The Neuroscience on the Web Series:
CMSD 620 Neuroanatomy of Speech, Swallowing and Language

CSU, Chico, Patrick McCaffrey, Ph.D.


Chapter 9. The Cranial Nerves (including lower motor neurons involved in swallowing and speech)


The cranial nerves innervate the muscles of the jaw, face, tongue, neck, pharynx, and larynx. Some of them are motor, some are sensory and some are mixed nerves, containing both sensory and motor fibers. Six of them are involved in speech and swallowing, and are therefore very important to the speech, language pathologist.

The motor nuclei and processes of the cranial nerves are lower motor neurons as they form a final common pathway for information descending from the cerebrum to the periphery. Because the motor roots of the cranial nerves are located in the brain stem, messages from the precentral and postcentral gyri reach the cranial nerves on the corticobulbar portion of the pyramidal tract. With the exception of part of CN VII (the facial nerve) and parts of CN XII the hypoglossal nerve, this innervation is bilateral.

Along with the spinal nerves, the cranial nerves are part of the peripheral nervous system.

There are twelve pairs of cranial nerves.

Cranial Nerves Involved in Smell and Vision

CN I, the olfactory nerve, is a purely sensory nerve. It has receptors within the mucous membrane of the nose. Information runs posteriorly along the olfactory tract and through the olfactory bulb to the temporal lobe where it is processed. Remember that this is the only sense not mediated by the thalamus.

CN II, the optic nerve, is also a sensory nerve. Visual information from the retina is carried back to the superior colliculus of the midbrain on the optic tract which is immediately below the olfactory tract in the more anterior part of the brain. From the superior colliculus, messages are first passed on the lateral geniculate body in the thalamus and then to the cortex of the occipital lobe.

CN III is the oculomotor nerve. The nucleus of this motor nerve is located in the midbrain. It mediates movements of the eyeball and constriction and dilation of the pupil.

CN IV or the trochlear nerve, is a motor nerve. Its nucleus also lies in the mesencephalon. This nerve also innervates eye movements and damage to it will cause double vision.

CN VI is the abducens nerve. The nucleus of this motor nerve is found in the pons. This nerve provides innervation for movements of the eyeball.

The Cranial Nerves Involved in Speech, Hearing and Swallowing

CN V is the trigeminal nerve.

The motor nucleus of this large mixed nerve originates in the pons. It provides motor innervation to the muscles that control the mandible (jaw), the tensor veli palatini muscle of the velum, and the tensor tympani muscle of the middle ear.

It mediates sensation from the head, jaw, face, some of the sinuses and tactile sensation from the anterior two thirds of the tongue.

CN VII or the facial nerve, is most often classified as a motor nerve, but can also be considered a mixed nerve.

Its motor nucleus which is located in the junction of the pons and medulla innervates all of the muscles of facial expression including those in the forehead, cheeks, and lips, as well as the stapedius muscle of the middle ear. It also sends motor impulses to the rest of the ear; if you can wiggle your ears, this action is mediated by CN VII.

The part of the nucleus that sends commands to the upper part of the face receives bilateral (ipsalateral and contralateral) innervation from upper motor neuronal tracts. However, the portion that controls the lower part of the face receives only contralateral (unilateral) innervation. This means that unilateral lesions of the pyramidal tract may have noticeable effects on voluntary movements of the cheeks and lips. Note that involuntary facial expressions of emotion will not be impaired in the case of a pyramidal tract lesion as they are controlled by the extrapyramidal tract.so that if a patient smiles both sides of the mouth will lift normally. With lower motor neuron lesions the entire side of the face will be paralyzed and won't respond to either voluntary or involuntary upper motor neuron input.

The facial nerve mediates taste in the anterior two thirds of the tongue.

CN VIII is the vestibular/acoustic nerve. The two branches of this sensory nerve carry information from the cochlea and from the vestibular end organs in the inner ear. This nerve originates in the medulla. The vestibule which connects to the pons and then to the cerebellum regulates balance and equilibrium including the coordination of head and body movements.The auditory fibers go to the inferior colliculi of the midbrain and then to the medial geniculate bodies of the thalamus and finally project bilaterally to the temporal cortex. Lesions on the cell bodies in the medulla will not result in total hearing loss because of the bilateral auditory projections.

CN IX, or the glossopharyngeal nerve, is a mixed nerve.

Its motor aspect contributes to the action of the middle pharyngeal constrictor muscle and innervates the stylopharyngeus muscle.

Its sensory aspect carries input from the posterior one third of the tongue, the velum, and the pharynx including the tonsils.

CN X is the vagus nerve. This mixed nerve originates in the medulla.

One of the motor nuclei of the vagus innervates the majority of the viscera, including the heart, respiratory system, and digestive system.

Another motor nucleus sends motor commands to the pharyngeal constrictor muscles and completely controls the intrinsic musculature of the larynx. The superior branch of the vagus innervates the cricothyroid muscle and so is involved in pitch changes. Its recurrent branch innervates all of the other intrinsic laryngeal musculature.

