Stria Medullaris

The Split Brain

Patients with life-threatening, intractable epileptic seizures were treated in the past by surgical commissurotomy or cutting of the corpus callosum (see Fig. 7.7). This procedure effectively cut off most of the neuronal communication between the left and right hemispheres and vastly improved patient status because seizure activity no longer spread back and forth between the hemispheres.

There was a remarkable absence of overt signs of disability following commissurotomy; patients retained their original motor and sensory functions, learning and memory, personality, talents, emotional responding, and so on. This outcome was not unexpected because each hemisphere has bilateral representation of most known functions; moreover, those ascending (sensory) and descending (motor) neuronal systems that crossed to the opposite side were known to do so at levels lower than the corpus callosum.

Notwithstanding this appearance of normalcy, following commissurotomy, patients were shown to be impaired to the extent that one hemisphere literally did not know what the other was doing. It was further shown that each hemisphere processes neuronal information differently from the other, and that some cerebral functions are confined exclusively to one hemisphere.

In an interesting series of studies by Nobel laureate Roger Sperry and colleagues, these patients with a so-called split-brain were subjected to psychophysiological testing in which each disconnected hemisphere was examined independently. Their findings confirmed what was already known: Sensory and motor functions are controlled by cortical structures in the contralateral hemisphere. For example, visual signals from the left visual field were perceived in the right occipital lobe, and there were contralateral controls for auditory, somatic sensory, and motor functions. (Note that the olfactory system is an exception, as odorant chemicals applied to one nostril are perceived in the olfactory lobe on the same side.) However, the scientists were surprised to find that language ability was controlled almost exclusively by the left hemisphere. Thus, if an object was presented to the left brain via any of the sensory systems, the subject could readily identify it by the spoken word. However, if the object was presented to the right hemisphere, the subject could not find words to identify it. This was not due to an inability of the right hemisphere to perceive the object, as the subject could easily identify it among other choices by nonverbal means, such as feeling it while blindfolded. From these and other tests it became clear that the right hemisphere was mute; it could not produce language.

In accordance with these findings, anatomic studies show that areas in the temporal lobe concerned with language ability, including Wernicke's area, are anatomically larger in the left hemisphere than in the right in a majority of humans, and this is seen even prenatally. Corroborative evidence of language ability in the left hemisphere is shown in persons who have had a stroke, where aphasias are most severe if the damage is on the left side of the brain. Analysis of people who are deaf who communicated by sign language prior to a stroke has shown that sign language is also a left-hemisphere function. These patients show the same kinds of grammatical and syntactical errors in their signing following a left-hemisphere stroke as do speakers.

In addition to language ability, the left hemisphere excels in mathematical ability, symbolic thinking, and sequential logic. The right hemisphere, on the other hand, excels in visuospatial ability, such as three-dimensional constructions with blocks and drawing maps, and in musical sense, artistic sense, and other higher functions that computers seem less capable of emulating. The right brain exhibits some ability in language and calculation, but at the level of children ages 5 to 7. It has been postulated that both sides of the brain are capable of all of these functions in early childhood, but the larger size of the language area in the left temporal lobe favors development of that side during language acquisition, resulting in nearly total specialization for language on the left side for the rest of one's life.

ergic) and serotonergic nerve terminals emanating from brainstem nuclei that contain relatively few cell bodies compared to their extensive terminal projections. From neurochemical manipulation of monoaminergic neurons in the limbic system, it is apparent that they play a major role in determining emotional state.

Dopaminergic neurons are located in three major pathways originating from cell groups in either the midbrain (the substantia nigra and ventral tegmental area) or the hypothalamus (Fig. 7.11). The nigrostriatal system consists of neurons with cell bodies in the substantia nigra (pars compacta) and terminals in the neostriatum (caudate and putamen) located in the basal ganglia. This dopaminergic pathway is essential for maintaining normal muscle tone and initiating voluntary movements (see Chapter 5). The tuberoinfundibular system of dopaminergic neurons is located entirely within the hypothalamus, with cell bodies in the arcuate nucleus and periventricular nuclei and terminals in the median eminence on the ventral surface of the hypothalamus. The tuberoinfundibular system is responsible for the secretion of hypothalamic releasing factors into a portal system that carries them through the pituitary stalk into the anterior pituitary lobe (see Chapter 32).

