PDF Download

1. Identify and describe the major features of the brain.

The brain and the spinal cord comprise the central nervous system. It is important to be able to identify and describe the major features of the brain to be able to relate to the surrounding structures of the brain, such as the meninges, bone, and neurovasculature.

Cerebrum

The cerebrum is the largest component of the brain, and consists of the R. & L. cerebral hemispheres and basal ganglia (composed of subcortical nuclei - recall nuclei are collections of cell bodies of neurons in the CNS).

The cerebral hemispheres are the dominant structures of the cerebrum (and the brain as a whole), and in a superficial survey, the gyri (folds of cerebral cortex), fissures (prominent clefts), and sulci (grooves) are easily observable.

Of particular note, the longitudinal cerebral fissure (which runs in a sagittal plane) separates the R. & L. cerebral hemispheres, and also houses the falx cerebri.

The cerebral lobes (frontal, parietal, temporal, and occipital) do not correspond precisely with the similarly named bone.

Diencephalon

The diencephalon is often considered the ‘central core’ of the internal brain. As such, this portion of the brain is only seen in deep dissections or views of the brain. It connects the forebrain (cerebrum + diencephalon + lateral & 3rd ventricles) to the brainstem. It consists of several structures, such as the dorsal thalamus, hypothalamus, and epithalamus. 

Hypophysis (pituitary gland)

The hypophysis is located in the hypophysial fossa of the sphenoid bone. This gland is continuous with the infundibulum, a component of the hypothalamus.

Brainstem

The brainstem consists of the midbrain (proximal; rostral), pons (intermediate), and medulla oblongata (distal; caudal). At the level of the foramen magnum, the medulla oblongata will transition into the spinal cord. These divisions are often easiest to see in a hemisected brain or inferior view.

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.

2. Identify the cranial meninges and their features and differentiate between cranial and spinal dura mater.

The central nervous system (CNS) is surrounded and protected by three layers of meninges. From superficial to deep, they are arranged: dura, arachnoid, & pia mater. The dura mater of the brain is more specialized with 2 layers, whereas there is only 1 layer associated with the spinal cord. At the foramen magnum, the cranial and spinal dura mater are continuous. 

Dura mater (‘tough mother’)

The two layers of the cranial dura mater are:

• Periosteal (endosteal) dura - lines the cranial cavity

• Meningeal dura - inner layer of dura that is sometimes separated from the periosteal layer, forming dural partitions and dural venous sinuses. Dural venous sinuses will drain mostly deoxygenated blood from the brain, and will be discussed in more detail in an upcoming learning objective.

In certain areas, there are elongations of the meningeal layer of the cranial dura mater, referred to as dural partitions, which serve to separate and create compartments within the cranial cavity, as well as stabilize the brain.

The two largest dural partitions include the:

• Falx cerebri - located in the longitudinal fissure between the cerebral hemispheres and is associated with the superior & inferior sagittal sinuses

• Tentorium cerebelli - located between the occipital lobe of the cerebrum and the cerebellum, so in essence separating the forebrain from the hindbrain

  • Its internal (anterior) concave border is free and defines the tentorial notch, where the midbrain sits

    • Tentorial herniation: pathological condition where portions of the brain (such as the temporal lobe of cerebrum) herniate into tentorial notch compressing contents, such as midbrain, oculomotor nn. (CN III), and posterior cerebral aa.

Arachnoid mater (‘spidery mother’)

The arachnoid mater is a filmy and transparent layer that is adhered to the dura mater and connected to the pia mater via trabeculae. It is separated from the pia mater by the subarachnoid space, which is filled with cerebrospinal spinal fluid (CSF).

Pia mater (‘delicate mother’)

The pia mater is a delicate investment that is closely adhered to the brain, and enmeshes blood vessels on the surface of the CNS.

3. Diagram the location (with respect to meninges) of epidural (extradural), subdural (dural border), and subarachnoid hemorrhages/hematomas.

In order to understand the basics of some of the more common cranial hemorrhages/hematomas, it is important to understand the anatomical basis of the spaces created by cranial meninges.

Some of these spaces are naturally occurring (real spaces), and some are potential spaces, which may become a real/actual space with the entrance of extravasated blood that separate normally adherent structures.

