NeuroScience Week #7 Cranial Nerves

Visual Reflexes

Direct and Consensual Light Reflexes Accommodation Reflex Corneal ReflexVisual Body Reflexes Pupillary Skin Reflex

Visual Body Reflexes

- automatic scanning movements of the eyes and head that are made when reading - automatic movement of the eyes, head, and neck toward the source of the visual stimulus - protective closing of the eyes and even the raising of the arm for protection are reflex actions that involve the following reflex arcs. The visual impulses follow the optic nerves, optic chiasma, and optic tracts to the superior colliculi. Here, the impulses are relayed to the tectospinal and tectobulbar (tectonuclear) tracts and to the neurons of the anterior gray columns of the spinal cord and cranial motor nuclei.

Accommodation Reflex

1) contraction of the medial recti brings about convergence of the ocular axes for distant sight; 2) the lens thickens to increase its refractive power by contraction of the ciliary muscle; 3) and the pupils constrict to restrict the light waves to the thickest central part of the lens. The afferent impulses travel through the optic nerve, the optic chiasma, the optic tract, the lateral geniculate body, and the optic radiation to the visual cortex. The visual cortex is connected to the eye field of the frontal cortex.

Direct and Consensual Light Reflexes I

A small number of fibers leave the optic tract and synapse on nerve cells in the pretectal nucleus, close to the superior colliculus. The impulses are passed by axons of the pretectal nerve cells to the parasympathetic nuclei (Edinger- Westphal nuclei) of the third cranial nerve on both sides. Here, the fibers synapse and the para-sympathetic nerves travel through the third cranial nerve to the ciliary ganglion in the orbit. Finally, postganglionic parasympathetic fibers pass through the short ciliary nerves to the eyeball and the constrictor pupillae muscle of the iris. Both pupils constrict in the consensual light reflex because the pretectal nucleus sends fibers to the para-sympathetic nuclei on both sides of the midbrain

I: Olfactory nerve

A: Distribution of olfactory nerves on the lateral wall of the nose. B: Connections between the olfactory cells and the neurons of the olfactory bulb. C: Connections between the olfactory cell and the rest of the olfactory system. olfactory receptor cells consist of a small bipolar nerve cell with a coarse peripheral process from which short cilia arise, the olfactory hairs, projecting into the mucus covering the surface of the mucous membrane. These projecting hairs react to odors in the air and stimulate the olfactory cells.

Cranial nerve - head structures connection

Although muscular actions in the head and neck are thought of as far removed from the origin and course of the cranial nerves, fetal nerves that supply the branchial arch derivatives (structures derived from each branchial arch) only have a short distance to travel from the brain. The trigeminal (V) nerve supplies the skin covering the parts of the face derived from the first branchial arch maxillary and mandibular divisions (the ophthalmic division does not make a contribution). The facial (VII) nerve supplies the muscles derived from the first arch. The nerve of the third branchial arch is the glossopharyngeal (IX) nerve.The superior laryngeal nerve innervates derivatives of the sixth branchial arch. Two branches of the vagus (X) nerve supply the remaining branchial arches.

Distribution of the facial nerve.

Below: Branches of the facial nerve within the petrous part of the temporal bone; the taste fibers are shown in black. The glossopharyngeal nerve is also shown.

Descending Auditory Pathways

Descending fibers originating in the auditory cortex and in other nuclei in the auditory pathway accompany the ascending pathway. These fibers are bilateral and end on nerve cells at different levels of the auditory pathway and on the hair cells of the organ of Corti. It is believed that these fibers serve as a feedback mechanism and inhibit the reception of sound. They may also have a role in the process of auditory sharpening, suppressing some signals and enhancing others.

Accomodation reflex I

From the eye field of the frontal cortex, cortical fibers descend through the internal capsule to the oculomotor nuclei in the midbrain. The oculomotor nerve travels to the medial recti muscles. Some of the descending cortical fibers synapse with the parasympathetic nuclei (Edinger-Westphal nuclei) of thethird cranial nerve on both sides. Here, the fibers synapse, and the parasympathetic nerves travel through the third cranial nerve to the ciliary ganglion in the orbit. Finally, postganglionic parasympathetic fibers pass through the short ciliary nerves to the ciliary muscle and the constrictor pupillae muscle of the iris (Fig. 11-3).

Hypoglossal Nerve (Cranial Nerve XII)

Hypoglossal Nucleus The 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 Function The hypoglossal nerve 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 thecervical plexus.

