Written Learning Objectives (PDF) Learning Objective Video Slides (PDF)

1. Briefly locate and describe the bones that make up the cranium.

The thirteen (13)  bones comprising the cranium can be divided into the neurocranium, or bones that house the brain, and the viscerocranium, or bones that make up the facial skeleton. Several of these bones have fused into a single bone that spans both right and left halves of the cranium while others remain paired bilaterally – these differences are represented below by either a (1) for singular bones or a (2) for paired bones.

Neurocranium:

1. Frontal (1): The most anteriorly situated component of the neurocranium. Its inferior portion forms the supraorbital ridge and the bulk of the superior orbital wall, then flattens into a broad plate that extends superiorly and posteriorly to form roughly ⅓ of the cranial vault.

2. Sphenoid (1): An irregularly shaped bone that sits directly inferior to the frontal bone and contributes to both the cranial cavity and the posterior orbital walls.

3. Ethmoid (1): An irregularly shaped bone with an unusually large proportion of air sinuses contained in its walls. It contributes to the anterior cranial cavity as well as the nasal cavity and medial orbit.

4. Parietal (2): Thin, plate-like bones that form the superolateral portion of the neurocranium

5. Temporal (2): Forms the inferolateral portion of the cranial cavity. Its lateral portion is thin and fan-like, while its inferior petrous portion is quite solid and houses the middle and inner ear.

6. Occipital (1): Form the posteroinferior aspect of the neurocranium. Includes the foramen magnum, through which the spinal cord travels to meet the brainstem.

Viscerocranium:

7. Nasal (2): Form the bridge of the nose and upper margin of the bony nasal opening.

8. Lacrimal (2): Small bones that form a portion of the medial orbit and house the nasolacrimal duct.

9. Zygomatic (2): Colloquially referred to as the “cheek bones,” these contribute to the lateral orbit and the lateral face.

10. Maxilla (2): Forms most of the central aspect of the face, with contributions to the inferior orbit, nasal cavity, and hard palate. Houses the superior toothrow; all teeth here are deemed “maxillary teeth.”

11. Mandible (1): Often referred to as the jaw bone, the mandible is unique in being connected to the viscerocranium via a synovial joint instead of a suture. It houses the lower toothrow, hence all teeth here are called “mandibular teeth.”

12. Vomer (1): A triangular bone that forms the posteroinferior aspect of the nasal septum.

13. Palatine bone (2): An L-shaped bone that contributes posteriorly to the hard palate, nasal cavity, and orbit.

2. What are the major parts of the brain?

The brain may be divided into two cortex areas, deeper ganglia, and a brainstem that is contiguous with the spinal cord. The cortex areas – the cerebrum and cerebellum – are characterized by a series of gyri (peaks) and sulci (valleys) on their surface. These folds provide greater surface area to house the neuron cells bodies that make up the surface gray matter. 

The cerebrum is the larger of the two cortex areas and is further separated into right and left hemispheres by a longitudinal fissure that extends from the anterior limit to the posterior limit. Neural fibers (axons) passing between the right and left cerebral hemispheres do so via the corpus callosum, which runs through the deepest portion of the fissure. Each hemisphere can be regionally divided into 4 lobes – frontal, parietal, occipital, and temporal – that sit approximately deep to (but do not share exact borders with) the bones that share their names. The smaller cerebellum sits inferior to the occipital lobe of the cerebrum. It is also divided physically into 2 hemispheres, which are connected by a white matter vermis.

The diencephalon sits deep to the cerebrum and is home to a number of structures involved with integration of various bodily functions, including the thalamus, hypothalamus, subthalamus, and epithalamus. The pituitary gland also called the hypophysis is suspended by the infundibulum from the hypothalamus.

The diencephalon is contiguous with the brainstem, which consists of the midbrain, pons, and medulla oblongata. The medulla oblongata is contiguous with the spinal cord.

3. Understand the basics of cranial nerves. Describe the name, number, modality (sensory/motor/both) and general regions/structures innervated by each. Which cranial nerves carry preganglionic parasympathetic fibers?

