Lecture 6: CRANIAL NERVES:PERIPHERAL DISTRIBUTION

Question 1

SPINAL NERVES

Answer 1

Spinal nerves arise from the spinal cord and innervate almost everything below the head

Question 2

CRANIAL NERVES

Answer 2

Cranial nerves arise from the brainstem and innervate structures related to the head and neck.

On exception: One cranial nerve innervates thoracic and abdominal viscera. It is called the vagus nerve (L: wanderer).

Question 3

11 of the 12 cranial nerves are visible in a ventral view of the brainstem.

Answer 3

They are indicated by Roman numerals II – XII.

Question 4

OVERVIEW OF FUNCTIONAL COMPONENTS OF CRANIAL NERVES

Answer 4

As in spinal nerves, cranial nerves may contain axons that carry motor and/or sensory information. As for spinal nerves, motor information is subdivided into axons that innervate skeletal or smooth muscle (somatic vs. autonomic systems).

There are a few additional components in cranial nerves.

There is a unique group of axons that carry sensory information from receptors that are only found in the head such as those relaying the sensations of taste, vision, hearing, or balance. These are Special Sensory Afferents.

One additional component is related to innervation of a subset of skeletal muscles that have a unique embryological origin and are called Special Visceral Efferents.

Cranial nerves may contain only a single component, or multiple components.

Any nerve that has either a general or special sensory component has a sensory ganglia located in the periphery related to it. These are analagous to the DRGs in spinal nerves.

The VS designation for cranial nerves IX and X refers to a special visceral sensory component carried by these nerves..

Question 5

Somatic motor

Answer 5

Innervate skeletal muscle.

III, IV, VI, XII

Oculomotor, Trochlear, Abducens, Hypoglossal

Question 6

BRANCHIAL MOTOR (SVE)

Answer 6

Innervate skeletal muscle with unique embryological origin. These are muscles involved in
movement of the facial muscles, jaws, pharynx and larynx.

V, VII, IX, X, XI

Trigeminal, Facial, Glossopharyngeal, Vagus, Accessory

Question 7

VISCERAL MOTOR

Answer 7

Innervate glands, smooth muscle in organs (parasympathetic portion of autonomic nervous system).

III, VII, IX, X

Oculomotor, Facial, Glossopharyngeal, Vagus

Question 8

GENERAL SENSORY

Answer 8

Carries general sensations from target area (e.g., touch, pressure, pain).

V, VII, IX, X

Trigeminal, Facial, Glossopharyngeal, Vagus

Question 9

Special Sensory

Answer 9

Sensations unique to the head (e.g., taste, hearing, vision, olfaction, balance).

I, II, VII, VIII, IX, X

Olfactory, Optic, Facial, Vestibulocochlear, Glossopharyngeal, Vagus

Question 10

If a cranial nerve has a motor component….

Answer 10

If a cranial nerve has a motor component (somatic or visceral) the neuronal cell body will be in a NUCLEUS located INSIDE the CNS (i.e., brain).

Question 11

If a cranial nerve has a sensory component…..

Answer 11

If a cranial nerve has a sensory component, the neuronal cell body will be located in a ganglia located outside the CNS. These are analogous to dorsal root ganglia.

Question 12

THERE ARE TWO TYPES OF GANGLIA IN THE HEAD –

Answer 12

SENSORY -These contain cell bodies that convey general and special sensory information. General rule: Any neuron conveying sensory information from the body or head will be located in a peripheral ganglion.

AUTONOMIC - Those containing postganglionic parasympathetic (visceral motor) neurons. These are part of the autonomic nervous system which controls smooth muscle and glands in the head.

Question 13

OLFACTORY

Answer 13

CN I

Special Sensory – Olfactory Information

Sensory “Ganglion” In Epithelium In Upper Part Nasal Cavity Contains Sensory Neurons.

Cranial nerve I is the olfactory nerve. It has one functional component which is special sensory or SSA. It’s function is to detect odors and to convert them to electrical signal that are interpreted by the brain.

