rise to fibers that descend and terminate on (form synapses at) the spinal cord. These descending pathways provide facilitory signals which maintain tone of the "anti-gravity" or "postural" muscles. These descending pathways also maintain appropriate excitability of the neurons that form the spinal reflex arcs.
The degree to which the RAS "facilitates" the cortex is manifested in the pattern of brain waves (the EEG). For example, the brain wave pattern of a subject who is awake but with eyes closed and in a relaxed state is dominated by alpha waves (low amplitude waves which occur at frequencies of 8-13 waves/sec). If the subject begins to cerebrate (think), opening the eyes and concentrating on some mental task, the EEG changes to a beta wave pattern (desynchronized, very low amplitude waves with frequencies of 14-24 waves/sec). It is known that alpha and beta wave frequencies originate from the brainstem component of the RAS. On the other hand, it has been shown that when a space occupying lesion, such as a brain tumor, intervenes between the thalamus and the cortex, the area of cortex lacking connections with the RAS will chronically demonstrate brain wave activity in the delta wave category (high amplitude waves with frequencies of 3.5 per second or less). This suggests that the intrinsic electrical pattern of the cortex itself is the delta wave pattern and that delta wave patterns reflect a lack of facilitation such as when a person is in deep surgical anesthesia.
During a normal nights sleep, the RAS alternately releases the cortex then paradoxically strongly facilitates the cortex while the subject remains asleep. This alternating pattern is responsible for the two normal forms of sleep we experience each night. The initial sleep form is termed slow wave or delta wave sleep. This is a deep and restorative form of sleep when the cortex is not being facilitated and the EEG would of course be dominated by delta waves. After approximately 90 minutes of delta wave activity, the brain wave pattern changes dramatically from the delta wave to the beta wave pattern. This change in brain wave pattern signals the onset of what is termed "rapid eye movement" or REM sleep. During this REM or "paradoxical" sleep, there is a high incidence of dreaming accompanied by strong inhibition of skeletal muscle tone throughout the body.
The cortex is highly facilitated by the RAS, but obviously the channeling of impulses that supports REM sleep differs from the channeling of impulses that supports the alert waking state.
Facilitation of the RAS: Factors Promoting the Waking State of Consciousness
Once a person achieves the waking state of consciousness, consciousness itself appears to be self-promoting. There are two important mechanisms that excite and facilitate the RAS. The first of these is activity in the sensory pathways that ascends through the brainstem, especially the somatosensory pathways (dorsal column and spinothalamic pathways ). These somatosensory pathways have colaterals (that is branches that split off the main sensory pathway to the thalamus) that terminate on the brainstem component of the RAS. The somatosensory colaterals provide an important source of stimulation to the RAS. Put simply, activity in the sensory pathways to the cortex helps activate the RAS, which in turn facilitates the cortex which allows processing of the sensory input. A second source of facilitation resides in the cortex itself. There are extensive bidirectional connections between the cortex and the thalamus. Once the cortex is facilitated, it reciprocates by facilitating the thalamus. Thus, the waking state is promoted by a positive feedback mechanism between the cortex and the thalamus.
What all of this means is that, when you start to become physically tired... it gets harder to stay awake... but, if you are sufficiently interested in some sensory stimulus, you may manage to stay awake. For example, you get together with some of your buddies for a Friday night Freddy Kruger "all nighter." By the time you get to Nightmare on Elm Street Part 8, you are really sleepy. Your RAS will help you stay awake a bit longer than usual, because you are receiving interesting sensory input. But, the bottom line is, your RAS is going to begin to drift off... and now your cerebrum is becoming increasingly dependent upon itself to stay awake. You may find yourself fighting to stay awake... turning up the volume of the television, drinking a lot of coffee, running back and forth to the bathroom, running outside to get fresh air, etc. Without the help of your RAS, it is really tough to stay awake!
In addition to the widespread activation of the brain provided by the RAS, activation of the brain is also provided by a neurohormonal system. This system consists of brainstem nuclei which send projections upward to many brain regions and also downward to the spinal cord (just like the RAS). Activation of these nuclei brings about the release of neurohormonal agents causing effects which persist for a time frame of minutes to hours. These neurohormonal centers are important for their influence on various states of consciousness.
Here are some examples of members of the neurohormonal system:
The Norepinephrine System
This system arise as the locus ceruleus (Latin for "blue spot"), which is a cluster of several hundred nerve cell bodies located near the floor of the 4th ventricle at the junction of the pons and midbrain. These are noradrenergic neurons which, despite their small numbers, send projections to literally all parts of the CNS. Although the function of the locus ceruleus is not known for certain, there are several clues to its role in consciousness. For example, a high rate of discharge correlates with a high level of arousal and alertness and as arousal decreases and drowsiness ensues, activity of the locus ceruleus also diminishes. With deep sleep, the locus ceruleus is inactive. This nucleus or center is also related to the occurrence of REM sleep. Recall that REM sleep is a time when the cortex is being highly facilitated. It has been noted that lesions in the region of the locus ceruleus are associated with abnormalities or an absence of REM sleep. Also, it has been noted that lesions in this region can render a person incapable of experiencing the waking state of consciousness, as occurs in African Sleeping Sickness or lethargicum encephalitis which is the neural form of African Trypanosomiasis.
The Serotonin System
This system arises as a group of eight thin plates of primarily serotonergic neurons located on the midline of the brainstem. These are referred to as the midline raphe nuclei. Like the locus ceruleus, there are relatively few cells but they have extensive projections to the diencephalon and cortex of the brain and also to the spinal cord. Certain of the raphe nuclei are most certainly important "sleep centers," as electrical stimulation of these areas will initiate near normal sleep while lesions placed in these nuclei render an animal incapable of experiencing normal sleep. Projections to the spinal cord from certain of the raphe nuclei can inhibit transmission of pain signals over the spinothalamic tracts; it appears, then, that the raphe nuclei are a part of the brains analgesic system.
The Dopamine System
The dopaminergic neurons are found mainly in the upper part of the mesencephalon where they form the darkly pigmented area called the substantia nigra. Most of the fibers of this dopamine system project into the basal ganglia (putamen and caudate nuclei) where their inhibitory effects are an important controlling factor in the voluntary motor system. The reader is reminded of Parkinsonís disease associated with degenerative changes in the substantia nigra. Other neurons from this region project to various areas of the forebrain including the nucleus accumbens, hypothalamus, septal area, amygdala, and the cerebral cortex. The extensive dopaminergic projections to these areas suggest that dopamine plays a role in motivation and cognition. There is some evidence that abnormality of this system may play a role in certain types of mental illness such as schizophrenia. Some of the antipsychotic drugs that alleviate symptoms of schizophrenia are known to block some of the dopamine receptors.
In summary of this complex topic we note that the various states of consciousness are the result of an interaction between two systems. One system arises in the brainstem reticular formation and sends projections to the cortex via non-specific thalamic nuclei; the other system consists of brainstem nuclei which release neurohormones to various brain regions via ascending projections.