So, as an example, preganglionic parasympathetic innervation to the lacrimal gland would originate with the preganglionic cell body in the nucleus for the facial n. (CN VII). The preganglionic axon would travel with the facial n. and synapse in the pterygopalatine ganglion with its postganglionic partner’s cell body. Finally, the postganglionic axon would join a branch of the maxillary n. (CN V2) to reach the lacrimal gland.

5. What are the two main arteries supplying the brain? What is the clinical importance of the cerebral arterial circle (Circle of Willis?)

The arterial supply of the brain derives from the internal carotid arteries (ICA) anteriorly and the vertebral arteries posteriorly. The internal carotid arises from the bifurcation of the common carotid artery in the neck and travels superiorly to enter the cranium through the carotid canal. The vertebral arteries arise from the subclavian artery and ascend the neck by passing through the transverse foramina of vertebrae C1 – C6 and entering the cranium through the foramen magnum alongside the spinal cord.

The anterior portions of the brain – comprising the anterior and middle cerebrum – are supplied by branches of the internal carotid artery. After entering the cranium, the ICA ascends through the cavernous sinus to the base of the brain, lateral to the origin of the optic nerve (CN II), and begins dividing into branches.

Internal Carotid Branches

• Ophthalmic a.: first branch of the ICA, travels through the optic canal to supply the orbit

• Anterior cerebral a.: travels in the longitudinal fissure between right and left hemispheres to supply anterior and superior portion of the cerebrum

  • Anterior communicating br.: anastomosis between right and left anterior cerebral aa.

• Middle cerebral a.: travels in the lateral fissure between temporal and parietal lobes to supply the lateral portion of the cerebrum

• Posterior communicating br.: anastomosis between the ICA and the posterior cerebral a.; connects ICA branches to vertebral a. branches

The posterior brain – including the posterior cerebrum, cerebellum, and brainstem – are supplied by branches of the vertebral arteries. As the vertebral arteries pass through the foramen magnum, they give rise to the posterior inferior cerebellar artery (PICA) and anterior spinal artery before merging into the basilar artery. The basilar artery then travels anteriorly across the surface of the brainstem and further divides into branches that will serve the posterior brain.

Vertebral Artery Branches

• Posterior inferior cerebellar a. (PICA): supplies inferior cerebellum, medulla oblongata

• Anterior spinal a.: right and left branches merge to form a single artery that travels along and supplies the anterior aspect of the spinal cord

Basilar Artery Branches

• Anterior inferior cerebellar a.: supplies anterior aspect of inferior cerebellum, inferior pons

• Superior cerebellar a.: supplies superior cerebellum, superior and middle pons

• Posterior cerebral a.: travels along the inferior surface of the cerebrum to supply the occipital lobe and inferior portions of the temporal lobe

Posterior communicating br.: anastomosis between the posterior cerebral a. and ICA

The cerebral arterial circle (Circle of Willis) provides collateral arterial circulation for the brain by forging connections between the anterior and posterior arteries, as well as between right- and left-sided arteries. The posterior communicating branches connect the ICA posteriorly to the posterior cerebral arteries, which are the terminal branches of the basilar artery. The basilar artery itself is formed by the merger of the right and left vertebral arteries, and the anterior communicating branch completes the circle by connecting the right and left anterior cerebral arteries. This circular structure provides collateral blood flow in the event of a blockage, but also increases the risk of formation of saccular (Berry) aneurysms for its components due to its multiple sources and consistent changes in blood flow direction.

6. Map out the dural venous sinuses. Where do the majority of the sinuses eventually drain?

Individual veins draining the parenchyma of the brain empty into dural venous sinuses before exiting the cranium via the internal jugular vein. These sinuses are formed between the two layers of cranial dura mater: the periosteal layer remains attached to the cranial skeleton while the meningeal layer pulls away to follow the contours and fissures of the brain. This creates a space for venous blood to collect. In addition to veins from the brain, the venous sinuses may also receive blood via the ophthalmic vein and emissary veins that connect externally to veins of the scalp.

