Cranial Cavity, Brain, and Spinal Cord 

Learning Objectives


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:


Viscerocranium:


"OpenStax AnatPhys fig.7.9 - Posterior View Skull - English labels" by OpenStax, license: CC BY. Source: book 'Anatomy and Physiology', https://openstax.org/details/books/anatomy-and-physiology.

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?

4. How is the parasympathetic nervous system structured? How do parasympathetics travel within the head and neck to reach their targets? What are the 4 ganglia associated with the parasympathetic supply of head and neck structures? 

The parasympathetic nervous system is an autonomic (involuntary) motor system. Its generalized function is often described as “rest and digest” – that is: it slows bodily functions to conserve existing energy stores and accelerates the process of bringing new energy into the body. Examples of this include decreasing heart rate, constricting the bronchi, stimulating digestive processes, constricting the pupil, and increasing exocrine gland secretion.


The primary function of the parasympathetic nervous system in the head and neck is to stimulate exocrine gland secretion. Its targets include:



Parasympathetic pathways involve a chain of two synapsing neurons that travel through the periphery to reach target structures. The first of these – the preganglionic neuron – has a cell body located in the central nervous system and an axon that travels peripherally as part of a nerve. For head and neck pathways, these cell bodies are found in the brainstem and their axons travel as part of cranial nerves. For distal organs of the digestive tract and pelvis, these preganglionic neurons originate in the sacral spinal cord and travel via sacral splanchnic nerves. Because of this dual origin, the parasympathetic system is said to have “cranio-sacral outflow.” We will focus on the head and neck pathways for this course.


Preganglionic neuron summary:



The second neuron in the chain – called the postganglionic neuron – is located entirely in the peripheral nervous system. The cell bodies of postganglionic neurons cluster together to form ganglia, and synapse between parasympathetic neurons of the head and neck occur here. In the head, these ganglia are large and individually named. After synapse, the postganglionic axon then travels from the ganglion to the target structure via cranial nerves.


Postganglionic neuron summary:

Each named ganglion therefore is associated with a specific parasympathetic pathway. In practice, this means that each ganglion receives preganglionic axons from only one cranial nerve, and sends postganglionic axons along only one (potentially different) cranial nerve to reach their targets. “Hitchhiking” occurs when the parasympathetic axons traveling with one cranial nerve branch off to join a second cranial nerve. 

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


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



Basilar Artery Branches

Posterior communicating br.: anastomosis between the posterior communicating 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 blow 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:


Tentorium cerebelli:


Falx cerebelli:

Other sinuses:

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

Middle Cranial Fossa

Posterior Cranial Fossa

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:








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.


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.


"Blausen 0896 - Ventricular system of the brain - English labels " by Bruce Blaus, license: CC BY. Source: "Wikimedia Commons: category: Images from Blausen Medical Communications" https://commons.wikimedia.org/wiki/Category:Images_from_Blausen_Medical_Communications

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


CSF Circulation Pathway

"OpenStax AnatPhys fig.13.18 - CFS Circulation - English labels " by OpenStax, license: CC BY. Source: book 'Anatomy and Physiology', https://openstax.org/details/books/anatomy-and-physiology.

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.

"KnowledgeWorks - Drawing Spinal nerves and plexuses - English labels" at AnatomyTOOL.org by KnowledgeWorks Global Ltd., license: Creative Commons Attribution

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:


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). 



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


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:

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.

a. C5 – C6

b. Source of postganglionic sympathetic fibers to the thyroid gland

c. Often absent

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: