Myelencephalon

The myelencephalon is a brain vesicle that gives rise to the medulla oblongata. It differs from the spinal cord in that its lateral walls are everted (Fig. 19.18, B and C). Alar and basal plates separated by the sulcus limitans can be clearly distinguished. The basal plate, similar to that of the spinal cord, contains motor nuclei. These nuclei are divided into three groups: (a) a medial somatic efferent

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Sulcus Limitans Function

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Figure 19.17 Lateral view of the brain vesicles in an 8-week embryo (crown-rump length approximately 27 mm). The roof plate of the rhombencephalon has been removed to show the intraventricular portion of the rhombic lip. Note the origin of the cranial nerves.

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Figure 19.17 Lateral view of the brain vesicles in an 8-week embryo (crown-rump length approximately 27 mm). The roof plate of the rhombencephalon has been removed to show the intraventricular portion of the rhombic lip. Note the origin of the cranial nerves.

group, (b) an intermediate special visceral efferent group, and (c) a lateral general visceral efferent group (Fig. 19.18C).

The first group contains motor neurons, which form the cephalic continuation of the anterior horn cells. Since this somatic efferent group continues rostrally into the mesencephalon, it is called the somatic efferent motor column. In the myelencephalon it includes neurons of the hypoglossal nerve that supply the tongue musculature. In the metencephalon and the mesencephalon, the column contains neurons of the abducens (Fig. 19.19), trochlear, and oculomotor nerves (see Fig. 19.23), respectively. These nerves supply the eye musculature.

The special visceral efferent group extends into the metencephalon, forming the special visceral efferent motor column. Its motor neurons supply striated muscles of the pharyngeal arches. In the myelencephalon the column is represented by neurons of the accessory, vagus, and glossopharyngeal nerves.

The general visceral efferent group contains motor neurons that supply involuntary musculature of the respiratory tract, intestinal tract, and heart.

The alar plate contains three groups of sensory relay nuclei (see Fig. 19.18C). The most lateral of these, the somatic afferent (sensory) group, receives impulses from the ear and surface of the head by way of the vestibu-locochlear and trigeminal nerves. The intermediate, or special visceral afferent, group receives impulses from taste buds of the tongue and from the palate, oropharynx, and epiglottis. The medial, or general visceral afferent, group receives interoceptive information from the gastrointestinal tract and heart.

The roof plate of the myelencephalon consists of a single layer of ependy-mal cells covered by vascular mesenchyme, the pia mater (Figs. 19.5 and

Meten-cephalon

Myelen-cephalon

Meten-cephalon

Myelen-cephalon

Metencephalon And Myelencephalon

Mesencephalon

Abducens nerve

Rhombic lip

Basal plate

Alar plate

Attachment of roof plate

4th ventricle

Roof plate

Mesencephalon

Abducens nerve

Rhombic lip

Basal plate

4th ventricle

Roof plate

Sulcus Limitans

Basal plate

Sulcus limitans

Alar plate

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