Cranial nerve

There are 12 pairs of cranial nerves, which leave the brain and pass through foramina and fissures in the skull. All the nerves are distributed in the head and neck, except cranial nerve X, which also supplies structures in the thorax and abdomen. The cranial nerves are named as follows:1. Olfactory2. Optic3. Oculomotor4. Trochlear5. Trigeminal6. Abducent7. Facial8. Vestibulocochlear9. Glossopharyngeal10. Vagus11. Accessory12. Hypoglossal

The olfactory, optic, and vestibulocochlear nerves are entirely sensory.

The oculomotor, trochlear, abducent, accessory, and hypoglossal nerves are entirely motor.

The trigeminal, facial, glossopharyngeal, and vagus nerves are both sensory and motor nerves.

Human vision is binocular, although one sees with both the eyes, the inverted images are seen as one and straight only.

Human vision is stereoscopic, i.e. one sees height, width and thickness of the objects

Human vision is coloured, one sees different colours put up by nature.

When one looks at an object, both eyes are focused on it. Right eye sees a little additional of right side whereas left eye sees a little additional of left side of the object. These visions are monocular visions. Main part is the binocular vision.

Origin of the Optic Nerve

The fibers of the optic nerve are the axons of the cells in the ganglionic layer of the retina.

They converge on the optic disc and exit from the eye, about 3 or 4 mm to the nasal side of its center, as the optic nerve .

The fibers of the optic nerve are myelinated, but the sheaths are formed from oligodendrocytes rather than Schwann cells, since the optic nerve is comparable to a tract within the central nervous system.

The optic nerve leaves the orbital cavity through the optic canal and unites with the optic nerve of the opposite side to form the optic chiasma.

Optic Chiasma

The optic chiasma is situated at the junction of the anterior wall and floor of the third ventricle.

Its anterolateral angles are continuous with the optic nerves, and the posterolateral angles are continuous with the optic tracts .

In the chiasma, the fibers from the nasal (medial) half of each retina, including the nasal half of the macula,1 cross the midline and enter the optic tract of the opposite side, while the fibers from the temporal (lateral) half of each retina, including the temporal half of the macula, pass posteriorly in the optic tract of the same side.

Optic TractThe optic tract emerges from the optic chiasma and passes posterolaterally around the cerebral peduncle. Most of the fibers now terminate by synapsing with nerve cells in the lateral geniculate body, which is a small projection from the posterior part of the thalamus.

A few of the fibers pass to the pretectal nucleus and the superior colliculus of the midbrain and are concerned with light reflexes .

Lateral Geniculate BodyThe lateral geniculate body is a small, oval swelling projecting from the pulvinar of the thalamus. It consists of six layers of cells, on which synapse the axons of the optic tract. The axons of the nerve cells within the geniculate body leave it to form the optic radiation .

Optic RadiationThe fibers of the optic radiation are the axons of the nerve cells of the lateral geniculate body. The tract passes posteriorly through the retrolenticular part of the internal capsule and terminates in the visual cortex (area 17), which occupies the upper and lower lips of the calcarine sulcus on the medial surface of the cerebral hemisphere.The visual association cortex (areas 18 and 19) is responsible for recognition of objects and perception of color.

Neurons of the Visual Pathway and Binocular Vision

Four neurons conduct visual impulses to the visual cortex: (1) rods and cones, which are specialized receptor neurons in the retina; (2) bipolar neurons, which connect the rods and cones to the ganglion cells; (3) ganglion cells, whose axons pass to the lateral geniculate body; and (4) neurons of the lateral geniculate body, whose axons pass to the cerebral cortex.In binocular vision, the right and left fields of vision are projected on portions of both retinae

The image of an object in the right field of vision is projected on the nasal half of the right retina and the temporal half of the left retina.

In the optic chiasma, the axons from these two retinal halves are combined to form the left optic tract. The lateral geniculate body neurons now project the complete right field of vision on the visual cortex of the left hemisphere and the left visual field on the visual cortex of the right hemisphere.

The lower retinal quadrants (upper field of vision) project on the lower wall of the calcarine sulcus, while the upper retinal quadrants (lower field of vision) project on the upper wall of the sulcus.

Note also that the macula lutea is represented on the posterior part of area 17, and the periphery of the retina is represented anteriorly.

How to test optic nerve

Visual Reflexes

Direct and Consensual Light ReflexesIf a light is shone into one eye, the pupils of both eyes normally constrict.

