Head and Neck Lab 2B

Lab 2B: Disarticulation, Orbit, Eye and Posterior Pharynx

Learning Objectives

Preform a cervical disarticulation to view the posterior pharynx and associated structures.
Enter the orbit to identify the nerves and muscles relevant to the eye.
Trace the pathways of neurovasculature along the posterior pharynx.

Short List of Structures - 2B

Disarticulation of the Head

The goal is to separate the head from the neck, but to preserve the connections between the cervical viscera and the base of the skull.

First, turn your cadaver supine and use blunt dissection only to isolate the cervical viscera.

Big Picture: Once your hand is able to fit in the retropharyngeal space, all of the structures you want to keep (not cut) will be above your hand. Anything underneath your hand will be cut.

You will separate the cervical viscera from the vertebral column and pre-vertebral muscles. From both side of the neck, work your fingers into the pre-vertebral space. The goal is to separate the trachea, esophagus, carotid aa., internal jugular vv., and cranial nerves from their attachment to the cervical vertebrae and the pre-vertebral muscles, this will require some force. Once you’ve made that separation, continue it up to the base of the skull, and down toward the thoracic vertebrae.

If you successfully completed this step you should be able to slide your hand anteriorly until you can feel the base of the skull and inferiorlyas low as possible. With your hand in place, as in the picture on the right, only things on top of the palm of your hand will be preserved when we make cuts later.

Before moving on, call a member of the teaching staff over to make sure you have done this correctly.

Figure 1.3 Separating the cervical viscera from the pre-vertebral muscles.

Once a member of the teaching staff has given you the OK, you may now turn your cadaver prone and begin disarticulating the base of the skull from the C1 vertebrae.

To do this, grab a chisel and place it between the base of the skull and C1 vertebrae, as in the dotted line in the image to the right—do keep in mind the articulation is not a straight line, it has some shape to it, try to wedge the chisel into this irregularly shaped joint.

You will now completely separate the skull base from the C1 vertebrae, the chisel will cut through most of the tissue in the joint, but you mwill likely need a scalpel as well. Call a member of the teaching staff over if you have any difficulties, this is not a transferable skill, we are happy to step in and help here.

Once the skull base is completely separated from the vertebrae, the only thing attaching the skull to the rest of the body will be soft tissue and musculature of the neck. Use a scalpel to join the space you just made with the chisel to the space you made via blunt dissection. Be sure to safeguard the cervical viscera.

Figure 1.4 Disarticulating the skull from the vertebral column.

Figure 1.5 Anterior view of the pre-vertebral muscles.

Posterior Pharynx

Return to the cervical spine of your cadaver from which the the head has been disarticulated (not decapitated, which would involve severing all connections with the body) and you’ll have a clear view of the anterior surface of the vertebral column (Fig. 1.5 above; the prevertebral muscles).

Locate the scalene muscles again. Recall that the roots of the brachial plexus pass between the anterior and middle scalenes.

Medial to the scalene muscles look for the longus colli mm. The slips of longus colli mm. connect vertebrae to vertebrae, and function to flex and/or stabilize the cervical spine.

Superior to the longus colli mm. are the longus capitis mm. The longus capitis mm. attach to the transverse processes of cervical vertebrae and to the occipital bone. Since we cut through the soft tissue around the cervical spine to disarticulate the head, we will have cut through the longus capitis mm.

Now turn your attention to the posterior view of the cervical viscera. Spend some time cleaning off connective tissue and looking for the structures you can see from this posterior view.

Begin by identifying the broad, thin, flat pharyngeal constrictor mm. These muscles wrap around the back of the pharynx, and initiate swallowing. The three pharyngeal constrictors attach anteriorly to bony and cartilagenous elements of the pharynx, and the left and right halves of each muscle meet posteriorly at a midline raphe. You’ll probably need to pull connective tissue off the posterior surface of the muscles to get a clear view of them. The superior and inferior constrictor mm. are not too difficult to find. To differentiate the middle constrictor m. you’ll have to clean off connective tissue to get a clear view of the muscle fascicles, and then look for changes in the fascicle orientation to find the superior and inferior borders of the middle constrictor.

