There are 12 pairs of cranial nerves, out of these two pairs arise from the forebrain and 10 pairs from the brainstem (see related text on page 47 and Figs 6.9, 8.1). The cranial nerves are designated by Roman numerals in order from before backwards in which they are attached on to the brain:
I – Olfactory
II – Optic
III – Oculomotor
IV – Trochlear
V – Trigeminal
VI – Abducent
VII – Facial
VIII – Vestibulocochlear
IX – Glossopharyngeal
X – Vagus
XI – Accessory
XII – Hypoglossal
N.B. A pair of small nerves closely related to the olfactory nerves has been described as the thirteenth pair (or “O” pair) of cranial nerves (the nervi terminalis). Each nerve is a minute bundle of mainly unmyelinated nerve fibres which is attached to the cerebrum posterior to olfactory stria close to anterior perforated substance and septal areas. The nerve runs forward along the medial side of corresponding olfactory tract and its branches traverse the cribriform plate of ethmoid and are distributed lo the nasal mucous membrane, but its exact function is unknown.
It is thought to: (a) provide a special chemosensory pathway of olfaction and affects the secretion of luteinizing hormone releasing factor (LHRF) from hypothalamus, (b) play an important role in smell mediated sex-behaviour.
Morphological Classification of Cranial Nerves
The cranial nerves can be classified into the following three morphological groups:
• Those supplying the muscles derived from cranial myotomes, viz. oculomotor (III), trochlear (IV), abducent (VI), and hypoglossal (XII) nerves.
• Those supplying the muscles derived from branchial arches, viz. trigeminal (V), facial (VII), glossopharyngeal (IX), vagus (X), and accessory (XI) nerves.
• Those associated with special sense organs, viz. olfactory (I), optic (II) and vestibulocochlear (VIII) nerves.
Functional Columns and Nuclei of Cranial Nerves
A cranial nerve consists of motor fibres (motor nerve) or sensory fibres (sensory nerve) or both motor and sensory fibres (mixed nerve).
The questions on cranial nerve nuclei and their functional components are frequently asked in the examinations. The arrangement of cranial nerve nuclei, and the functional columns they represent are linked with the development of the brain; hence the following text discusses in brief, the development of functional columns and nuclei of cranial nerves.
Development of Functional Columns and Nuclei of Cranial Nerves
During the development of spinal cord due to the appearance of a longitudinal groove (the sulcus limitans), the lateral wall of the neural tube is divided into two parts or laminae: (a) a dorsal alar lamina, and (b) a ventral basal lamina. The cells of basal lamina are motor while those of alar lamina are sensory in function. The cells of each of these two laminae, gets arranged into two longitudinal columns: the somatic and the visceral, the visceral components lying close to the sulcus limitans. Thus, there are four functional components/columns in the lateral wall of the spinal cord from ventral to dorsal side, viz.
1. General somatic efferent column.
2. General visceral efferent column.
3. General visceral afferent column.
4. General somatic afferent column.
During the development of hindbrain two significant events occur: (a) the neural tube widens to form the fourth ventricle, as a result the alar laminae are splayed apart like opening a book, consequently the functional components undergo a directional change from ventrodorsal to mediolateral, and (b) an extra branchial (special) column appears between somatic and visceral columns in both basal and alar laminae to supply the derivatives of pharyngeal arches.
Apart from this, an extra special somatic column appears in the most lateral part of the alar lamina to receive sensations of hearing and balance.
Thus, there are seven functional columns in the brainstem (Fig. 9.1A), from medial to lateral side they are:
FIG. 9.1 (A) Functional columns of grey matter in brainstem (of embryo). (B) Schematic diagram to show different cranial nerve nuclei in adult brain derived from seven functional columns of grey matter. (N = nucleus, Mes = mesencephalic, Spi = spinal, Chi Sen = chief sensory, S = superior, L = lateral, Med = medial, G = gustatory nucleus, EW = Edinger-Westphal, Sal = salivatory nuclei, 3 = oculomotor, 4 = trochlear, 5 = trigeminal, 6 = abducent, 7 = facial, 8 = glossopharyngeal, 9 = vagus.)
As the development proceeds, these columns differentiate into one or more discrete cranial nerve nuclei. Nuclei derived from various functional columns are enumerated in Table 9.1 and shown in Figure 9.1B. The surface projection of these nuclei on the dorsal aspect of the brainstem is shown in Figure 9.2.
Table 9.1
Cranial nerve nuclei derived from various functional columns in the brainstem
FIG. 9.2 Surface projection of cranial nerve nuclei on the dorsal aspect of the brainstem. Motor nuclei are shown on the right side and sensory nuclei on the left side.
Thus, in total there are seven functional components to which the fibres of cranial nerve may belong:
• 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).
General Somatic Efferent Nuclei
General somatic efferent nuclei supply the striated muscles of somatic origin.
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 seepage 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.
Unique features of the trochlear nerve
• It is the smallest and most slender cranial nerve.
• It decussates before emerging from the brain.
• It emerges on the dorsal aspect of the brain.
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).
Special Visceral (Branchial) Efferent Nuclei
Special visceral efferent nuclei supply the striated muscles derived from branchial arches.
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 seepage 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.
General Visceral Efferent Nuclei
The cells of these nuclei give origin to preganglionic fibres that constitute the cranial parasympathetic outflow. These fibres end in the peripheral parasympathetic ganglia. The postganglionic fibres arising in these ganglia supply smooth muscles or glands.
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.
General and Special Visceral Afferent Nuclei
General and special visceral afferent nuclei are represented by only one nucleus, the nucleus of solitary tract.