The mesolimbic/mesocortical system of dopaminergic neurons originates in the ventral tegmental area of the midbrain region of the brainstem and innervates most structures of the limbic system (olfactory tubercles, septal nuclei, amygdala, nucleus accumbens) and limbic cortex (frontal and cingulate cortices). This dopaminergic system plays an important role in motivation and drive. For example, dopaminergic sites in the limbic system, particularly the more ventral structures such as the septal nuclei and nucleus accumbens, are associated with the brain's reward system. Drugs that increase dopaminergic transmission, such as cocaine, which inhibits dopamine reuptake, and amphetamine, which promotes dopamine release and inhibits its reuptake, lead to repeated administration and abuse presumably because they stimulate the brain's reward system. The mesolimbic/mesocortical dopaminergic system is also the site of action of neuroleptic drugs, which

Cingulate gyrus Corpus callosum

Anterior nucleus of thalamus r Fornix f Stria medullaris

, Longitudinal stria

Septal nuclei Prefrontal cortex

Olfactory bulb

Septal nuclei Prefrontal cortex

Olfactory bulb

Stria Terminalis And Stria Medullaris

Habenula

Stria terminalis

Mammillothalamic tract Hippocampal formation

Amygdaloid complex

Mammillary body

Parahippocampal gyrus

Habenula

Amygdaloid complex

Stria terminalis

Mammillothalamic tract Hippocampal formation

Mammillary body

Parahippocampal gyrus

The cortical and subcortical structures of the limbic system extending from the cerebral cortex to the dien-cephalon. The fiber tracts that interconnect the structures of the limbic system are also shown. (Modified from Truex RC, Carpenter MB. Strong and Elwyn's Human Neuroanatomy. 5th Ed. Baltimore: Williams & Wilkins, 1964.)

are used to treat schizophrenia (discussed later) and other psychotic conditions.

Noradrenergic neurons (containing norepinephrine) are located in cell groups in the medulla and pons (Fig. 7.12). The medullary cell groups project to the spinal cord, where they influence cardiovascular regulation and other autonomic functions. Cell groups in the pons include the lateral system, which innervates the basal forebrain and hy-

Limbic System Rage And Placidity

Hypothalamus

Tuberoinfundibular system Ventral tegmental area Midbrain

The main circuit of the limbic system.

pothalamus, and the locus ceruleus, which sends efferent fibers to nearly all parts of the CNS.

Noradrenergic neurons innervate all parts of the limbic system and the cerebral cortex, where they play a major role in setting mood (sustained emotional state) and affect (the emotion itself; e.g., euphoria, depression, anxiety). Drugs that alter noradrenergic transmission have profound effects on mood and affect. For example, reserpine, which depletes brain norepinephrine (NE), induces a state of depression. Drugs that enhance NE availability, such as monoamine oxidase inhibitors (MAOIs) and inhibitors of reuptake, reverse this depression. Amphetamines and cocaine have effects on boosting noradrenergic transmission similar to those described for dopaminergic transmission; they inhibit reuptake and/or promote the release of norepinephrine. Increased noradrenergic transmission results in an elevation of mood, which further contributes to the po

Mesolimbic/ mesocortical system

Cingulate gyrus

Basal ganglia Thalamus

Frontal cortex

Cingulate gyrus

Basal ganglia Thalamus

Mesolimbic/ mesocortical system

Frontal cortex

Mesocortex

Nigrostriatal system

Substantia nigra

Medulla

The origins and projections of the three major dopaminergic systems. (Modified from Heimer L. The Human Brain and Spinal Cord. New York: Springer-Verlag, 1983.)

Nigrostriatal system

Hypothalamus

Tuberoinfundibular system Ventral tegmental area Midbrain

Substantia nigra

Medulla

The main circuit of the limbic system.

The origins and projections of the three major dopaminergic systems. (Modified from Heimer L. The Human Brain and Spinal Cord. New York: Springer-Verlag, 1983.)

Frontal cortex

Cingulate gyrus

Basal ganglia Thalamus

Frontal cortex

Stria Medullaris Thalami

Midbrain Locus ceruleus

Pons Medulla

To spinal cord

The origins and projections of five of seven cell groups of noradrenergic neurons of the brain. The depicted groups originate in the medulla and pons. Among the latter, the locus ceruleus in the dorsal pons innervates most parts of the CNS. (Modified from Heimer L. The Human Brain and Spinal Cord. New York: Springer-Verlag, 1983.)

Hypothalamus

Midbrain Locus ceruleus

Pons Medulla

To spinal cord

The origins and projections of five of seven cell groups of noradrenergic neurons of the brain. The depicted groups originate in the medulla and pons. Among the latter, the locus ceruleus in the dorsal pons innervates most parts of the CNS. (Modified from Heimer L. The Human Brain and Spinal Cord. New York: Springer-Verlag, 1983.)

tential for abusing such drugs, despite the depression that follows when drug levels fall. Some of the unwanted consequences of cocaine or amphetamine-like drugs reflect the increased noradrenergic transmission, in both the periphery and the CNS. This can result in a hypertensive crisis, myocardial infarction, or stroke, in addition to marked swings in affect, starting with euphoria and ending with profound depression.