The constriction of the pupil on which the light is shone is called the direct light reflex; the constriction of the opposite pupil, even though no light fell on that eye, is called the consensual light reflex .

Accommodation Reflex

The accommodation reflex is a reflex action of the eye, in response to focusing on a near object, then looking at distant object (and vice versa), comprising coordinated changes in convergence, lens shape and pupil size (accommodation).

It is dependent on cranial nerve II (afferent limb of reflex), superior centres (interneuron) and cranial nerve III (efferent limb of reflex). The change in the shape of the lens is controlled by the ciliary muscles inside the eye.

Changes in contraction of the ciliary muscles alter the focal distance of the eye, causing nearer or farther images to come into focus on the retina; this process is known as accommodation.

The reflex, controlled by the parasympathetic nervous system, involves three responses; pupil accommodation, lens accommodation, and convergence.

Corneal Reflex

Light touching of the cornea or conjunctiva results in blinking of the eyelids. The corneal reflex, also known as the blink reflex, is an involuntary blinking of the eyelids elicited by stimulation of the cornea (such as by touching or by a foreign body), or bright light, though could result from any peripheral stimulus. Stimulation should elicit both a direct and consensual response (response of the opposite eye). The reflex occurs at a rapid rate of 0.1s. The purpose of this reflex is to protect the eyes from foreign bodies and bright lights (the latter known as the optical reflex). The blink reflex also occurs when sounds greater than 40-60 dB are made.

Visual Body Reflexes

The automatic scanning movements of the eyes and head that are made when reading, the automatic movement of the eyes, head, and neck toward the source of the visual stimulus, and the protective closing of the eyes and even the raising of the arm for protection are reflex actions that involve the following reflex arcs .

Pupillary Skin ReflexThe pupil will dilate if the skin is painfully stimulated by pinching. The afferent sensory fibers are believed to have connections with the efferent preganglionic sympathetic neurons in the lateral gray columns of the first and second thoracic segments of the spinal cord. The white rami communicates of these segments pass to the sympathetic trunk, and the preganglionic fibers ascend to the superior cervical sympathetic ganglion. The postganglionic fibers pass through the internal carotid plexus and the long ciliary nerves to the dilator pupillae muscle of the iris.

CN5 Trigeminal Nerves

CN5 Trigeminal NervesFifth cranial nerve is the largest cranial nerve. It comprises three branches, two of which are purely sensory and third, the largest branch is mixed nerve.

The trigeminal nerve is the largest cranial nerve and contains both sensory and motor fibers. It is the sensory nerve to the greater part of the head and the motor nerve to several muscles, including the muscles of mastication

Trigeminal Nerve NucleiThe trigeminal nerve has four nuclei: (1) the main sensory nucleus, (2) the spinal nucleus,(3) the mesencephalic nucleus, (4) the motor nucleus.

Main Sensory NucleusThe main sensory nucleus lies in the posterior part of the pons, lateral to the motor nucleus. It is continuous below with the spinal nucleus.

Spinal NucleusThe spinal nucleus is continuous superiorly with the main sensory nucleus in the pons and extends inferiorly through the whole length of the medulla oblongata and into the upper part of the spinal cord as far as the second cervical segment .

Mesencephalic NucleusThe mesencephalic nucleus is composed of a column of unipolar nerve cells situated in the lateral part of the gray matter around the cerebral aqueduct. It extends inferiorly into the pons as far as the main sensory nucleus

Motor NucleusThe motor nucleus is situated in the pons medial to the main sensory nucleus

Sensory Components of the Trigeminal Nerve

The sensations of pain, temperature, touch, and pressure from the skin of the face and mucous membranes travel along axons whose cell bodies are situated in the semilunar or trigeminal sensory ganglion

Proprioceptive impulses from the muscles of mastication and from the facial and extraocular muscles are carried by fibers in the sensory root of the trigeminal nerve that have bypassed the semilunar or trigeminal ganglion

Course of the Trigeminal Nerve

The trigeminal nerve leaves the anterior aspect of the pons as a small motor root and a large sensory root. The nerve passes forward out of the posterior cranial fossa and rests on the upper surface of the apex of the petrous part of the temporal bone in the middle cranial fossa.

The large sensory root now expands to form the crescent-shaped trigeminal ganglion, which lies within a pouch of dura mater called the trigeminal or Meckel cave.

The ophthalmic, maxillary, and mandibular nerves arise from the anterior border of the ganglion .