Because these sinuses form where the meningeal and periosteal layers of the dura mater separate, the majority are closely related to dural infoldings and partitions:

Falx cerebri:

• Superior sagittal sinus

• Inferior sagittal sinus

Tentorium cerebelli:

• Transverse sinus

• Straight sinus

• Confluens of sinuses

• Superior petrosal sinus

Falx cerebelli:

• Occipital sinus

Other sinuses:

• Sigmoid sinus

• Cavernous sinus

• Inferior petrosal sinus

The venous sinuses join to form two pathways for venous blood to flow to the internal jugular veins. The internal jugular vein forms at the jugular foramen in the posterior cranial fossa and drains the majority of blood from the cranial cavity.

7. Identify the anterior, middle, and posterior cranial fossae. What are the pertinent foramina/fossae/canals in each, and what do they transmit?

Anterior Cranial Fossa

• Boundaries:

  • Anterolateral: frontal bone

  • Posterior: lesser wing of sphenoid

  • Floor: frontal, ethmoid, lesser wing of sphenoid

• Contents:

  • Frontal lobe of brain

  • Olfactory bulb, tracts, and nerve (CN I)

Middle Cranial Fossa

• Boundaries:

  • Anterior: lesser wing of sphenoid

  • Lateral: temporal and parietal bones

  • Posterior: petrous portion of temporal bone

  • Floor: parietal bone, temporal bone, greater wing of sphenoid

• Contents:

  • Brain:

    • Temporal lobe of brain

    • Pituitary gland

  • Vasculature:

    • Internal carotid a. 

    • Ophthalmic a. & vv.

    • Middle meningeal a.

    • Cavernous sinus

  • Cranial nerves:

    • Optic n. (CN II)

    • Oculomotor n. (CN III)

    • Trochlear n. (CN IV)

    • Trigeminal n. (CN V)

    • Abducens n. (CN VI)

Posterior Cranial Fossa

• Boundaries:

  • Anterolateral: petrous portion of temporal bone

  • Posterolateral: occipital bone

  • Floor: occipital bone

• Contents:

  • Brain:

    • Cerebellum

    • Brainstem

  • Vasculature:

    • Vertebral and basilar aa.

    • Dural venous sinuses

    • Internal jugular vein

  • Cranial nerves

    • Facial n. (CN VII)

    • Vestibulocochlear n. (CN VIII)

    • Glossopharyngeal n. (CN IX)

    • Vagus n. (CN X)

    • Accessory n. (XI)

    • Hypoglossal n. (XII)

8. What are the three types of spinal and cranial meninges? What is the difference between cranial and spinal dura mater? Where are dural infoldings/partitions located?

The spinal and cranial meninges are a series of three (3) membranes that envelope and protect the spinal cord and brain. These meninges are separated by either actual or potential spaces, both from one another and the vertebral canal. From superficial to deep, these are:

1. Vertebral (spinal) canal

• Epidural space: 

  • Actual space extending from foramen magnum to the sacrum

  • Filled with fat, vasculature, lymphatics

2. Dura mater (“tough mother”): thickest of the meninges, made of dense connective tissue

   • Cranial dura has two distinct layers that separate to form dural infoldings and sinuses

     • Periosteal layer: lines the bone of the cranial cavity

     • Meningeal layer: lines the brain and follows its major contours (ie: into fissures between hemispheres and cerebrum/cerebellum, but not into smaller sulci)

• Subdural space: potential space, may fill will blood after injury

3. Arachnoid mater (“spidery mother”): thin, transparent membrane

   • Arachnoid trabeculae: thin projections through subarachnoid space to connect with pia mater

   • Arachnoid granulations: tufts of elaborately folded arachnoid that project into dural venous sinuses

     • Used to return cerebrospinal fluid, which is a filtrate of blood, back into the circulatory system

• Subarachnoid space:

  • Actual space surrounding entire spinal cord and brain

  • Filled with cerebrospinal fluid (CSF), vasculature

4. Pia mater (“delicate mother”): also a thin, transparent membrane; adheres to the surface of the brain and spinal cord and follows their contours (ie: fissures, sulci, etc.); is not dissectable away from the surface of the CNS

The meningeal layer of the cranial dura mater follows the major fissures of the brain. In doing so, it creates several dural infoldings (two layers of meningeal dura folded against one another) that assist in anchoring and stabilizing the brain. These infoldings are also often associated with dural venous sinuses.

