Cranial Nerve Nuclei

Cranial Nerve Nuclei





A. Development:

The cranial nerve nuclei (along with other neurons of the brainstem) form in the periventricular zone within medial (motor) and lateral (sensory) regions in the developing brain.

Legend: Red: motor neurons (efferents) Green: sensory relay neurons (afferents)

The motor and sensory nuclei are further differentiated for the special muscles and senses of the head.

Legend of nuclear groups: (Motor nuclei are described first, in a medial to lateral direction, then the sensory nuclei continuing laterally.) <BLOCKQUOTE> Red: Somatic motor nuclei (or GSE) efferents (large motor neurons), which lie medially. They innervate striate muscles (of the tongue and eyes). Orange: Branchiomeric motor (or SVE) nuclei which initially develop in the periventricular gray lateral to the somatic motor nuclei and then migrate along a radial column to occupy a ventrolateral position in the adult. Like somatic motor neurons, they innervate striate muscles (of the neck, larynx, pharynx, jaw and face). Yellow: Autonomic or visceral motor (or GVE corresponding to preganglionic parasympathetic) neurons develop in the periventricular gray lateral to the branchiomeric neurons and also in some cases, migrate out to a ventrolateral position. They innervate smooth muscles and glands (of the abdominal and thoracic viscera and the head). Blue: Visceral sensory (or GVA and SVA) relay neurons lie lateral to the visceral motor. They receive afferent input from chemo- and baro-ceptors (of the abdominal and thoracic viscera and of the neck and head, including the tongue for taste). Green: Somatic sensory (or SA) relay neurons form lateral to the visceral sensory neurons and migrate into a more ventro-lateral position. They receive afferent input from mechano-, noci- or thermo-receptors (in the skin, muscles and joints (of the head). Purple: Special somatic sensory (or SSA) relay neurons lie most lateral in the periventricular zone. They receive special afferent input carrying vestibular and auditory signals (from the ear).</BLOCKQUOTE>

At the level of the medulla, all 6 types of nuclei are present, whereas more rostrally in the pons and midbrain, only certain types are present.




B. Cranial Nerve Nuclei:

Ten of the 12 cranial nerve nuclei are in the brainstem, the other two are in the forebrain:

• Olfactory nerve (I)	• Trigeminal nuclei (V)	• Glossopharyngeal nuclei (IX)
• Optic nerve (II)	• Abducens nuclei (VI)	• Vagus (X)
• Oculomotor nuclei (III)	• Facial nuclei (VII)	• Accessory nuclei (XI)
• Trochlear nuclei (IV)	• Vestibulocochlear nuclei (VIII)	• Hypoglossal nuclei (XII)

The different nuclei will be presented from caudal to rostral, starting medially at each level (medulla -> pons -> midbrain). First, somatic motor nuclei (XII, IV, and III) are presented, then branchiomeric motor nuclei (XI, X, IX, VII, and V), and finally visceral motor nuclei (X, IX, VII, and EW). Second, visceral sensory nuclei (X, IX, and VII), then somatic sensory nuclei (Sp5, M5, and Mes5, all part of V), and the special sensory nucleus (VIII) are presented.

(posterior view)

<BLOCKQUOTE> <BLOCKQUOTE> Legend: <BLOCKQUOTE> 1. GSE: General Somatic Efferent 2. GVE: General Visceral Efferent 3. SVE: Special Visceral Efferent 4. GVA: General Visceral Afferent 5. SVA: Special Visceral Afferent 6. GSA: General Somatic Afferent 7. SSA: Special Somatic Afferent </BLOCKQUOTE></BLOCKQUOTE></BLOCKQUOTE>

