Anatomy of the Brain and Meninges

Question 1

What are the meninges and what do they form?

Answer 1

The meninges are membranous coverings of the brain which not only protect the brain and enclose the fluid filled cavity, but also form the supporting network for arteries, veins and venous sinuses.

The meninges are layers of connective tissue that envelope the central nervous system. Which is the only part of the brain that lies outside the meninges? The meninges consist of 3 layers: dura, arachnoid and pia mater. They separate the brain from the skull (slide 2).

Question 2

What is the pia mater and where is it located?

Answer 2

Pia mater is medieval Latin meaning “tender mother”. The pia mater (slide 3) is the innermost layer of meninges that is firmly attached to the surface of the spinal cord (spinal pia) and brain (cranial pia) by astrocytes in the brain itself. It is microscopically thin and fragile layer composed of fibrous connective tissue (imagine draping wet kitchen towel/ tissue over a brain).

Because of its attachment to the actual cells of the brain and spinal cord, it follows their contours closely into the grooves, fissures and sulci unlike other layers of the meninges. At its caudal end, the filum terminale (find on slide 1) is an extension of the pia mater that is attached to the coccygeal segments, whose function is to suspend the cord in CSF (cerebrospinal fluid).
Small blood vessels run on the pia mater (slide 4) and send perforating capillaries through it to supply the brain.

Question 3

What is the arachnoid mater? What is it connected?

Answer 3

The arachnoid mater (slide 5) is the middle layer of meninges, and is also thin and fragile (imaging draping wet paper over a brain). It is avascular.

There is a real space that exists between the arachnoid and the pia called the subarachnoid space and is filled with cerebrospinal fluid (CSF). The subarachnoid space contains a network of connective tissue strands, blood vessels and nerves and CSF.

The arachnoid and pia mater are connected by fibrous filaments called trabecula that cross the sub-arachnoid space (slide 6) and become continuous with the pia matter. This web-like appearance gives the arachnoid mater its name (“Arachne” (“spider”), the suffix “-oid” (“in the image of”).

Question 4

What is the dura mater? What is it attached to and how is it different to the other meningeal layers?

Answer 4

The dura mater is the outermost layer of the meninges (slide 7). It is a tough fibrous later (Latin: tough mother, imagine a thick layer of card surrounding the brain that can hold its own structure). On its inside surface, arachnoid clings to the dura and neither follow the contours of the brain (unlike the pia). Its outside surface is in direct opposition to the skull. In dissection, when seperating the skull from the brain, the dura is more strongly attached to the skull and often remains attached to it during removal whereas the pia and arachnoid mater remain covering the brain.

The dura mater is much more stiff and robust than the other meningeal layers, and it provides structure for the brain and its vasculature within the skull that will be demonstrated later in this module (including folds, reflections and venous sinuses).

Question 5

What are the dural folds? What are their different names and what do they do?

Answer 5

The dura mater is the outermost layer of the meninges and is a dense fibrous membrane that can invaginate to form dural folds (septa) that separate different brain regions from each other. The largest of these septa is the cerebral falx. It separates the 2 cerebral hemispheres. The falx becomes continuous with the cerebellum tentorium in the midline. The cerebellum tentorium separates the cerebellum from the occipital lobe of the cortex. It covers the posterior fossa structures (hindbrain) and supports the temporal and occipital lobes. It contains a gap – the tentorial notch – through which the brainstem and blood vessels pass to enter the middle cranial fossa. Tumours that occupy this space raise the intracranial pressure and may cause herniation of the temporal lobe (uncus) through this space.

Question 6

What are the dural venous sinuses?

Answer 6

The dura mater can be split into an outer periosteal layer (that lines the bone) and an inner meningeal layer (that lines the arachnoid mater). The dural venous sinuses are endothelium-lined spaces between the periosteal and the meningeal layers of the dura. Large veins from the surface of the brain empty into these sinuses and most of the blood from the brain ultimately drains through them into the internal jugular vein.