• Falx cerebri: a sickle-shaped (falx: sickle, Latin), dural partition spanning the longitudinal fissure between the right and cerebral hemispheres

  • Attachments: 

    • Anterior: crista galli (of ethmoid)

    • Posterior: tentorium cerebelli and internal occipital protuberance

  • Dural sinuses: 

    • Superior sagittal sinus (upper margin)

    • Inferior sagittal sinus (lower, free margin)

• Falx cerebelli: a small, sickle-shaped partition between the cerebellar hemispheres

  • Attachments: occipital bone

  • Dural sinuses: occipital sinus

• Tentorium cerebelli: a crescent-shaped dural partition between the occipital lobe of the cerebrum and the cerebellum. Its internal border is free and defines the tentorial notch.

  • Attachments: occipital and temporal bones

  • Dural sinuses: 

    • Transverse sinus

    • Superior petrosal sinus

• Diaphragma sellae: a horizontally oriented partition that covers the sella turcica of the sphenoid bone and the pituitary gland. A central aperture allows the hypophyseal stalk (infundibulum) to pass through

  • Attachments: sella turcica (of sphenoid)

  • Dural sinuses: none

9. What is cerebrospinal fluid (CSF)? What is CSF’s function and where is it located? Describe the ventricular system and discuss the role of arachnoid villi/granulations in maintaining flow of CSF.

Cerebrospinal fluid (CSF) is an ultrafiltrate of blood plasma produced by the choroid plexus. The choroid plexus is a vascular network that lines the wall of the brain's ventricular system (hollow spaces within the brain). CSF is secreted into and fills the ventricles, then flows into the subarachnoid space, which envelopes the entirety of the brain and spinal cord. CSF provides protection to the central nervous system, especially from impacts, and also carries nourishment to and waste away from nervous tissue. CSF is continually secreted and reabsorbed into the bloodstream through arachnoid granulations that protrude into the dural venous sinuses.

The ventricular system is an interconnected series of hollow spaces within the brain. Its walls support the choroid plexus and its connected chambers provide a pathway for CSF to enter the subarachnoid space. There are four ventricles and their associated communications.

• Lateral ventricles (first and second ventricles)

  • Location: deep cerebral hemispheres

  • Communication: each lateral ventricle has an interventricular foramen that opens to the third ventricle

• Third ventricle

  • Location: midline, between the right and left halves of the diencephalon

  • Communication:

    • Interventricular foramina from lateral ventricles

    • Cerebral aqueduct to fourth ventricle

• Fourth ventricle

  • Location: midline between the pons, cerebellum, and medulla oblongata

  • Communication: 

    • Cerebral aqueduct from third ventricle

    • Median (1) and lateral (2) apertures to the subarachnoid space

Cerebrospinal fluid is reabsorbed back into the bloodstream through the dural venous sinuses. Elaborately folded tufts of arachnoid mater called arachnoid granulations project into the dural sinuses and allow CSF to flow from the subarachnoid space back into the venous blood. 

CSF Circulation Pathway

1. Lateral ventricle

2. Interventricular foramen

3. Third ventricle

4. Cerebral aqueduct

5. Fourth ventricle

6. Medial or lateral apertures

7. Subarachnoid space

8. Arachnoid granulations

9. Dural venous sinus

10. Describe the major regions of the spinal cord and spinal gray matter. What types of neurons are housed in each region?

The spinal cord is contiguous with the brain at the medulla oblongata and travels inferiorly through the foramen magnum and vertebral canal. Thirty-one pairs of spinal nerves exit the vertebral canal through intervertebral foramina (spaces created between the posterior aspects of two consecutive vertebrae). Each spinal nerve is assigned a letter denoting the region it originates and a number indicating its position in that region. There are 8 cervical spinal nerves, C1 – C8.