Since this is a sensory nerve, there must be a ganglion located in the periphery that detects odors coming into the nasal cavity. In the case of the olfactory nerve, there is not a defined ganglion. Rather sensory neurons are located in the roof of the nasal cavity in the olfactory epithelium. Odorants entering the nose reach receptors on these sensory neurons leading to depolarization or excitation of the cells. This signal is sent through small openings in the roof of the nasal cavity called the cribiform plate where they enter a large structure within the skull called the olfactory bulb. The primary sensory neurons synapse on neurons in the olfactory bulb which then relay the information to olfactory related areas of the cerebral cortex.

Question 14

OLFACTORY BULB AND TRACT

Answer 14

Odorants entering the nose reach receptors on these sensory neurons leading to depolarization or excitation of the cells. This signal is sent through small openings in the roof of the nasal cavity called the cribiform plate where they enter a large structure within the skull called the olfactory bulb. The primary sensory neurons synapse on neurons in the olfactory bulb which then relay the information to olfactory related areas of the cerebral cortex.

Question 15

OPTIC NERVE

Answer 15

Cranial nerve II

Special sensory: visual information

Sensory cell bodies are in the retina in the ganglion cell layer

The optic nerve has only one functional component which special sensory for transmission of visual information. The sensory cell bodies are located within the retina in the ganglion cell layer. The axons of these neurons leave the eye via the optic nerve which leaves through the posterior part of the orbit. Within the skull, axons arising from ganglion neurons on the nasal side of the retina cross the midline to form the optic chiasm. These contralateral axons, together with ipsilateral axons arising from retinal ganglion cells on the temporal side of the retina unite to form the optic tract which carries the visual signal into the CNS for further processing.

Question 16

OCULOMOTOR NERVE

Answer 16

Cranial nerve III

Somatic Motor and Visceral Motor components

Neurons giving rise to these axons are located in nuclei within the brainstem.

Somatic Motor Component: innervates all but 2 of the muscles that allow you to voluntarily move your eye.

Cranial nerve III has 2 motor components, namely somatic motor or GSE and visceral motor of GVE.

Consider the somatic motor component of cranial nerve III. Cranial nerve III arises from the midbrain and enters the orbit through a slit called the Superior Orbital Fissure.

You do not need to know the names of these muscles with the exception of the medial rectus m. !!

On entering the eye socket, it divides into multiple branches that innervate all but of 2 of the 7 muscles that allow us to move our eyes or elevate our eye lids. These muscles are shown in yellow and include the Levator palpebrae (which elevates the eyelid, the superior rectus (which moves the eye straight upward), the medial rectus (which moves the eye toward the nose; think crossing your eyes), the Inferior rectus (which moves the eye straight downward), and the Inferior Oblique, which moves the eye down and inward). Arising from the nerve is a small branch that carries axons into a small ganglion located just lateral to the optic nerve.

Question 17

OCULOMOTOR NERVE

Answer 17

Cranial nerve III

Somatic Motor and Visceral Motor components

Neurons giving rise to these axons are located in nuclei within the brainstem.

Somatic Motor Component: innervates all but 2 of the muscles that allow you to voluntarily move your eye.

Cranial nerve III has 2 motor components, namely somatic motor or GSE and visceral motor of GVE.

Consider the somatic motor component of cranial nerve III. Cranial nerve III arises from the midbrain and enters the orbit through a slit called the Superior Orbital Fissure.

You do not need to know the names of these muscles with the exception of the medial rectus m. !!

On entering the eye socket, it divides into multiple branches that innervate all but of 2 of the 7 muscles that move or elevate our eyes and eye lids. These muscles include the Levator palpebrae (which elevates the eyelid, the superior rectus (which moves the eye straight upward), the Medial Rectus (which moves the eye toward the nose; think crossing your eyes), the Inferior rectus (which moves the eye straight downward), and the Inferior Oblique, which moves the eye down and inward). Arising from the nerve is a small branch that carries axons into a small ganglion located just lateral to the optic nerve.

