1. General somatic efferent column.

2. General visceral efferent column.

3. General visceral afferent column.

4. General somatic afferent column.

• The general somatic efferent fibres arise from the nuclei of general somatic efferent column, and supply the striated muscles of the limbs and body wall developing from somites (‘soma’ = body wall).

• The special visceral (branchial) efferent fibres arise from the nuclei of special visceral (branchial) efferent column and supply the striated muscles developing from branchial or pharyngeal arches, viz. muscles of facial expression, muscles of palate, pharynx and larynx.

• The general visceral efferent fibres arise from the nuclei of general visceral efferent column and supply the glands and the smooth muscles of vessels and viscera. These fibres form the cranial outflow of the parasympathetic nervous system.

• The general visceral afferent fibres carry general sensations (sense of distention and ischaemia) from the viscera, viz. lung, heart and upper part of GIT and associated glands to the general visceral afferent column.

• The special visceral (branchial) afferent fibres carry special sensation of taste from tongue, etc. to the nuclei of special visceral afferent column (this is because the epithelium of tongue including taste buds develop from endoderm (visceral).

• The general somatic afferent fibres carry general sensations (pain, touch and temperature from skin) and proprioceptive sensations (vibration, muscle and joint sense) to the nuclei of general somatic afferent column.

• The special somatic afferent fibres carry special sensations of hearing and equilibrium to the nuclei of special somatic afferent column (this is because the organs concerned with these sensations develop from ectoderm of the body wall).

Oculomotor nucleus

Oculomotor nucleus is located in the central grey matter of midbrain, ventral to cerebral aqueduct at the level of superior colliculi (for details see page 85).

Trochlear nucleus

Trochlear nucleus is located in the central grey matter of midbrain, ventral to cerebral aqueduct and close to midline at the level of inferior colliculi. It is just caudal to the oculomotor nucleus and its ventral aspect is closely related to medial longitudinal fasciculus. Fibres from each trochlear nucleus course dorsally and then medially around the central grey matter to reach the cranial end of superior medullary velum, wherein they decussate to emerge on the lateral side of frenulum veli on the dorsal aspect of the midbrain (Figs 8.3, 8.12), The trochlear nerve fibres have an unusual course and this is the only nerve which emerges from the dorsal aspect of brainstem. It has been suggested that this nerve originally supplied the muscles of pineal eye, which would account for its dorsal course.

Abducent nucleus

Abducent nucleus is located in the lower part of the pons beneath the facial colliculus in the floor of fourth ventricle, a short distance from the median plane and in line with the nuclei of IIIrd and IVth cranial nerves above, and hypoglossal nerve below. Medial longitudinal fasciculus is closely related to its ventromedial aspect. Fibres from abducent nucleus pass ventrally downwards through the reticular formation intersecting the trapezoid body and medial lemniscus and traversing the basilar part of pons to emerge at the junction of pons and pyramid of medulla. The abducent nerve supplies the lateral rectus muscle of eyeball.

The cells in the reticular formation adjacent to the abducent nucleus constitute a “para-abducent nucleus” which functions as a “centre for lateral gaze.” These cells send fibres to the ipsilateral abducent nucleus and through the medial longitudinal fasciculus, to those cells of contralateral oculomotor nucleus that supply the medial rectus muscle. The actions of medial and lateral recti muscles are thus coordinated in horizontal movements of the eye.

Hypoglossal nucleus

Hypoglossal nucleus is an elongated column consisting of motor cells like those of anterior horn cells of the spinal cord. It extends throughout the length of medulla oblongata in the paramedian plane. The upper part of nucleus lies deep to hypoglossal triangle in the floor of IVth ventricle. The medial longitudinal bundle lies immediately ventral to it. In the closed part (lower part) of medulla, the nucleus lies in the central grey matter ventral to the central canal. The fibres from the hypoglossal nucleus course ventrally on the lateral side of medial lemniscus and emerge on the ventral aspect of medulla as a series of about 12 rootlets in the sulcus between pyramid and olive to form hypoglossal nerve (Fig. 8.6).

Motor nucleus of trigeminal nerve

Motor nucleus of trigeminal nerve is situated in the upper part of the pons, in its dorsal region, medial and just cranial to chief sensory nucleus of trigeminal nerve. Fibres from the motor nucleus constitute the motor root of trigeminal nerve which emerges on the ventral aspect of pons medial to the sensory root. The motor root crosses the superior border of petrous temporal bone and passes posterior to trigeminal ganglion. Then it passes through the foramen ovale and immediately joins the sensory root of the mandibular nerve (Figs 8.9, 9.4).

