Gse In The Brain

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Hypoglossal nucleus

Tongue movements

aGSA, general somatic afferent; GSE, general somatic efferent; GVA, general visceral afferent; GVE, general visceral efferent; SSA, special somatic afferent; SVA, special visceral afferent; SVE, special visceral efferent.

bNote: Input from stretch receptors in the extraoccular and facial muscles may be mediated by cells in the trigeminal ganglion and similar input from lingual muscles by inconstant ganglion cells along the hypoglossal rootlets.

location in the sheet. As to function, each column is stimulus-specific. All of the neurons in a given column respond preferentially to some stimulus parameter: sensory modality, stimulus orientation, ocular dominance, etc. Like the many floors and floorplans of a tall office building, the orderly laminar and columnar features of cortex facilitate integrative, progressive analytic, and comparative functions.

Beneath the cortex, in the white matter, lie millions of myelinated axons of varying caliber, so closely packed as to seem a solid mass of myelin. All of the axons are associated with the cortex. Some go to it,

Figure 6 Autonomic nervous system (ANS) showing target organs: preganglionic sympathetic fibers (solid lines), postganglionic (dotted lines); preganglionic parasympathetic fibers (dashed lines), postganglionic (solid lines). From The Life of Mammals, copyright 1957 by Oxford University Press, Inc. Used by permission of Oxford University Press, Inc. (original painting by Frank Netter, modified by Jane deVere).

Figure 6 Autonomic nervous system (ANS) showing target organs: preganglionic sympathetic fibers (solid lines), postganglionic (dotted lines); preganglionic parasympathetic fibers (dashed lines), postganglionic (solid lines). From The Life of Mammals, copyright 1957 by Oxford University Press, Inc. Used by permission of Oxford University Press, Inc. (original painting by Frank Netter, modified by Jane deVere).

whereas others depart from it. The total length of axons involved in cortical connectivity is estimated at more than 100,000 km. These fibers are crucial to cortical function. They allow parts of the two cortical sheets to influence one another and parts of the underlying brain. Three types of fibers are found (see later discussion). For rapid communication, most are large, well-myelinated axons.

Association fibers interconnect cortical areas in one cerebral hemisphere. They are short, looping from one gyrus to the next, or long, extending the entire frontooccipital length of the hemisphere. They may pass from one cortical area to another in linear sequence or bypass areas, diverge, or converge in a selective manner. These cascades provide profound connectivity in the cortical sheet. Outward or

Figure 7 Lateral (top) and medial (bottom) aspects of brain, showing major fissures and lobes (except central lobe; see Fig. 4, 9 months) and principal regions of the brain stem. From J. Nolte and J. B. Angevine, Jr., The Human Brain. In Photographs and Diagrams, 2nd ed., Mosby, St. Louis, 2000 (photograph by Biomedical Communications, The University of Arizona College of Medicine).

Figure 7 Lateral (top) and medial (bottom) aspects of brain, showing major fissures and lobes (except central lobe; see Fig. 4, 9 months) and principal regions of the brain stem. From J. Nolte and J. B. Angevine, Jr., The Human Brain. In Photographs and Diagrams, 2nd ed., Mosby, St. Louis, 2000 (photograph by Biomedical Communications, The University of Arizona College of Medicine).

feedforward connections originate and terminate in specified cortical layers. Backward or feedback connections begin and end in other specific cortical "landing strips.'' Wherever signals enter the network, they eventually filter through to the orbitofrontal cortex and limbic system, where emotional colorations and affect may be factored in. This network makes for unified cortical function: in analysis and integration, wherein multiple data are processed, coincidences detected, and sensory and motor data reduced and combined in schemes of reference, such as our world and ourselves within it.

Commissural fibers interconnect cortical areas across the midline. There they form a huge curvilinear bridge of white matter, the corpus callosum (''hard body''), the largest assembly of nerve fibers in the human brain at 700 mm long and 50-100 mm thick, comprising over 300 million well-myelinated axons (Fig. 7). Most of these make mirror-image connections between the two hemispheres, but some cross to areas different from the ones at which they arise. Much of the temporal lobe interconnects in the anterior commissure (Fig. 12; small oval structure just beneath the inter-ventricular foramen).

The traditional concept is that the corpus callosum allows information sharing and teamplay between the two hemispheres. This idea is being reevaluated. Callosal connections involve a time lag (about 30 msec at 6.5m/sec to travel the roughly 175 mm between origin and termination) that could interfere with cooperative feature analysis. Not excluding hemispheric cooperation, new ideas posit callosal connections permitting hemispheric competition. Callosal connections may effect largely inhibitory, not excitatory, influences and express true hemispheric dominance, albeit moment to moment.

Projection fibers lead to, or come from, subcortical structures. By analogy, they ''cross state lines.'' They convey impulses between structures in different principal regions of the CNS (corticospinal, spinocerebel-lar, cerebellothalamic, etc.). The neocortex of mammals (as contrasted with the olfactory cortex of all vertebrates) is unique in its direct lines to every level of the neuraxis. Association and commissural fibers are direct lines too, from one part of the cortex to another, near or far. No region of the CNS is beyond the reach of the cerebral cortex (Table III). Cortical efferents modulate activity in all parts of sensory and motor neurons, visceral and somatic. Most cortical projections go to the thalamus, which reciprocates them. The work of the cortex would be impossible but for the thalamus. A concept of thalamocortical resonance may offer a better understanding of mechanisms of attention and perseverative thought processes.

