Behavioral Neuroanatomy
The brain, and the brain alone, is the source of our pleasures, joys, laughter, and amusement, as well as our sorrow, pain, grief, and tears. It is especially the organ we use to think and learn, see and hear, to distinguish the ugly from the beautiful, the bad from the good, and the pleasant from the unpleasant. The brain is also the seat of madness and delirium, of the fears and terrors which assail by night or by day, of sleeplessness, awkward mistakes and thoughts that will not come, of pointless anxieties, forgetfulness and eccentricities. —HippocratesThe human nervous system consists of the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS contains the brain (cerebral hemispheres, basal ganglia, and thalamus); brainstem (pons, medulla, and midbrain); and the spinal cord. The PNS consists of somatosensory (afferent) neurons, motor (efferent) neurons, and autonomic neurons.
The cerebral cortex
The area of the brain most closely associated with behavior is the cerebral cortex, although subcortical areas are also involved. The activity of the cortex can be segregated functionally into sensory, motor, and association areas that act together to ultimately affect behavior.
The cortex also can be divided anatomically into frontal, temporal, parietal, and occipital lobes, as well as limbic lobes that contain the medial parts of the frontal, temporal, and parietal lobes.
Frontal lobes
The frontal lobes have four major subdivisions. The first two, the motor strip and the supplemental motor area, are involved in motor behavior; the third (Broca's area) in language. The fourth division is the prefrontal cortex. Clinically occurring events, surgically imposed changes, and neuroimaging studies provide evidence for the behavioral and personality functions of the prefrontal cortex. The famous case of Phineas Gage, a man who received a large prefrontal lobe lesion in an accident in the mid-nineteenth century, first demonstrated the personality functions of the frontal lobes. Although he had remarkably few obvious neurological problems, Gage demonstrated a significant personality change after his brain lesion healed. A formerly nonaggressive person, Gage showed outbursts of anger after the accident. A respectful, energetic, persistent, and organized person before his accident, he began to show an inability to carry out plans and a lack of self-control and concern for others. In a similar way, some patients who have had bilateral prefrontal lobotomy, a surgical procedure used in the past to treat serious psychiatric illness, retain intellectual functioning but show uncharacteristic apathy and lack of goal-directed behavior after the surgery.
Perseveration, engaging in repeated unnecessary behavior and thought, disinhibition, and sudden outbursts of temper, as well as reinstatement of the infantile sucking ,palmomental and rooting reflexes (frontal release signs) are seen in patients with prefrontal lobe damage; this is now known as prefrontal lobe syndrome. Interestingly, schizophrenia and obsessive-compulsive disorder (OCD), both of which are characterized by personality and affective changes, are associated with decreased bilateral prefrontal cortical activity as measured by functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) .
Although personality changes are associated with damage to the entire prefrontal cortex, clinical and other evidence indicates that the three major prefrontal subdivisions”the orbitofrontal region, the dorsolateral convexity, and the medial region”have specialized behavioral functions. The dorsolateral convexity influences behavior and personality and has executive responsibilities involving activities like formulating plans, maintaining attention and concentration, and changing problem-solving strategies when needed. The orbitofrontal cortex is a center for the biological control of inhibition, emotions, and drive states. It is also part of the dopamine-driven reward circuit of the brain and is activated in addicts exposed to drug-related cues . The medial region has connections to the basal ganglia and accessory cortical motor areas and is involved primarily in motor activity. Damage to each of these subdivisions results in characteristic behavioral abnormalities The emotional-behavioral functions of the frontal lobes are lateralized. Lesions of the left prefrontal area, both cortical and subcortical, can result in depression, whereas lesions of the right are more likely to produce manifestations of elevated mood. Similarly, fMRI studies reveal that positive mood is associated with activation of the left prefrontal cortex and stress with activation of the right prefrontal cortex.
Limbic lobe
Because the neurons within the limbic lobe form circuits that play a major role in emotions, the limbic lobe has been called the limbic system. Its primary functions are to mediate between the hypothalamus and cerebral cortex and to modulate the activity of the autonomic nervous system . First described by Papez in 1937 and expanded later to include other areas, the limbic system or Papez circuit contains the hippocampus, fornix, amygdala, septum, part of the thalamus, the cingulate gyrus, and related structures . The limbic system also acts on the hypothalamus, which in turn influences endocrine control of emotions via secretion of hormones. Damage to the limbic lobe, particularly the amygdala and hypothalamus, results in behavioral abnormalities. Recently, neuroimaging studies indicate that, like the prefrontal cortex, the volume of limbic structures like the amygdala and hippocampus are reduced in patients with schizophrenia.
