The master control switchboard – Cranial nerve nuclei

The cranial nerve nuclei are a collection of neurons in the brainstem. They are either motor (efferent) or sensory (afferent), and both can be somatic or visceral. The cranial nerves arise from those nuclei. The cranial nuclei are motor or sensory, while the cranial nerves can be motor, sensory, or mixed. Motor nuclei are closer to the anterior (ventral), and sensory nuclei and neurons are closer to the posterior (dorsal) sides of the brainstem.

The olfactory nerve (CN I) does not arise from a cranial nerve nucleus. It arises from the olfactory bulb. The olfactory bulb is a structure located in the forebrain (Prosencephalon) and is responsible for olfaction.

The optic nerve (CN II) does not arise from a cranial nerve nucleus either. It arises from the retina of the eye and transmits the visual impulses to the visual cortex at the occipital lobe.

The oculomotor nucleus is in the upper part of the midbrain at the level of the superior colliculus. From the oculomotor nucleus, the oculomotor nerve fibers arise to supply most of the extraocular muscles of the eye (superior, medial, and inferior rectus, inferior oblique, and levator palpebrae superioris muscles).

The accessory oculomotor nucleus (Edinger-Westphal nucleus) is in the midbrain. Fibers that originate from this nucleus pass through the oculomotor nerve reach the ciliary ganglion to supply the sphincter pupillae and the ciliary muscle.

The trochlear nucleus is in the inferior part of the midbrain at the level of the inferior colliculus. The fibers of the trochlear nerve (CN IV) originate from this nucleus and innervate the superior oblique muscle of the eye.

The trigeminal nerve (CN V) originates from the motor nucleus of the trigeminal nerve and three sensory nuclei (mesencephalic, principal sensory and the spinal nucleus of the trigeminal nerve). They expand from the midbrain to the medulla.

The motor nucleus of the trigeminal nerve (CN V) is located at the posterior (dorsal) part of the pons, medially to the principal sensory nucleus of the trigeminal nerve. The motor nucleus of the trigeminal nerve innervates the muscles of mastication (masseter, temporalis, medial and lateral pterygoid). Also, the mylohyoid muscle is innerved by the mylohyoid nerve which is one of the branches of the inferior alveolar nerve (branch of the mandibular nerve (CN V3). Lastly, the tensor palati muscle is supplied by the medial pterygoid nerve, a branch of the mandibular nerve (CN V3).

The main or principal sensory nucleus of the trigeminal nerve is in the upper part of the pons. It is located lateral to the motor nucleus of the trigeminal nerve. The superior sensory nucleus receives information about discriminative sensation and proprioception of the face.

The mesencephalic nucleus of the trigeminal nerve extends superiorly from the main sensory nucleus in the pons to the midbrain. It is involved in the proprioception of the muscles of the face (mostly from the mastication muscles). The mesencephalic nucleus is the center for the masseter reflex (jaw jerk).

The spinal nucleus of the trigeminal nerve expands from the superior sensory nucleus to the medulla ended up at the upper spinal cord. It receives information about deep touch, pain, and temperature from the ipsilateral face. Additionally, it receives pain information from the facial (CN VII), glossopharyngeal (CN IX), and vagus nerves (CN X).

The abducens nucleus lies in the inferior part of the pons. The abducent nerves originate from it and innervate the lateral rectus muscle.

The facial nucleus. This nucleus lies in the inferior part of the pons, anterolateral to the abducens nucleus. The fibers from this nucleus innervate the facial expression muscles and the stapedius muscle.

The salivatory nuclei, superior and inferior, lie in the posterior (dorsal) part of the pons above its border with the medulla. From the superior salivatory nucleus, facial nerve fibers through the submandibular ganglion innervate the submandibular and sublingual salivary glands. From the inferior salivary nucleus, the glossopharyngeal nerve fibers are originated. Those fibers through the otic ganglion innervate the parotid gland.

The solitary tract nucleus receives sensory fibers through the vagus and glossopharyngeal nerves. Through those afferents of the solitary tract plays an important role in the control of respiratory and cardiovascular functions. The upper part of the nucleus is the gustatory nucleus where sensory afferent fibers by facial, glossopharyngeal and vagus nerves turn out.

The vestibulocochlear nuclei are subdivided into the cochlear nuclei and vestibular nuclei. They are two cochlear nuclei, the anterior (ventral) and the posterior (dorsal). They are respectively located anterior and posterior at the level of the junction between the pons and medulla. The two nuclei are being connected by nerve fibers. They receive input from the cochlea of the ear via the auditory nerve.

The vestibular nucleus is extended posteriorly from the medulla to the pons. It is divided into the superior, medial, lateral, inferior vestibular nuclei. The fibers that arise from this nucleus are responsible for maintaining equilibrium and posture, the perception of head position and acceleration, and the general muscle tone.

The nucleus ambiguus is located in the medulla. Inferiorly, it is continuous with the spinal accessory nucleus. It contributes fibers to the glossopharyngeal, vagus, and accessory nerves. These fibers innervate the ipsilateral muscles of the soft palate, pharynx, larynx, and upper esophagus. They are mainly responsible for swallowing and speaking.

The posterior (dorsal) or motor nucleus of the vagus nerve (CN X) is vertically located posteriorly in the medulla. Its upper end lies deep to the vagal trigone in the floor of the fourth ventricle. The fibers that arise from this nucleus supply the heart, lungs, bronchi, esophagus, stomach, small intestine and large intestine up to the right two-thirds of the transverse colon.

The spinal nucleus of accessory nerve (CN XI) extends from the medulla to approximately C5. Fibers from this nucleus innervate sternocleidomastoid and trapezoid muscles responsible for flexion and lateral rotation of the head and movement of the scapula, respectively.

The hypoglossal nucleus (CN XII) is an extended column located between the posterior (dorsal) nucleus of the vagus and the midline of the medulla. The fibers arising from this nucleus innervate the muscles of the tongue.

A brain stem injury, stroke, or malignancy can cause multiple issues from insomnia, dizziness, nystagmus, nausea, vomiting, swallowing, eating and drinking difficulties, balance issues, motor control issues, cough or gag incapability, abnormal breathing and heart rate, speech issues, paralysis, coma, or death in more severe cases.

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