Direct and Consensual Light Reflexes

If 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 réflex

Corneal Reflex

Light touching of the cornea or conjunctiva results in blinking of the eyelids. Afferent impulses from the cornea or conjunctiva travel through the ophthalmic division of the trigeminal nerve to the sensory nucleus of the trigeminal nerve. Internuncial neurons connect with the motor nucleus of the facial nerve on both sides through the medial longitudinal fasciculus. The facial nerve and its branches supply the orbicularis oculi muscle, which causes closure of the eyelids.

Olfactory Tract

Narrow band of white matter made of the central axons of the mitral and tufted cells of the bulb and some centrifugal fibers from the opposite olfactory bulb. As the olfactory tract divides into medial and lateral olfactory striae.The lateral stria carries the axons to the olfactory area of the cerebral cortex, namely, the periamygdaloid and pre-piriform areas known as the primary olfactory cortex. The entorhinal area (area 28) of the parahippocampal gyrus, which receives numerous connections from the primary olfactory cortex, is called the secondary olfactory cortex. These areas of the cortex are responsible for the appreciation of olfactory sensations. *Note that in contrast to all other sensory pathways, the olfactory afferent pathway has only two neurons and reaches the cerebral cortex without synapsing in one of the thalamic nuclei.* The primary olfactory cortex sends nerve fibers to many other centers within the brain to establish connections for emotional and autonomic responses to olfactory sensations.

Sensory nerve supply to the skin of the head and neck

Notice that the skin over the angle of the jaw is supplied by the great auricular nerve (C2 and C3) and not by branches of the trigeminal nerve.

Optic Nerve (Cranial Nerve II)

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. The fibers of the optic nerve are myelinated, but the sheaths are formed from oligodendrocytes rather than Schwann cells. 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.

Parasympathetic Nuclei

Parasympathetic nuclei lie posterolateral to the main motor nucleus. They are the superior salivatory and lacrimal nuclei. The 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.

Modalities

Simplistically, each cranial nerve can be described as being sensory, motor or both. They can more specifically transmit seven types of information; three are unique to cranial nerves (SSA, SVA and SVE). See table 1 for a summary of the cranial nerves, their modalities and functions.

Abducent Nerve (Cranial Nerve VI)

The abducent nerve is a small motor nerve that supplies the lateral rectus muscle of the eyeball.

Accessory Nerve (Cranial Nerve XI)

The accessory nerve is a motor nerve that is formed by the union of a cranial and a spinal root. The cranial root joins the vagus nerve and is distributed in its pharyngeal and recurrent laryngeal branches to the muscles of the soft palate, pharynx, and larynx. The spinal root (part) is formed from axons of nerve cells in the spinal nucleus, which is situated in the anterior gray column of the spinal cord in the upper five

Cochlear Nuclei

The anterior and posterior cochlear nuclei are situated on the surface of the inferior cerebellar peduncle. They receive afferent fibers from the cochlea through the cochlear nerve. The cochlear nuclei send axons (second-order neuron fibers) that run medially through the pons to end in the trapezoid body and the olivary nucleus. Here, they are relayed in the posterior nucleus of the trapezoid body and the superior olivary nucleus on the same or the opposite side. The axons now ascend through the posterior part of the pons and midbrain and form a tract known as the lateral lemniscus. Each lateral lemniscus, therefore, consists of third-order neurons from both sides. As these fibers ascend, some of them relay in small groups of nerve cells, collectively known as the nucleus of the lateral lemniscus. On reaching the midbrain, the fibers of the lateral lemniscus either terminate in the nucleus of the inferior colliculus or are relayed in the medial geniculate body and pass to the auditory cortex of the cerebral hemisphere through the acoustic radiation of the internal capsule. The primary auditory cortex (areas 41 and 42) includes the gyrus of Heschl on the upper surface of the superior temporal gyrus. The recognition and interpretation of sounds on the basis of past experience take place in the secondary auditory area. Nerve impulses from the ear are transmitted along auditory pathways on both sides of the brainstem, with more being projected along the contralateral pathway. Many collateral branches are given off to the reticular activating system of the brainstem. The tonotopic

Cochlear Nerve

The cochlear nerve conducts nerve impulses concerned with sound from the organ of Corti in the cochlea. The fibers of the cochlear nerve are the central processes of nerve cells located in the spiral ganglion of the cochlea. They enter the anterior surface of the brainstem at the lower border of the pons on the lateral side of the emerging facial nerve and are separated from it by the vestibular nerve. On entering the pons, the nerve fibers divide, with one branch entering the posterior cochlear nucleus and the other branch entering the anterior cochlear nucleus.