The meningeal spaces include (from superficial to deep):

• Epidural space: between bone and periosteal dura mater

  • Potential space in cranial region; actual space in spinal region

  • Epidural (extradural) hemorrhage/hematoma: 

    • Blood typically arterial in origin, commonly middle meningeal a. branches

• Subdural space: between dura mater and arachnoid mater

  • Potential space

  • Subdural (dural border) hemorrhage/hematoma: 

    • Blood typically venous in origin; commonly bridging veins associated with superior cerebral v. or superior sagittal sinus

• Subarachnoid space: between arachnoid mater and pia mater among arachnoid trabeculae

  • Actual space

  • Contents include: cerebrospinal fluid (CSF), arteries & veins near surface of brain, intracranial portion of cranial nerves

  • Subarachnoid hemorrhage/hematoma: blood source varies, but commonly a result of ruptured aneurysm 

4. Define dural venous sinus. Identify the superior sagittal & cavernous sinuses, and relate the location where the majority of these sinuses drain?

The cranial dura mater is composed of two, typically fused layers: periosteal (outer) and meningeal (inner). In certain areas, the layers separate, forming dural venous sinuses. Dural venous sinuses allow drainage of blood from the veins of the brain, such as the cerebral veins, to the internal jugular vein at the jugular foramen where the majority of these sinuses drain. The venous sinuses are similar to veins in that they drain relatively de-oxygenated blood, but differ in wall composition.

There are 13 named (some paired, some not) dural venous sinuses, and many of these are associated with dural partitions.

Two of the largest and most observable sinuses include:

• Superior sagittal sinus: runs the length of the superior falx cerebri, the dural partition that runs the between the cerebral hemispheres in the longitudinal fissure.

• Cavernous sinuses (paired): relatively expansive sinuses lateral to the body of the sphenoid.

  • Many structures traverse the cavernous sinus, including the internal carotid a. (ICA) and the closely associated carotid plexus of sympathetic nerves, as well as the abducens n. (CN VI)

    • The oculomotor n. (CN III), trochlear n. (CN IV), and ophthalmic division of the trigeminal n. (CN V1) run in the lateral walls of the cavernous sinus, with the maxillary division of trigeminal n. (CN V2) running deep to the dura in this area.

    • Cavernous sinus thromboses may endanger these structures.

5. Diagram the ventricular system, and describe the location and generalized flow of cerebrospinal fluid (CSF).

The ventricular system of the brain is composed of a series of spaces lined with ependymal cells (a type of neuroglia) and their connections within the brain that allows for production and flow of cerebrospinal fluid (CSF) .

Lateral ventricles 

1st & 2nd ventricles

Hollow spaces within each cerebral hemispheres

C-shaped with a central body and 3 horns (anterior, posterior, & inferior)

↓

Interventricular foramina

Connect the lateral ventricles to the 3rd ventricle

↓

3rd ventricle

Located in midline of portions of the diencephalon 

↓

Cerebral aqueduct

Connects the 3rd ventricle to the 4th ventricle

↓

4th ventricle

Located between the pons/medulla and cerebellum

↓

Subarachnoid space (via apertures of 4th ventricle) OR central canal of spinal cord 

(mostly vestigial remnant of the ventricular system of spinal cord)

• Apertures include the median aperture (foramen of Magendie) and lateral apertures (foramina of Luschka) that lead into subarachnoid cisterns. 

• Subarachnoid cisterns are comparatively expansive subarachnoid spaces (larger span between the arachnoid and pia mater) that are prevalent in the region of the brain. These cisterns are continuous with the more generalized subarachnoid space.

• The subarachnoid space of the brain and spinal cord are continuous allowing CSF to flow around all components of the CNS.

Cerebrospinal fluid (CSF) is formed in the brain from arterial blood supplying the choroid plexuses located on the walls of the ventricles, and is reabsorbed mostly into the venous system via arachnoid granulations. Arachnoid granulations are extensions of arachnoid mater and associated subarachnoid space that project into the dural venous sinuses, in particular the superior sagittal sinus and can be grossly visible in this area and its lateral projections, the lateral lacunae. If normal, the CSF is clear and continually flowing and draining.