Locate the posterior belly of the digastric m., it may help to look anteriorly and find the anterior belly of the digastric m. which is attached to it by a common intermediate tendon. The posterior belly attaches to the mastoid process of the temporal bone, you will see it medial to the attachment of the sternocleidomastoid m.

Sympathetic trunks and ganglia - you may find separate inferior, middle, and superior ganglia. You will definitely see the superior cervical ganglion, because it’s the largest. Recall that the superior cervical ganglion is the “last stop” for preganglionic sympathetic neurons to synapse with post-ganglionic sympathetic neurons that are going to the face of the inside of the skull.

The superior cervical ganglion and sympathetic trunk may have been reflected with the skull and cervical viscera as in Figure 1.6, or it may have stayed with the vertebral column as in the image below:

Dissection of the orbit

Dissecting the orbit:

We’ll take a superior approach to dissecting the right orbit. If you like, you may take the same approach to dissect the left orbit as well, or try a surgical approach (from the front) on the left orbit.

There’s a lot of stuff in the orbit, small things in a small space. Take your time pulling the bone off the roof of the orbit, and give yourself plenty of room to work. There’s a fat pad between the bone and the nerves, vessels and muscles, so unless you’re especially ham-fisted with your dissection you need not worry about going too deep too soon.

To open the orbit from above:

Peel the dura off the frontal bone.

Use a chisel and mallet to gently break (like cracking a thick egg-shell) the approximate center of the roof of the orbit.

Use Rongeurs and forceps to remove the pieces of bone. Use the Rongeurs and chisel and mallet to open the roof of the orbit broadly to give yourself plenty of room to work. Also remove the top of the optic canal and the top of the superior orbital fissure to follow structures into the orbit from the cranial cavity.

Figure 5.11 Superior approach to the orbit.

A quick inventory of the contents of the orbit includes:

Eyeball
CN II
CN III and the ciliary ganglion (para. part of CN III)
CN IV
CN V1
CN VI
Ophthalmic a. and its branches
Ophthalmic vv.
Lacrimal gland
7 muscles that move the eyelid (1 muscle) and eyeball (6 muscles, collectively called the extra-ocular muscles)

After removing the bone that forms the roof of the orbit you’ll have to pick through the orbital fat pad. The more superior structures in the orbit are:

The ophthalmic n. is the first branch of the 3 main branches of the trigeminal nerve (V1). The ophthalmic n. has 3 main branches in the orbit.

The frontal n. is the largest of the 3 branches, and will be the first large structure you should see as you clear adipose from the orbit. The frontal n. will split to form the supratrochlear n. and supraorbital n., which pass out of the orbit and provide sensory innervation to some of he skin of the forehead.
The nasociliary n. is another branch of the frontal n., it runs medially toward the ethmoid air cells and the superior part of the nasal cavity.
The lacrimal n. is the smallest of the three branches of V1. It provides somatic sensory innervation to the lacrimal gland.

The levator palpebrae superioris m., which raises the upper eyelid.

The superior rectus m., one of the 6 extra-ocular muscles, lies just underneath the levator palpebrae superioris m.

The superior oblique m. is the most medial of the extra-ocular m. Follow it out toward the front of the orbit and find the trochlea, a pulley that the muscle tendon passes through that allows the muscle to produce a rotational movement of the eyeball.

Figure 5.12 Superior view of the orbits. Right side of the figure shows the more superior structures in the orbit. Left side of the figure has the superior muscles cut and reflected to view more inferior structures.

The superior oblique muscle is innervated by CN IV, the Trochelear n. The nerve is small and enters the substance of the superior oblique m. near its origin from the sphenoid bone.

Opposite the trochlea, in the lateral anterior corner of the orbit, find the lacrimal gland (tear gland).