Visceral Motor (Parasympathetic) Component.

Postganglionic Parasympathetic Fibers Innervate:

• Constrictor (Sphincter) Pupilae M.
• Ciliary Muscle (Accommodation)

The second motor component of cranial nerve III are axons that innervate smooth muscles in the eye, muscles that we do not voluntarily control. These are controlled by the ANS. Be aware that the GVE axons in cranial nerve III are part of the parasympathetic division of the ANS.

The axons that innervate these smooth muscle structures accompany the GSE axons as they enter the bony orbit. Within the orbit they leave the main nerve and enter a small ganglion located on the lateral side of the orbit called the ciliary ganglion where they synapse on neurons located within the structure. These axons are called preganglionic .

This is not a sensory ganglion. Rather this ganglion contains motor neurons that will innervate smooth muscle structures within the eye. The axons of the neurons in the ganglion leave the structure and enter the eyeball*** to innervate a muscle that constricts the iris (constrictor pupilae M.) and a muscle that adjusts the shape of the lens to adapt to viewing objects that are near vs. far. (ciliary m.).

Innervation of these smooth muscles, requires 2 neurons - a pregagnlionic located in the CNS that exits with cranial nerve III to synapse on neuron in the ciliary ganglion. This second neuron within the ganglion gives rise to a postganglionic axon that innervates the target muscles.

Cranial nerve III has axons that arise from distinct populations of neurons that either innervate skeletal or smooth muscle (i.e., they are either part of the somatic or ANS.

Question 18

TROCHLEAR NERVE

Answer 18

Cranial nerve IV

Somatic Motor: innervates one muscle that moves eye, the superior oblique m.

This Is The Only Cranial Nerve That Arises from the Dorsal Aspect of the Brainstem and the Only One That Is Completely Crossed.

The trochlear nerve has only one component: somatic motor. Cranial nerve IV has some unique characteristics. First it is the only cranial nerve that arises from the dorsal surface of the brainstem.

It is the only cranial nerve that crosses and projects contralaterally. This nerve enters the orbit and innervates a single muscle, the superior oblique. The tendon of this small muscle hooks around a bony projection called the trochlea and attaches to the posterior and lateral aspect of the eye. This muscle rotates the eye downward and laterally.

Question 19

ABDUCENS NERVE

Answer 19

cranial nerve VI

Somatic Motor: innervates one muscle that moves eye = lateral rectus m.

The abducens nerve is also involved in controlling movements of the eye. It has a single functional component which is somatic motor. It leaves the brainstem at the junction of the pons and medulla. On entering the orbit, it innervates a single muscle, the lateral rectus. This muscle rotates the eye laterally.

Question 20

3 Cranial nerves control eye movements:

Answer 20

III – oculomotor,
IV – trochlear
VI – abducens

One, CN – III, also innervates smooth muscle in the eye to regulate pupillary diameter and accommodation.

Question 21

TRIGEMINAL NERVE

Answer 21

Cranial nerve V

General Sensory and Branchial Motor (Only V3)

PRIMARY GENERAL SENSORY NERVE to the face and skull. Carries both touch and pain information.

Sensory Ganglion: Trigeminal (Semilunar) Ganglion

The trigeminal nerve is the primary sensory nerve to the face carrying all general sensations such as touch, pressure, pain, temperature, etc. from the face.

The ganglion containing the sensory cell bodies is called the trigeminal or the semilunar ganglion. It is located in the middle cranial fossa.

There is one exception. Sensory axons that carry sensory information from spindles in muscles that close the mandible (muscles of mastication) have their cell bodies within the pons itself.
This is a rare exception to the rule that all sensory cell bodies are located outside the CNS.

The trigeminal nerve also has a small motor component that innervates the muscles that close the jaw during mastication. These muscles are derived from a branchial arch and therefore the axons are designated Special Visceral Efferents.

