Structural Organization of the Brain 190

I. During embryonic development, five regions of the brain are formed: the telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon.

A. The telencephalon and diencephalon constitute the forebrain; the mesencephalon is the midbrain, and the hindbrain is composed of the metencephalon and the myelencephalon.

B. The CNS begins as a hollow tube, and thus the brain and spinal cord are hollow. The cavities of the brain are known as ventricles.

Cerebrum 192

I. The cerebrum consists of two hemispheres connected by a large fiber tract called the corpus callosum.

A. The outer part of the cerebrum, the cerebral cortex, consists of gray matter.

B. Under the gray matter is white matter, but nuclei of gray matter, known as the basal nuclei, lie deep within the white matter of the cerebrum.

C. Synaptic potentials within the cerebral cortex produce the electrical activity seen in an electroencephalogram (EEG).

II. The two cerebral hemispheres exhibit some specialization of function, a phenomenon called cerebral lateralization.

A. In most people, the left hemisphere is dominant in language and analytical ability, whereas the right hemisphere is more important in pattern recognition, musical composition, singing, and the recognition of faces.

B. The two hemispheres cooperate in their functions; this cooperation is aided by communication between the two via the corpus callosum.

III. Particular regions of the left cerebral cortex appear to be important in language ability; when these areas are damaged, characteristic types of aphasias result.

A. Wernicke's area is involved in speech comprehension, whereas Broca's area is required for the mechanical performance of speech.

B. Wernicke's area is believed to control Broca's area by means of the arcuate fasciculus.

C. The angular gyrus is believed to integrate different sources of sensory information and project to Wernicke's area.

IV. The limbic system and hypothalamus are regions of the brain that have been implicated as centers for various emotions.

V. Memory can be divided into short-term and long-term categories.

A. The medial temporal lobes—in particular the hippocampus and perhaps the amygdaloid nucleus— appear to be required for the consolidation of short-term memory into long-term memory.

B. Particular aspects of a memory may be stored in numerous brain regions.

C. Long-term potentiation is a phenomenon that may be involved in some aspects of memory.

Diencephalon 204

I. The diencephalon is the region of the forebrain that includes the thalamus, epithalamus, hypothalamus, and pituitary gland.

A. The thalamus serves as an important relay center for sensory information, among its other functions.

B. The epithalamus contains a choroid plexus, where cerebrospinal fluid is formed. The pineal gland, which secretes the hormone melatonin, is also part of the epithalamus.

C. The hypothalamus forms the floor of the third ventricle, and the pituitary gland is located immediately inferior to the hypothalamus.

II. The hypothalamus is the main control center for visceral activities.

A. The hypothalamus contains centers for the control of thirst, hunger, body temperature, and (together with the limbic system) various emotions.

B. The hypothalamus regulates the secretions of the pituitary gland. It controls the posterior pituitary by means of a fiber tract, and it controls the anterior pituitary by means of hormones.

Midbrain and Hindbrain 206

I. The midbrain contains the superior and inferior colliculi, which are involved in visual and auditory reflexes, respectively, and nuclei that contain dopaminergic neurons that project to the corpus striatum and limbic system of the forebrain.

II. The hindbrain consists of two regions: the metencephalon and the myelencephalon.

A. The metencephalon contains the pons and cerebellum. The pons contains nuclei for four pairs of cranial nerves, and the cerebellum plays an important role in the control of skeletal movements.

B. The myelencephalon consists of only one region, the medulla oblongata. The medulla contains centers for the regulation of such vital functions as breathing and the control of the cardiovascular system.

Spinal Cord Tracts 209

I. Ascending tracts carry sensory information from sensory organs up the spinal cord to the brain.

II. Descending tracts are motor tracts and are divided into two groups: the pyramidal and the extrapyramidal systems.

A. Pyramidal tracts are the corticospinal tracts. They begin in the precentral gyrus and descend, without synapsing, into the spinal cord.

B. Most of the corticospinal fibers decussate in the pyramids of the medulla oblongata.

C. Regions of the cerebral cortex, the basal nuclei, and the cerebellum control movements indirectly by synapsing with other regions that give rise to descending extrapyramidal fiber tracts.

D. The major extrapyramidal motor tract is the reticulospinal tract, which has its origin in the reticular formation of the midbrain.

Cranial and Spinal Nerves 212

I. There are twelve pairs of cranial nerves. Most of these are mixed, but some are exclusively sensory in function.

II. There are thirty-one pairs of spinal nerves. Each pair contains both sensory and motor fibers.

A. The dorsal root of a spinal nerve contains sensory fibers, and the cell bodies of these neurons are contained in the dorsal root ganglion.

B. The ventral root of a spinal nerve contains motor fibers.

III. A reflex arc is a neural pathway involving a sensory neuron and a motor neuron. One or more association neurons also may be involved in some reflexes.

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Essentials of Human Physiology

Essentials of Human Physiology

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