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 tractsrespectively. 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).
FIG. 9.3 Connections of nucleus tractus solitarius. The inset on the right side shows that the lower ends of the nuclei of two sides fuse to form the commissural nucleus of the vagus nerve.
FIG. 9.4 Schematic illustration to show connections of trigeminal nerve nuclei. M = motor nucleus. Inset on the right upper corner shows the subdivisions of spinal nucleus of trigeminal nerve and disposition of afferent fibres from its ophthalmic maxillary and mandibular divisions. (PR = pars rostralis, PI = pars inter-polaris, PC = pars caudalis, V1 = ophthalmic nerve, V2 = maxillary nerve, V3 = mandibular nerve, M = motor nucleus, SVE = special visceral efferent, GSA = general somatic afferent.)
FIG. 9.5 Nuclei, functional components and course of the oculomotor nerve. (A) Functional components, (B) course and distribution.
General Somatic Afferent Nuclei
General somatic afferent nuclei include the three sensory nuclei of the trigeminal nerve.
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 lemniscuswhich 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).
N.B. Out of the three sensory nuclei of the trigeminal nerve, the chief sensory nucleus is mainly responsible for sense of touch, the spinal nucleus for sense of pain and temperature, and mesencephalic nucleus for sense of proprioception.
Special Somatic Afferent Nuclei
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).
• Inferior vestibular nucleus lies in the medulla, just medial to the inferior cerebellar peduncle. It is continuous rostrally with the lateral vestibular nucleus and related medially to the medial vestibular nucleus. It extends from the cranial end of nucleus gracilis to the pontomedullary junction.
• Lateral vestibular nucleus lies immediately cranial to inferior vestibular nucleus and extends upwards in the pons almost to the level of nucleus of abducent nerve. It is composed of large multipolar cells resembling typical motor neurons. The cells of this nucleus give origin to the fibres of lateral vestibulospinal tract (Fig. 8.15).
• Superior vestibular nucleus is smaller in size and located entirely within the pons above the medial and lateral vestibular nuclei.
• Medial vestibular nucleus extends from medulla at the level of olive to the lower part of the pons. It is bounded laterally and rostrally by the other three vestibular nuclei. Its medial border is near the midline of the brainstem. The caudal end of this nucleus is near the caudal limit of the fourth ventricle.
Connections of the vestibular nuclei
Afferents
• Fibres of the vestibular nerve (main afferents): Most of the vestibular nerve terminate in the four vestibular nuclei, however, few pass directly to the cerebellum by way of inferior cerebellar peduncle to the flocculonodular lobe.
• Cerebellovestibular fibres: The fibres from cerebellar cortex (flocculonodular lobe) relay in the nucleus fastigius which give rise to the fastigiobulbar tract. The fibres of this tract mostly pass through inferior cerebellar peduncle, some fibres, however, as they pass from cerebellum, loop around the superior cerebellar peduncle to form the uncinate fasciculus (tract of Russell) before joining the main tract which terminate in the vestibular nuclei.
Efferents
• To the cerebellum (vestibulocerebellar fibres): Most of these fibres arise from vestibular nuclei, however, few are the direct fibres of vestibular nerve from cells of vestibular ganglion as noted above. These fibres pass through the medial portion of the inferior cerebellar peduncle (juxta-restiform body) of the same side to relay into the cortex of archicerebellum.
• To the spinal cord: The principal connections between the vestibular nuclei and spinal cord are mediated through the vestibulospinal tract and the descending portion of the medial longitudinal fasciculus.
– Vestibulospinal tract (also called lateral vestibulospinal tract) (Fig. 8.15): Fibres of this tract arise exclusively from lateral vestibular nucleus. They descend downwards in the medulla dorsal to the inferior olivary nucleus and continue so in the anterior funiculus of the spinal cord, where they terminate on the anterior horn cells at all level of spinal cord especially in regions of cervical and lumbosacral enlargements. The vestibulospinal tract is uncrossed and regulates the muscle tone throughout the body in such a manner that the balance is maintained.
– Descending portion of medial longitudinal fasciculus: Fibres from vestibular nuclei, mainly from medial, project towards the midline and then turn caudally in the medial longitudinal fasciculi of both the sides which continue downward into the sulcomarginal fasciculus of anterior funiculus of spinal cord and terminate on anterior horn cells throughout the cervical part of the spinal cord. These connections provide for changes in the tone of neck muscles as required to support the head in various positions and during various movements.
• To the cranial nerve nuclei: Fibres from vestibular nuclei first project towards the midline than ascend up in ascending portions of the medial longitudinal fasciculi of both the sides and synapse with the cells of IIIrd, IVth and VIth cranial nerve nuclei; and downwards in the descending portions of the MLFs and synapse with spinal nucleus of XIth cranial nerve. These connections provide for the synchronized conjugate movements of the eyes, coordinated with the movements of the head (such coordination relies heavily on the information required by vestibular nuclei from semicircular canals or kinetic labyrinth).
Having considered the cranial nerve nuclei and there connections, it is now possible to workout the functional components of the individual cranial nerves.
Nuclei, Functional Components and Distribution of Individual Cranial Nerves
I Olfactory Nerve (Nerve of Smell)
The fibres of this nerve are classified as special somatic afferent (SSA). They carry sense of smell from olfactory epithelium of nose which is derived from ectoderm (of nasal placodes). For details see olfactory system in Chapter 18.
