Functional systems of the brain
Functional brain systems are networks of neurons that work together but span relatively large distances in the brain, so they cannot be localized to specific regions. Two of the best examples of this are the limbic system and reticular formation.
The limbic system
The limbic system is a group of structures located on the medial aspect of each cerebral hemisphere and diencephalon. Parts of the rhinencephalon (septal nuclei, cingulate gyrus, parahippocampal gyrus, dentate gyrus, and hippocampus) and the amygdala. The fornix and other fiber tracts links the limbic system together. The limbic system is our emotional brain. Two parts are especially important: the amygdala and the anterior part of the cingulate gyrus. The amygdala recognizes angry or fearful facial expressions, assesses danger, and creates the fear response. The cingulate gyrus plays a role in expressing emotions with gestures and in resolving mental conflicts.
The sense of smell is highly tied to emotion due to much of the limbic system residing in the primitive rhinencephalon, the old part of the brain once responsible for smell. Reactions to smells are rarely neutral, and odors often recall memories of emotionally-laden events. Most limbic output is relayed through the hypothalamus. Because the hypothalamus is also responsible for visceral function, acute or unrelenting emotional stress often can cause visceral illnesses, called psychosomatic illness.
The limbic system is also tied strongly with the prefrontal lobes, which explains why we react emotionally to things we consciously understand to be happening and why we are aware of the emotional richness of life. This connection also explains why emotion sometimes overrides reason and why, other times, reason stops us from expressing emotion in improper ways. The hippocampus and amygdala also play a role in memory.
The sense of smell is highly tied to emotion due to much of the limbic system residing in the primitive rhinencephalon, the old part of the brain once responsible for smell. Reactions to smells are rarely neutral, and odors often recall memories of emotionally-laden events. Most limbic output is relayed through the hypothalamus. Because the hypothalamus is also responsible for visceral function, acute or unrelenting emotional stress often can cause visceral illnesses, called psychosomatic illness.
The limbic system is also tied strongly with the prefrontal lobes, which explains why we react emotionally to things we consciously understand to be happening and why we are aware of the emotional richness of life. This connection also explains why emotion sometimes overrides reason and why, other times, reason stops us from expressing emotion in improper ways. The hippocampus and amygdala also play a role in memory.
the reticular formation
The reticular formation extends through the central core of the medulla oblongata, pons, and midbrain. In what is otherwise white matter, it is basically loosely-clustered neurons. These neurons form three broad columns along the length of the brain stem: the raphe nuclei, the medial (large cell) group, and the lateral (small cell) group of nuclei. Reticular neurons are unique in their far-flung axonal connections: individual neurons project to the hypothalamus, thalamus, cerebral cortex, cerebellum, and spinal cord. This makes the reticular neurons ideal for governing the arousal of the brain as a whole. For example, the reticular activating system, unless inhibited by other brain areas, sends a continuous stream of impulses to the cerebral cortex, keeping it alert, conscious, and highly excitable. Interestingly, impulses from all the great ascending sensory tracts synapse with RAS neurons, keeping them active and enhancing their arousing effect on the cerebrum (this may explain why many students prefer to study in a crowded area or listen to music). The RAS also acts as a filter for sensory inputs: repetitive, familiar, or weak signals are filtered out, allowing only unusual, significant, or strong ones to reach consciousness. It is believed that upwards of 99% of all sensory stimuli are disregarded by either the RAS or cerebral cortex as "unimportant." The drug LSD interferes with these sensory dampers, which is what causes its "sensory overload" sensation.
The RAS in inhibited by sleep centers in the hypothalamus and other neural regions, as well as sedatives. Severe injury to this system (swift, rapid twisting of the brain stem) results in permanent unconsciousness (sometimes called "irreversible coma").
The RAS in inhibited by sleep centers in the hypothalamus and other neural regions, as well as sedatives. Severe injury to this system (swift, rapid twisting of the brain stem) results in permanent unconsciousness (sometimes called "irreversible coma").