Except for minor differences in sulcal pattern, the hemispheres look much alike, but show functional asymmetry. Each makes the same functional contributions, but more pronounced than those of the other side. For example, one hemisphere (usually the left) is better in speech and calculation and has a stronger link with consciousness and deliberate, analytical thought processes. The other (usually the right) is better at spatial relations, nonverbal ideation, and holistic

Golgi Nissl Weigert

Neurons Cell bodies Myelinated axons

(Golgi method) (Nissl stain) (Weigert stain)

Figure 8 Cerebral cortex showing six principal layers, constituent neurons (pyramidal, stellate, multiform), and myelinated axons. Horizontal lamination is evident, but a vertical or columnar plan of organization is also prominent (see text). From Principles of Neuroanatomy by J. B. Angevine, Jr., and C. W. Cotman, copyright 1981 by Oxford University Press, Inc. Used by permission of Oxford University Press, Inc. (illustration by Konstantin Brodmann).

Neurons Cell bodies Myelinated axons

(Golgi method) (Nissl stain) (Weigert stain)

Figure 8 Cerebral cortex showing six principal layers, constituent neurons (pyramidal, stellate, multiform), and myelinated axons. Horizontal lamination is evident, but a vertical or columnar plan of organization is also prominent (see text). From Principles of Neuroanatomy by J. B. Angevine, Jr., and C. W. Cotman, copyright 1981 by Oxford University Press, Inc. Used by permission of Oxford University Press, Inc. (illustration by Konstantin Brodmann).

thinking. But in both hemispheres, all abilities are well-represented, and neural integration and pattern synthesis are equally profound.

2. The Basal Ganglia

The basal ganglia are large nuclei within each hemisphere, joined here and separated there, by cortical efferents. They contribute to motor control, cogni tion, motivation, selection and initiation of behavior, emotion, and perhaps other higher functions (Fig. 9). Only relatively recently have we begun to understand them.

A huge mass of gray matter, the striatum ("striped" by myelinated inputs), has four parts. A lateral, bulky part, the putamen (''a shell''), is separated by projection fibers from an inner, curving one, the caudate nucleus (with head, body, and tail). Below, in the

Table III

temporal lobe, is the amygdala ("almond"), now part of the limbic system. It mediates learning, expression, and the cognitive experience of emotion. A fourth part is the nucleus accumbens (not shown), also annexed by the limbic system.

Medial to the putamen is the globuspallidus, a major output center to the thalamus and midbrain. Two smaller centers are now considered basal ganglia: the subthalamic nucleus and the mesencephalic substantia nigra. The former provides an excitatory sidearm between the outer and inner segments of the globus pallidus (all other circuits of the basal ganglia are inhibitory). The lower reticular part of the nigra serves, like the inner pallidum, as an output center. Its upper compact part synthesizes and ships dopamine up to the caudate and putamen for use as a neurotransmitter in their activities.

The contributions of the basal ganglia have long eluded understanding. The results of their injury, however, are dramatic, expressing something terribly wrong with the teamwork of motor control. Examples include Parkinson's disease and Huntington's chorea. Great strides are now being made regarding the functions of the basal ganglia, the connections that underlie their interplay with the cortex, and the chemistry and molecular biology of their complex synaptic relationships.

The basal ganglia receive widespread cortical inputs and project back, via the thalamus, to motor cortex

(Fig. 10). The circuit details are daunting, but the advantage is clear. Almost all of the cortex is involved in movements in the making. The same is true of the cerebellum. Projections from the basal ganglia and cerebellum to the motor cortex are integrated in the rostral thalamus en route.

3. The Limbic System

The limbic system (Fig. 11) is the most controversial subsystem of the human brain. It does not represent one sensory modality, nor one effector mode. Many disagree as to its components. Its functional contributions are not as defined as those of other subsystems or of new ones postulated. The system comprises closely interconnected structures with components in all brain regions, not just the forebrain as conceived. It has multimodal inputs: sensory and associative, and dual outputs: neural and endocrine. It is postulated to mediate memories, drives, and rewards underlying motivation, regulate visceral function and emotional expression, and influence and emotionally color high functions essential to perception and perhaps thoughts.

Nineteenth century neurosurgeon Paul Broca's limbic lobe (the cingulate and parahippocampal gyri, bordering the sulcus of the corpus callosum) gave the system its name. The hippocampus, fornix, mammil-lary body, and anterior thalamic nuclei projecting to the cingulate gyrus were key links in neuroanatomist James Papez's 1937 sulcal circuit. Then other structures were added: the septal area, temporoamygdaloid complex, habenula and its somatovisceral connections; preoptic area and hypothalamus with visceromotor and endocrine outputs; the striatal nucleus accumbens and ventral pallidum; and the ventral tegmental area and other brain stem nuclei serving visceral afferents energizing the system. Some include the orbitofrontal cortex and the thalamic medial dorsal nucleus projecting to it. This cortex has direct lines to most of the preceding places for cortical oversight and regulation.

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