Head and Neck

Reading Assignment - Exam I

Chapters 1, 2, 3

The head and neck region has several characteristics that make it difficult to comprehend, while at the same time having several that make it easy - especially at the level associated with this course. The major concept that adds difficulty is the amount of detail that is found in the head and neck regions. Fortunately, for an introductory-level course such as this, we will not be going into tremendous amounts of detail - only basic reference structures.

Additional good news is the fact that the head and neck have more symmetry than any other areas of the body. This symmetry is not only useful for study purposes, but also for diagnostics. Generally speaking, if you see (or don't see) something on one side of the head/neck, and you see (or don’t see) it on the other, then something is wrong somewhere.

Some of the “applied anatomy" concepts associated with the head and neck are as follows:

The entire CNS (central nervous system) is enclosed in bone, with protection being the probable evolutionary driving force associated with this feature. The brain resides within the cranial cavity of the skull, while the spinal cord is found within the vertebral canal formed by the various vertebrae. While this arrangement offers some significant advantages from a protective point of view, it also presents certain problems or shortcomings.

From an imaging point of view, the presence of the protective bone layer decreases the number of imaging modalities that can be used for those regions, i.e., ultrasound can generally not be used to image the brain or spinal cord. (An exception is found in infants, where the cranial fontanelles have been used as "windows" to allow access to the cranial contents for sonography.) And while the bone may help protect the CNS from certain types of low-level traumatic forces applied to the head, additional problems arise when some degree of damage does occur, and when the CNS tissues begin to swell as a result of that trauma. A classic clinical problem is herniation of the cerebellum through the foramen magnum, secondary to cerebral edema following trauma. In general, the contents of the cranial cavity can be divided into three main compartments - brain and brain-associated tissue, blood, and CSF (cerebrospinal fluid). These are also the entities that are said to "occupy space" inside of the cranial cavity. Brain-associated tissue would include the vessels within the brain, as well as the meninges (dura mater, arachnoid mater, pia mater - from superficial to deep). Accumulation or enlargement of any of the three compartments will result in a concomitant rise in ICP (intracranial pressure). In some instances, such as the images shown below, the complete fracture of the skull allows for at least some degree of pressure relief for a certain period of time.

In the following case the skull integrity has been maintained, resulting in retention of blood associated with the epidural hematoma (blood trapped between the dura mater and the inner table of the skull). If the pressure secondary to this hematoma is not stopped or relieved, significant damage to the brain could occur, including herniation through the foramen magnum. (Note: Increase in retained blood (occupying more space than normal) will result in an increased pressure on, or displacement of, the other two compartments of the cranial cavity.)

CLINICAL NOTE: As previously mentioned, note the loss of symmetry associated with the above image. The blood accumulation is occurring in the right parietal/occipital region. In addition to the pocket of blood associated with the hematoma, note that the right lateral ventricle has been compressed to the point where it is not visible. As the cranial nerve I (optic nerve) leaves the cranial cavity and then courses to the posterior globe of each eye, it is reasonable that an elevated ICP could possibly be transmitted along the path of this nerve. Thus, a fundic exam (examination of the back wall of the eye by way of using an ophthalmoscope to visualize the posterior wall) often reveals a "bulging" optic disc in cases of elevated ICP. (Recall that the optic disc is the region where the optic nerve makes contact with the posterior wall of the globe of the eye.) Elevated ICP can also result in significant changes in mental status of the patient.

Major arterial blood supply to the head is via the common carotid arteries, and the vertebral arteries. At the angle of the mandible, the common carotid artery bifurcates into the internal and external carotid arteries. The external carotid artery supplies blood to extra-cranial structures, while the internal carotid artery supplies blood to the brain. The two vertebral arteries combine to form the basilar artery, which also supplies blood to the brain. As explained below, the internal carotid arteries and the basilar artery join with the Circle of Willis prior to distributing blood to the brain. Extra-cranial blood is drained away from the head by the external jugular vein system, while the brain is drained by the internal jugular system. Both internal and external jugulars drain into the brachiocephalic veins of the thorax.

Clinical note: The common carotid arteries are common sites for accumulation of atherosclerotic plaque. This plaque can result in three major complications associated with the brain.

• As the plaque accumulates, there is a concomitant diminution of blood flow to the circle of Willis and, therefore, the brain.

• Atherosclerotic plaque is somewhat "rough" in texture, increasing the possibility of an intravascular blood clot (thrombus).