As it’s name suggests, the trigeminal nerve has 3 divisions each of which innervates specific regions of the face. ***V1 or the opthalmic N which supplies the forehead and the area of the face around the orbits.

• **V2 is the maxillary nerve and it supplies the area of the face between the bottom of the orbit and the zygomatic arch. Divisions V1 and V2 only contain sensory axons.
• **V3 is the mandibular nerve. This division of the trigeminal nerve carries sensory information from the region around the mandible, including the teeth in the jaw as well as motor fibers that innervate the muscles of mastication.

Question 22

Trigeminal Ganglion

Answer 22

V1: Sensory innervation of orbit including eyeball. Touch, pressure, pain. Also area around orbit externally including forehead.

V2: Sensory innervation of area around zygomatic arch, nasal cavity and upper teeth.

V3: Sensory innervation area around mandible and chin, including lower teeth and tongue.

Immediately posterior to the orbit on both sides, is the trigeminal ganglion. This large structure contains sensory neurons that send axons into the 3 divisions of the trigeminal nerve. Some ganglion neurons send their axons into V1. These axons enter the orbit above the muscles that elevate the eyelid or move the eye. They carry the sensory information from the orbit as well as the eye itself. Think of touching the eye with your finger or putting in a contact lens. Also, putting drops in your eyes. The touch and cool sensations are carried to the CNS via branches of V1. Other sensory neurons in the ganglion send their axons into V2. These axons leave the skull through a separate opening and innervate structures such as the upper teeth, the area of the face over the zygomatic arch, the nasal cavity, and the hard and soft palates. Other neurons send their axons into V3 which leaves through another opening. This branch innervates the teeth in the mandible as well as the skin over the cheek, the tongue (general sensory only) and the chin. This branch of the trigeminal nerve also contains axons that innervate muscles of mastication. The neurons giving rise to these axons are located within the brainstem. They will be shown on a subsequent slide. In addition to these peripheral processes, the ganglion cells give rise to a central process that enters the CNS carrying sensory information from different regions of the face.

Question 23

V3: MANDIBULAR NERVE – Branchial motor branches to muscles of mastication.

Answer 23

Neuronal cell body for these motor axons will be within the brainstem.

Axons pass through the trigeminal ganglion but do not synapse on any neurons within it.

The trigeminal ganglion contains the neurons that carry sensory information from the face. The cell bodies that give rise to the motor axons are located within the CNS. The axons of these motor neurons leave the brainstem in the trigeminal nerve, their axons pass through the trigeminal ganglion, but they do not for synaptic contacts. They enter the V3 branch and continue to the periphery where they innervate muscles of mastication, that is muscles that actively close the jaw. In addition, a few small muscles that control the palate (tensor veli palatani), the ear drum (tensor tympani), and 2 muscles that open the jaw (mylohyoid and the anterior belly of the digastric) are also innervated by motor axons in the trigeminal nerve.

Question 24

4 Cranial nerves arise from the rostral medulla:

Answer 24

VII – Facial
VIII – Vestibulocochlear
IX – Glossopharyngeal
X – Vagus

Question 25

FACIAL nerve

Answer 25

Cranial nerve VII

BRANCHIAL MOTOR: Motor to Muscles of facial expression

VISCERAL MOTOR: Parasympathetic to lacrimal gland; Parasympathetic to salivary glands

SPECIAL SENSORY: Special Sensory – taste anterior 2/3 of tongue

GENERAL SENSORY: General sensory to skin around ear

Cell Bodies of Origin for both sensory components are in Geniculate Ganglion

The facial nerve contains 4 functional components including branchial motor to muscles of facial expression. Remember, these are voluntary skeletal muscles but they are derived from branchial arches rather than the mesoderm. There are also visceral motor axons that innervate salivary glands and the lacrimal gland. Cranial nerve VII also carries special sensory information in the form of taste from the anterior 2/3 of the tongue. Finally, the smallest component are the General Sensory Afferents which carry general sensory information from the area just posterior to the ear.