Serotonergic neurons also innervate most parts of the CNS. Cell bodies of these neurons are located at the midline of the brainstem (the raphe system) and in more laterally placed nuclei, extending from the caudal medulla to the midbrain (Fig. 7.13). Serotonin plays a major role in the defect underlying affective disorders (discussed later). Drugs

Cingulate gyrus

Basal ganglia

Frontak ^Thalamus cortex

Brain Area Pituitary Locus Cingulate

To spinal cord

The origins and projections of the nine cell groups of the serotonergic system of the brain. The depicted groups originate in the caudal medulla, pons, and midbrain and send projections to most regions of the brain. (Modified from Heimer L. The Human Brain and Spinal Cord. New York: Springer-Verlag, 1983.)

Hypothalamus

Midbrain

Pons Medulla

To spinal cord

The origins and projections of the nine cell groups of the serotonergic system of the brain. The depicted groups originate in the caudal medulla, pons, and midbrain and send projections to most regions of the brain. (Modified from Heimer L. The Human Brain and Spinal Cord. New York: Springer-Verlag, 1983.)

that increase serotonin transmission are effective antide-pressant agents.

The Brain's Reward System. Experimental studies beginning early in the last century demonstrated that stimulating the limbic system or creating lesions in various parts of the limbic system can alter emotional states. Most of our knowledge comes from animal studies, but emotional feelings are reported by humans when limbic structures are stimulated during brain surgery. The brain has no pain sensation when touched, and subjects awakened from anesthesia during brain surgery have communicated changes in emotional experience linked to electrical stimulation of specific areas.

Electrical stimulation of various sites in the limbic system produces either pleasurable (rewarding) or unpleasant (aversive) feelings. To study these findings, researchers use electrodes implanted in the brains of animals. When electrodes are implanted in structures presumed to generate rewarding feelings and the animals are allowed to deliver current to the electrodes by pressing a bar, repeated and prolonged self-stimulation is seen. Other needs—such as food, water, and sleep—are neglected. The sites that provoke the highest rates of electrical self-stimulation are in the ventral limbic areas, including the septal nuclei and nucleus accumbens. Extensive studies of electrical self-stimulatory behavior indicate that dopaminergic neurons play a major role in mediating reward. The nucleus accumbens is thought to be the site of action of addictive drugs, including opiates, alcohol, nicotine, cocaine, and amphetamine.

Aggression and the Limbic System. A fight-or-flight response, including the autonomic components (see Chapter 6) and postures of rage and aggression characteristic of fighting behavior, can be elicited by electrical stimulation of sites in the hypothalamus and amygdala. If the frontal cortical connections to the limbic system are severed, rage postures and aggressiveness become permanent, illustrating the importance of the higher centers in restraining aggression and, presumably, in invoking it at appropriate times. By contrast, bilateral removal of the amygdala results in a placid animal that cannot be provoked.

Sexual Activity. The biological basis of human sexual activity is poorly understood because of its complexity and because findings derived from nonhuman animal studies cannot be extrapolated. The major reason for this limitation is that the cerebral cortex, uniquely developed in the human brain, plays a more important role in governing human sexual activity than the instinctive or olfactory-driven behaviors in nonhuman primates and lower mammalian species. Nevertheless, several parallels in human and nonhuman sexual activities exist, indicating that the limbic system, in general, coordinates sex drive and mating behavior, with higher centers exerting more or less overriding influences.

Copulation in mammals is coordinated by reflexes of the sacral spinal cord, including male penile erection and ejaculation reflexes and engorgement of female erectile tissues, as well as the muscular spasms of the orgasmic response. Copulatory behaviors and postures can be elicited in animals by stimulating parts of the hypothalamus, olfactory system, and other limbic areas, resulting in mounting behavior in males and lordosis (arching the back and raising the tail) in females. Ablation studies have shown that sexual behavior also requires an intact connection of the limbic system with the frontal cortex.

Olfactory cues are important in initiating mating activity in seasonal breeders. Driven by the hypothalamus' endogenous seasonal clock, the anterior and preoptic areas of the hypothalamus initiate hormonal control of the gonads. Hormonal release leads to the secretion of odorants (pheromones) by the female reproductive tract, signaling the onset of estrus and sexual receptivity to the male. The odorant cues are powerful stimulants, acting at extremely low concentrations to initiate mating behavior in males. The olfactory system, by virtue of its direct connections with the limbic system, facilitates the coordination of behavioral, endocrine, and autonomic responses involved in mating.

Although human and nonhuman primates are not seasonal breeders (mating can occur on a continual basis), vestiges of this pattern remain. These include the importance of the olfactory and limbic systems and the role of the hypothalamus in cyclic changes in female ovarian function and the continuous regulation of male testicular function. More important determinants of human sexual activity are the higher cortical functions of learning and memory, which serve to either reinforce or suppress the signals that initiate sexual responding, including the sexual reflexes coordinated by the sacral spinal cord.

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