Look at the diagram of the dural venous sinuses, paying particular attention to the cavernous sinus. Note in the diagrams how the internal carotid artery and a number of cranial nerves pass through the cavernous sinus. Also note the position of the pituitary gland and the optic chiasma/ optic tracts.

Question 7

What are examples of the dural venous sinuses?

Answer 7

Superior sagital sinus
Inferior sagital sinus
Straight sinus
Confluence of sinuses
Transverse sinuses
Sigmoid sinuses
Cavernous sinuses
Superior petrosal sinuses
Inferior petrosal sinuses

Question 8

What is the tentorium cerebelli?

Answer 8

Separating the cerebellum from the cerebrum. Note how it tents up in the middle to meet the falx cerebri (hence the name - tent of the cerebellum)

Question 9

What is the falx cerebri?

Answer 9

Separates the two cerebral hemispheres

Question 10

What is meningitis and how is it caused? What are its effects?

Answer 10

Meningitis is an acute inflammation of the meninges. Patients clasically experience a combination of fever, headache, neck stiffness, altered consciousness, vomiting, and photosensitivity. It is most commonly caused in the UK by viral infections and can also be caused by bacteria, fungi, parasites and drug reactions. A lumbar puncture can be used to sample cerebrospinal fluid (CSF), to diagnose or exclude bacterial meningitis. Bacterial meningitis is very dangerous as it can be associated with septicaemia and death from septic shock. Other forms of meningitis may not cause septic shock, but the meningeal inflammation alone can cause cerebral oedema, hydrocephalus, raised intracranial pressure cerebral infarction and brain death.

Question 11

What does hydrocephalus look like on a brain scan?

Answer 11

The visualised ventricles are enlarged - this is known as hydrocephalus (or ventriculomegaly).

Question 12

What is the function of the CSF within the cranium?

Answer 12

CSF provides many functions within the cranium:

1. Buoyancy - the CSF that surrounds the brain acts to reduce the weight of the brain. This can also enable to to work as a shock absorber.
2. Physical buffer - the CSF can acts as a physical buffer for the brain. If the pressure within the cranium increases, CSF can be displaced first to help to prevent ischaemia.
3. Homeostasis - acting as a chemical buffer as well, CSF can help to regulate various substances around the brain.
4. Excretion of waste - metabolites excreted by the brain into the CSF can be recirculated into the bloodstream to be excreted via bodily mechanisms.

Question 13

How is CSF generated and where does it circulated?

Answer 13

Cerebrospinal fluid (CSF) is generated by ependymal cells which are found in the chorionic villi within the ventricles, predominantly within the lateral ventricles.

From the lateral ventricles, the CSF passes through the interventricular foramina into the third ventricle.

CSF then flows through the cerebral aqueduct into the fourth ventricle where it can pass into the central canal or leave via the median or lateral apertures.

CSF is then later resorbed into the bloodstream through the arachnoid granulations.

Around 500ml of CSF is generated per day, however only around 150ml of CSF is within the ventricles and surrounds the brain. Therefore, CSF is constantly being produced and absorbed back into the bloodstream.

Question 14

What problems may there be with CSF resorption?

Answer 14

There can be problems with CSF resorption through the arachnoid granulations, as well as obstructions within the system. These situations are pathological and can lead to swelling of the ventricular system called hydrocephalus (or ventriculomegaly).

Communicating hydrocephalus occurs once CSF has left the ventricular system - hence the CSF can still ‘communicate’ or flow between the ventricles.

Non-communicating hydrocephalus (also known as obstructive hydrocephalus) occurs where there is a blockage at one point within the ventricular system (normally where there is a narrower region) so flow cannot occur leading to a proximal build-up of CSF and ventricular enlargement.

Question 15

What are the components of the ventricular system (from top to bottom)?