A small lacrimal n. (a branch of CN V1) runs out to the lacrimal gland. The main part of the lacrimal n. (the part from CN V1) is somatic sensory to the lacrimal gland. Along its course the lacrimal n. picks up sympathetic (from the superior cervical ganglion) and parasympathetic (from the pterygopalatine ganglion) nerve fibers that supply secretomotor (visceral motor) innervation to the lacrimal gland. Both sympathetic and parasympathetic stimulation result in tear production, but the parasympathetic system dominates lacrimation.

Branches of the ophthalmic a. include the supraorbital a. and the lacrimal a.

The ophthalmic v. has superior and inferior branches that unite at the back of the orbit and empty into the cavernous sinus.

To access structures in the more inferior part of the orbit, cut the levator palpebrae superioris m. and the superior rectus m. near their mid-point and reflect the cut ends superiorly.

You may find branches of the superior division of the oculomotor n. entering the bellies of the levator palbebrae superioris and the superior rectus mm.

The most lateral of the extra-ocular muscles is the lateral rectus m. You may be able to find CN VI entering the belly of the lateral rectus m. The medial rectus m. is the antagonist of the lateral rectus m.

You can’t miss the optic nerve tract entering the back of the eyeball.

Figure 5.13 The ophthalmic artery and its branches.

You may also see multiple small nerve branches entering the back of the eyeball, the long ciliary nn. and the short ciliary nn.

Both the long and short ciliary nn. contain somatic sensory fibers of CN V1 that provide somatic sensory innervation from the eyeball.
Both long and short ciliary nn. also contain post-ganglionic sympathetic neurons from the superior cervical ganglion that traveled along with arteries. The sympathetic neurons innervate the smooth muscle in the inta-ocular blood vessels, the dilator pupillae m. (dilate the pupils), and the superior tarsal m., a smooth muscle in the upper eyelid that helps hold the eye open. The sympathetic neurons also cause relaxation of the ciliary m., which leads to the lens flattening out to facilitate far vision.
The short ciliary nn. contain postganglionic parasympathetic neurons from CN III that synapsed in the ciliary ganglion. The only neurons that synapse in the ciliary ganglion are the pre- and postganglionic fibers of CN III. Somatic sensory and postganglionic sympathetic neurons pass through the ciliary ganglion but do not synapse there. The parasympathetic neurons innervate the sphincter pupillae m. (constricts the pupil) and cause contraction of the ciliary muscles, which allows the lens to thicken to facilitate near vision.

Inferior to the optic nerve tract you’ll also see small branches of CN III innervating extraocular mm. Move the eyeball around and look for the inferior oblique m. and the inferior recuts m., they will be the toughest to see. Feel free to transect the optic nerve tract to better see the inferior structures in the orbit.

Figure 5.14 Schematic of the long and short ciliary nerves.

Figure 5.15 Right lateral view of structures in the orbit.

Clinical Correlations

Orbital Blow-out Fracture:

A blowout fracture of the orbit occurs when trauma causes increased pressure within the orbit and one or more of the walls of the orbit fractures. The most common causes are sports injuries and fisticuffs (or some combination of the two). Orbital material is often pushed into the paranasal sinus(es) due to the pressures involved. Although the medial wall of the orbit is thinnest, the floor of the orbit is most commonly fractured in a blow-out fracture. Fracture of the floor of the orbit may result in entrapment of the inferior rectus muscle, and a resultant deficit in upward gaze due to increased tension in the muscle.

Illustrations of orbital blow-out fractures in which orbital material is forced into the maxillary sinus

Coronal CT. Blow-out fracture of left orbit with entrapment of the inferior rectus muscle in the maxillary sinus.

Orbital blow-out fracture.

Ophthalmic artery and branches.

Normal retina above, retina of a person with CRAO below.

Oculomotor Nerve Palsy (Neuropathy):

As its name implies, the oculomotor nerve supplies most of the muscles that move the eyeball, and so damage to this nerve leads to a general lack of normal eye movements, resulting in dipolpia (double vision).