The ophthalmic nerve (V1) contains only sensory fibers and leaves the skull through the superior orbital fissure to enter the orbital cavity.

The maxillary nerve (V2) also contains only sensory fibers and leaves the skull through the foramen rotundum. The mandibular nerve (V3) contains both sensory and motor fibers and leaves the skull through the foramen ovale.

The axons of the neurons in the main sensory and spinal nuclei and the central processes of the cells in the mesencephalic nucleus now cross the median plane and ascend as the trigeminal lemniscus to terminate on the nerve cells of the ventral posteromedial nucleus of the thalamus.

The axons of these cells now travel through the internal capsule to the postcentral gyrus (areas 3, 1, and 2) of the cerebral cortex.

Motor Component of the Trigeminal Nerve

The motor nucleus receives corticonuclear fibers from both cerebral hemispheres.

It also receives fibers from the reticular formation, the red nucleus, the tectum, and the medial longitudinal fasciculus. In addition, it receives fibers from the mesencephalic nucleus, thereby forming a monosynaptic reflex arc.

The cells of the motor nucleus give rise to the axons that form the motor root.

The motor nucleus supplies the muscles of mastication, the tensor tympani, the tensor veli palatini, and the mylohyoid and the anterior belly of the digastric muscle.

Course of the Trigeminal Nerve

The trigeminal nerve leaves the anterior aspect of the pons as a small motor root and a large sensory root.

The nerve passes forward out of the posterior cranial fossa and rests on the upper surface of the apex of the petrous part of the temporal bone in the middle cranial fossa.

The large sensory root now expands to form the crescent-shaped trigeminal ganglion, which lies within a pouch of dura mater called the trigeminal or Meckel cave.

The ophthalmic, maxillary, and mandibular nerves arise from the anterior border of the ganglion. The ophthalmic nerve (V1) contains only sensory fibers and leaves the skull through the superior orbital fissure to enter the orbital cavity.

The maxillary nerve (V2) also contains only sensory fibers and leaves the skull through the foramen rotundum. The mandibular nerve (V3) contains both sensory and motor fibers and leaves the skull through the foramen ovale.

Cranial nerve VII

Facial Nerve Nuclei

The facial nerve has three nuclei: (1) the main motor nucleus, (2) the parasympathetic nuclei, and (3) the sensory nucleus.

Main Motor NucleusThe main motor nucleus lies deep in the reticular formation of the lower part of the pons.

The part of the nucleus that supplies the muscles of the upper part of the face receives corticonuclear fibers from both cerebral hemispheres. The part of the nucleus that supplies the muscles of the lower part of the face receives only corticonuclear fibers from the opposite cerebral hemisphere.

Parasympathetic Nuclei

Parasympathetic nuclei lie posterolateral to the main motor nucleus. They are the superior salivatory and lacrimal nuclei . Superior salivatory nucleus receives afferent fibers from the hypothalamus through the descending autonomic pathways.

Information concerning taste also is received from the nucleus of the solitary tract from the mouth cavity.

The lacrimal nucleus receives afferent fibers from the hypothalamus for emotional responses and from the sensory nuclei of the trigeminal nerve for reflex lacrimation secondary to irritation of the cornea or conjunctiva.

Sensory NucleusThe sensory nucleus is the upper part of the nucleus of the tractus solitarius and lies close to the motor nucleus.

Sensations of taste travel through the peripheral axons of nerve cells situated in the geniculate ganglion on the seventh cranial nerve.

The central processes of these cells synapse on nerve cells in the nucleus.

Efferent fibers cross the median plane and ascend to the ventral posterior medial nucleus of the opposite thalamus and to a number of hypothalamic nuclei.

From the thalamus, the axons of the thalamic cells pass through the internal capsule and corona radiata to end in the taste area of the cortex in the lower part of the postcentral gyrus

Course of the Facial NerveThe facial nerve consists of a motor and a sensory root. The fibers of the motor root first travel posteriorly around the medial side of the abducent nucleus .

They then pass around the nucleus beneath the colliculus facialis in the floor of the fourth ventricle and, finally, pass anteriorly to emerge from the brainstem .

The sensory root (nervus intermedius) is formed of the central processes of the unipolar cells of the geniculate ganglion. It also contains the efferent preganglionic parasympathetic fibers from the parasympathetic nuclei.

The two roots of the facial nerve emerge from the anterior surface of the brain between the pons and the medulla oblongata.