The basal ganglia
The basal ganglia are a group of nuclei that receive information from the entire cerebral cortex and project it to the frontal lobes via the thalamus. There are four structural components of the basal ganglia:Striatum (containing the caudate nucleus and the putamen)
Pallidum (also called the globus pallidus)
Substantia nigra
Subthalamic nucleus
The basal ganglia function to translate the desire to execute movement into actual movement. Conditions causing damage to its nuclei can result in neuropsychiatric illnesses with motor symptoms. For example, over activity of the striatum or damage to the substantia nigra results in Parkinson's-like symptoms like the inability to initiate movement (bradykinesia). Underactivity of the striatum and shrinking of the caudate nucleus are associated with the symptoms of Huntington's disease. Damage to the caudate is associated also with Tourette's syndrome, whereas lesions of the pallidum and subthalamic nucleus result in conditions characterized by sudden, uncontrolled limb movements(hemi….?)
Hemispheric specialization
The left side of the brain controls the right side of the body. Because approximately 90% of the population preferentially uses the right hand, the left hemisphere of the brain is referred to as the dominant hemisphere. Communication between the cerebral hemispheres occurs via structures including the corpus callosum, anterior commissure, hippocampal commissure, and habenular commissure.
Because the left hemisphere is associated with language function, damage to this hemisphere results in impairment of skills such as speech, writing, and reading in almost all right-handed people and in most left-handed people. The right, or nondominant hemisphere, is associated primarily with perception and also with spatial relations, body image, recognition of faces and music, puzzle-solving, map-reading, and musical and artistic ability. Damage to the right hemisphere has motor sequelae and indirect effects on behavior but does not usually affect intelligence or personality directly.
There are sex differences in functional organization of the brain. For example, women generally have a larger corpus callosum and anterior commissure and appear to have better interhemispheric communication than men. When doing a verbal task, women typically use both hemispheres, whereas men show activation of only one hemisphere. The better-developed right hemispheres of men may in part explain the consistently documented male advantage in executing spatial tasks
Consciousness, coma, and brain death
The thalamus and reticular formation, a network of neurons in the brainstem, are the brain regions most closely involved in arousal and consciousness. In contrast to cortical lesions (particularly left-sided lesions), which must be extensive to cause loss of consciousness, relatively small, localized lesions of either of these structures, particularly the reticular formation, can cause profound loss of consciousness or coma, nonsleep loss of consciousness that extends for a prolonged period. Similarly, lesions that disrupt connections between the brainstem and thalamus can result in coma.
A patient in a profound coma has no conscious cognitive function. If this condition is not reversible, the person is said to be in a persistent vegetative state. Whether to maintain a person who is in this state on life support is an important ethical issue in medicine
TABLE Neuropsychiatric Anatomy: Function and Dysfunction
REGION/DIVISION
MAJOR FUNCTIONS
EFFECTS OF LESION ON BEHAVIOR
Frontal lobes
Dorsolateral convexity
Planning for future action (executive functions)
Decreased motivation, concentration and attention
Disorientation
Mood disturbances
Orbitofrontal cortex
Control over biological drives
Disinhibition and inappropriate behavior
Poor judgment
Lack of inhibition or remorse (pseudopsychopathic behavior)
Medial cortex
Control of movement
Apathy
Decreased spontaneous movement (akinesia)
Gait disturbances
Incontinence
Temporal lobes
Memory
Learning
Emotion
Auditory processing
Impaired memory
Psychomotor seizures
Changes in aggressive behavior
Inability to understand language (i.e., Wernicke's aphasia [left-side lesions])
LIMBIC LOBES
Hippocampus
Memory storage
Poor new learning
Amygdala
Coordination of emotional states, particularly anger and aggression, with somatic responses
Klأuer-Bucy syndrome (decreased aggression, increased sexuality, hyperorality)
Decreased conditioned fear response
Inability to recognize facial and vocal expressions of anger in others
Parietal lobes
Somatic sensation and body image
Impaired IQ
Impaired processing of visual-spatial information, (i.e., cannot copy a simple line drawing or a clock face correctly [right-sided lesions])
Gerstmann's syndrome (i.e., cannot name fingers, write, tell left from right, or do simple math, and impaired processing of verbal information [left-sided lesions])
Occipital lobes
Vision
Visual hallucinations and illusions
Inability to identify camouflaged objects
Blindness