A complete palsy (in which the entire nerve is knocked out) leads to the affected eye assuming the “down and out position”, in which the gaze is directed inferiorly and laterally. This is because the lateral rectus and superior oblique muscles are still innervated, and since their actions are unopposed they draw the gaze laterally (lateral rectus) and inferiorly (superior oblique). The oculomotor nerve also supplies the levator palpebrae superioris, so ptosis (drooping eyelid) is also typically present.

Parasympathetic nerve fibers that run in CN III are superficial and thus may be affected first by a compression neuropathy, before any movement disorder occurs, such that the first symptom may be mydriasis (dilated pupils, or a “blown pupil”) due to lack of innervation of the constrictor pupillae.

The causes of aquired third nerve palsy are diverse, and include space occupying lesions or tumors, inflammation and infection, trauma, demyelinating diseases such as Multiple Sclerosis, autoimmune disorders such as Myasthenia Gravis, cavernous sinus thrombosis, and any of a number of vascular disorders.

One of the vascular disorders than may lead to third nerve palsy is aneurysm, especially of the posterior communicating artery (PCA). The most common site of PCA aneurysm is where the PCA branches off the posterior cerebral artery. That point is typically right next to the oculomotor nerve, and so an aneurysm at that site may exert pressure on the nerve and lead to compression neuropathy.

Oculomotor nerve palsy.

Position of CN III relative to the posterior cerebral and superior cerebellar arteries.

Dysphagia

Dysphagia is the term used to describe difficulty swallowing. It is often considered a sign or symptom, but may be used as a distinct condition in and of itself. The main types of dysphagia include oral, pharyngeal, esophageal, and functional. The first 3 types describe where the problem is, while functional dysphagia describes a case of dysphagia for which no cause can be found (idiopathic).

There are many possible causes of dysphagia, including:

Oral – tonsillitis or peritonsillar abcess, tongue cancer, Bell's palsy, xerostomia

Pharyngeal – an impacted foreign body within the pharynx, inflammation of the pharyngeal wall (pharyngitis), or a thyroid malignancy

Esophageal – an impacted foreign body within the esophagus, strictures of the esophagus due to reflux disease, or aortic aneurysm.

Diagnosis is by flouroscopy of a barium swallow, in lateral or AP view.

Excellent explanations and images of normal swallowing and various types of dysphagia can be found here:

http://rad.desk.nl/en/440bca82f1b77

Eustachian tube blockage

The eustachian tube (pharyngotympanic tube) connects the middle ear (petrous part of temporal bone) to the nasopharynx. The medial part of the tube is cartilagenous, and terminates in the torus tubarius, which you should see in the nasopharynx.

3 muscles involved in the pharyngeal phase of swallowing attach to the cartilagenous part of the eustachian tube, and the tube is pulled open as a result of swallowing. This allows the air pressure in the middle ear to equilibrate with atmospheric pressure; "clearing" the ears.

The muscles involved are:

tensor palatini – contacts to stiffen (tense) the soft palate
levator palatini – elevates the stiffened soft palate to close off the nasal cavity from the oropharynx
salpingopharyngeus – pulls the pharynx upwards and forwards during the pharyngeal phase of swallowing.

Inflammation of the nasal mucosa surrounding the torus tubarius can cause blockage of the eustachian tube, which may result in fluid build-up in the tube and middle ear and eventual infection of the middle ear.

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Tonsils

The tonsils form a ring of lymphoid tissue (Waldeyer's tonsilar ring) in the pharynx, and are typically described as the first line of defense against ingested or inhaled pathogens (though their exact immunologic role is not known).

In common use, "tonsils" most often refers to the palatine tonsils, though there are 3 main sets of tonsilar tissue:

Most inferior, the lingual tonsils are posterior to the terminal sulcus of the tongue, on the posterior 1/3 of the tongue.
Moving laterally, the palatine tonsils sit between the palatoglossal and palatopharyngeal arches.
Superiorly, the tubal tonsils and pharyngeal tonsils (adenoids) are on the roof of the pharynx (nasopharynx), behind the soft palate.

Tonsils reach their largest size during puberty, and atrophy with age. Relative to the diameter of the throat, the tonsils are largest in young children.

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