S4 Cranial Cavity, Venous Sinuses and Brain - Learning Objectives
1. Identify the anterior, middle, and posterior cranial fossae. What are the pertinent foramina/fossae/canals in each, and what do they transmit?
Boundaries of the cranial fossae:
Middle cranial fossa
Posterior cranial fossa
2. What are the major parts of the brain?
Cerebrum: The cerebrum is the largest component of the brain, and consists of the R & L cerebral hemispheres (separated by the longitudinal cerebral fissure) and basal ganglia. The cerebral lobes (frontal, parietal, temporal, and occipital) do not correspond precisely with the similarly named bone.
Brainstem: The brainstem consists of the midbrain, pons, and medulla oblongata. Midbrain is the most proximal portion of the brainstem, pons is intermediate, and medulla oblongata is the most distally-oriented.
Cerebellum: The cerebellum is inferior to the cerebrum and posterior to the brainstem. Similar to the cerebrum, the cerebellum has two hemispheres connected by the vermis.
Hypophysis (pituitary gland): Hypophysis is located within the hypophyseal fossa of the sphenoid bone. This gland is continuous with the infundibulum, a component of the hypothalamus.
3. What are the three types of cranial meninges? What is the difference between cranial and spinal dura mater? Where are dural infoldings/partitions located?
- Pia mater ("delicate mother")
- is a delicate investment that is closely applied to the brain and dips into fissures and sulci.
- enmeshes blood vessels on the surface of the brain.
- Arachnoid mater ("spidery mother")
- is a filmy, transparent, layer that is connected to the pia mater via trabeculae.
- is separated from the pia mater by the subarachnoid space, which is filled with cerebrospinal fluid (CSF). The subarachnoid space may contain blood after hemorrhage of a cerebral artery (formation of subarachnoid hematoma).
- projects into the superior sagittal sinus to form arachnoid villi, which serve as sites where CSF diffuses into the blood.
- Arachnoid granulations
- are tuft-like collections of highly folded arachnoid that project into the superior sagittal sinus and its lateral lacunae (lateral extensions of the superior sagittal sinus).
- release CSF into the superior sagittal sinus and often produce erosion or pitting (granular foveolae) of the inner surface of the calvaria.
- Arachnoid granulations
- Dura mater ("tough mother")
- outermost covering
- two layers in the cranium (one in the spinal canal)
- periosteal dura - lines the cranial cavity
- meningeal layer - inner layer that is sometimes separated from the periosteal layer, forming dural venous sinuses and partitions
Dural partitions are elaborations of the meningeal layer of cranial dura mater. There are 4 partitions: falx cerebri, falx cerebelli, tentorium cerebelli, and diaphragma sellae.
The tentorial notch is a distinct curved notch in the tentorium cerebelli. The notch also separates the tentorium cerebelli from the diaphragma sellae.
4. What is cerebrospinal fluid (CSF)? What is CSF’s functions, and where is it located? Describe the ventricular system.
CSF is formed in the brain from arterial blood supplying the choroid plexuses of the ventricles, and drains via arachnoid granulations projecting into the superior sagittal sinus.
- Parts of the ventricular system
- The cerebral hemispheres are hollow, each containing a lateral ventricle. The ventricles contain secretory tissue, choroid plexuses, which convert arterial blood into cerebrospinal fluid (CSF). The lateral ventricles communicate with the midline third ventricle by way of the interventricular foramen. A thin membrane with an attached choroid plexus roofs the third ventricle. In the midbrain, the narrow cerebral aqueduct connects the third and fourth ventricles.
- The fourth ventricle lies between the pons, cerebellum, and the medulla. It communicates with the cerebral aqueduct, the central canal of the spinal cord, and the subarachnoid space. The roof of the fourth ventricle also has a choroid plexus. The roof of the fourth ventricle is perforated by a small median aperture and two lateral apertures that allow cerebrospinal fluid to exit the ventricular system and bathe the brain and spinal cord.
- The flow of CSF from production to reabsorption:
- CSF is secreted (produced) by the choroidal epithelial cells of the choroid plexuses in the lateral, third, and fourth ventricles.
- CSF leaves the lateral ventricles through the interventricular foramen and enters the third ventricle. CSF then passes through the cerebral aqueduct into the fourth ventricle. CSF leaves this ventricle through its median and lateral apertures and enters the subarachnoid space, which is continuous around the spinal cord and brain. The arachnoid forms borders of cisterns (spaces around the brain), filled with CSF.
- Reabsorption of CSF (reabsorption into the venous system) - the main site of CSF absorption (reabsorption) into the venous system is through arachnoid granulations. The subarachnoid space containing CSF extends into the arachnoid granulations, which in turn project upward through the dura into the superior sagittal sinus and lateral projections from it called lateral lacunae.
5. Map out the dural venous sinuses. Where do the majority of the sinuses eventually drain?
6. What are the two main arteries supplying the brain? What is the clinical importance of the cerebral arterial circle (circle of Willis)? What arteries make up the arterial circle?
- Internal carotid a. - give rise to the:
- Ophthalmic a,
- Posterior communicating a,
- Anterior cerebral a, and
- Middle cerebral a.
- Vertebral a.
- Anterior spinal a. - paired branches that unite in the midline.
- Posterior inferior cerebellar aa., from which arise the
- Posterior spinal arteries
- Basilar a. - formed by union of the vertebral arteries, it gives rise to:
- Anterior inferior cerebellar aa.,
- Superior cerebellar aa., and bifurcates into the
- Posterior cerebral aa.
- Cerebral arterial circle (Circle of Willis) - forms an important means of collateral circulation in case of obstruction. The circle itself is an example of collateral circulation, but branches of the circle are end arteries and there is little collateral circulation in the brain itself. Formed by the union of the anterior cerebral, anterior communicating, posterior communicating, and posterior cerebral arteries.
7. Understand the basics of the cranial nerves. Name, number, sensory (afferent)/ motor (efferent)/both, and what general structures do they innervate?
8. Understand the difference between epidural (extradural), subdural (dural border), and subarachnoid hemorrhage/hematoma. What is a brain infarction?
The term hemorrhage is an umbrella term for blood loss, while hematoma indicates a hemorrhage within an existing tissue space.
Brain infarction is an area of dead (necrotic) tissue in the brain, often areas of the cerebrum. This is typically caused by stenosis of an artery (such as the common carotid aa., internal carotid aa., or vertebral aa.), usually by an accumulation of plaque within the artery. A portion of the plaque (an embolus) may dislodge and travel in the bloodstream until it cannot pass (gets stuck) in a smaller, intracranial branch, typically causing insufficient arterial supply to that region of the brain (acute cortical infarction).