The brain is an assembly of interrelated neural systems that regulate their own and each other’s activity in a dynamic, complex fashion largely through intercellular chemical neurotransmission.

Macrofunctions of the Brain Regions

The large anatomical divisions provide a superficial classification of the distribution of brain functions.

Cerebral Cortex

The two cerebral hemispheres constitute the largest division of the brain. Regions of the cortex are classified in several ways: (1) by the modality of information processed (e.g., sensory, including somatosensory, visual, auditory, and olfactory, as well as motor and associational); (2) by anatomical position (frontal, temporal, parietal, and occipital); and (3) by the geometric relationship between cell types in the major cortical layers (“cytoarchitectonic” classifications). The specialized functions of a cortical region arise from the interplay between connections from other regions of the cortex (corticocortical systems) and noncortical areas of the brain (subcortical systems) and a basic intracortical processing module of approximately 100 vertically connected cortical columns (Mountcastle, 1997). Varying numbers of adjacent columnar modules may be functionally, but transiently, linked into larger information-processing ensembles. The pathology of Alzheimer’s disease, for example, destroys the integrity of the columnar modules and the corticocortical connections.

Cortical areas termed association areas process information from primary cortical sensory regions to produce higher cortical functions such as abstract thought, memory, and consciousness. The cerebral cortices also provide supervisory integration of the autonomic nervous system and integrate somatic and vegetative functions, including those of the cardiovascular and gastrointestinal systems.

Limbic System

The “limbic system” is an archaic term for an assembly of brain regions (hippocampal formation, amygdaloid complex, septum, olfactory nuclei, basal ganglia, and selected nuclei of the diencephalon) grouped around the subcortical borders of the underlying brain core to which a variety of complex emotional and motivational functions have been attributed. Modern neuroscience avoids this term because those ill-defined regions of the “limbic system” do not function consistently as a system. Parts of these limbic regions also participate individually in functions that can be more precisely defined. Thus, the basal ganglia or neostriatum (the caudate nucleus, putamen, globus pallidus, and lentiform nucleus) form an essential regulatory segment of the extrapyramidal motor system. This system complements the function of the pyramidal (or voluntary) motor system. Damage to the extrapyramidal system depresses the ability to initiate voluntary movements and causes disorders characterized by involuntary movements, such as the tremors and rigidity of Parkinson’s disease or the uncontrollable limb movements of Huntington’s chorea. Similarly, the hippocampus may be crucial to the formation of recent memory, since this function is lost in patients with extensive bilateral damage to the hippocampus. Memory also is disrupted by Alzheimer’s disease, which destroys the intrinsic structure of the hippocampus as well as parts of the frontal cortex.

Diencephalon

The thalamus lies in the center of the brain, beneath the cortex and basal ganglia and above the hypothalamus. The neurons of the thalamus are arranged into distinct clusters, or nuclei, which are either paired or midline structures. These nuclei act as relays between the incoming sensory pathways and the cortex, between the discrete regions of the thalamus and the hypothalamus, and between the basal ganglia and the association regions of the cerebral cortex. The thalamic nuclei and the basal ganglia also exert regulatory control over visceral functions; aphagia and adipsia, as well as general sensory neglect, follow damage to the corpus striatum or to selected circuits ending there. The hypothalamus is the principal integrating region for the entire autonomic nervous system and regulates body temperature, water balance, intermediary metabolism, blood pressure, sexual and circadian cycles, secretion of the adenohypophysis, sleep, and emotion. Recent advances in the cytophysiological and chemical dissection of the hypothalamus have clarified the connections and possible functions of individual hypothalamic nuclei.

Midbrain and Brainstem

The mesencephalon, pons, and medulla oblongata connect the cerebral hemispheres and thalamus-hypothalamus to the spinal cord. These “bridge portions” of the CNS contain most of the nuclei of the cranial nerves, as well as the major inflow and outflow tracts from the cortices and spinal cord. These regions contain the reticular activating system, an important but incompletely characterized region of gray matter linking peripheral sensory and motor events with higher levels of nervous integration. The major monoamine-containing neurons of the brain (see below) are found here. Together, these regions represent the points of central integration for coordination of essential reflexive acts, such as swallowing and vomiting, and those that involve the cardiovascular and respiratory systems; these areas also include the primary receptive regions for most visceral afferent sensory information. The reticular activating system is essential for the regulation of sleep, wakefulness, and level of arousal, as well as for coordination of eye movements. The fiber systems projecting from the reticular formation have been called “nonspecific” because the targets to which they project are relatively more diffuse in distribution than those of many other neurons (e.g., specific thalamocortical projections). However, the chemically homogeneous components of the reticular system innervate targets in a coherent and functional manner despite their broad distribution.

Cerebellum

The cerebellum arises from the posterior pons behind the cerebral hemispheres. It also is highly laminated and redundant in its detailed cytological organization. The lobules and folia of the cerebellum project onto specific deep cerebellar nuclei, which in turn make relatively selective projections to the motor cortex (by way of the thalamus) and to the brainstem nuclei concerned with vestibular (position-stabilization) function. In addition to maintaining the proper tone of antigravity musculature and providing continuous feedback during volitional movements of the trunk and extremities, the cerebellum also may regulate visceral function (e.g., heart rate, so as to maintain blood flow despite changes in posture). In addition, the cerebellum plays a significant role in learning and memory.

Spinal Cord

The spinal cord extends from the caudal end of the medulla oblongata to the lower lumbar vertebrae. Within this mass of nerve cells and tracts, the sensory information from skin, muscles, joints, and viscera is locally coordinated with motoneurons and with primary sensory relay cells that project to and receive signals from higher levels. The spinal cord is divided into anatomical segments (cervical, thoracic, lumbar, and sacral) that correspond to divisions of the peripheral nerves and spinal column. Ascending and descending tracts of the spinal cord are located within the white matter at the perimeter of the cord, while intersegmental connections and synaptic contacts are concentrated within the H-shaped internal mass of gray matter. Sensory information flows into the dorsal cord, and motor commands exit via the ventral portion. The preganglionic neurons of the autonomic nervous system are found in the intermediolateral columns of the gray matter. Autonomic reflexes (e.g., changes in skin vasculature with alteration of temperature) can be elicited within local segments of the spinal cord, as shown by the maintenance of these reflexes after the cord is severed.

Source: Goodman & Gilman’s The Pharmacological Basis of Therapeutics

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