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The pons (metencephalon or ''behind-brain'') and medulla oblongata (myelencephalon or ''medulla-brain'') are the two most caudal divisions of the brain, lying between the mesencephalon and the spinal cord. Seen from the ventral surface (Fig. 1A), the boundaries between mesencephalon and pons (pontomesencepha-lic sulcus) and between pons and medulla oblongata (pontomedullary sulcus) are clearly demarcated by the massive population of transversely oriented pontocerebellar fibers. In contrast, there is a smooth transition from medulla oblongata to spinal cord the only indication being the pyramidal decussation. Most of the dorsal surface of the pons and medulla oblongata

Encyclopedia of the Human Brain Volume 2

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Figure 1 (A) Ventral view of the human brain stem showing the major structural features of the pons and medulla oblongata, including the cranial nerves (modified from Kandel et al, 1992, Principles of Neural Science, 3rd ed., with permission of the McGraw-Hill Companies). (B) Organization of cranial nerve-associated nuclei into afferent (sensory) and efferent (motor) columns. The locations of nuclei relative to mesencephalic (midbrain), pontine, and medullary divisions are approximate. Phylogenetic variants on this theme exist. Because of longitudinal migrations of certain nuclear groups, embryonic origins may not correspond to final locations. For example, the trochlear nucleus (IV) has been shown to derive from the pontine division in lower mammals [redrawn and modified from Martin (1989), Neuroanatomy, with permission of the McGraw-Hill Companies].

Figure 1 (A) Ventral view of the human brain stem showing the major structural features of the pons and medulla oblongata, including the cranial nerves (modified from Kandel et al, 1992, Principles of Neural Science, 3rd ed., with permission of the McGraw-Hill Companies). (B) Organization of cranial nerve-associated nuclei into afferent (sensory) and efferent (motor) columns. The locations of nuclei relative to mesencephalic (midbrain), pontine, and medullary divisions are approximate. Phylogenetic variants on this theme exist. Because of longitudinal migrations of certain nuclear groups, embryonic origins may not correspond to final locations. For example, the trochlear nucleus (IV) has been shown to derive from the pontine division in lower mammals [redrawn and modified from Martin (1989), Neuroanatomy, with permission of the McGraw-Hill Companies].

underlies the fourth ventricle as its floor. The floor of the fourth ventricle and the ventral surface of the medulla oblongata show a relief of underlying longitudinal fiber tracts and nuclei. The lateral recesses of the fourth ventricle are bounded by a rim of neural tissue called the rhombic lip, which is continuous with a thin velum overlying the fourth ventricle. The caudal portion of the velum differentiates into choroid plexus. In the intact brain, the fourth ventricle is hidden by the cerebellum.

Eight of the 12 cranial nerves originate from the pons or medulla oblongata, issuing from specific sites on the ventral and lateral aspects (Fig. 1A). The trigeminal nerve issues from among the pontocerebel-lar fibers on the lateral aspect of the pons. The abducens, facial, and vestibulocochlear nerves issue from respectively medial to lateral positions at the pontomedullary border. The glossopharyngeal, vagus, and spinal accessory nerves issue from respectively rostral to caudal sites along the ventrolateral aspect of the medulla oblongata, dorsal to the inferior olive. The hypoglossal nerve issues from the ventral aspect of the medulla oblongata between the inferior olive and the pyramid.

The nuclei associated with the cranial nerves are organized into longitudinal motor and sensory columns that are subdivided according to which peripheral structures are innervated (Fig. 1B). The organization is similar to that in the spinal cord except for the presence of special columns that innervate structures specific to the head. The motor columns lie more medially and innervate either striated muscle (somatic and branchial columns) or parasympathetic ganglia (visceral column). The sensory columns lie more laterally and transmit tactile, proprioceptive, pain, or temperature signals (general somatic and visceral columns) or other specific sensory modalities such as audition and balance (special somatic column) and taste and olfaction (special visceral column). The sulcus limitans, a shallow longitudinal groove visible in the floor of the fourth ventricle, separates the motor from the sensory columns (Fig. 2).