II Optic Nerve (Nerve of Vision)
Optic nerve fibres are also regarded as special somatic afferent (SSA). The retina develops from optic vesicle of the forebrain and contains two types of receptors for light, viz. rods and cones. The visual impulses from these receptors run through bipolar cells to the ganglion cells. The axons of ganglion cells converge towards the optic disc. They pierce the choroid and sclera to leave the eyeball and form the optic nerve. In view of its structure and development the optic nerve is regarded as the tract rather than a peripheral nerve. For details see visual system in Chapter 18.
Unique features of the optic nerve
• It is not a peripheral nerve but a tract (a prolongation of white matter of the brain) as the optic nerve develops from the stalk of optic vesicle.
• It is covered by three meninges (pia, arachnoid and dura) of the brain.
• It is devoid of neurilemmal sheath.
• Its fibres are myelinated by oligodendrocytes.
Clinical Correlation
The optic nerve, if damaged, cannot regenerate because its constituent fibres are devoid of neurilemmal sheath.
III Oculomotor Nerve
Functional components
This nerve has following functional components (Fig. 9.5A):
• General somatic efferent fibres (GSE). These fibres arise from main oculomotor nucleus and supply all the extrinsic muscles of the eyeball including levator palpebrae superioris except lateral rectus and superior oblique. These fibres form the main component of the oculomotor nerve.
• General visceral efferent fibres (GVE). These fibres arise from Edinger-Westphal nucleus and pass through the ocul omotor nerve to terminate in the ciliary ganglion. These are preganglionic parasympathetic fibres. The postganglionic fibres from ciliary ganglion, run through short ciliary nerves to supply the sphincter pupillae and ciliary muscles.
Course and distribution (Fig. 9.5B)
The oculomotor nerve arises from medial aspect of cerebral peduncle. It continues forwards in the interpeduncular cistern between the posterior cerebral and superior cerebellar arteries. It pierces pia and arachnoid as it enters the back of cavernous sinus. Here it lies above the trochlear nerve in the lateral wall of the cavernous sinus. In the anterior part of the cavernous sinus, it divides into upper and lower divisions. Both of these divisions of oculomotor nerve enter the orbit through the middle part of the superior orbital fissure, within the common tendinous ring (of Zinn). The distribution of upper and lower divisions of occulomotor nerve is presented in the Table 9.2.
Table 9.2
Distribution of oculomotor nerve
|
Division |
Functional components |
Distribution |
|
Upper |
• GSE |
– Superior rectus (GSE) |
|
• GVE |
– Levator palpebrae |
|
|
– Superioris (GSE + GVE) |
||
|
– Dilator pupillae (GVE) |
||
|
Lower |
• GSE |
– Inferior rectus (GSE) |
|
– Inferior oblique (GSE) |
||
|
– Medical rectus (GSE) |
||
|
• GVE (Parasympathetic) |
– Constrictor pupillae (GVE) |
|
|
– Ciliary muscle (GVE) |
GSE = general somatic efferent fibres, GVE = general visceral efferent fibres.
Along its course in the cavernous sinus the oculomotor nerve picks up the sympathetic fibres (GVE) from the sympathetic plexus around the internal carotid artery. These fibres are postganglionic and arise from superior cervical sympathetic ganglion. The preganglion fibres arise from first thoracic (T1) spinal segment. The sympathetic fibres supply dilator pupillae and smooth part of the levator palpebrae superioris (muscle of Muller).
The oculomotor nerve plays an important role in the movements of the eyeball, and is responsible for accommodation. In addition it forms parts of pathways involved in pupillary reflexes.
Clinical Correlation
In oculomotor nerve palsy: (a) the eye is fixed in lateral and downward position (lateral squint/strabismus) due to unopposed action of the lateral rectus and superior oblique, (b) the pupil is dilated due to unopposed action of dilator papillae (which is supplied by sympathetic fibres accompanying the nasociliary branch of the ophthalmic nerve), and (c) the upper eyelid droops down (ptosis) due to paralysis of levator palpebrae superioris.
IV Trochlear Nerve
Functional components
It consists of only general somatic efferent (GSE) fibres which arise from the trochlear nucleus and supply only one muscle, the superior oblique muscle of the eyeball.
Course and distribution
The trochlear nerve arise on the dorsal aspect of midbrain below the inferior colliculus just lateral to the frenulum veli. It courses ventrally around the cerebral peduncle, along and just below the free margin of the tentorium cerebelli. It pierces pia and arachnoid and enters the back of cavernous sinus lateral to the third nerve. In cavernous sinus, it crosses to the medial side of the oculomotor nerve and occupies a more cranial position. It enters orbit through the lateral part of the superior orbital fissure outside the common tendinous ring (of Zinn) to supply superior oblique muscle (Fig. 9.6).
FIG. 9.6 Distribution of IIIrd (oculomotor), IVth (trochlear), and VIth (abducent) cranial nerves. (LPS = levator palpebrae superioris, SR = superior rectus, IR = inferior rectus, MR = medial rectus, LR = lateral rectus, ON = optic nerve, OA = ophthalmic artery, NC = nasociliary nerve, CG = ciliary ganglion, SOV = superior ophthalmic vein, IOV = inferior ophthalmic vein, TN = trochlear nerve.)
Clinical Correlation
The superior oblique is the only depressor of the eyeball during adduction, therefore in trochlear nerve palsy (the isolated injury of trochlear nerve is uncommon) the eye is extorted and elevated due to unopposed action of inferior oblique. The vertical diplopia occurs in this position as the image falls on the upper half of the retina. The diplopia worsens when patient looks down, when the eye is adducted.
V Abducent Nerve
Functional components
This nerve consists of general somatic efferent (GSE) fibres only which arise from the abducent nucleus and supply only lateral rectus muscle of the eyeball.