Numbering order

The cranial nerves are numbered by their location on the brain stem (superior to inferior, then medial to lateral) and the order of their exit from the cranium (anterior to posterior)

Optic Radiation

The 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.

Olfactory Nerves (Cranial Nerve I)

The fine central processes form the olfactory nerve fibers. Bundles of these nerve fibers pass through the openings of the cribriform plate of the ethmoid bone to enter the olfactory bulb.

Glossopharyngeal Nerve (Cranial Nerve IX)

The glossopharyngeal nerve is a motor and a sensory nerve. Glossopharyngeal Nerve Nuclei The glossopharyngeal nerve has three nuclei: (1) the main motor nucleus (2) the parasympathetic nucleus (3) the sensory nucleus. Main Motor Nucleus The main motor nucleus lies deep in the reticular formation of the medulla oblongataand is formed by the superior end of the nucleus ambiguus. It receivescorticonuclear fibers from both cerebral hemispheres. The efferent fibers supply the stylopharyngeus muscle. Parasympathetic Nucleus The 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

Distribution of the trigeminal nerve

The large sensory root 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.

Lateral Geniculate Body

The 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.

left vagus nerve

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.

Main Motor Nucleus

The 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. These pathways explain the voluntary control of facial muscles. However, another involuntary pathway exists; it is separate and controls mimetic or emotional changes in facial expression. This other pathway forms part of the reticular formation.

Main Motor Nucleus of the Vagus

The 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.

mandibular nerve (V3)

The mandibular nerve (V3) contains both sensory and motor fibers and leaves the skull through the foramen ovale. The sensory fibers to the skin of the face from each division supply a distinct zone, with little overlap of the dermatomes.

maxillary nerve (V2)

The maxillary nerve (V2) also contains only sensory fibers and leaves the skull through the foramen rotundum.

Oculomotor Nerve (Cranial Nerve III)

The oculomotor nerve is entirely motor in function. The oculomotor nerve supplies the following extrinsic muscles of the eye: - the levator palpebrae superioris, superior rectus - medial rectus, inferior rectus - inferior oblique. It also supplies parasympathetic nerve fibers to the following intrinsic muscles: - the constrictor pupillae of the iris - ciliary muscles. the oculomotor nerve is entirely motor and is responsible for: - lifting the upper eyelid - turning the eye upward, downward, and medially - constricting the pupil - accommodating the eye

Origin of the Cranial Nerves

The olfactory nerve (CN I) and optic nerve (CN II) originate from the cerebrum. Cranial nerves III - XII arise from the brain stem. They can arise from a specific part of the brain stem (midbrain, pons or medulla), or from a junction between two parts: •Midbrain - the trochlear nerve (IV) comes from the posterior side of the midbrain. It has the longest intracranial length of all the cranial nerves. •Midbrain-pontine junction - oculomotor (III).•Pons - trigeminal (V).•Pontine-medulla junction - abducens, facial, vestibulocochlear (VI-VIII). •Medulla oblongata - posterior to the olive: glossopharyngeal, vagus, accessory (IX-XI). Anterior to the olive: hypoglossal (XII).

Ophthalmic nerve (V1)

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

Parasympathetic Nucleus of the Vagus

The parasympathetic nucleus forms the dorsal nucleus of the vagus and lies beneath the floor of the lower part of the fourth ventricle postero-lateral 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.

Pupillary Skin Reflex (Ciliospinal reflex)

The 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 communicantes 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.

Sensory Nucleus of the glossopharyngeal

The 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 Afferent impulses from the carotid sinus, a baroreceptor situated at the bifurcation of the common carotid brainstem through the artery, also travel with the glossopharyngeal nerve. They terminate in the nucleus of the tractus solitarius superior ganglion of the glossopharyngeal nerve but ends aindthaerespcionnanlencutceldeutos tohfethdeortsraiglemoitnoarlnuecrlveeu.s of the vagus nerve. The carotid sinus reflex that involves the glossopharyngeal and vagus nerves assists in the regulation of arterial blood pressure.

Sensory Nucleus of the Vagus

The 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.

