Anatomy: Head and Neck Part 2

Cranial Nerves

• Hypoglossal nerve (CN XII) is a motor nerve that emerges on the anterior surface of the medulla oblongata between the pyramid and the olive.
• It crosses the posterior cranial fossa and leaves the skull through the jugular foramen.
• The cranial root of the accessory nerve joins the vagus nerve and is distributed in its branches to the muscles of the soft palate and pharynx.

Systematic Examination of Cranial Nerves

• Systematic examination of the 12 cranial nerves is an important part of the examination of every neurologic patient.
• It may reveal a lesion of a cranial nerve nucleus or its central connections, or it may show an interruption of the lower motor neurons.

Testing Olfactory Nerve Integrity

• The olfactory nerve can be tested by applying substances with different odors to each nostril.
• It should be remembered that food flavors depend on the sense of smell and not on the sense of taste.
• Fractures of the anterior cranial fossa or cerebral tumors of the frontal lobes may produce lesions of the olfactory nerves, with consequent loss of the sense of smell (anosmia).

Testing Optic Nerve Integrity

• The optic nerve is evaluated by first asking the patient whether any changes in eyesight have been noted.
• A small object is then moved in an arc around the periphery of the field of vision, and the patient is asked whether they can see the object.
• Blindness in one half of each visual field is called hemianopia.
• Lesions of the optic tract and optic radiation can cause hemianopia.

Testing Trigeminal Nerve Integrity

• The trigeminal nerve has sensory and motor roots.
• The sensory root passes to the trigeminal ganglion, from which emerge the ophthalmic (V1), maxillary (V2), and mandibular (V3) divisions.
• The motor root joins the mandibular division.
• The sensory function can be tested by using a cotton wisp over each area of the face supplied by the divisions of the trigeminal nerve.

Testing Glossopharyngeal Nerve Integrity

• The glossopharyngeal nerve supplies the stylopharyngeus muscle and sends secretomotor fibers to the parotid gland.
• Asking the patient to clench the teeth can test the motor function.

Anatomy of the Middle Ear

• The mastoid antrum is an important structure in the middle ear.
• The relations of the mastoid antrum are important in understanding the spread of infection.
• The anterior wall of the mastoid antrum is related to the tympanic cavity and contains the aditus to the mastoid antrum.
• The posterior wall separates the antrum from the sigmoid venous sinus and the cerebellum.
• The lateral wall forms the floor of the suprameatal triangle and is 1.5 cm thick.
• The medial wall is related to the posterior semicircular canal.
• The superior wall is a thin plate of bone, the tegmen tympani.

Middle Ear Function

• The auditory tube serves as a route for equalizing pressure and draining the middle ear.
• Opening of the auditory tube occurs during palatal activity, such as during swallowing and yawning.
• The continuity of the mucous membrane from nasopharynx to auditory tube to middle ear allows the spread of infection from the nasopharynx to the middle ear.

Vagus Nerve (CN X)

• Composed of motor and sensory fibers, with the most extensive distribution of all cranial nerves
• Emerges from the medulla oblongata between the olive and the inferior cerebellar peduncle
• Passes laterally through the posterior cranial fossa and leaves the skull through the jugular foramen
• Has both superior and inferior ganglia (Fig. 12.65)
• Pharyngeal branch contains nerve fibers from the cranial part of the accessory nerve and supplies all muscles of the pharynx (except the stylopharyngeus) and of the soft palate (except the tensor veli palatini)

Branches of Vagus Nerve

• Auricular branch supplies the pinna, external auditory meatus, and tympanic membrane
• Pharyngeal branch joins the pharyngeal plexus and supplies all muscles of the pharynx (except the stylopharyngeus) and of the soft palate (except the tensor veli palatini)
• Superior laryngeal nerve divides into the internal and external laryngeal nerves
• Internal laryngeal nerve is sensory to the mucous membrane of the piriform fossa and the larynx down to the vocal cords
• External laryngeal nerve is motor and supplies the cricothyroid muscle

Testing Cranial Nerve Integrity

• Testing vagus nerve integrity: assesses the nerve's motor and sensory functions
• Testing trigeminal nerve integrity: assesses the nerve's sensory and motor functions
• Testing glossopharyngeal nerve integrity: assesses the nerve's function in the stylopharyngeus muscle and the parotid gland

Autonomic Nervous System

• Consists of both sympathetic and parasympathetic components
• Sympathetic part has a simpler organizational scheme, while the parasympathetic part has more components and more complex pathways