Course and distribution
The abducent nerve emerges at pontomedullary junction above the pyramid of the medulla. It runs forwards to the dura on the clivus. It then pierces dura mater lateral to the dorsum sellae below the petroclinoid ligament (ligament of Grubar). Here its crosses sharp superior border of petroustemporal bone. It enters the back of cavernous sinus. In the sinus it runs along the posterolateral aspect of the internal carotid artery. It enters the orbit, through the intermedial part of superior orbital fissure within the common tendinous ring (of Zinn); to supply the lateral rectus muscle (Fig. 9.7).
FIG. 9.7 Origin, course and distribution of abducent nerve.
Clinical Correlation
The abducent nerve palsy is the most common isolated palsy occurring due to increased intracranial pressure.
The abducent nerve lesion causing paralysis of lateral rectus results in the medial or convergent squint/strabismus (i.e. eyeball is fixed in the medial position) due to unopposed action of the medial rectus. The horizontal diplopia occurs, which increases when patient attempts to look towards the paralysed side.
The distribution of IIIrd (oculomotor), IVth (trochlear) and VIth (abducent) cranial nerves is shown in Figure 9.6.
VI Trigeminal Nerve
Functional components
This nerve has following functional components:
• Special visceral efferent (SVE) fibres arise from motor nucleus and supply the muscles derived from the first pharyngeal arch mesoderm, viz. muscles of mastication, tensor tympani, tensor palati, anterior belly of digastric and mylohyoid.
• General somatic afferent (GSA) fibres are divided into two groups:
– Fibres carrying exteroceptive sensations from skin of the face and mucous membrane of the mouth and nose. The cell bodies of these neurons lie in the trigeminal ganglion. Most of the central processes of these neurons bifurcate, the ascending branches terminate in the chief sensory nucleus whereas descending branches end in the spinal nucleus.
– Fibres carrying proprioceptive sensations from muscles of mastication, temporomandibular joint, teeth and tongue. The cell bodies of these neurons lie in the mesencephalic nucleus.
The peripheral processes of nerve cells located in the trigeminal ganglion and mesencephalic nucleus are arranged into three divisions of the trigeminal nerve, viz. ophthalmic, maxillary and mandibular. The functional components of these divisions are listed in Table 9.3.
Table 9.3
Functional components in the three divisions of trigeminal nerve
|
Division |
Functional components |
|
Ophthalmic |
GSA fibres |
|
Maxillary |
GSA fibres |
|
Mandibular |
GSA and SVE fibres |
Course and distribution (Fig. 9.8)
The trigeminal nerve arises by two roots from pons at its junction with the middle cerebellar peduncle. The two roots are: (a) a very large lateral sensory root, and (b) a small medial motor root.
FIG. 9.8 The distribution of trigeminal nerve. (TG = trigeminal ganglion, V1 = ophthalmic division, V2 = maxillary division, V3 = mandibular division, 1 = ciliary ganglion, 2 = pterygopalatine ganglion, 3 = otic ganglion, 4 = submandibular ganglion, NS = nervus spinosus, NM = nerve to medial pterygoid, M = masseteric n, D = deep temporal ns, NL = nerve to lateral pterygoid, AT = auricu-lotemporal n, CT = chorda tympani n., SO = supraorbital n., ST = supratrochlear n., T = infratrochlear n., AE = anterior ethmoidal n., SDP = superior dental plexus). Note: All the parasympathetic ganglia (1, 2, 3, and 4) of head and neck region are morphologically associated with the trigeminal nerve.
The two roots run forward and laterally over the apex of petrous temporal bone to enter the middle cranial fossa. Here the sensory root exhibits an enlargement—the trigeminal ganglion. The trigeminal ganglion divides into branches: ophthalmic, maxillary and mandibular.
The ophthalmic nerve runs forwards in the lateral wall of the cavernous sinus and divides into three branches: lacrimal, frontal and nasocilliary before entering the orbit through superior orbital fissure. Through these branches ophthalmic nerve supplies the eyeball, conjunctiva, upper part of nasal cavity, lacrimal gland, the skin of forehead, the external nose and eyelids. The ophthalmic nerve also forms the afferent limb of the corneal reflex.
The maxillary nerve leaves the skull through foramen rotundum to enter the pterygopalatine fossa, then it enters the orbit through inferior orbital fissure and acquires name infraorbital nerve. The infraorbital nerve leave the orbit through infraorbital foramen to emerge on the face. Thus, maxillary nerve traverses four regions: middle cranial fossa, pterygopalatine fossa, orbit and face. The branches of maxillary nerve are: (a) meningeal branch in the middle cranial fossa, (b) ganglion branches to pterygopalatine ganglion, (c) zygomatic nerve, and (d) posterior superior alveolar nerve in the pterygopalatine fossa, (e) middle superior alveolar, nerves in orbit; and palpebral, lateral nasal and labial branches in the face. Through these branches, it supplies, nasal cavity, palate, upper teeth and gums and skin of the middle third of the face. It provides sensory supply to the mucous membrane of the nasopharynx and the maxillary sinus. It conveys secretomotor fibres to the lacrimal gland.
The mandibular nerve leave the skull through the foramen ovale to enter the infratemporal fossa. Here, just below the basal skull it is joined by motor root of the trigeminal nerve. Motor root accompanies the sensory root, these passes deep at the side of trigeminal ganglion and leaves the skull through mandibular foramen to join the mandibular division of trigeminal nerve to form the trunk of mandibular nerve.
The trunk of mandibular nerve terminates into two large divisions—anterior and posterior. The following branches arise from mandibular nerve:
• Meningeal branch (nervus spinosus).