Sensory Nucleus

The 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

Abducent Nerve Nucleus

The small motor nucleus is situated beneath the floor of the upper part of the fourth ventricle and receives afferent corticonuclear fibers from the tectobulbar tract from the superior colliculus, and from the medial longitudinal fasciculus, by which it is connected to the nuclei of the third, fourth, and eighth cranial nerves. The abducent nerve is entirely a motor nerve and supplies the lateral rectus muscle and, therefore, is responsible for turning the eye laterally.

Trigeminal Nerve Nuclei

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

Trochlear Nerve (Cranial Nerve IV)

The trochlear nerve is entirely motor in function.

Trochlear Nerve Nucleus

The trochlear nucleus receives corticonuclear fibers from both cerebral hemispheres. It receives the tectobulbar fibers, which connect it to the visual cortex through the superior colliculus (Fig. 11-6). It also receives fibers from the medial longitudinal fasciculus, by which it is connected to the nuclei of the third, sixth, and eighth cranial nerves.

Pathways of the Vagi nerves

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.

Vagus Nerve (Cranial Nerve X)

The vagus nerve is a motor and a sensory nerve. Vagus Nerve Nuclei The vagus nerve has three nuclei: (1)the main motor nucleus (2)the parasympathetic nucleus (3)the sensory nucleus.

Vestibulocochlear Nerve (Cranial Nerve VIII)

This nerve consists of two distinct parts, the vestibular nerve and the cochlear nerve, which are concerned with the transmission of afferent information from the internal ear to the central nervous system. Vestibular Nerve The vestibular nerve conducts nerve impulses from the utricle and saccule that provide information concerning the position of the head; the nerve also conducts impulses fromthe semicircular canals that provide information concerning movements of the head. The nerve fibers of the vestibular nerve are the central processes of nerve cells locatedin the vestibular ganglion, which is situated in the internal acoustic meatus. They enter the anterior surface of the brainstem in a groove between the lower border of thepons and the upper part of the medulla oblongata. When they enter the vestibular nuclear complex, the fibers divide into short ascending and long descending fibers; a small number of fibers pass directly to the cerebellum through the inferior cerebellar peduncle, bypassing the vestibular nuclei.

Olfactory Bulb

This ovoid structure possesses several types of nerve cells, the largest of which is the mitral cell. The incoming olfactory nerve fibers synapse with the dendrites of the mitral cells and form rounded areas known as synaptic glomeruli. Smaller nerve cells, called tufted cells and granular cells, also synapse with the mitral cells. The olfactory bulb, in addition, receives axons from the contralateral olfactory bulb through the olfactory tract.

trigeminal nerve

Types of fibers: •special visceral efferent (SVE, branchial motor) •general somatic afferent (GSA) •location of neurons: •SVE (branchial motor): trigeminal motor nucleus •GSA:•trigeminal ganglion: contains sensory neurons that transmit touch, pressure, pain, and temperature information•mesencephalic trigeminal nucleus: contains sensory neurons that transmit proprioceptive information (only sensory neurons located within CNS)

Branchial Arches: transient embryonal structures during development

branchial apparatus = four branchial arches visible on the surface of the embryo, plus a fifth and sixth arches that cannot be seen on the surface. Branchial pouches and clefts are numbered craniocaudally. The first branchial arch (Meckel's) cartilage is the position of the future mandible, as well as the eventual malleus and incus. The second branchial arch cartilage produces the stapes, the styloid process, the stylohyoid ligament, and the superior portion of the body of the hyoid. The other branchial arch cartilages contribute to the inferior portion of the hyoid as well as the thyroid cartilage. Myoblasts differentiate and migrate to various parts of the head and neck, where they form the muscles of mastication and facial expression, each retaining their original nerve supply.

Facial Nerve (Cranial Nerve VII)

controls the surface muscles involved with facial expression.

Optic pathway

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.

Motor (efferent) Modalities:

•General somatic motor (GSE) - skeletal muscles. •General visceral motor (GVE) - smooth muscles of gut and autonomic motor. •Special visceral motor (SVE) - muscles derived from pharyngeal arches.

Sensory (afferent) Modalities:

•General somatic sensory (GSA) - general sensation from skin. •General visceral sensory (GVA) - general sensation from viscera. •Special somatic sensory (SSA) - senses derived from ectoderm (e.g. sight, sound, balance). •Special visceral sensory (SVA) - senses derived from endoderm (e.g. taste, smell).