Question 26

Cell Bodies of Origin for both sensory components of the facial nerve are in Geniculate Ganglion

Answer 26

The cell bodies of origin for the sensory branches of the Facial nerve are located in the Geniculate Ganglion which is located outside the CNS. Neurons in the geniculate ganglion give rise to peripheral processes that carry both general and special sensory information. The general sensory neurons carry sensory information from the skin behind the ear and is a minor component of the nerve. The special sensory axons carry taste information from the anterior part of the tongue. These special sensory afferents form a branch called the chorda tympani as they course through the middle ear, emerging through a small opening on the skull. They then continue to the tongue. This branch passes just medial to the tympanic membrane. The central proccesses of the neurons in the geniculate ganglion enter the brain.

Question 27

FACIAL- BRANCHIAL MOTOR and VISCERAL MOTOR

Answer 27

The cell bodies of origin for the Branchial Motor and Visceral Motor branches of the facial nerve are located within the CNS. Their axons leave the brain and enter the area of the skull occupied by the inner ear via the internal auditory meatus. The axons of the branchial motor neurons bypass the geniculate ganglion which contains the cell bodies of sensory neurons related to VII, and leave the skull via small opening at the base of the ear lobe to supply muscles of facial expression. The axons of visceral motor cell bodies that innervate salivary and lacrimal glands also leave the brainstem and enter the area of the skull occupied by the inner ear. Some axons leave the main nerve and course to the lacrimal gland as well as nasal and palatal glands. Others join the branch of VII that carries taste information from the tongue and course toward the tongue. These axons are part of the ANS and are called preganglionic parasympathetic axons. There is another ganglion located adjacent to the tongue where these axons synapse. Axons arising from neurons in this ganglion innervate the salivary glands and are called postganglionic axons.

Question 28

FACIAL NERVE - CHORDA TYMPANI - REVIEW

Answer 28

The branch of the facial nerve that carries the taste fibers and the preganglionic parasympathetic axons is called chorda tympani because it courses through the middle ear. The cell bodies that carry taste information are located within the geniculate ganglion. Their axons innervate taste buds on the anterior 2/3 of the tongue. The cell bodies that innervate salivary glands are located within the CNS. Their axons course with those carrying taste information. These axons are called preganglionic parasympathetic axons. They synapse on neurons in the submandibular ganglion located at the base of the tongue. This is an autonomic ganglion. Neurons in the submandibular ganglion send their axons to the submandibular and sublingual salivary glands. These are called postganglionic axons. These are part of the ANS.

Question 29

vestibulocochlear nerve

Answer 29

Cranial nerve VIII

SPECIAL SENSORY Ganglia:

• Cochlear (Spiral) Ganglion In Cochlea
• Vestibular (Scarpa’s) Ganglion - Semicircular Canals And Otolith Organs

The vestibulocochlear nerve has one component which is special sensory. However, this special sensory function is divided into 2 different modalities. One is auditory information, the other is vestibular or balance. The cell bodies for these sensory functions are located in peripheral ganglia within the inner ear. For auditory information, the cell bodies are located within the cochlear or spiral ganglion. The cell bodies of neurons carrying vestibular information are related to the peripheral organs that perceive balance. The ganglia is the vestibular or Scarpa’s ganglion. The axons arising from these neurons leave the inner ear and enter the brainstem at the junction of the pons and medulla.

Question 30

GLOSSOPHARYNGEAL

Answer 30

Cranial nerve IX

Like the facial nerve, the glossopharyngeal nerve has multiple components including branchial motor to a single muscle that elevates the pharynx during swallowing called the stylopharyngeus muscle. The largest component of the glossopharyngeal nerve is its general and special sensory components. The cell bodies for this component are located in peripheral ganglia located in the area of the jugular foramen. These are called the superior and inferior ganglia.

BRANCHIAL MOTOR, VISCERAL MOTOR,
GENERAL SENSORY, SPECIAL SENSORY

Question 31

IX – GLOSSOPHARYNGEAL N. – General and Special Sensory Components

Answer 31

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