• Nerve to medial pterygoid from undivided trunk.
• Masseteric nerve.
• Deep temporal nerves (2 in number).
• Lateral pterygoid nerve.
• Buccal nerve from anterior division.
• Auriculotemporal nerve.
• Lingual nerve.
• Inferior alveolar nerve from posterior division.
Through these branches mandibular nerve supplies the muscles of mastication (temporalis, masseter, medial and lateral pterygoids), mylohyoid, anterior belly of diagastric, tensor veli palatine and tensor tympani. The sensory fibres supply auricle, temporal region, skin of the lower one-third of face except over angle of mandible.
Clinical Correlation
• Lesion of trigeminal nerve presents following clinical features:
– Loss of general sensation from face and mucous membrane of oral and nasal cavities.
– Loss of corneal reflex.
– Flaccid paralyses of muscles of mastication. Jaw deviates to the side of lesion due to unopposed action of lateral pterygoid muscle.
– Hypoacusis (partial deafness to low pitched sounds due to paralysis of tensor tympani muscle).
• Trigeminal neuralgia (tic douloureux) is a paroxysmal severe pain of sudden onset and short duration in the area of cutaneous distribution of one or more of the divisions of the trigeminal nerve, usually affecting the 2nd and 3rd divisions (Fig. 9.9).
FIG. 9.9 Areas of the face supplied by three divisions of the trigeminal nerve.
VII Facial Nerve
Functional components
Facial nerve has the following functional components (Fig. 9.10):
FIG. 9.10 Functional components and nuclei of the facial nerve. (GSA = general somatic efferent, SVA = somatic visceral afferent, GVE = general visceral efferent, SVE = special visceral efferent.)
• Special visceral efferent (SVE) fibres arise from motor nucleus and supplies the muscles derived from the mesoderm of 2nd pharyngeal arch, viz. muscles of facial expression, etc.
• General visceral efferent (GVE) fibres are preganglionic parasympathetic fibres to the pterygopalatine and submandibular ganglia for lacrimation and salivation respectively. These fibres arise from the lacrimatory and the superior salivatory nuclei respectively.
– The preganglionic fibres arising from lacrimatory nucleus terminate in the pterygopalatine ganglion, from which postganglionic fibres arise and supply the lacrimal gland through zygomatic branch of trigeminal nerve.
– The preganglionic fibres arising from superior salivatory nucleus relay in the submandibular ganglion, from which postganglionic fibres arise and supply the submandibular and sublingual salivary glands.
• Special visceral afferent (SVA) fibres are concerned with the taste sensations. The cell bodies of these fibres lie in the geniculate ganglion. The peripheral processes of ganglion cells carry taste sensations from the taste buds on the anterior two-third of the tongue except vallate papillae. The central processes of ganglion cells carry these sensations to the upper part of the nucleus of tractus solitarius.
• General somatic afferent (GSA) fibres have their cell bodies in the geniculate ganglion. The peripheral processes of these cells innervate part of the skin of the external ear, while the central processes terminate in the spinal nucleus of trigeminal nerve.
Course and distribution (Fig. 9.11)
The facial nerve arises by two roots—motor and sensory on the ventral aspect brainstem from the lower border of the pons opposite the groove between the olive and inferior cerebellar peduncle. The main trunk, i.e. motor root enters the external auditory meatus accompanied by the small sensory root (nervous intermedius of Wrisberg), vestibule cochlear nerve and labyrinthine vessels. At the lateral end of meatus two roots unite to form the trunk of the facial canal where it first runs above the bony labyrinth of internal ear and then bends posteriorly in the medial wall of the middle ear, forming genu of facial nerve. It runs posteriorly in the middle ear medial wall below the lateral semicircular canal. Finally the nerve turns 90° and runs in the posterior wall of the middle ear till it reaches the stylomastoid foramen at the base, through which it leaves the cranial cavity. Finally, it runs anterolaterally to enter the parotid gland, where it divides into five terminal branches. The facial nerve gives off following branches:
FIG. 9.11 The course and distribution of the facial nerve.
• Greater petrosal nerve, which is joined by deep petrosal nerve to form nerve of pterygoid canal. It provides secretomotor supply to lacrimal, nasal and palatal glands.
• Nerve to stapedius.
• Chorda tympani nerve, which joins lingual nerve. It carry taste fibres from anterior two-third of the tongue and provides preganglion fibres to the submandibular ganglion.
• Posterior auricular nerve to supply occipitalis and posterior auricular muscles.
• Nerve to posterior belly of diagastric and stylohyoid muscles.
• Terminal branches (temporal, zygomatic, buccal, mandibular and cervical to the muscles of facial expression.
Clinical Correlation
The part of motor nucleus of facial nerve supplying the muscles of the lower part of the face receives the corticonuclear fibres from the opposite cerebral hemisphere while the part of motor nucleus of facial nerve which supplies the muscles of the upper part of the face (frontalis, orbicularis oculi) receives corticonuclear fibres from both cerebral hemispheres. As a result in supranuclear lesions (i.e. lesions involving the UMNs) of the facial nerve the upper half of the face on both sides is spared and the lower half of the face is affected on the opposite side, on the other hand in nuclear and infranuclear lesions, i.e. lower motor neuron (LMN) lesions whole of the face is affected on the side of lesion (Fig. 9.12).
FIG. 9.12 Effects of UMN (corticonuclear fibres) and LMN (nuclear and infranuclear) lesions of the facial nerve.
N.B. That is why upper face escapes in certain hemiplegias due to the supranuclear lesions.
