147. The Brainstem: Myelencephalon (fifth Vesicle) – Basal Motor Plate

  1. Introduction: the external shape of the cephalic portion of the neural tube changes markedly with the appearance of brain vesicles and the development of cervical and cephalic flexures. Despite changes, some morphologic characteristics seen in the spinal cord are recognizable in most of the brain vesicles
    1. DISTINCT BASAL AND ALAR PLATES, representing the motor and sensory areas, respectively, are seen on each side of the midline in most of the brain vesicles
    2. THE SULCUS LIMITANS, which formed the boundary line between the alar and basal plates in the cord, is present in the rhombencephalon and mesencephalon, where it also forms the divider between sensory and motor areas
  2. The myelencephalon is the most caudal brain compartment and extends from the first spinal nerve to the pontine flexure and gives rise to the medulla oblongata
    1. THE MEDULLA differs from the cord in that its lateral walls rotate around an imaginary long axis in the floor plate, like opening a textbook, and as a result, the roof plate stretches and becomes a single layer of cells covering an enlarged central cavity called the fourth ventricle
      1. The lateral wall structure is similar to that of the spinal cord, and one sees alar and basal plates separated by the sulcus limitans
    2. THE BASAL MOTOR PLATE of the myelencephalon
      1. Like the spinal cord, the basal plate contains the motor nuclei, but these are divided into 3 groups
        1. A medial somatic efferent group forms the cephalic continuation of the anterior horn cells containing the motor neurons which innervate striated muscle (derived from myotomes in the cephalic region)
          1. Since the somatic efferent group continues rostrally into the mesencephalon (through the metencephalon), it is referred to as the somatic efferent motor column
          2. It is represented in the myelencephalon by the neurons of the hypoglossal (XII) cranial nerve which supplies the 4 occipital myotomes for the tongue musculature
          3. In the metencephalon and mesencephalon, the column is represented by the neurons of the abducens (VI), trochlear (IV), and oculomotor (III) cranial nerves, supplying eye muscles thought to be derived from preoptic myotomes
          4. The neurons of nerves III, IV, VI, and XII are all located near the midline
        2. An intermediate special visceral efferent group extends into the metencephalon and forms the special visceral efferent motor column which contains motor neurons supplying striated muscles derived from the mesenchyme of the pharyngeal or branchial arches
          1. In the myelencephalon, the column is represented by neurons of the accessory (XI), vagus (X), and glossopharyngeal (IX) cranial nerves
          2. In the adult, motor neurons of the above nerves are formed by the nucleus ambiguus and the bulbar portion of the accessory nerve
        3. A lateral general visceral efferent group contains the neurons whose axons grow out as preganglionic fibers to synapse in the parasympathetic ganglia supplying involuntary muscles of the heart, respiratory tract, and intestinal tract as well as innervating the salivary glands
        4. In the myelencephalon, this group is represented by the dorsal nucleus of the vagus (X) and the inferior salivatory nucleus which, by way of the cranial nerve IX (glossopharyngeal), innervates the parotid gland

the brainstem: myelencephalon  (fifth vesicle) – basal motor plate: image #1