Hydrocephalus (abnormal accumulation of CSF leading to enlargement of portions of the ventricular system) typically occurs with an obstruction leading to accumulation of CSF. These obstructions are most common in the narrow channels or foramina. A blockage of the intervertebral foramen would lead to an accumulation of CSF in the lateral ventricles. A blockage of the cerebral aqueduct would lead to an accumulation of CSF in both the 3rd ventricle and lateral ventricles, and an obstruction of the apertures associated with the 4th ventricle may result in overall enlargement of the entire ventricular system. Occasionally, hydrocephalus can be caused by ineffective absorption of CSF.

6. Recognize the two major arteries that supply the brain and diagram the cerebral arterial circle (of Willis).

There are two major arterial pairs that supply the brain: internal carotid aa. (which supply most of the cerebrum/forebrain region) and vertebral aa. (which supply the brainstem and cerebellum, as well as more posterior portions of the cerebrum).

Internal carotid aa. (ICA)

• The internal carotid aa. arise at the bifurcation of the common carotid aa. in the neck. There are no ICA branches in the neck.

• The ICA will enter the skull via the carotid canal of the temporal bone.

• The ICA (with branches) is often referred to as the anterior circulation of the brain.

• Will give rise to numerous branches, including:

  • Ophthalmic aa. (supplies orbit and eye region)

  • Posterior communicating aa. (communicating branch between ICA and posterior cerebral aa., which is a branch of the basilar a.)

    • Connection between the ICA and vertebrobasilar system

  • Anterior cerebral aa. - the smaller terminal branches

    • There will be a small anterior communicating a. that will connect the R. & L. anterior cerebral aa. 

    • Supplies anterior, medial, and superior surfaces of the cerebrum

  • Middle cerebral aa. - the larger terminal branches

    • Supplies the middle or lateral portions of the cerebrum

Vertebral aa.

• The vertebral aa. are a branch of the 1st division of the subclavian a., will ascend through the transverse foramina of the C6-C1 vertebrae, and will enter the skull via the foramen magnum.

• Will give rise to numerous branches, including:

  • Posterior inferior cerebellar aa. (PICA) (supplies portions of the cerebellum and distal brainstem)

  • Basilar a. - this formed by the union of the vertebral aa. approximately at the junction of the pons & medulla oblongata

    • Often the area supplied by branches of the vertebral and basilar aa. is considered supplied by the vertebrobasilar system and can be called posterior circulation

    • Will give rise to numerous branches including:

      • Anterior inferior cerebellar aa. (supplies cerebellum and small portions of brainstem)

      • Superior cerebellar aa. (supplies cerebellum and more proximal portions of the brainstem)

      • Posterior cerebral aa. - terminal branches

        • Supplies the more inferior surfaces of the brain, in particular the occipital lobe of the cerebrum

Circle of Willis (Cerebral arterial circle)

• The circle of Willis forms an important means of collateral circulation at the base of the brain.

  • Will serve to connect the ICA and vertebrobasilar system as well as between branches of ICA

  • There can be significant variation in caliber of the arteries as well as overall pattern of the circle

• Formed by:

  • Posterior communicating aa. - connecting the internal carotid aa. and posterior cerebral aa. (branches of the basilar a.)

  • Anterior communicating aa. - connecting the anterior cerebral aa. (branches of ICA)

Lack or reduced blood flow to a region of the brain (ischemic stroke) can be caused by a multitude of factors. This is not to be confused with a hemorrhagic stroke, which is commonly caused by a subarachnoid hemorrhage caused by a ruptured aneurysm.

7. List the cranial nerves (Roman numeral & name) and basic characteristics of each (sensory/motor/both and general target organs).

Cranial nerves are a component of the peripheral nervous system (PNS), which are composed of sensory (afferent) fibers, motor (efferent) fiber, or both sensory and motor (similar to what occurs with spinal nerves). 

There are 12 pairs of cranial nerves, which have both a name and associated Roman numeral. You will hear these used interchangeably, so it is important to know both.

Fibers of the cranial nerves will connect to specific nuclei in the brain, which are groups of neurons where efferent fibers originate or afferent fibers terminate.

In this learning objective, we will introduce very basic characteristics of the 12 pairs of cranial nerves, including which fibers compose the nerve (sensory/motor/both) and general target organs.