The Neuroscience on the Web Series:
CMSD 642 Neuropathologies of Swallowing and Speech

CSU, Chico, Patrick McCaffrey, Ph. D


Chapter 3 Anatomy of the Swallow
(Zemlin, 1997; Logemann, 1989, 1998; Cherney, 1994; Morrell, 1984 )


Basic anatomical structures for swallowing and speech

The Oral Cavity

The Oral Cavity is defined as the space between the lips and pharynx.

The muscles involved in chewing are all innervated by the trigeminal nerve. They include: the temporalis which elevates, retracts, and assists in closing the mandible, the masseter which elevates and closes the mandible, the medial which also elevates the mandible and aids in its closure, the lateral pterygoid which depresses, opens, and protrudes the mandible, as well as moving it laterally.

Other muscles involved in chewing are the obicularis oris and the zygomaticus. Both are lip muscles. The buccinator holds food in contact with the teeth. All three are innervated by the facial nerve (CN. VII).

Five muscles control the movements of the velum. The palatoglossal and the levator veli palatini both raise the velum. They are innervated by the vagus nerve (CN. X). The tensor veli palatini tenses the velum. It receives innervation from the trigeminal (CN. V).

The palatopharyngus depresses the velum and constricts the pharynx. It is innervated by the spinal accessory (CN. XI). The muscularis uvula shortens the velum. It is also innervated by the spinal accessory.

Poor velopharyngeal closure will affect speech but is not a matter of great concern in regard to swallowing. Patients may be concerned about it and believe that it is very important. While the entrance of food into the nasopharynx may be unpleasant, it is certainly not life-threatening.

The Pharynx

The hypopharynx is the lower portion of the pharynx. It is also known as the laryngopharynx.

There are three pharyngeal recesses: food boluses can lodge in these recesses.

The vallecula is the space or depression between the base of the tongue and the epiglottis.

The two pyriform sinuses are located in the pharynx, beside the larynx. They are formed by the shape of muscle attachments to the pharyngeal walls.

The superior, middle, and inferior pharyngeal constrictor muscles make up the external circular layer of the pharynx.

The stylopharyngus m. and the salpingopharyngus m. make up the internal longitudinal layer of the pharynx.

The pharyngeal constrictor muscles help move food down toward the esophagus via a stripping action. (This process should not be confused with peristalsis which is the wave-like motions of muscles that occur in the esophagus. In some of the literature the action of the pharyngeal constrictor muscles is mistakenly called peristalsis.)

The latest research on swallowing suggests that the action of the pharyngeal constrictor muscles is not the most critical factor in the movement of food down the pharynx. It seems that the plunger action of the tongue, or the tongue driving force, plays a major role in this process.

The cricopharyngus m. or pharyngeal-esophageal (P.E) segment separates the pharynx from the esophagus. At the end of the pharyngeal stage of the swallow, it must relax to allow the bolus to enter the esophagus. (It is normally closed to prevent the reflux of food and to keep air out of the digestive system.) If the P.E. segment does not relax, food will build up in the pharynx and eventually spill over the top of the larynx into the airway. The cricopharyngus is innervated by the vagus (CN. X).

Problems with the P.E. segment are rare. According to Logemann (1983, 1989,1997), only 5% of dysphagias are caused by malfunction of the cricopharyngus muscle. In the past, physicians frequently treated all types of swallowing problems by cutting the P.E. segment. This procedure is called myotomy.

The Neuroanatomy of Swallowing 
(Zemlin, 1997; Logemann, 1997, 1989; Morrell, 1984; Dobie, 1978)

Definition

In the past, the swallow was classified as a reflex. Now most sources agree that swallowing is a pattern-elicited response.

The gag reflex, in contrast, is a good example of a true reflex. It is "triggered" whenever a noxious substance touches the back of the tongue, back of the pharynx, or soft palate. The swallow response, on the other hand, cannot be initiated by touching any particular area in the oral cavity. The gag reflex and the swallow response also differ in terms of neurological control. The gag reflex is completely controlled by the brain stem. The swallow, on the other hand, is only partially controlled by the brain stem. It also receives cortical input, and input from muscle spindles, including feedback from tongue movements.

(It is important to note that the gag reflex and the swallow response are not related. In the past, many physicians would determine feeding status based on the presence or absence of a patient's gag. Actually, the presence or absence of a gag reflex does not predict the status of the swallow response.)

