Medical brain notes 24

Cranial Nerve Nuclei The seven functional components to which the fibres of a cranial nerve may belong are as follows. (1)            Somatic efferent.  (2)  General visceral efferent.  (3)  Special visceral efferent.  (4)  Generalsomatic afferent.  (5)  Special somatic afferent.  (6)  General visceral afferent.  (7)  Special visceral afferent.         Each functional component has its own nuclei of origin (in the case of efferent fibres) or of termination (in the case of afferent fibres). In the embryo the nuclei related to the various components are arranged in vertical rows (or columns) in a definite sequence in the grey matter related to the floor of the fourth ventricle (Fig. 10.1). The sequence is easily remembered if the following facts are kept in mind. a.              ...

  Somatic Efferent Nuclei         The somatic efferent column consists of the following nuclei that supply striated (skeletal) muscle of somatic origin. 1.                The oculomotor nucleus is situated in the upper part of the midbrain at the level of thesuperior colliculus (Figs. 10.1, 10.2 and 10.3F). The nuclei of the two sides form a single complex that lies in the central grey matter, ventral to the aqueduct. 2.                The trochlear nucleus is situated in the lower part of the midbrain at the level of the inferiorcolliculus (Figs. 10.1, 10.2 and 10.3E). The nucleus lies ventral to the aqueduct in the central grey matter. 3.                The abducent nucleus is situated in the lower part of the pons. It lies in the grey mat...

  Special Visceral Efferent Nuclei         These nuclei are also called branchial efferent or branchiomotor nuclei. They supply striated (skeletal) muscle derived from branchial arches. 1.                The motor nucleus of the trigeminalnerve  lies in the upper part of the pons, in itsdorsal part (Figs. 10.1, 10.2, and 10.3D). It is situated in the lateral part of the reticular formation, medial to the main sensory nucleus of the trigeminal nerve . 2.                The nucleus of thefacial nerve  lies in thelower part of the pons and occupies a position similar to that of the motor nucleus of the trigeminal nerve. The spinal nucleus and tract of the trigeminal nerve lie lateral to it. (Figs. 10.1, 10.2 and 10.3C). 3.              ...

  General Visceral Efferent Nuclei         The nuclei of this column give origin to preganglionic fibres that constitute the cranial parasympathetic outflow. These fibres end in peripheral ganglia. Postganglionic fibres arising in these ganglia supply smooth muscle or glands. The nuclei are as follows. 1.                The Edinger-Westphal nucleus (or accessory oculomotor nucleus ) lies in the midbrain(Figs. 10.1, 10.2 and 10.3F). It is closely related to the oculomotor complex. Fibres arising in this nucleus pass through the oculomotor nerve. They relay in the ciliary ganglion to supply the sphincter pupillae and the ciliaris muscle. 2.                The salivary (or salivatory ) nuclei (superior and inferior) lie in the dorsal part of the pons,just above its junction with the medulla (Figs. 10.1, 10.2). Th...

  General and Special Visceral Afferent Nuclei          Both these columns are represented by the  nucleus of the solitary tract , present in the medulla (Figs. 10.1, 10.2). Like other cranial nerve nuclei of the medulla, the cells of this nucleus form an elongated column lying deep in the reticular formation. Its upper part lies ventrolateral to the dorsal nucleus of the vagus (Fig. 10.3B). When traced downwards it extends into the closed part of the medulla: here it lies dorsomedial to the vagal nucleus (Fig. 10.3A). The lower ends of the nuclei of the two sides fuse to form the commissural nucleus  of the vagus. The nucleus solitarius receives fibres carrying general visceral sensations through the vagus and glossopharyngeal nerves. Through these afferents, and through connections with the reticular formation, the nucleus plays an important role in reflex control of respiratory and cardiovascular functions.    ...

  General Somatic Afferent Nuclei         The general somatic afferent column is represented by the sensory nuclei of the trigeminal nerve. These are as follows. 1.                The main (or superior ) sensory nucleus lies in the upper part of the pons, in the lateral partof the reticular formation. It lies lateral to the motor nucleus of the trigeminal (Figs 10.1, 10.2 and 10.3D). It is mainly concerned in mediation of proprioceptive impulses, touch and pressure (from the region to which the trigeminal nerve is distributed). 2.                The spinal nucleus extends from the main nucleus down into the medulla (Figs 10.1, 10.2 and10.3 A,B,C) and into the upper two segments of the spinal cord. Its lower end is continuous with the substantia gelatinosa of the cord. In addition to fibres of the trigeminal ne...

  Other Connections of Cranial Nerve Nuclei 1.                The somatic efferent and special visceral efferent nuclei of cranial nerves supply striated (skeletal)muscle. Their connections are, therefore, similar to those of ventral horn cells of the spinal cord (Fig. 10.4). a.                These nuclei are under cortical control through corticonuclear fibres. Cortical control of the hypoglossal nucleus is contralateral (i.e., from the opposite cerebral hemisphere). Corticonuclear fibres to the part of the facial nucleus innervating the lower part of the face is contralateral; but fibres for the rest of the nucleus are bilateral. The trochear nucleus receives ipsilateral fibres (i.e., from the same side). All other nuclei are influenced by both cerebral hemispheres, but the fibres to the abducent nerve are predominantly crossed. Corticonucle...

  Pathway of Hearing & Connections of Cochlear Nuclei          The cochlear nuclei are cell stations in the pathway of hearing. The first neurons of this pathway are located in the spiral ganglion, present in intimate relationship to the cochlea. These neurons are bipolar. Their peripheral processes reach the hair cells in the spiral organ of Corti (which is the end organ for hearing). The central processes of the neurons form the cochlear nerve, and terminate in the dorsal and ventral cochlear nuclei. The neurons in these nuclei are, therefore, second order neurons. Their axons pass medially in the dorsal part of the pons. Most of them cross to the opposite side, but some rema in uncrossed.         The crossing fibres of the two sides form a conspicuous mass of fibres called the  trapezoid body.  Some crossing fibres run separately in the dorsal part of the pons and do not form part of the tra...