Answer 15

Right and left lateral ventricles
Interventricular foramen
Third ventricle
Cerebral aqueduct
Fourth ventricle
Central canal

Question 16

What does the cerebral aqueduct do?

Answer 16

Connects the 3rd and 4th ventricles

Question 17

What does the interventricular foramen do?

Answer 17

Connects the lateral ventricles and 3rd ventricle

Question 18

What does the arachnoid granulations do?

Answer 18

Resorption of CSF

Question 19

What does the choroid plexus do?

Answer 19

Production of CSF

Question 20

What would be the location of a brain tumour if the lateral and third ventricles were enlarged?

Answer 20

This would suggest that the obstruction is distal to that, and therefore the most likely answer is the cerebral aqueduct.

Question 21

What is the function of the cerebral cortex?

Answer 21

The cerebral cortex is our secret weapon. It processes complex motor and sensory information and grants us the consciousness to interact with our environment by planning, interpreting, and reacting. The cerebral cortex can be mapped into different regions, which work cooperatively to perform different functions.

Question 22

What does the prefrontal cortex do?

Answer 22

Occupying the frontal pole of the brain, the prefrontal cortex is typically associated with ‘executive function’, including planning and decision making, as well as behaviour and personality

Question 23

What does the premotor cortex do and where is it?

Answer 23

Sits just anterior to the primary motor cortex, and is an area involved in planning movements

Question 24

Where and what is the precentral gyrus?

Answer 24

Lying just anterior to the centrus sulcus, this is the most posterior part of the frontal lobe, and houses the primary motor cortex

Question 25

What and where is the postcentral gyrus?

Answer 25

Lies just posterior to the central sulcus, making it the anterior-most part of the parietal lobe. This is the site of the primary somatosensory cortex

Question 26

What and where is the somatosensory association cortex?

Answer 26

Also found within the parietal lobe. This region receives input from the primary somatosensory cortex for further processing of sensory information

Question 27

What and where is the visual association cortex?

Answer 27

Lies just anterior to the primary visual cortex in the occipital lobe. Responsible for further processing of visual information from the primary visual cortex

Question 28

What and where is the primary visual cortex?

Answer 28

The final destination of the optic pathway, the primary visual cortex processes raw visual information. It lies at the occipital pole of the brain

Question 29

Where is Wernicke’s area and what does it do?

Answer 29

Found posteriorly in the superior temporal gyrus, this is one of the key language areas of the brain. Wernicke’s area is responsible for language comprehension

Question 30

Where and what is the primary auditory complex?

Answer 30

Found in the superior temporal gyrus and extending deep through the lateral fissure, the primary auditory cortex processes auditory information

Question 31

What and where is the secondary auditory cortex?

Answer 31

Anteroinferior to the primary auditory cortex in the superior temporal gyrus, this region assists with further processing of auditory information

Question 32

What and where is Broca’s area?

Answer 32

Found posteriorly in the inferior frontal gyrus, this is one of the key language areas of the brain. Broca’s area is responsible for language production

Question 33

What is an example of a structure located in the frontal lobe?

Answer 33

Broca’s area

Question 34

What is an example of a structure located in the parietal lobe?

Answer 34

Primary somatosensory cortex

Question 35

What is an example of a structure located in the temporal lobe?

Answer 35

Auditory cortex

Question 36

What is an example of a structure located in the occipital lobe?

Answer 36

Visual cortex

Question 37

On which side of the brain are Broca’s and Wernicke’s region usually located?

Answer 37

Left (cerebral cortex)

Question 38

What is the result of damage to a discrete area of cortex?

Answer 38

Damage to a discrete area of cortex will lead to sensory or motor deficit in the part of the body mapped to that area of cortex. This applies to all areas of cortex, including auditory, visual etc.. Damage to the cerebral cortex globally will cause an impairment in consciousness.

Question 39

How might the primary motor and somatosensory cortices be further mapped?

