Brain Circulation, Meninges, and CSF

Question 1

Carotid Artery and Internal/external

Answer 1

• Main ones shown to supply the whole head are the two common carotid arteries (left and right)
• main supply to the whole head region
• derives from aortic arch
• -common carotid artery ends by forming into 2: an internal carotid artery and an external carotid artery
• beyond this, the external carotid artery goes to form many branches to supply much of the head structures, the main exception being the brain
• the brain is supplied by the internal carotid artery. Doesn’t give off branches until it reaches its termination and supplies the brain
• due to this formation, we have a trunk root of blood supply to the brain
• From the heart (or the aortic arch) through the common carotid arteries and internal carotids which don’t have significant branches along the way–> all the blood in these vessels need to supply the brain!

Question 2

Cerebral Arterial Circle

(Loop of Willis)

Answer 2

• Can see an obvious circular arrangement of arteries lying on the ventral surface of the diencephalon which is called the Cerebral Arterial Circle
• circle of arteries supplying the brain
• many branches in a circle going to supply various regions in the brain
• named quite specifically by location
• MUCH of the brain is supplied by this arterial circle
• The Cerebral arterial circle is being supplied by the internal carotid artery on either side of the brain and the Basilar Artery!

Question 3

Basilar Artery

Answer 3

• Midline artery running over the ventral aspect of the brain stem and reaching caudal end of the arterial circle –> Basilar Artery
• Also contributes blood supply to the arterial circle
• the Basilar artery also gives rise to some branches supplying the Caudal Cerebellar region and other parts of the hindbrain
• supplies a relatively small amount of the brain compared to branches coming off the arterial circle
• there are two vertebral arteries that fuse to form the basilar artery

Question 4

Supply TO the arterial circle

Answer 4

1. Internal carotid a. (2)
2. Basilar a. (formed by union of 2 vertebral aa.)-terminally unite to form the single basilar artery

• This is the arrangement in many mammals including primates, horse, dog and pig
• Not the case in ALL mammals

Question 5

Vertebral Artery

Answer 5

Vertebral a.

• Branch of subclavian a. in thorax
• Runs through transverse foramina in cervical vertebrae
• Then enters cranial cavity via foramen magnum
• In the cranial cavity, the vertebral arteries unite to form the midline vessel, the basilar a.

Question 6

Variation of Cerebral Arterial Circle

Answer 6

In some other species including CATS and RUMINANTS:

• The internal carotid develops, but regresses postnatally and conveys NO BLOOD to the brain
• SO, other channels to arterial circle must exist

Question 7

Venous Drainage of the Brain

Answer 7

• Cerebral veins lack valves: there is potential for venous blood ot flow in either direction
• Drain to venous sinuses of dura mater (which are venous channels, but not veins due to differing structure)

Question 8

3 layers of the Meninges (and spaces between)

Answer 8

1. Dura mater (outer)- then subdural space
2. Arachnoid (middle)-then subarachnoid space
3. Pia mater (inner)

Question 9

Epidural Space

Answer 9

• Lies between periosteum and dura in vertebral canal (NOT CRANIAL CAVITY)
• COntains- loose connective tissue, veins and lymphatics
• Cushions the cord as it flexes (as vertebral column flexes)
• Can be used for nerve blocks (epidural anasthesia)
• NOTE: Cranial periosteum and dura mater are fused- in effect one single layer, no space

Question 10

Fax Cerebri

Answer 10

• A fold (projection) of dura mater that leaves the periosteum of the skull and projects into the brain between the two cerebral hemispheres

Question 11

Dura Mater

Answer 11

• Dense connective tissue
• fused with the periosteum of the cranium
• contains the venous sinuses

Question 12

Subdural Space

Answer 12

• Lies between dura mater and arachnoid
• Potential space containing only lymph-like fluid
• Site of subdural haematomata

Question 13

Arteries to Brain

Answer 13

Brain= largest mass of nervous tissue in the body

• Needs rich blood supply (due to requirements of nervous tissue)
• Must be maintained throughout life

Question 14

Common carotid a.

Answer 14

= main supply to head (from the aortic arch)

• Internal carotid a. (unbranched) – main supply to brain (direct)
• External carotid a. (many branches) – supply head other than brain

Question 15

Cerebral Arterial Circle

Answer 15

Circle of arteries lying on the ventral surface of the brain, circling around the diencephalon

• Supply TO circle- The paired internal carotid arteries
• Basilar artery (formed by union of paired vertebral* aa.)-feeds into caudal extremity of the CAC
• This is arrangement in many mammals including primates, horse, dog and pig

*Vertebral artery- Branch of subclavian a. (main arterial trunk in forelimb), runs through transverse foramina in cervical vertebrae cranial cavity via foramen magnum–> unite to form basilar a.

Question 16

Species Variations

(arteries)

Answer 16

• In cats & ruminants- Internal carotid (2) – develop but regress to fibrous strands postnatally & convey no blood to the brain. Instead their supply comes from:
• Maxillary artery (anastomosing branch–> CAC) derived from external carotid
• In large ruminants, the CAC is also supplied by:
• Vertebral artery (anastomosing branch–> CAC)- important in ritual
• Too deep to incise when cutting tissues of ventral neck, only cuts common carotid- controversial

Question 17

Venous Drainage of the Brain

Answer 17

• Cerebral veins lack valves- to facilitate blood drainage at all times, allows blood to flow either way according to differential pressures
• Many of the cerebral veins flow not into other veins, but–> to venous sinuses of dura mater- don’t have the same wall structure as veins & contain no valves.