The pons and medulla oblongata contain many fiber tracts that relay information between the cerebrum, cerebellum, and the spinal cord. The transversely oriented pontocerebellar projection is one of the largest of these and is the dominant external feature of the pons (Fig. 1A). Within the pons, longitudinal fiber tracts course internally to or intermingled with the pontocerebellar fibers. Within the medulla oblongata, most longitudinal tracts course at or near the outer (pial) surface. Some longitudinal tracts,

Figure 2 Series of transverse sections through the medulla oblongata at different stages of development. Different nuclei originate from basal and alar plates in a specific pattern. Motor and sensory cranial nerve-associated columns, for example, derive from the basal and alar plates, respectively. Note that inferior olive neurons derive from the alar plate but emigrate to the basal plate where they coalesce to form the nucleus [modified from Kandel et al., (1992), Principles of Neural Science, 3rd ed., with permission from the McGraw-Hill Companies].

Figure 2 Series of transverse sections through the medulla oblongata at different stages of development. Different nuclei originate from basal and alar plates in a specific pattern. Motor and sensory cranial nerve-associated columns, for example, derive from the basal and alar plates, respectively. Note that inferior olive neurons derive from the alar plate but emigrate to the basal plate where they coalesce to form the nucleus [modified from Kandel et al., (1992), Principles of Neural Science, 3rd ed., with permission from the McGraw-Hill Companies].

however, course at deeper locations near the ventral midline of the medulla oblongata. These include several important descending tracts to the spinal cord. The largest longitudinal tract, the corticospinal tract, is visible along the ventral medullary surface as the bilaterally paired pyramids, one of the dominant external features of the ventral medulla oblongata (Fig. 1A).

Like the cranial nerve nuclei, the longitudinal fiber tracts are differentially situated according to functional modality. Ascending tracts conveying sensory information from spinal and medullary centers to higher centers generally course at more dorsal and dorsolateral locations, whereas descending tracts generally course at more ventral and ventromedial locations. Exceptions to this general rule are the rubrospinal tract, which attains a lateral position within the medulla oblongata as it descends to the spinal cord; the medial lemniscus and ventral trigem-inal tract, two sensory tracts that initially have a ventromedial course; and the spinal trigeminal tract, a sensory tract that descends along the dorsolaterally located spinal trigeminal nucleus.

Several well-defined nuclei exist within the pons and medulla oblongata in addition to those associated with the cranial nerve nuclei. The largest of these is the inferior olive, which produces a prominant bulge along the ventrolateral surface of the rostral medulla (Fig. 1A), and whose characteristically convoluted appearance in transverse sections makes it an unmistakable landmark (Fig. 2). The inferior olive is the source of climbing fiber afferents to the contralateral cerebellum.

The bulk of the pontine and medullary core is filled by a more diffusely organized population of neurons that make up the reticular formation, so named because it appears highly reticulated when stained nonspecifically for nerve fibers. This appearance, along with early physiological findings that demonstrated a lack of modality-specific activity in reticular efferents, led to the early notion that much of the reticular formation functioned as a distributed network involved in general activation and arousal. Recent anatomical and physiological studies, on the other hand, have demonstrated a higher degree of anatomical and functional mosaicism within the reticular formation than was previously appreciated. Anatomical subdivisions with characteristic patterns of connections and neurotransmitter profiles exist, and a number of well-defined premotor networks have been identified that organize and integrate specific goal-directed movements.

Certain reticular neuron populations are distinct enough anatomically to be defined as nuclei, even though most of these are not sharply delimited from the rest of the reticular formation. Two of the most distinct reticular nuclei are of special interest because of their neurotransmitter phenotypes. The raphe nuclei are clusters of neurons located within the ventral raphe, the majority of which are serotonergic. The locus coeruleus is a collection of noradrenergic neurons in the pons. The raphe nuclei and the locus coeruleus have exceptionally widespread projections and terminations and exert modulatory effects on a variety of neural systems.

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