The vagus also innervates the glossopalatine and levator veli palatine muscles, making it primarily responsible for palatal functioning.

CN XI is the spinal accessory nerve, a motor nerve that originates in the medulla. It innervates the trapezius and sternocleidomastoid muscles of the neck. It also sends some motor messages to the uvula and the levator veli palatine (raises the velum).

CN XII, is the hypoglossal nerve, another motor nerve that originates in the medulla. It controls tongue movement, innervating both the intrinsic and extrinsic tongue muscles. The part of the nucleus that innervates the genioglossus, the muscle involved in tongue protrusion, is connected only to contralateral fibers from the pyramidal tract. Thus, unilateral upper motor lesions can affect this type of movement. All other tongue muscles receive bilateral innervation for voluntary movements.

If there is damage to the hypoglossal nerve itself, the tongue may reveal the presence of the lower motor neuron lesion by fasciculating (twitching).

The Six Cranial Nerves Involved in Speech and Swallowing  

CN V - - the trigeminal nerve

CN VII - - the facial nerve

CN IX - - the glossopharyngeal nerve

CN X - - the vagus nerve

CN XI - - the spinal accessory nerve

CN XII - - the hypoglossal nerve

Stages of Deglutition (Logemann,1989, 1994, 1998. Morrell,1984. In Groher,1984.)

In the Oral-preparatory stage, food is moved around the mouth, chewed and tasted. Time needed for this stage is variable.

During the oral stage, food is moved to the back of the mouth by the tongue. This stage lasts for about 1 second, ending when the bolus touches the back of the oropharynx.

In the first part of the pharyngeal stage, or laryngeal substage, a number of things happen simultaneously. The larynx moves up and forward, the vocal folds approximate and the epiglottis falls over the top of the larynx.

During the second part of this stage, the bolus of food is moved down the pharynx by the stripping action of the pharyngeal constrictor muscles. This phase of the pharyngeal stage ends when the cricopharyngus muscle, also known as the p.e. segment, opens, allowing the food to enter the esophagus. It is not known what leads the p.e. segment to open at the right time during the swallowing sequence. Some think that it opens due to stretching caused by the elevation of the larynx. The total time required for the entire pharyngeal phase is 1 second.

During the esophageal stage, food travels down the esophagus via a wave-like motion called peristalsis. This phase lasts between 8 and 20 seconds.

Note that peristalsis occurs only in the esophagus and colon, not in the pharynx. In some older literature on swallowing, there is confusion about this, and the stripping action of the pharyngeal constrictor muscles is mislabeled as peristalsis.

Swallowing is not just a motor reflex; it requires a combination of sensory and motor control. The swallowing center, which is located in the nucleus ambiguous of the medulla, recognizes a pattern that consists of both sensory and motor elements and then triggers the swallowing response. This is why the swallow is considered to be a type of patterned response or pattern recognition system.

Sensory input that initiates the swallow response comes from the trigeminal, facial and glossopharyngeal nerves.

The trigeminal nerve is involved in the oral preparatory and oral stages. It provides the innervation that controls jaw movement for chewing and it also mediates tactile sensation in the anterior two thirds of the tongue.

During the oral preparatory and oral stages, taste is carried from the anterior two thirds of the tongue by the facial nerve.

The glossopharyngeal nerve is responsible for taste in the posterior one third of the tongue and for tactile sensation to the posterior part of the oral cavity, including the velum, tonsils, and walls of the oropharynx. It provides the feedback that is most important in the elicitation of the swallow.

Feedback from motor movements, especially tongue movements which are mediated by the hypoglossal nerve, also help to trigger the swallow.

Input from both the cerebral cortex and the cerebellum is responsible for the coordination and timing of the motor movements involved in swallowing.

Mnemonic for the Cranial Nerves

On

(olfactory)

Some

(sensory)

Old

(optic)

Say

(sensory)

Olympus'

(oculomotor)

Marry

(motor)

Towering

(trochlear)

Money

(motor)

Top

(trigeminal)

But

(both)

A

(abducens)

My

(motor)

Finn

(facial)

Mother*

(motor)

And

(auditory)

Says

(sensory)

German

(glossopharyngeal)

Bad

(both)

Vended

(vagus)

Business

(both)

At

(accessory)

Marry

(motor)

Hopps

(hypoglossal)

Money

(motor)

The facial nerve could also be classified as both sensory (taste for anterior two thirds of tongue) and motor, in which case the word in this part of the rhyme would change to "brother." It is usually classified as a motor nerve.


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Other courses in the Neuroscience on the Web series:
CMSD 636 Neuropathologies of Language and Cognition | CMSD 642 (Neuropathologies of Swallowing and Speech)

Copyright, 1998/2001. Patrick McCaffrey, Ph.D. This page is freely distributable.