The spinal cord proper typically ends between the L1 and L2 vertebrae, however, the spinal nerves for lower levels continue to travel within the vertebral canal to the lowest vertebral levels of the sacrum and coccyx. 

The circumference of the spinal cord varies across its length and becomes markedly larger in the cervical and lumbar regions. These swellings are named the cervical expansion (C5 – T1) and lumbar expansion (L2 – S3), respectively, and are closely related to the origin of the upper and lower limbs. Muscular limbs require greater motor and sensory inputs than the trunk alone, so the nuclei at these vertebral levels are accordingly larger to accommodate the additional neurons.

The gray matter (composed of neuron cell bodies) of the spinal cord is centrally positioned and surrounded by white matter (neuron axons). The gray matter is arranged in an approximate “H” shape with two dorsally oriented horns and two ventrally oriented horns. The right and left sides of the body are each served by the ipsilateral dorsal horn and one ventral horn. Motor nuclei are housed in the ventral horns and sensory nuclei in the dorsal horns. 

11. What is the “big picture” structure of a single spinal nerve, and what are the various functions of the parts of a spinal nerve?

Thirty one (31) spinal nerves originate from the spinal cord and are named for the one vertebrae they travel between to exit the vertebral canal. For the thoracic, lumbar, and sacral regions, this namesake is the vertebra the spinal nerve exits inferior to. In the cervical region, however, the opposite is true: the 8 cervical spinal nerves take the name of the vertebra they exit superior to. This is because the first spinal nerve, C1, exits between the occipital bone of the cranium and the C1 vertebra.

General Structure

Spinal nerves are a collection of motor and sensory neurons traveling from the spinal cord that innervate a specific, spatially adjacent region of the body, or dermatome. Their structure is similar to that of a tree, which collects input from multiple roots into a single trunk, then divides into branches. 

Roots

The roots of a spinal nerve are formed of several smaller rootlets that originate from one of the two ipsilateral horns of the spinal cord. The dorsal root, therefore, comprises axons of somatic sensory neurons while the ventral root is made up of motor neurons (somatic and sympathetic motor).

Dorsal Root Ganglion

Sensory neurons are pseudounipolar and have a branched axon. One branch of the axon communicates with the periphery via a spinal nerve and its branches, the other communicates with the sensory nuclei found in the dorsal horn of the spinal cord. The cell bodies of peripheral sensory neurons that contribute to a spinal nerve, then, do not sit within the CNS, but rather are clustered together on that spinal nerve’s dorsal root. A cluster of neuron cell bodies in the PNS is termed a ganglion, and these specifically take the name dorsal root ganglia (= pl. ganglion).

Trunk

The trunk of a spinal nerve receives both sensory fibers from the dorsal root and motor fibers from the ventral root. Branches, or rami, of the spinal nerve are also mixed modality. 

Primary Rami

The trunk of a spinal nerve is very short – typically less than 1 cm. Its first division produces two rami: the large primary ventral ramus (VPR) and the smaller dorsal primary ramus (DPR). These are named for the regions of the body wall they innervate. The territory of the DPR is limited to the intrinsic muscles of the spine and the skin overlying them. The VPR covers a much larger territory, including the anterior trunk and all 4 limbs.

12. How is the sympathetic nervous system structured? What are the four basic pathways of preganglionic sympathetic neurons? What are the major sources of sympathetics to the head and neck, and how do they travel within the head and neck to reach their targets? What is Horner’s Syndrome and what are its symptoms? 