1) Motor Efferents:a. Somatic motor:
Medially and caudally located, the hypoglossal nucleus (XII) projects axons ventrally through cranial nerve XII to terminate on the muscles of the tongue. The tongue also receives corticobulbar projections (as the muscles are under voluntary control) which are predominantly crossed.
Rostrally, at the level of the caudal pons, the abducens nuclei (VI) projects axons ventromedially through the abducens nerve to terminate on the lateral rectus muscle of the eyeball. The lateral rectus muscle is responsible for abduction of the eye on lateral gaze.
At the level of the inferior colliculus (lower midbrain), motor axons project contralaterally from the trochlear nucleus (IV) to innervate the superior oblique muscle of the eyeball.
Finally, at the level of the superior colliculus, the oculomotor nucleus (III) projects axons ventro-medially in the III nerve to innervate the medial rectus, superior, inferior rectus and inferior oblique muscles of the eyeball.
b. Branchiomeric motor:
Within the brainstem, branchiomeric motor nuclei are positionned ventrally and laterally to somatic motor nuclei. The most caudal of the branchiomeric motor nuclei is the accessory nucleus (XI), which sends efferent motor neurons to striated muscles of the head and neck (eg. trapezius and sternocleidomastoid) via the accessory nerve.
At the medullary level, the nucleus ambiguus sends alpha motor fibers (in IX and X) to innervate the branchiomeric muscles of the pharynx and larynx. These are large neurons similar to surrounding cells in the reticular formation and few in number, thus being difficult to identify. They are involved in the control of striated muscles. The nucleus ambiguus sends motor axons into both the vagus nerve (innervating the larynx and pharynx) and the glossopharyngeal nerve (innervating the upper pharynx). The nucleus ambiguus receives corticobulbar inputs as these muscles are under voluntary control.
At the level of the lower pons, the facial nucleus projects motor axons to the branchiomeric muscles of the face, mediating facial expression (eg. frowning). These fibers project medially and loop around the rostral 2/3 of the abducens nucleus before turning ventro-laterally to join the facial nerve (this particular path is consequential to the embryological origin of the branchiomeric facial nucleus, as illustrated above). Each facial nucleus controls a complete half of the face and receives cortico-bulbar projections for voluntary control (more on the facial nucleus in the Motor System section).
Finally, at the mid-pons, the trigeminal motor nuclei of V (Mot5) send axons through the motor Vth nerve to the muscles of the jaw for mastication.

c. Visceral motor:
More caudally, at the level of the medulla, the dorsal motor nucleus of the vagus (X) sends preganglionic parasympathetic axons to visceral organs of the thorax and abdomen through cranial nerve X.
The inferior salivatory nucleus sends preganglionic fibers (in IX) to the parotid gland. The superior salivatory nucleus sends preganglionic fibers (in VII) to the sublingual and submandibular salivatory glands and to the lacrimal gland.
Finally, at the level of the rostral midbrain, the Edinger-Westphal nuclei (EWN) sends visceral motor fibers into the oculomotor nerve (III) to terminate within the ciliary ganglion. This ganglion innervates the muscles of the pupil and lens and is thus involved in the control pupillary constriction and accommodation.

2) Sensory Afferents:
a. Visceral sensory:
Chemoreceptors and baroreceptors inputs reach the nucleus solitarius bringing and processing sensory visceral inputs to cranial nerve nuclei of X and IX (eg. from the carotid body and sinus) via the solitary tract.
The nucleus solitarius also processes taste afferents. These visceral sensory inputs enter at the medullary level to reach the solitary tract nucleus from VII (ant 2/3 portion of tongue), IX (posterior 1/3 of tongue), and X (back of throat, including the epiglottis). Some somatic input (eg. touch) also travels in through these nerves to terminate in the spinal trigeminal nucleus. It is also interesting to note that the nucleus solitarius collaborates with the reticular formation and is the major sensory component of the medullary respiratory center and of the ill-defined cardiovascular center.
b. Somatic sensory:
Somatic sensations from the face and part of the mouth enter at the level of the pons via the Vth nerve from the trigeminal ganglion. First order somatic sensory axons conveying pain, temperature and crude touch take a 90º turn and descend toward the medulla and spinal cord in the spinal trigeminal tract (see figure above), synapsing along the way onto the spinal trigeminal nucleus (Sp5). Axons from Sp5 will cross the midline and ascend thereafter to the thalamus.
Some fibers from the trigeminal nerve travel up to reach, at the midbrain level, the mesencephalic nuclei of V (Mes5) made up of ganglion cells. The peripheral processes of these cells comprise muscle receptors in the muscles of mastication as well as mechanoreceptors around the teeth, gums, and hard palate. The central axons of the Mes5 project onto the motor trigeminal nucleus(Mot5), underlying the jaw jerk reflex.
Fibers from the trigeminal ganglion also reach the main sensory nuclei of V (M5) (analogous to the dorsal column nuclei gracilis and cuneatus) at the mid-pontine level.
c. Special sensory:
Special sensory afferents bring auditory (lateral) and vestibular (medial) inputs to the cochlear nuclei (CN) and vestibular complex (VC) via cranial nerve VIII. More specifically, fibers from the vestibular apparatus (important for a sense of balance and position) terminate dorsal and lateral to the nucleus solitarius. Primary auditory fibers terminate in the cochlear nuclei, on the lateral dorsal surface of the inferior cerebellar peduncle.


Clinical Problems:

1) Describe the events that occur when you move your eyes to look at an object to your left.
2) How would you test for the status of both spinal trigeminal nerves?
3) What is the jaw jerk reflex?
4) Describe the gag reflex.
5) Describe how the baroreceptors within the carotid sinus mediate a decrease in blood pressure.