Neurological Control

Both sensory and motor information are necessary for the initiation of the swallow response; swallowing is dependent on both sensory and motor control or on information from both afferent and efferent systems. Sensory feedback plays a more important role in swallowing than it does in speech. Sensory input involved in the initiation in the swallow comes from the trigeminal, facial, and glossopharyngeal nerves. Information about motor movement is received from the muscle spindles in the tongue via the hypoglossal nerve.

Sensory and motor information from these sources is carried to the swallowing center, which is believed to be located in the medulla, within the nuclei of the reticular formation; specifically the nucleus ambiguous. When the swallow response is initiated, this center causes messages to be sent to the glossopharyngeal, the vagus, and the hypoglossal nerves. The glossopharyngeal is considered the major nerve for the swallowing center.

Six of the cranial nerves provide the innervation for both swallowing and speech. The link is to CMSD 620, Unit 9 cranial nerves:

  1. CN. V The Trigeminal Nerve
  2. CN. VII The Facial Nerve
  3. CN. IX The Glossopharygeal Nerve
  4. CN. X The Vagus Nerve
  5. CN. XI The Spinal Accessory Nerve
  6. CN. XII The Hypoglossal Nerve

The Trigeminal Nerve (CN. V):

Motor Component

The efferent portion of the trigeminal nerve innervates the muscles involved in chewing. These include the temporalis, the masseter, the medial, and the lateral pterygoid.

The trigeminal nerve also innervates the tensor veli palatine muscle, which tenses the velum.

In addition, the trigeminal assists the glossopharyngeal nerve in raising the larynx and pulling it forward during the laryngeal substage of the pharyngeal swallow.

Sensory Components

The trigeminal nerve carries feedback about all kinds of sensation except taste from the anterior 2/3 of the tongue.

CN. V also carries sensory information from the face, mouth and mandible.

The Facial Nerve (CN. VII)

Motor Components

The facial innervates the lip muscles including the orbicularis oris and the zygomaticus. The muscles must contract during the oral preparatory and oral transport stages of the swallow to prevent food from dribbling out of the mouth.

The facial also innervates the buccinator muscles of the cheeks. These must remain tense during the oral component of the swallowing process to prevent the pocketing of food between the teeth and the cheeks.

Sensory Component

The facial carries information about taste from the anterior 2/3 of the tongue.

The Glossopharyngeal Nerve (CN. IX)

Motor Components

It innervates the 3 salivary glands in the mouth. The saliva from these glands mixes with the chewed up food to form a bolus.

CN. IX has motor, sensory, and autonomic nervous system nerve fibers. It, along with the vagus (CN. X), provides some innervation to the upper pharyngeal constrictor muscles (Zemlin, 1997).

It innervates the stylopharyngeus muscle which elevates the larynx and pulls it forward during the pharyngeal stage of the swallow. This action also aids in the relaxation and opening of the cricopharyngeus muscle.

Sensory components

The glossopharyngeal nerve mediates all sensation, including taste, from the posterior 1/3 of the tongue (The facial carries information about taste from the anterior 2/3 of the tongue).

CN. IX also carries sensation from the velum and the superior portion of the pharynx. A lesion may have impaired the gag reflex unilaterally (Zemlin, 1997).

The Vagus Nerve (CN. X)

Motor Components

The vagus is responsible for raising the velum as it innervates the glossopalatine and the levator veli palatine muscles.

The vagus along with CN. IX innervates the pharyngeal constrictor muscles.

The vagus along with CN. XI innervates the intrinsic musculature of the larynx. It is responsible for vocal fold adduction during the swallow.

The vagus also innervates the cricopharyngeus muscle.

The vagus controls the muscles involved in the esophageal stage of the swallow as well as those that control respiration. (This is the only cranial nerve that influences structures inferior to the neck.)

Sensory Component

The vagus carries sensory information from the velum and posterior and inferior portions of the pharynx.

The vagus also mediates sensation in the larynx.

The Spinal Accessory Nerve (CN. XI)

Motor Components

CN. XI innervates the palatopharyngeus muscle which depresses the velum and constricts the pharynx.

It also innervates the muscularis uvula which tenses the velum. It, along with CN.X, innervates the levator veli palatini.
(CN. XI is strictly a motor nerve.)

The Hypoglossal Nerve (CN. XII)

Motor Components

The hypoglossal innervates all extrinsic and intrinsic tongue muscles
(It is strictly a motor nerve.)


Other courses in the Neuroscience on the Web series: next
CMSD 620 Neuroanatomy | CMSD 636 Neuropathologies of Language and Cognition

Copyright, 1998-2013. Patrick McCaffrey, Ph. D.