Anatomy and nomenclature
The forebrain or prosencephalon includes the telencephalon (cerebrum) and diencephalon (thalamus and hypothalamus).1
Level of consciousness (cerebrum, brainstem, especially front half)
One of the critical functions of the reticular formation of the brainstem is activation of the cerebral cortex for the awake
state. This component of the formation, known as the ascending reticular activating system (ARAS), is an ill-defined meshwork
of cells concentrated in the rostral brainstem that receives afferent input from all parts of the CNS and projects excitatory
stimuli cortically.2 Focal to extensive lesions anywhere in the brainstem may reduce the level of consciousness, whereas cortical injury must
be diffuse to cause noticeable obtundation. The ARAS also is involved in the initiation and maintenance of sleep. Abnormalities
of production or action of hypothalamic arousal peptides (hypocretins/orexins)5 or imbalances of brainstem neurotransmitters may result in narcolepsy/cataplexy sleep disorders.
Behavior (limbic system, temporal lobes)
Normal behavior requires integration of signals from the entire CNS but principally involves the forebrain. Most important
in controlling intrinsic behavior is the limbic system – a connected series of structures in the cerebrum and diencephalon.
A minor component is also found in the midbrain. Included are the amygdala, hippocampus, fornix, cingulate gyrus, and septal
area. A closely associated region, which is important in primate behavior, is the temporal lobe of the cerebrum. It is thought
that behavior based on conditioning and experience (i.e., learning) is controlled by the temporal lobes. Structural, metabolic,
or psychological disturbances affecting these areas may result in behavioral abnormalities (i.e., dementia). Dementia can
be defined as changes in normal habits, personality, attitude, reaction to the environment, or loss of learned skills. Some
of the signs that may be seen include disorientation in a familiar environment, failure to recognize a handler or object,
loss of the ability to be led, frequent yawning, head-pressing, irritability, unprovoked kicking or biting, compulsive walking
or circling, and dramatic changes in eating or drinking habits.
Almost any disturbance of the forebrain potentially can cause dementia. Encephalitis, head trauma, space-occupying lesion,
malformation, infarct, and metabolic disorders all are likely to cause changes in behavior. It is likely that structural or
metabolic forebrain disease is the cause of dementia if other neurologic abnormalities are found by neurologic examination
or imaging studies. In the absence of such supportive findings, abnormal behavior such as self-mutilation6 may have a psychological basis.
Seizures are sudden, transient attacks of abnormal motor and/or behavioral activity attributable to paroxysmal depolarization
of part to all of the brain. Depolarization occurs either simultaneously throughout the brain or originates from a hyperirritable
focus somewhere in the forebrain. Seizures originating from a focus likely will initially have asymmetric clinical signs and
there may be additional signs of forebrain disease between seizures that are revealed by neurologic examination. Seizures
frequently originate in the frontal (motor) cortex and involve muscle fasciculations and tremors around the head or abnormal
movements of the jaws and tongue ("chewing gum fits"). Convulsions characteristic of neonatal encephalopathy often are of
this type. In their severest (generalized) form, seizures manifest as sudden recumbency, with a brief phase of extensor tonus,
followed by clonic ("galloping") movements of the legs, loss of consciousness, and a variety of signs of autonomic discharge
(e.g., sweating, urination, defecation, pupillary dilation). Mild motor seizures are often accompanied by behavioral signs
such as obtundation, compulsive walking, hyperresponsiveness to stimuli, or other signs of dementia. Seizure foci in the forebrain
may occur at sites of previous or current trauma or inflammation. Syndromes of more than multiple seizure episodes without
interictal evidence of brain disease often are described as epilepsy.