They pass laterally in the posterior cranial fossa with the vestibulocochlear nerve and enter the internal acoustic meatus in the petrous part of the temporal bone.

At the bottom of the meatus, the nerve enters the facial canal and runs laterally through the inner ear.

On reaching the medial wall of the tympanic cavity, the nerve expands to form the sensory geniculate ganglion and turns sharply backward above the promontory.

At the posterior wall of the tympanic cavity, the facial nerve turns downward on the medial side of the aditus of the mastoid antrum, descends behind the pyramid, and emerges from the stylomastoid foramen.

Distribution of the Facial Nerve

The motor nucleus supplies the muscles of facial expression, the auricular muscles, the stapedius, the posterior belly of the digastric, and the stylohyoid muscles.

The superior salivatory nucleus supplies the submandibular and sublingual salivary glands and the nasal and palatine glands. The lacrimal nucleus supplies the lacrimal gland.

The sensory nucleus receives taste fibers from the anterior two-thirds of the tongue, the floor of the mouth, and the palate.

Cranial nerve IX

The glossopharyngeal nerve is a motor and a sensory nerve.

Glossopharyngeal Nerve NucleiThe glossopharyngeal nerve has three nuclei: (1) the main motor nucleus, (2) the parasympathetic nucleus, and (3) the sensory nucleus.

Main Motor NucleusThe main motor nucleus lies deep in the reticular formation of the medulla oblongata and is formed by the superior end of the nucleus ambiguus . It receives corticonuclear fibers from both cerebral hemispheres. The efferent fibers supply the stylopharyngeus muscle.

Parasympathetic NucleusThe parasympathetic nucleus is also called the inferior salivatory nucleus. It receives afferent fibers from the hypothalamus through the descending autonomic pathways.

It also is thought to receive information from the olfactory system through the reticular formation. Information concerning taste also is received from the nucleus of the solitary tract from the mouth cavity.

The efferent preganglionic parasympathetic fibers reach the otic ganglion through the tympanic branch of the glossopharyngeal nerve, the tympanic plexus, and the lesser petrosal nerve. The postganglionic fibers pass to the parotid salivary gland.

Sensory NucleusThe sensory nucleus is part of the nucleus of the tractus solitarius .

Sensations of taste travel through the peripheral axons of nerve cells situated in the ganglion on the glossopharyngeal nerve.

The central processes of these cells synapse on nerve cells in the nucleus.

Efferent fibers cross the median plane and ascend to the ventral group of nuclei of the opposite thalamus and a number of hypothalamic nuclei.

From the thalamus, the axons of the thalamic cells pass through the internal capsule and corona radiata to end in the lower part of the postcentral gyrus.

Afferent information that concerns common sensation enters the brainstem through the superior ganglion of the glossopharyngeal nerve but ends in the spinal nucleus of the trigeminal nerve.

Afferent impulses from the carotid sinus, a baroreceptor situated at the bifurcation of the common carotid artery, also travel with the glossopharyngeal nerve.

They terminate in the nucleus of the tractus solitarius and are connected to the dorsal motor nucleus of the vagus nerve.

The carotid sinus reflex that involves the glossopharyngeal and vagus nerves assists in the regulation of arterial blood pressure.

Course of the Glossopharyngeal NerveThe glossopharyngeal nerve leaves the anterolateral surface of the upper part of the medulla oblongata as a series of rootlets in a groove between the olive and the inferior cerebellar peduncle.

It passes laterally in the posterior cranial fossa and leaves the skull through the jugular foramen. The superior and inferior glossopharyngeal sensory ganglia are situated on the nerve here.

The nerve then descends through the upper part of the neck in company with the internal jugular vein and the internal carotid artery to reach the posterior border of the stylopharyngeus muscle, which it supplies.

The nerve then passes forward between the superior and middle constrictor muscles of the pharynx to give sensory branches to the mucous membrane of the pharynx and the posterior third of the tongue.

Cranial nerve X

The vagus nerve is a motor and a sensory nerve.

Vagus Nerve NucleiThe vagus nerve has three nuclei: (1) the main motor nucleus, (2) the parasympathetic nucleus, and (3) the sensory nucleus.

Main Motor NucleusThe main motor nucleus lies deep in the reticular formation of the medulla oblongata and is formed by the nucleus ambiguus. It receives corticonuclear fibers from both cerebral hemispheres.

The efferent fibers supply the constrictor muscles of the pharynx and the intrinsic muscles of the larynx.