• The previously mentioned thrombus can have two possible effects on the brain. First, the clot can cause an overall diminution of blood flow to the brain, i.e., cerebral ischemia. Secondly, the thrombus might break loose and travel (then known as an embolus or embolism) to the brain. If the embolus remains intact, a fairly large region of the brain might infarct (die) when the embolus arrests at a given location in the cerebral vascular system. Whatever function is associated with that specific portion of the brain will cease. If this occurs in a life supporting area, the results can be fatal. Otherwise, a specific function (e.g., speech, gross motor control, etc.) will be lost. An alternative situation occurs when the embolus originating in the common carotid artery fragments at the time it dislodges from the artery. Instead of one large embolus traveling to one part of the brain, many smaller emboli travel to many different locations throughout the brain. When these emboli arrest at various locations, they cause many small infarcts. These infarct areas are often not of a life-threatening nature. However, the "overall" diminishment of CNS tissue and function often results in a noticeable "change" in the overall presentation of the patient. Thus, it is often common to have a person diagnosed with Alzheimer's disease when, in fact, they are suffering from this condition (MID or multi-infarct dementia).

Associated with a high utilization of oxygen and glucose, the brain has a significant requirement for blood. Evolutionarily, this could have been met in a number of ways. As previously mentioned, the two internal carotid arteries and the vertebral/basilar artery system are the main vessels supplying blood to the brain. Instead of having each vessel supply its own individual portion of the brain, all of the aforementioned arteries empty into the Circle of Willis, which acts like a reservoir of blood from which the various cerebral vessels emanate. Thus, occlusion of any one of the main vessels supplying the brain will generally not cause infarction of a specific region of the brain. Overall flow will diminish at first, with a compensatory increase often seen following the initial occlusion.

Blood returning from the brain is collected in various venous sinuses, and then returned to the internal jugular veins and on to the superior vena cava. The major vascular sinus, the superior sagittal sinus (SSS), is also the main drainage route for CSF. Occlusion of the SSS by way of a thrombus can result in one form of hydrocephalus, i.e., increased fluid accumulation within the cranial cavity. While the intracranial and extracranial blood systems are generally separated for the most part, there is some communication via what are termed emissary vessels that do connect intra and extracranial regions.

The brain is organized into two hemispheres - the left and the right. As a general statement, each hemisphere controls the contralateral side of the body. Each hemisphere is anatomically and functionally divided in lobes, with the lobe names referring to the most adjacent portion of the skull. Thus, each side has a frontal, parietal, occipital, and temporal lobe. (See following diagram.)

The brain is not a solid organ. Instead, it contains several spaces known as ventricles. Within these ventricles reside the choroid plexus capillaries, which are responsible for producing the CSF that eventually surrounds the entire CNS. As long as the production and drainage (via the superior sagittal sinus) of CSF remain constant, so remains the intracranial pressure. The CSF is held in contact with the CNS by a series of membranes known as the meninges. Traveling from superficial to deep, these membranes are the dura mater, the arachnoid (mater), and the pia mater. Infection or inflammation of these membranes is commonly known as meningitis.

In times prior to MRIs, CTs, and the like - standard radiographs did not demonstrate the ventricles very well, as the density of the CSF and the CNS tissue are quite similar. In an effort to delineate the ventricles, they were often injected with air as a contrast medium. This procedure, known as a pneumoencephalogram, was both dangerous as well as painful. Modern imaging modalities have eliminated the need for this procedure.

The neck is a very complex region from an anatomical point of view. In addition to the musculoskeletal support structures, components of the respiratory, digestive, nervous, and vascular systems all pass through the neck in one form or another. Thus, injury to the neck region can have serious and life-threatening consequences.

Although the human head does not weigh much more than a small bowling ball, the fact of the matter is that it must be held in an upright orientation for most of the day. Thus, a significant portion of neck anatomy is dedicated to the musculoskeletal components necessary to do this. A quick view of a cross section of the neck shows that the "default" setting of the head is to fall forward. Thus, a significant portion of the neck musculature is designed to extend the neck and keep the head in an upright position. (Clinical note: Those individuals who experience "muscle tension headaches" while working at computers are well aware of the significant amount of musculature found in the posterior aspect of the human neck.)

1) The carotid sheath contains the common carotid artery, the internal jugular vein, and the vagus nerve. These structures are all deep to the sternocleidomastoid muscle.

2) The external jugular vein is superficial to the sternocleidomastoid muscle and is, therefore, easily accessible for invasive procedures.

- the respiratory system (larynx or trachea, depending on location) is the most anterior of the midline structures.

- the digestive system (pharynx or esophagus, depending on location) is posterior to the respiratory system.

- the nervous system (spinal cord, surrounded by the vertebral column) is the most posterior system.

You should now begin studying the cross-sectional images from your text. Remember - it is best if you:

• Look at the diagrams in your text first, and highlight those structure names (from your Structure List) that are required for Exam I.

• Once you feel that you are familiar with the structures in your text, go to the Head & Neck Anatomy section of this webpage and begin looking at the unlabelled images. Name as many structures as you can on each image, and then check your text to confirm (hopefully:) that you named them correctly.