VIII Vestibulocochlear Nerve
Functional components
This nerve consists of two divisions: cochlear and vestibular. Both these divisions consist of special somatic afferent (SSA) fibres.
The fibres of the cochlear nerve are the central processes of bipolar neurons of the spiral ganglion. The peripheral processes of these cells innervate the ‘organ of Corti’ (hearing receptor) in the cochlea of the inner ear.
The fibres of the vestibular nerve are the central processes of bipolar neurons in the vestibular ganglion. The peripheral processes of these neurons innervate the vestibular receptors in the semicircular ducts (for kinetic balance) and in the saccule and utricle (for static balance) of the inner ear.
Course and distribution
The vestibulocochlear nerve emerges from the lateral aspect of the pontomedullary junction, passes through the cerebellopontine angle to enter the internal acoustic meatus along with facial nerve and labyrinthine vessels, where the cochlear component—the cochlear nerve separates and pierces the fundus of the meatus in the anteroinferior quadrant. Then it runs in the cochlear modiolus where it terminates by supplying the sensory receptor of hearing the spiral organ of corti of membranous labyrinth.
The vestibular component—the vestibular nerve divides into superior and inferior division and singular nerve. They pierce the posterosuperior and posteroinferior quadrants of the fundus to innervate the sensory receptors of the equilibrium—the maculae and cristae ampullaris of membranous labyrinth.
For details, see the auditory and vestibular systems in Chapter 18.
IX Glossopharyngeal Nerve
Functional components
This nerve consists of following functional components (Fig. 9.13).
FIG. 9.13 Functional components and nuclei of the glossopha-ryngeal nerve.
• Special visceral efferent (SVE) fibres arise from nucleus ambiguus and supply only one muscle, the stylopharyngeus (the muscle of third pharyngeal arch).
• General visceral efferent (GVE) fibres arise from inferior salivatory nucleus and relay in the otic ganglion. The postganglionic fibres arising from ganglion supply the parotid gland via auriculotemporal nerve (a branch of mandibular nerve).
• General visceral afferent (GVA) fibres. The cell bodies of these fibres lie in the superior ganglion of the glossopharyngeal nerve. The peripheral processes of these cells carry general sensations (touch, pain and temperature) from posterior one-third of the tongue, pharynx, carotid body and carotid sinus to the ganglion. The central processes carry these sensations to the spinal nucleus of the trigem-inal nerve.
• Special visceral afferent (SVA) fibres have their cell bodies in the inferior ganglion of the glossopharyngeal nerve. The peripheral processes of these cells carry taste sensations from the posterior one-third of the tongue and circumvallate papillae to the ganglion. The central processes convey these impulses to the nucleus of tractus solitarius.
Course and distribution (Fig. 9.14)
The glossopharyngeal nerve arise from the upper part of the lateral aspect of the medulla posterior to the olive, as three of four rootlets. The rootlets soon fuse to form a single nerve trunk which passes anterolaterally to leave the cranial cavity through the anterior compartment of the jugular foramen. The nerve passes two ganglia—a smaller superior and a larger inferior ganglion as it passes through the jugular foramen. The nerve passes downwards and forwards between the internal carotid artery and internal jugular vein. Then it runs inferolaterally looping around the lateral aspect of the stylopharyngeus which it supplies. Now it runs deep to hyoglossus to terminate into lingual branches.
FIG. 9.14 Course and distribution of the glossopharyngeal nerve.
The branches of glossopharyngeal nerve are:
• Tympanic branch (Jacobson's nerve), which supplies sensory fibres via the tympanic tube. Its secretomotor fibres run through lesser petrosal nerve to the otic ganglion when they synapse. The postganglionic fibres from ganglion supplies parotid gland via auriculotemporal nerve.
• Carotid nerve, to carotid sinus and body. It carries sensory fibres from these structures.
• Pharyngeal branch, takes part in the formation of pharyngeal plexus.
• Nerve to stylopharyngeus, provides motor supply to this muscle.
• Tonsillar branches, supply mucous membrane of tonsil, fauces and palate.
• Lingual branches, supply posterior one-third of tongue including vallate papillae and carry taste and general sensations.
Clinical Correlation
Lesions of glossopharyngeal nerve produce following clinical features:
– Loss of gag-reflex, due to interruption of the afferent limb.
– Loss of general sensations in pharynx, tonsils, fauces and posterior one-third of tongue.
– Loss of taste sensations from posterior one-third of the tongue.
– Hypersensitive carotid sinus syndrome (syncope).
X Vagus Nerve
Functional components
The functional components of this nerve are as follows (Fig. 9.15):
FIG. 9.15 Functional components and nuclei of the vagus nerve.
• Special visceral efferent (SVE) fibres arise from nucleus ambiguus and supply the muscles of pharynx and larynx.
• General visceral efferent (GVE) fibres arise from dorsal nucleus of vagus as preganglionic parasympathetic fibres. They supply heart, lungs, GIT up to the junction of right two-third and left one-third of the transverse colon through postganglionic fibres which arise from small ganglia situated close to or within the walls of the viscera.
• General visceral afferent (GVA) fibres. The cell bodies of these fibres are located in the inferior ganglion of the vagus nerve. The peripheral processes of these cells carry sensations from the pharynx, larynx, trachea, oesophagus and from the thoracic and abdominal viscera to the ganglion, from where they are conveyed to the dorsal nucleus of vagus and nucleus of tractus solitarius through central processes.