Sympathetic Part

• Preganglionic neurons originate in the lateral horn of the first through fourth spinal cord segments
• Axons enter the sympathetic trunk at those levels (via white rami communicantes) and ascend through it
• Cervical part of the sympathetic trunk extends from the base of the skull down to the neck of the first rib
• Middle cervical ganglion lies at the level of the cricoid cartilage and has branches that include:
  • Gray rami communicantes to the anterior rami of the fifth and sixth cervical nerves
  • Thyroid branches, which pass along the inferior thyroid artery to the thyroid gland
  • Middle cardiac nerve, which descends in the neck and ends in the cardiac plexus in the thorax

Cervical Plexus

• Supplies skin and muscles of the head, neck, and shoulder, plus the diaphragm and other thoracic structures
• Four cutaneous nerves (lesser occipital, great auricular, transverse cervical, and supraclavicular) branch off the cervical plexus and supply the skin over the lower head and the front and sides of the neck
• The lesser occipital nerve (C2) supplies the back of the scalp and the auricle
• The great auricular nerve (C2 and 3) supplies the skin over the angle of the mandible
• The transverse cervical nerve (C2 and 3) supplies the skin over the front of the neck

Ear

• Tympanic membrane is circular and measures about 1 cm in diameter
• The circumference is thickened and is slotted into a groove in the bone
• The groove (tympanic sulcus) is deficient superiorly, which forms a notch
• From the sides of the notch, two bands, termed the anterior and posterior malleolar folds, pass to the lateral process of the malleus
• The small triangular area on the tympanic membrane that is bounded by the folds is slack and is called the pars flaccida

Labyrinth

• The labyrinth consists of the membranous labyrinth and the bony labyrinth.
• The membranous labyrinth is filled with endolymph and surrounded by perilymph.
• The bony labyrinth has three semicircular canals, which are connected to the vestibule.
• The vestibule has five openings that connect to the semicircular canals.
• The duct of the cochlea is located in the bony cochlea.

Vestibular Apparatus

• The utricle is the larger of the two vestibular sacs.
• The utricle is connected to the saccule and the ductus endolymphaticus.
• The vestibular nerve expands to form the vestibular ganglion.
• The branches of the nerve supply the utricle, the saccule, and the ampullae of the semicircular ducts.

Teeth

• The teeth are placed in the alveolar margins of the maxillae (upper jaw) and mandible (lower jaw).
• The teeth begin to erupt at 6 months after birth and are fully erupted by the end of 2 years.
• The 32 permanent teeth consist of 4 incisors, 2 canines, 4 premolars, and 6 molars in each jaw.
• The teeth of the lower jaw appear before those of the upper jaw.

Oral Cavity Development

• The oral cavity is formed from two sources: a depression from the ectoderm and a part from the cephalic end of the foregut lined with endoderm.
• The buccopharyngeal membrane separates the two parts at first, but breaks down and disappears during the 3rd week of development.
• The epithelium of the hard palate, sides of the mouth, and enamel of the teeth are ectodermal structures.

Tongue

• The tongue is a mass of striated muscle covered with mucous membrane.
• The tongue is divided into right and left halves by a median fibrous septum.
• Muscles attach the tongue to the styloid process and the soft palate above, and to the mandible and the hyoid bone below.
• The tongue is supplied by the lingual artery and drained by the internal jugular vein.

Lymph Drainage

• The tip of the tongue drains into the submental lymph nodes.
• The sides of the anterior two-thirds of the tongue drain into the submandibular and sublingual lymph nodes.
• The posterior third of the tongue drains into the deep cervical lymph nodes.

Sensory Innervation

• The anterior two-thirds of the tongue is innervated by the lingual nerve branch of the mandibular division of the trigeminal nerve (general sensation) and the chorda tympani branch of the facial nerve (taste).
• The posterior third of the tongue is innervated by the glossopharyngeal nerve (general sensation and taste).

Hypoglossal Nerve Testing

• Asking the patient to protrude their tongue is a basic test of the function of the hypoglossal nerve.
• Deviation of the tip of the tongue indicates a lesion of the nerve on the side to which the deviation occurs.

Head and Neck

• The sublingual salivary gland lies beneath the sublingual fold of the floor of the mouth and opens into the mouth through numerous small ducts.

Cyst Formation

• Blockage of one or more of these ducts is believed to cause cysts under the tongue.

Pharynx

• The pharynx is situated behind the nasal cavities, mouth, and larynx, and consists of three parts: nasopharynx, oropharynx, and laryngopharynx.

Structure of Pharynx

• The pharynx is funnel-shaped, with its upper, wider end lying under the base of the skull, and its lower, narrower end continuous with the esophagus opposite the sixth cervical vertebra.

Musculomembranous Wall

• The pharynx has a musculomembranous wall, which is deficient anteriorly, where it is replaced by the posterior openings into the nose (choanae), the opening into the mouth, and the inlet of the larynx.

