The Brain

Three cavities, called the primary brain vesicles, form during the early embryonic development of the brain. These are the forebrain (prosencephalon), the midbrain (mesencephalon), and the hindbrain (rhombencephalon).

During subsequent development, the three primary brain vesicles develop into five secondary brain vesicles. The names of these vesicles and the major adult structures that develop from the vesicles follow (see Table 1 ):

• The telencephalon generates the cerebrum (which contains the cerebral cortex, white matter, and basal ganglia).

• The diencephalon generates the thalamus, hypothalamus, and pineal gland.

• The mesencephalon generates the midbrain portion of the brain stem.

• The metencephalon generates the pons portion of the brain stem and the cerebellum.

• The myelencephalon generates the medulla oblongata portion of the brain stem
  

  TABLE 1 The Vesicles and Their Components Primary Vesicles Secondary Vesicles Adult Structure Important Components or Features prosencephalon (forebrain) telencephacerebrum cerebral (cerebral hemispheres) cerebral cortex (gray matter): motor areas, sensory areas, association areas prosencephalon (forebrain) telencephacerebrum cerebral (cerebral hemispheres) cerebral white matter: association fibers, commisural fibers, projection fibers prosencephalon (forebrain) telencephacerebrum cerebral (cerebral hemispheres) basal ganglia (gray matter): caudate nucleus & amygdala, putamen, globus pallidus prosencephalon diencephalon diencephalon thalamus: relays sensory information prosencephalon (forebrain) diencephalon diencephalon hypothalamus: maintains body homeostasis prosencephalon (forebrain) diencephalon diencephalon mammillary bodies: relays sensations of smells to cerebrum prosencephalon (forebrain) diencephalon diencephalon optic chiasma: crossover of optic nerves prosencephalon (forebrain) diencephalon diencephalon infundibulum: stalk of pituitary gland prosencephalon (forebrain) diencephalon diencephalon pituitary gland: source of hormones prosencephalon (forebrain) diencephalon diencephalon epithalamus: pineal gland mesencephalon (midbrain) mesencephalon brain stem midbrain: cerebral peduncles, sup. cerebellar peduncles, corpora quadrigemina, superior colliculi rhombencephalon (hindbrain) metencephalon brain stem pons: middle cerebellar peduncles, pneumotaxic area, apneustic area rhombencephalon (hindbrain) metencephalon cerebellum sup. cerebellar peduncles, middle cerebellar peduncles, inferior cerebellar peduncles rhombencephalon (hindbrain) myelencephalon brain stem medulla oblongata: pyramids, cardiovascular center, respiratory center

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A second method for classifying brain regions is by their organization in the adult brain. The following four divisions are recognized (see Figure 1 ).

Figure 1 The four divisions of the adult brain.

• The cerebrum consists of two cerebral hemispheres connected by a bundle of nerve fibers, the corpus callosum. The largest and most visible part of the brain, the cerebrum, appears as folded ridges and grooves, called convolutions. The following terms are used to describe the convolutions:

  • A gyrus (plural, gyri) is an elevated ridge among the convolutions.

  • A sulcus (plural, sulci) is a shallow groove among the convolutions.

  • A fissure is a deep groove among the convolutions.

  The deeper fissures divide the cerebrum into five lobes (most named after bordering skull bones)—the frontal lobe, the parietal love, the temporal lobe, the occipital lobe, and the insula. All but the insula are visible from the outside surface of the brain.

  A cross section of the cerebrum shows three distinct layers of nervous tissue:

  • The cerebral cortex is a thin outer layer of gray matter. Such activities as speech, evaluation of stimuli, conscious thinking, and control of skeletal muscles occur here. These activities are grouped into motor areas, sensory areas, and association areas.

  • The cerebral white matter underlies the cerebral cortex. It contains mostly myelinated axons that connect cerebral hemispheres (association fibers), connect gyri within hemispheres (commissural fibers), or connect the cerebrum to the spinal cord (projection fibers). The corpus callosum is a major assemblage of association fibers that forms a nerve tract that connects the two cerebral hemispheres.

  • Basal ganglia (basal nuclei) are several pockets of gray matter located deep inside the cerebral white matter. The major regions in the basal ganglia—the caudate nuclei, the putamen, and the globus pallidus—are involved in relaying and modifying nerve impulses passing from the cerebral cortex to the spinal cord. Arm swinging while walking, for example, is controlled here.

• The diencephalon connects the cerebrum to the brain stem. It consists of the following major regions:

  • The thalamus is a relay station for sensory nerve impulses traveling from the spinal cord to the cerebrum. Some nerve impulses are sorted and grouped here before being transmitted to the cerebrum. Certain sensations, such as pain, pressure, and temperature, are evaluated here also.

  • The epithalamus contains the pineal gland. The pineal gland secretes melatonin, a hormone that helps regulate the biological clock (sleep-wake cycles).

  • The hypothalamus regulates numerous important body activities. It controls the autonomic nervous system and regulates emotion, behavior, hunger, thirst, body temperature, and the biological clock. It also produces two hormones (ADH and oxytocin) and various releasing hormones that control hormone production in the anterior pituitary gland.

  The following structures are either included or associated with the hypothalamus.

  • The mammillary bodies relay sensations of smell.

  • The infundibulum connects the pituitary gland to the hypothalamus.

  • The optic chiasma passes between the hypothalamus and the pituitary gland. Here, portions of the optic nerve from each eye cross over to the cerebral hemisphere on the opposite side of the brain.

• The brain stem connects the diencephalon to the spinal cord. The brain stem resembles the spinal cord in that both consist of white matter fiber tracts surrounding a core of gray matter. The brain stem consists of the following four regions, all of which provide connections between various parts of the brain and between the brain and the spinal cord. (Some prominent structures are illustrated in Figure 2 ).
  
  

  Figure 2 Prominent structures of the brain stem.

  • The midbrain is the uppermost part of the brain stem.

  • The pons is the bulging region in the middle of the brain stem.

  • The medulla oblongata (medulla) is the lower portion of the brain stem that merges with the spinal cord at the foramen magnum.

  • The reticular formation consists of small clusters of gray matter interspersed within the white matter of the brain stem and certain regions of the spinal cord, diencephalon, and cerebellum. The reticular activation system (RAS), one component of the reticular formation, is responsible for maintaining wakefulness and alertness and for filtering out unimportant sensory information. Other components of the reticular formation are responsible for maintaining muscle tone and regulating visceral motor muscles.

• The cerebellum consists of a central region, the vermis, and two winglike lobes, the cerebellar hemispheres. Like that of the cerebrum, the surface of the cerebellum is convoluted, but the gyri, called folia, are parallel and give a pleated appearance. The cerebellum evaluates and coordinates motor movements by comparing actual skeletal movements to the movement that was intended.

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