Nucleus of facial nerve

Nucleus of facial nerve is situated in the lower part of the pons, in the ventrolateral part of its tegmentum, more or less in line with the motor nucleus of the trigeminal nerve. Its position is anterolateral and caudal to abducent nucleus, and medial to the nucleus of spinal tract of trigeminal nerve. The fibres arising from the nucleus pursue an aberrant course. First they course dorsomedially towards the floor of fourth ventricle to loop behind the motor nucleus of the abducent nerve. The loop (internal genu of facial nerve) elevates the floor of fourth ventricle and forms the facial colliculus, and then course ventrolaterally passing between the nucleus of their origin and nucleus of spinal tract of trigeminal nerve to emerge through the pontomedullary junction on the ventral aspect of the brainstem lateral to the emergence of the abducent nerve.

The unusual course of motor fibres of facial nerve represents an example of neurobiotaxis (for details see page 80).

Nucleus ambiguus

Nucleus ambiguus is an elongated column of typical motor neurons, extending throughout the length of medulla. Nucleus ambiguus is so named because it is not clearly defined in sections of the medulla. It occupies a position dorsal to the inferior olivary nucleus and ventromedial to the nucleus of spinal tract of Vth nerve. Fibres from nucleus ambiguus are first directed dorsally and then turn sharply in the ventrolateral direction to mingle with other fibres of IXth, Xth, and XIth cranial nerves, which emerge from medulla along the posterolateral sulcus (Fig. 8.6). The fibres from nucleus ambiguus supply the muscles derived from third, fourth and sixth branchial arches.

Edinger-Westphal nucleus (visceral oculomotor nucleus)

Edinger-Westphal nucleus is located in the upper part of the midbrain dorsal to the rostral two-thirds of the main oculomotor nucleus (Fig. 9.5). Preganglionic para-sympathetic fibres arising from this nucleus reach the ciliary ganglion by way of oculomotor nerve, where they terminate.

Superior salivatory and lacrimatory nuclei

Superior salivatory and lacrimatory nuclei are seen as “indefinite clusters of small nerve cells” in the dorsal part of pons medial to the motor nucleus of facial nerve.

The preganglionic fibres from superior salivatory nucleus pass to the submandibular ganglion through the nervus intermedius, geniculate ganglion, facial nerve and its chorda tympani branch (see Fig. 20.10).

Preganglionic fibres from lacrimatory nucleus reach the pterygopalatine ganglion through nervus intermedius and greater superficial petrosal nerve (Fig. 20.10).

Inferior salivatory nucleus

Inferior salivatory nucleus is a small collection of nerve cells in the dorsolateral part of pons, just above its junction with the medulla. It lies immediately caudal to the superior salivatory nucleus and just above the upper end of the dorsal nucleus of the vagus nerve. Preganglionic fibres from this nucleus run in the glossopharyngeal nerve, enter its tympanic branch (nerve of Jacobson) and relay in the “otic ganglion” by way of tympanic plexus and lesser superficial petrosal nerve (Fig. 20.9).

Dorsal nucleus of vagus (also called motor nucleus of vagus)

Motor nucleus of vagus is a long vertical column of cells extending throughout most of the length of medulla. Its upper end lies deep to the vagal triangle in the floor of the fourth ventricle. When traced downwards in the closed part of medulla, it occupies a position in the lateral part of central grey matter, dorsal to the hypoglossal nucleus (Fig. 8.5). This nucleus is the main source of parasympathetic fibres of vagus nerve.

The dorsal nucleus of vagus is usually described as mixed nucleus representing the fused general visceral efferent and general visceral afferent columns. According to the other school of thought, the general visceral afferent column is incorporated in the special visceral afferent column representing the nucleus of tractus solitarius.

Nucleus of solitary tract (Fig. 9.3)

Nucleus of solitary tract is an elongated column of cells and is intimately related to a group of descending fibres which constitute the tractus solitarius. The upper part of the nucleus lies deep in the reticular formation, ventrolateral to the dorsal nucleus of vagus (Fig. 8.6). When traced downwards it lies in the dorsal part of central grey matter in the closed part of the medulla, dorsomedial to the dorsal nucleus of vagus (Fig. 8.5). The lower ends of the nuclei of two sides fuse to form the commissural nucleus of the vagus. The rostral portion of the nucleus is concerned with taste sensations and receives the special visceral afferent fibres from facial, glossopharyngeal and vagus nerves and is frequently referred to as gustatory nucleus. The nuclear terminations of VII, IX and X nerves are in rostrocaudal direction (Fig. 9.3). The caudal portion of the nucleus receives the general visceral sensations from pharynx (glossopharyngeal and vagus) and from oesophagus and abdominal part of alimentary canal up to right two-thirds of the transverse colon (vagus). It is presumed that axons from nucleus of tractus solitarius project to the thalamus and hypothalamus of the opposite side through the solitariothalamic and solitariohypothalamic tracts respectively. These tracts join the medial lemniscus of the opposite side on their way to thalamus and hypothalamus. The neurons from thalamus then project to the cerebral cortex (Fig. 9.3).