Nasopharynx

• The nasopharynx lies above the soft palate and behind the nasal cavities, with a collection of lymphoid tissue (pharyngeal tonsil) sitting in the submucosa of the roof.

Auditory Tube

• The opening of the auditory tube is on the lateral wall of the nasopharynx, with an elevated ridge (tubal elevation) capping the opening.

Laryngeal Inlet

• Two muscles modify the laryngeal inlet:
• Oblique arytenoid muscle (narrowing the inlet)
• Thyroepiglottis muscle (widening the inlet)

Vocal Fold Movement

• Five muscles move the vocal folds (cords):
• Cricothyroid muscle (tensing the vocal cords)
• Vocalis muscle (tensing the vocal cords)
• Thyroarytenoid muscle (relaxing the vocal cords)
• Lateral cricoarytenoid muscle (adducting the vocal cords)
• Posterior cricoarytenoid muscle (abducting the vocal cords)

Arytenoid Cartilages

• Rotating and sliding up and down on the sloping shoulder of the superior border of the cricoid cartilage
• The elastic tissue in the capsules of the cricoarytenoid joints keeps the arytenoid cartilages apart

Nerve Damage

• The left recurrent laryngeal nerve may be involved in bronchial or esophageal carcinoma or secondary metastatic deposits in the mediastinal lymph nodes
• Section of the external laryngeal nerve produces weakness of the voice because the cricothyroid muscle is paralyzed and the vocal fold cannot be tensed
• Unilateral complete section of the recurrent laryngeal nerve results in the vocal fold on the affected side assuming the position midway between abduction and adduction
• Bilateral complete section of the recurrent laryngeal nerve results in both vocal folds assuming the position midway between abduction and adduction, impaired breathing, and loss of speech

Laryngeal Mucous Membrane Edema

• The mucous membrane of the larynx is loosely attached to the underlying structures by submucous connective tissue
• The mucous membrane is firmly attached to the vocal ligaments in the region of the vocal folds

Important Anatomic Axes for Endotracheal Intubation

• Three upper airway axes must be brought into alignment for adequate visualization of the glottis through a laryngoscope:
• Axis of the mouth
• Axis of the pharynx
• Axis of the trachea

Reflex Activity Secondary to Endotracheal Intubation

• Stimulation of the mucous membrane of the upper airway during intubation may produce cardiovascular changes such as bradycardia and hypertension
• These changes are largely mediated through the branches of the vagus nerves

Brain Structure

• Axial image of the brain shows different parts of the lateral ventricle and the lateral sulcus of the cerebral hemisphere
• Coronal image through the frontal lobe shows the anterior horn of the lateral ventricle with improved contrast between gray and white matter

Ventricles and Sulci

• Lateral ventricle has anterior, posterior, and inferior horns
• Longitudinal fissure and falx cerebri are present
• Gray matter and white matter of the cerebral hemisphere and cerebellum can be differentiated

Skull and Head Structures

• Frontal lobe, occipital lobe, and corpus callosum are visible in the brain
• Nasal cavity, pituitary gland, and fourth ventricle are present
• Medulla oblongata and cerebellum are located in the posterior cranial fossa

Mandible and Neck Structures

• Mandible has a body, ramus, and angle
• Ramus has a posterior border overlapped by the parotid gland
• Masseter muscle covers the outer surface of the ramus
• Pterygomandibular ligament can be palpated as a tense band on the medial side of the ramus

Neck Anatomy

• Neck is subject to individual variation in body form and content
• Anterior and posterior triangles of the neck can be examined
• Thyrohyoid membrane fills the interval between the hyoid bone and the thyroid cartilage
• Upper border of the thyroid cartilage lies opposite the fourth cervical vertebra

Posterior Aspect of the Neck

• External occipital protuberance lies in the midline at the junction of the head and neck
• Nuchal groove and cervical spines can be palpated
• Carotid sheath contains the carotid arteries, internal jugular vein, vagus nerve, and deep cervical lymph nodes

Lateral Aspect of the Neck

• Sternocleidomastoid muscle can be palpated throughout its length
• Accessory nerve is relatively superficial as it emerges from the posterior border of the sternocleidomastoid

Digestive System

• Oral cavity, pharynx, and esophagus are the major components of the digestive system in the head and neck
• Nasal cavity has nasal conchae, nasal meati, and the olfactory region
• Paranasal sinuses (maxillary, frontal, sphenoid, and ethmoid) drain to specific spots on the lateral wall of the nasal cavity

Larynx

• Thyroid, cricoid, arytenoid, and epiglottic cartilages are the major skeletal elements
• Extrinsic and intrinsic muscles act on the cartilages to produce abduction/adduction of the vocal folds or tension/relaxation of the vocal ligaments
• CN X supplies the entire larynx via its superior laryngeal and recurrent laryngeal branches