-Blood eventually drains from these–> external jugular vein etc

Question 18

Meninges

Answer 18

• Outer layer: Dura mater
• Middle: Arachnoid
• Innermost: Pia mater

Question 19

Epidural Space

Answer 19

• Lies between periosteum of vertebral bone & dura in vertebral canal
• Contains - loose connective tissue, veins and lymphatics
• Cushions the cord as it flexes
• Can be used for nerve blocks

-N.B. In the cranial cavity dura mater
Is directly fused with periosteum in
effect one single layer, so no epidural space

Question 20

Dura Mater

Answer 20

A thick sheet of dense, fibrous connective tissue

• Fused with the periosteum of the cranial bones- no epidural space
• Contains venous sinuses.
• Folds of dura mater project between major sub-divisions of the brain- help hold brain in position
• Falx cerebri- midline fold between the 2 cerebral hemispheres
• Tentorium cerebelli- Oblique transverse fold between the cerebral hemisphere & the cerebellum
• Diaphragma sellae- forms a collar around neck of pituitary, forms roof of hypophyseal fossa

Question 21

Subdural Space

Answer 21

• Lies between dura & arachnoid
• Potential space containing only lymph-like fluid- lubricating layer
• Site of subdural haematoma- Leakage of blood may accumulate here

Question 22

Subarachnoid Space

Answer 22

• Between arachnoid and pia
• Contains CSF leaking from ventricular system
• Varying thickness in different places
• Enlargements of this space= cisterns:
• Cerebellomedullary cistern around foramen magnum - used for collection of CSF
• Lumbar cistern – near end of SC where it tapers. SC has more cranial ending in man (junction between L1 & L2)- used for lumbar puncture- easy to perform. More difficult/ dangerous in animals as spinal cord extends all the way through lumbar region to sacral spinal junction- not used routinely

Question 23

CSF

Answer 23

CSF is produced in all the main cavities (ventricles) of the brain, especially the 4th, by the choroid plexuses

• The two lateral ventricles–> the single, 3rd ventricle, which tapers down to the aqueduct (midbrain) –> the 4th ventricle (between the pons & cerebellum), which tapers down into the central canal of the SC
• Escapes ventricular system via 2 tiny apertures in the roof of the 4th ventricle–> subarachnoid space
• Drains from subarachnoid space–> venous sinuses of the dura mater via arachnoid villi

Question 24

The BBB

Answer 24

The blood-brain barrier- CNS capillary endothelium joined by tight junctions all fluid must pass through cells (not hypothalamus)- allows selective transfer of materials between blood stream & nervous tissue

• Surrounded by processes of astrocytes (neuroglial cells)- contribute to BBB

Question 25

The blood-CSF barrier

Answer 25

* **Ependymal epithelium** joined by tight junctions in choroid plexuses- fluid must pass

Question 26

Branches of the **Subclavian a.**

Answer 26

* becomes the main supply to the **forelimb** which then feeds into **axilliary artery** and so on * **Vertebral arteries** also branch of it and go through transverse foramina of the cervical vertebrae to then merge at brain stem to form **basilar a.** -Enter the cranial cavity via the foramen magnum (the hole in the base of the skull through which the spinal cord passes)

Question 27

Arachnoid Granulations

Answer 27

* **Arachnoid granulations**, also known as a Pacchionian granulation, are projections of the **arachnoid membrane** (villi) into the **dural sinuses** that ***allow CSF entrance from the subarachnoid space into the venous system***

Question 28

Cisterns of the Subarachnoid space

Answer 28

* **subarachnoid cisterns** are discrete named spaces within the subarachnoid space where the pia mater andarachnoid membrane are not in close approximation. * The subarachnoid tissue is not as abundant here as in the normal subarachnoid space and cerebrospinal fluid (CSF) gathers to form pools or cisterns * **Lumbar Cistern -** dangerous as in our domstic animals, the spinal cord runs through the lumbosacral junction! Risk damage to the spinal cord. Doesn't provide an easy route to get needle to lumbar system. Terminates much earlier in man! * **Cerebellomedullary cistern:** around foramen magnum, used for collection of CSF

Question 29

Choroid Plexus

Answer 29

* The **choroid plexus** is a plexus of cells that produces the **cerebrospinal fluid in the ventricles of the brain**. The choroid plexus consists of modified **ependymal cells**

Question 30

Role of CSF Fluid

Answer 30

* The clear, colorless ***cerebrospinal fluid*** is formed from teh blood plasma by ultrafiltration through the "blood cerebrospinal fluid barrier" at the choroid plexuses. fluid must be transported through the ependymal cells (CSF-blood barrier) to reach the ventricles 1. **Natural Buoyancy for weight of brain** 2. **protects the brain and spinal cord mechanically (shock absorber) and immunologically** 3. **Chemical buffering capacity** 4. **Transports nutrients** 5. **Flushes away waste products** 6. **Serves as a medium for the diffusion of neuroendocrine and neurotransmitter substances**