Answer 39

Remember also that the primary motor and somatosensory cortices may be further mapped somatotopically into motor and sensory homunculi (homunculus = little human), which describe the representation of different parts of the body over the cortex

Question 40

Looking at the axial CT images carefully, you see mass which has taken up the contrast and appears bright (on the central left hand side). Which of these cerebral cortical regions does this mass overlie?

Answer 40

Primary motor cortex
It is relatively straightforward to exclude the prefrontal cortex (which lies at the frontal pole) and the visual cortex (which lies at the occipital pole), leaving the auditory and primary motor cortices as possibilities. This cannot be the auditory cortex, however, as we know this is in the temporal lobe, which is at the same transverse level as the eyes and ears, which cannot be seen here. We are, therefore slightly higher, approximately at the midpoint of the right primary motor cortex.

Question 41

What kind of tumour would cause a focal/partial seizure?

Answer 41

The mass is probably closest to the upper limb region of the primary motor cortex. The lower limb is represented more medially and the face is represented more laterally.

Of course, the problem is in the right hemisphere, so this will manifest on the left side peripherally.

Clinically, this phenomenon of abnormal, unprovoked sensorimotor activity is known as a focal or partial seizure, and reflects involvement of distinct functional areas of cortex.

Question 42

What is the Circle of Willis?

Answer 42

The blood supply to the brain can be divided into its anterior and posterior circulation which are joined by a network of vessels known as the circle of Willis.

Circle of Willis
The anterior portion of the circulation is from the internal carotid arteries (ICA), which branches into the anterior cerebral arteries (ACA) and the middle cerebral arteries (MCA). A communicating vessel between the two anterior cerebral arteries is called the anterior communicating artery (AComm).

The posterior portion of the circulation is from the vertebral arteries, which forms the basilar artery. This later branches into the posterior cerebral arteries (PCA). Two communicating vessels exist between the two posterior cerebral arteries and the internal carotid arteries called the posterior communicating arteries (PComm).

Question 43

What does the posterior circulation supply?

Answer 43

As well as providing blood supply to the posterior aspect of the cerebral cortex, the posterior circulation also supplies the brainstem and cerebellum via several branches; the superior cerebellar arteries, the anterior inferior cerebellar arteries, the posterior inferior cerebellar arteries and the pontine arteries.

Question 44

What is an angiogram?

Answer 44

Dye can be injected into vessels so they can be imaged more clearly - this is called an angiogram.

The image on the right shows an angiogram of the vessels contributing to the circle of Willis. This imaging technique can be use to visualise and detect the location of aneurysms.

Question 45

What are the functional areas of the brain?

Answer 45

Each cerebral artery supplies a different functional region of the brain.

On the image on the right, we can see that the anterior cerebral artery (ACA) covers the medial (green) surface of the brain predominantly. The middle cerebral artery (MCA) covers the lateral (purple) surface of the brain. Finally, the posterior cerebral artery (PCA) covers the posterior (yellow) surface of the brain.

These arteries are therefore responsible for the oxygen supply to these functional areas.

Question 46

Through which foramen do the internal carotid arteries (ICA) enter the skull?

Answer 46

Carotid canal
The internal carotid arteries enter the skull through the carotid canal.
(Foramen lacerum is predominantly filled with cartilage)

Question 47

Through which foramen do the vertebral arteries enter the skull?

Answer 47

Foramen magnum

Question 48

Marie has a thrombus in her right middle cerebral artery, which functional areas would you expect to be affected?

Answer 48

Facial sensation
Facial movements

The MCA provides blood supply for the somatosensory and motor cortices, but only the regions dedicated to the face and upper limb. The lower limb is supplied by the ACA. Broca and Wernicke’s regions are usually located in the left cortex, and so are unlikely to be affected in this case.

However if the motor cortex to the face is affected Marie could still experience speech problems related to moving the tongue and larynx.