The sympathetic nervous system is a visceral (involuntary) motor system that provides visceral motor innervation to smooth muscle structures in both the body cavity and body wall. It is most commonly associated with the “flight or fight” response because it is active during periods where the body must expend energy at a higher rate than normal. Examples of sympathetic effects on viscera include:

• Increased heart rate

• Dilation of the conductive pathways of lungs 

• Reduced gut function

• Increased sweat secretion

• Dilation of pupils

• Activation of smooth muscles like arrector pili

General Structure

Sympathetic innervation is carried by paired, two-neuron chains in the peripheral nervous system (PNS). The cell body of the proximal, or preganglionic neuron is rooted in the intermediolateral (IML) horn of the spinal cord, which is present only between spinal levels T1 – L2. For this reason, sympathetic innervation is often referred to as “thoracolumbar” outflow. The postganglionic neuron may be found in one of several locations, depending on the region of the body they serve. For structures of the head and neck, postganglionic neurons are rooted in a longitudinal chain of sympathetic ganglia that sits parallel to the vertebral column. Because of their position, these are referred to as paravertebral ganglia, and the chain they form is alternately called the sympathetic trunk, or the sympathetic chain. Sympathetic neurons serving the head and neck will synapse within the sympathetic trunk, then travel to their targets via branches of spinal nerves.

Sympathetic Trunk

The sympathetic trunk is a series of consecutively connected sympathetic ganglia that allows the sympathetic nervous system to expand beyond its limited origin (spinal levels T1 – L2 only) to travel along every spinal nerve (C1 – Co1) and supply structures from the top of the head to the tips of the toes. The sympathetic trunk is connected to the VPR of spinal nerves directly by one or two short rami communicantes (singular: ramus communicans). 

1. White rami communicantes carry preganglionic sympathetic fibers from the VPR of each spinal nerve T1 – L2 into the paravertebral ganglion associated paravertebral ganglion of the sympathetic trunk.

2. Gray rami communicantes carry postganglionic sympathetic fibers from the sympathetic trunk back to the VPR of all spinal nerves.

Once the axon has arrived in the paravertebral ganglion, there are several routes it may take to reach head and neck viscera. 

1. Synapse in the same ganglion

a. The axon of the postganglionic neuron will then exit the sympathetic chain at this same level via the gray ramus communicans to rejoin the VPR of the same spinal nerve.

b. Can only occur between spinal levels T1 – L2

2. Descend the sympathetic trunk

a. Synapse will occur in a lower paravertebral ganglion. The postganglionic fibers will exit the sympathetic trunk via the gray ramus communicans associated with the VPR of the lower spinal nerve.

b. Can occur at any level below T1; spinal nerves L3 – Co1 can only be reached this way.

3. Ascend the sympathetic trunk

a. Synapse will occur in a higher paravertebral ganglion. The postganglionic fibers will exit the sympathetic trunk via the gray ramus communicans associated with the VPR of the higher spinal nerve.

b. Can occur at any level above L2; spinal nerves C1 – C8 can only be reached this way.

Cervical Sympathetic Trunk

The cervical sympathetic trunk is somewhat unique in that the paravertebral ganglia associated with these spinal levels fuse into larger ganglia. There are typically 3 large ganglia in the neck:

1. Superior cervical ganglion

a. C1 – C4

b. Source of postganglionic sympathetic fibers to the head

i. No spinal nerves in the head! These travel along the surface of the ICA as perivascular plexuses.

2. Middle cervical ganglion

a. C5 – C6

b. Source of postganglionic sympathetic fibers to the thyroid gland

c. Often absent

3. Inferior cervical ganglion

a. C7 – C8

b. Most often fuses with T1 as well to form the cervicothoracic (stellate) ganglion

Horner’s Syndrome

Horner’s syndrome is caused by interruption of the sympathetic innervation to the head. This can happen as a first order interruption within the brain and spinal cord, or as a second order interruption along the thoracic or cervical sympathetic trunks and ICA. The most common cause of Horner’s syndrome is injury to the head and neck during delivery.

Symptoms typically present ipsilaterally (same side as the interruption) and include:

• Miosis: constricted pupil from denervation of dilator pupillae m.

• Ptosis: drooping eyelid from denervation of smooth muscle serving palpebra

• Pseudo-enophthalmos: sunken globe appearance of eye from ptosis

• Hyperemia: flushed skin from denervation of vasomotor fibers

• Anhidrosis: lack of sweating from denervation of sudomotor fibers