Parasympathetic NucleusThe parasympathetic nucleus forms the dorsal nucleus of the vagus and lies beneath the floor of the lower part of the fourth ventricle posterolateral to the hypoglossal nucleus. It receives afferent fibers from the hypothalamus through the descending autonomic pathways. It also receives other afferents, including those from the glossopharyngeal nerve (carotid sinus reflex).

The efferent fibers are distributed to the involuntary muscle of the bronchi, heart, esophagus, stomach, small intestine, and large intestine as far as the distal one-third of the transverse colon.

Sensory NucleusThe sensory nucleus is the lower part of the nucleus of the tractus solitarius.

Sensations of taste travel through the peripheral axons of nerve cells situated in the inferior ganglion on the vagus nerve. The central processes of those cells synapse on nerve cells in the nucleus. Efferent fibers cross the median plane and ascend to the ventral group of nuclei of the opposite thalamus as well as to a number of hypothalamic nuclei.

From the thalamus, the axons of the thalamic cells pass through the internal capsule and corona radiata to end in the postcentral gyrus.

Afferent information concerning common sensation enters the brainstem through the superior ganglion of the vagus nerve but ends in the spinal nucleus of the trigeminal nerve.

Course of the Vagus NerveThe vagus nerve leaves the anterolateral surface of the upper part of the medulla oblongata as a series of rootlets in a groove between the olive and the inferior cerebellar peduncle.

The nerve passes laterally through the posterior cranial fossa and leaves the skull through the jugular foramen.

The vagus nerve possesses two sensory ganglia, a rounded superior ganglion, situated on the nerve within the jugular foramen, and a cylindrical inferior ganglion, which lies on the nerve just below the foramen.

Below the inferior ganglion, the cranial root of the accessory nerve joins the vagus nerve and is distributed mainly in its pharyngeal and recurrent laryngeal branches.

The vagus nerve descends vertically in the neck within the carotid sheath with the internal jugular vein and the internal and common carotid arteries..

The right vagus nerve enters the thorax and passes posterior to the root of the right lung, contributing to the pulmonary plexus.

It then passes on to the posterior surface of the esophagus and contributes to the esophageal plexus. It enters the abdomen through the esophageal opening of the diaphragm.

The posterior vagal trunk (which is the name now given to the right vagus) is distributed to the posterior surface of the stomach and, by a large celiac branch, to the duodenum, liver, kidneys, and small and large intestines as far as the distal third of the transverse colon.

This wide distribution is accomplished through the celiac, superior mesenteric, and renal plexuses.

The left vagus nerve enters the thorax and crosses the left side of the aortic arch and descends behind the root of the left lung, contributing to the pulmonary plexus.

The left vagus then descends on the anterior surface of the esophagus, contributing to the esophageal plexus.

It enters the abdomen through the esophageal opening of the diaphragm. The anterior vagal trunk (which is the name now given to the left vagus) divides into several branches, which are distributed to the stomach, liver, upper part of the duodenum, and head of the pancreas.

Cranial nerve XII

The hypoglossal nerve is a motor nerve that supplies all the intrinsic muscles of the tongue as well as the styloglossus, the hyoglossus, and the genioglossus muscles.

Hypoglossal NucleusThe hypoglossal nucleus is situated close to the midline immediately beneath the floor of the lower part of the fourth ventricle.

It receives corticonuclear fibers from both cerebral hemispheres. However, the cells responsible for supplying the genioglossus muscle only receive corticonuclear fibers from the opposite cerebral hemisphere.

The hypoglossal nerve fibers pass anteriorly through the medulla oblongata and emerge as a series of roots in the groove between the pyramid and the olive

Course of the Hypoglossal NerveThe hypoglossal nerve fibers emerge on the anterior surface of the medulla oblongata between the pyramid and the olive.

The nerve crosses the posterior cranial fossa and leaves the skull through the hypoglossal canal. The nerve passes downward and forward in the neck between the internal carotid artery and the internal jugular vein until it reaches the lower border of the posterior belly of the digastric muscle.

Here, it turns forward and crosses the internal and external carotid arteries and the loop of the lingual artery. It passes deep to the posterior margin of the mylohyoid muscle lying on the lateral surface of the hyoglossus muscle. The nerve then sends branches to the muscles of the tongue.

In the upper part of its course, the hypoglossal nerve is joined by C1 fibers2 from the cervical plexus. Thus, the hypoglossal nerve controls the movements and shape of the tongue.