• Special visceral afferent (SVA) fibres. The cell bodies of these fibres lie in the inferior ganglion of the vagus nerve. The peripheral processes of these cells carry taste sensations from the posteriormost part of the tongue and epiglottis to the ganglion. The central processes of the ganglion cells terminate in the upper part of the nucleus tractus solitarius.
• General somatic afferent (GSA) fibres. The cell bodies of these fibres are located in the superior ganglion of the vagus nerve. The peripheral processes of these neurons innervate the skin of the external ear and central processes terminate in the spinal nucleus of the trigeminal nerve.
Course and distribution (Fig. 9.16)
The vagus nerve arises from the lateral aspect of the medulla as a series of rootlets posterior to the olive between glossopharyngeal and cranial accessory rootlets. The rootlets unite to form a single nerve that leaves the cranial cavity through the intermediate compartment of the jugular foramen. Below the foramen it possesses two ganglia, a smaller superior ganglion and a larger inferior ganglion. The cranial root of accessory joins the vagus nerve just below the inferior jugular vein. The nerve passes vertically downwards within the carotid sheath lying between internal carotid artery and internal jugular vein. At the root of the neck on right side it enters the thorax by crossing in front of the right subclavian artery and on the left side by passing between the left common carotid and left subclavian arteries.
FIG. 9.16 Course and distribution of the vagus nerve.
1. Meningeal branch, to supply the dura mater of the posterior cranial fossa.
2. Sinus nerve, to carotid sinus and body.
3. Auricular branch to supply the exterior of tympanic membrane, the posterior wall of external auditory meatus and cranial surface of the auricle.
Clinical Correlation
Lesions of the vagus nerve produce following clinical features:
– Ipsilateral paralysis of the soft palate leading to sagging of palatal arch. The uvula deviates towards opposite, healthy side.
– Loss of the gag-reflex due to interruption of the efferent limb.
– Unilateral loss of cough-reflex due to anaesthesia of pharynx and larynx.
– Hoarseness of the voice due to unilateral paralysis of laryngeal muscles.
XI Accessory Nerve
Functional components
The functional components of this nerve (Fig. 9.17) as follows:
FIG. 9.17 Functional components and nuclei of the accessory nerve.
• Special visceral efferent (SVE) fibres form the cranial root of the accessory nerve. They arise from nucleus ambiguus and are distributed through the vagus nerve to supply:
– all the muscles of palate except tensor palati which is supplied by the mandibular nerve through nerve to medial pterygoid,
– all the muscles of pharynx except stylopharyngeus which is supplied by the glossopharyngeal nerve, and
– all the muscles of larynx.
• General somatic efferent (GSE) fibres form the spinal root of accessory nerve. They arise from elongated column of cells (spinal nucleus of accessory nerve) whose cell bodies lie in the lateral part of the anterior grey column of the upper five cervical spinal segments. These fibres supply trapezius and sternocleidomastoid muscles.
Course and distribution (Fig. 9.18)
The cranial root emerges from medulla as 4 to 6 rootlets posterior to olive immediately below to these of the vagus nerve. They soon fuse to form a single nerve. It passes out of posterior cranial fossa through the middle compartment of the jugular foramen. Below the foramen it fuses through this nerve.
FIG. 9.18 Course and distribution of the accessory nerve.
The spinal root arises by a series of rootlets from the lateral aspect of the upper five cranial segments, between the dorsal and ventral roots of the spinal nerves. These rootlets unite to form a single nerve which ascends up posterior to ligamenta denticulate and enters the cranial cavity through the foramen magnum behind the vertebral artery. It briefly adheres to the cranial root as it leaves through the jugular foramen. Leave the cranial root descends posterolaterally usually posterior to internal jugular vein, crosses over the lateral mass of the atlas (C1) to enter the deep surface of the sternocleidomastoid. It then emerges from the middle of the posterior border of sternocleidomastoid, crosses the posterior triangle to enter the trapezius about 5 cm above the clavicle where it terminates.
N.B. The motor neurons to sternocleidomastoid and trapezius muscles differentiate in the embryo near the cells that are destined to form the nucleus ambiguus but migrate into the spinal cord (segments C1 to C5) and take up position in the lateral part of the anterior grey horn forming spinal nucleus of accessory nerve, in line with the nucleus ambiguus. Therefore, spinal root is also said to consist of special visceral (branchial) efferent fibres.
Clinical Correlation
The unilateral peripheral lesions of spinal root (spinal accessory nerve) lead to paralysis of sternocleido-mastoid and trapezius muscles.
– The paralysis of sternocleidomastoid will result in turning of the face towards the same side and bending of the head to the opposite side due to unopposed action of opposite healthy muscle.
– The paralysis of trapezius results in drooping of the shoulder and inability to shrug the shoulder towards the side of injury.
XII Hypoglossal Nerve
Functional components (Fig. 9.19)
Hypoglossal nerve consists of general somatic efferent (GSE) fibres which take origin from the hypoglossal nucleus (Fig. 9.19) and supply all the intrinsic and extrinsic muscles of the tongue except the palatoglossus which is supplied by the cranial root of the accessory via vagus nerve.
FIG. 9.19 The functional component and nucleus of the hypo-glossal nerve.
Course and distribution (Fig. 9.20)
The hypoglossal nerve arises from anterolateral aspect of the medulla between olive and pyramid as a series of 10 to 15 rootlets. The rootlets soon fuse to form two roots which enter the hypoglossal canal, where they themselves fuse to form a single nerve and comes out of cranial cavity through this foramen laterally behind the vagus and glos-sopharyngeal nerves, passing forwards between the internal jugular vein and the internal carotid artery. Finally it runs forward superficial to internal and external carotid arteries and loop of lingual artery to reach above the hyoid bone. Here it supplies all the intrinsic and extrinsic muscles of the tongue except palatoglossus.