Chief sensory nucleus of trigeminal nerve

Chief sensory nucleus of trigeminal nerve lies in the dorsolateral region of the tegmentum of the upper part of the pons lateral to the motor nucleus (of trigeminal) and occupies an intermediate position between the mesencephalic nucleus above and the spinal nucleus below (Fig. 9.4). It is concerned only with the tactile sensibility.

Spinal nucleus of trigeminal nerve

Spinal nucleus of trigeminal nerve extends caudally from chief sensory nucleus in the pons to the second cervical spinal segment and lies just medial to the spinal tract of trigeminal nerve. The spinal nucleus and tract of trigeminal nerve are chiefly concerned with the pain and temperature sensations. Based on the cytoarchitecture, the spinal nucleus is divided into three parts (or subnuclei): In craniocaudal direction these are: (a) pars rostralis, (b) pars interpolaris, and (c) pars caudalis (Fig. 9.4).

Main afferents of chief sensory and spinal nuclei are the central processes of cells in the trigeminal ganglion (which makes up the large sensory root). After entering the pons many of these processes divide into ascending and descending branches. Others either ascend or descend without being branched.

The ascending fibres end in the chief sensory nucleus. The descending fibres from a large bundle of fibres called spinal tract of trigeminal nerve. Fibres of the spinal tract terminate in the subjacent spinal nucleus. The afferents from three trigeminal divisions rotate, so that the fibres of ophthalmic division terminate in the pars caudalis, the fibres of maxillary division in the pars interpolaris and the fibres of mandibular division in pars rostralis.

As described earlier, the fibres ending in the chief sensory nucleus are predominantly concerned with touch, and those ending in spinal nucleus are concerned predominantly with sensations of pain and temperature.

It is important to note at this juncture that in addition to trigeminal nerve, the spinal tract receives a small component of fibres from the VIIth, IXth and Xth cranial nerves which carry the general somatic sensations from external ear, mucosa of posterior third of tongue, pharynx and larynx.

Fibres arising from cells of chief sensory and spinal nuclei are the second order neurons (comparable to those of the spinothalamic tracts) cross to the opposite side and form a bundle called trigeminal lemniscus which ascends up and relay in the thalamus (ventral posteromedial nucleus) from where third order neurons arise and project to the sensory area of the cerebral cortex.

A separate bundle of more dorsally situated trigeminothalamic fibres (also called dorsal trigeminal lemniscus) is also described.

Mesencephalic nucleus of trigeminal nerve

Mesencephalic nucleus of trigeminal nerve extends from upper end of chief sensory nucleus in the pons to the midbrain where it lies in the central grey matter lateral to the cerebral aqueduct. Because it extends rostrally into the midbrain, it is called mesencephalic nucleus. Like dorsal root ganglia of spinal cord it is made up of pseudounipolar cells (1st order sensory neurons) and appears to have similar functions (the mesencephalic nucleus is unique in the sense that it is the only site in CNS which contains the cell bodies of first order sensory neurons). Peripheral processes of these cells carry proprioceptive impulses from the muscles of mastication, temporomandibular joint, teeth and possibly also from the extrinsic muscles of tongue. Central processes terminate in the motor nuclei of trigeminal nerve of the both sides. These connections establishes the stretch reflex originating in the neuromuscular spindles in masticatory muscles, together with a reflex for control of the force and accuracy of bite (Barr, N.L., 1972). These reflexes prevent the tongue from being bitten during chewing. Other central processes relay in the cells of reticular formation. From which fibres arise and run through dorsal trigeminal lemniscus to relay in the ventral posteromedial nucleus (VPM) of the thalamus (Fig. 9.4).

Cochlear nuclei

Cochlear nuclei are two in number, dorsal and ventral. They are placed on the dorsal and ventral aspects of the inferior cerebellar peduncle respectively in the upper part of the medulla.

The cochlear nuclei contain the cell bodies of the second order sensory neurons in the auditory pathway. Their connections are described in Chapter 18.

Vestibular nuclei

Vestibular nuclei are situated partly in the medulla and partly in the pons, immediately beneath the lateral part of the floor of the fourth ventricle called vestibular area. On the basis of cytoarchitecture and afferent and efferent connections, four distinct vestibular nuclei are recognised, viz. (a) inferior or spinal vestibular nucleus, (b) lateral vestibular nucleus (also called Dieter’s nucleus), (c) superior vestibular nucleus, and (d) medial vestibular nucleus (Fig. 8.15).

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Organization and functions of the nervous system Development of the nervous system Somatic motor and sensory pathways Autonomic nervous system Blood supply of the brain Special senses and their neural pathways

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