FIG. 9.20 Course and distribution of the hypoglossal nerve.
It is joined by a communication from the anterior primary ramus of C1spinal nerve, the fibres of which the hypo-glossal nerve distributes to the geniohyoid and thyroid muscles. The descending C1 fibres constitute the superior root of the ansa cervicalis (of the C2 and C3 fibres constitute the inferior root of the ansa cervicalis).
Clinical Correlation
• Upper motor neuron lesions of the hypoglossal nerve
Since the hypoglossal nucleus receives corticonuclear fibres only from the contralateral hemisphere, the supranuclear (UMN) lesions cause weakness of the opposite half of the tongue, and on protrusion, the tongue deviates to the side opposite to that of lesion.
• Lower motor neuron (nuclear and infranuclear) Lesions of the hypoglossal nerve result in paralysis of the ipsilateral half of the tongue and on protrusion the tongue deviates towards the side of lesion due to unopposed action of genioglossus of the healthy side.
The functional components, associated nuclei, distribution and functions of cranial nerves are summarized in Table 9.4.
Table 9.4
Functional components, nuclei, distribution and functions of cranial nerves
SE = somatic efferent, SVE = special visceral efferent, GVE = general visceral efferent, GVA = general visceral afferent, SVA = special visceral afferent, GSA = general somatic afferent, SSA = special somatic afferent.
Cortical Control of Cranial Nerves
All the motor cranial nerve nuclei are under the control of cerebral cortex through the corticonuclear fibres. Each nucleus is controlled by both the cerebral hemispheres with following exceptions:
• Part of hypoglossal nucleus innervating genioglossus is controlled by opposite cerebral cortex only.
• Part of facial nerve nucleus supplying lower part of the face is controlled by opposite cerebral cortex only.
• The trochlear nerve nucleus is controlled by the cerebral cortex of the same side only.
Reflexes Mediated by Cranial Nerves
The important reflexes mediated by cranial nerve/nerves as follows:
Corneal reflex: When cornea is touched with cotton wool, the person blinks due to contraction of palpebral part of orbicularis oculi.
Conjunctional reflex: When conjunctiva is touched with cotton wool the eye is closed rapidly due to contraction of orbicularis oculi.
Lacrimation reflex: Irritation of conjunctiva and cornea results in reflex lacrimation.
Oculocardiac reflex: Pressure on the eyeball slows the heart rate (bradycardia).
Gag reflex: Tickling of the oropharynx either with finger or with swab stick results in reflex contraction of pharyn-geal muscles, causing gagging and retching.
Carotid sinus reflex: Pressure on carotid sinus slows heart rate (bradycardia).
Sneezing reflex: When nasal mucosa is irritated, after a sharp inhalation, explosive exhalation occurs with closure of the oropharyngeal-isthmus by the action of palatoglossus, which diverts air through nasal cavity and thus expels the irritant.
Jaw jerk: If the muscles that closes the jaw (masseter, medial pterygoid and temporalis) are rapidly stretched, the jaw is reflexly closed.
The afferent and efferent limbs of these reflexes are summarized in the Table 9.5.
Table 9.5
Important cranial nerve reflexes
*Jaw-jerk reflex is the only monosynaptic reflex mediated by the cranial nerves.
Clinical Problems
1. A 50-year-old patient complained of double vision. On physical examination, the ophthalmologist found that his right eye, when at rest, was turned medially and when he was asked to turn it laterally, he failed to do so. Identify which cranial nerve is involved and also explain what is double vision and discuss, how it occurs.
2. Explain ‘internal and external ophthalmoplegias’ in relation to the third nerve lesions?
3. Explain, why in upper motor neuron type of facial palsy, the muscles of the upper part of the face are spared.
4. When a physician presses the tongue with a spatula to examine oropharynx, the patient often gags. Give its anatomical basis.
5. In upper motor neuron lesions of the hypoglossal nerve the tongue deviates towards the opposite side while in case of lower motor neuron lesions the tongue deviates towards the side of the lesion. Why?
Clinical Problem Solving
1. The patient is suffering from medial squint (also called medial strabismus) in the right eye due to lesion of right abducent nerve (seepage 99). In double vision (diplopia) the patient sees two images of the same object. The mechanism is as follows:
When the eyes look at an object, the image of an object falls on the macula of the retina of the both eyes. If there is faulty movement of one eye, the image of object will fall on the peripheral part of the retina of that eye. As a result two images will be seen, one true image located on the macula of the normal eye and the second false image seen by the peripheral retina of the abnormal eye.
2. The incomplete lesions of oculomotor nerve are common and may spare either the extraocular muscles or the intraocular muscles.
The oculomotor nerve consists of somatic motor fibres which supply extraocular muscles and parasym-pathetic motor fibres which supply intraocular muscles.
The parasympathetic autonomic fibres are superficially placed within the oculomotor nerve and are likely to be first affected by compression, viz. lateral tentorial herniation, resulting into internal ophthalmoplegia.
In diabetic neuropathy with impaired nerve conduction, the autonomic fibres remain unaffected whereas the somatic motor fibres are affected producing external ophthalmoplegia.
3. See Clinical correlation on page 103.
4. When physician presses the tongue with spatula, the sensory receptors in the mucous membrane of the oropharynx are stimulated producing gag-reflex. The afferent impulses of the reflex run through the glosso-pharyngeal nerve, and the efferent impulses travel through the glossopharyngeal and vagus nerves to the muscles pharynx.
5. See Clinical correlation on page 109.