Lecture 8 Material

Question 1

Mechanisms for support of the nervous system

Answer 1

Body encasement, meninges, CSF, blood supply

Question 2

Body encasement

Answer 2

The skull and vertebrae provide protection of the CNS from external injury

Question 3

Meniges

Answer 3

There are three connective tissue layers. They provide a mechanical suspension that is anchored to the bones to protect the CNS from internal injury, and they provide a space for cerebrospinal fluid circulation

Question 4

Cerebrospinal fluid

Answer 4

(CSF) this provides bouyancy (shock absorption) to the brain, which is mostly composed of fat.
The effective weight of the brain in CSF is 0.05kg, compared to 1.5kg in air, a 97% reduction.
CSF also provides a source of nutrients and assists in waste removal.

Question 5

Blood supply

Answer 5

This provides oxygenation, nutrition, and waste removal.

Question 6

Pia mater (general features)

Answer 6

This is a thin membrane covering the brain and spinal cord. It follows the major surface contours and dips into sulci and other depressions.
Surface blood vessels travel just above the pia; their smaller capillary branches penetrate the pia into the brain.

Question 7

Arachnoid (general features)

Answer 7

(Spider web appearance) This is a slightly thicker middle layer that is closely apposed to the dura.
It does not follow surface contours but bridges over major depressions. This creates the subarachnoid space, within which CSF circulates.

Question 8

Cisterns

Answer 8

Enlargements of the subarachnoid space

Question 9

Arachnoid trabeculae

Answer 9

Thin connective tissue threads that connect the pia and arachnoid layers

Question 10

Subdural space

Answer 10

The minimal space between teh dura and arachnoid layers

Question 11

Dura mater (general features)

Answer 11

This is much thicker than the other meningeal layers. It is relatively inelastic with the consistency of plastic wrap.
Consists of two sublayers. The outer periosteal dura is closely adhered to the bone and to the inner meningeal dura.

Question 12

Sinus

Answer 12

The sublayers are normally fused, but at certain sites, the layers separate to form a fluid-filled sinus. CSF and venous blood drains into the sinuses

Question 13

Superior sagittal sinus

Answer 13

The prominent space at the midline

Question 14

Dural reflections

Answer 14

When the meningeal layers of the dura fold inward and fuse.
The dural reflections create a sling, or hammock-like suspension system, that restricts movement of the brain within the skull and prevents the largest brain structures from impacting each other with blows to the head.

Question 15

Falx cerebri

Answer 15

(sickle of the brain) this lies within the longitudinal fissure and keeps the two cerebral hemispheres from impacting each other

Question 16

Tentorium cerebelli

Answer 16

(tent of the cerebellum) this covers the cerebellum and brainstem and ensures that bows to the bead do now drive the cortex into the cerebellum or brainstem

Question 17

Denticulate ligaments

Answer 17

Meningral layers lining the spinal cord form concentric tubes.
An additional connective tissue structure called the denticulate ligaments attach the layers to each other.

Question 18

Ventricles and their locations

Answer 18

Lateral ventricles - exhibit the characteristic backwards C-shape of the telencephalon
Third ventricle - within the diencephalon
Cerebral aqueduct - within the mesencephalon
Fourth ventricle - within the pons and medulla
Within the lateral, third, and fourth ventricles is the choroid plexus where CSF is produced

Question 19

Choroid plexus

Answer 19

Formed by invaginations of the pia mater and capillaries into the ventricular space early in development when the wall of the neural tube is thin. This invagination creates branched protrusions or villi that increase the surface area
Each choroid plexus is supplied with at least one artery and one vein.
Within each choroid villus is a single capillary.

Question 20

Capillaries within the choroid plexus

Answer 20

They are lined with endothelial cells that are leaky (fenestrated)

Question 21

Choroid epithelial cells

Answer 21

These are specialized ependymal cells of the ventricle.
Blood pressure forces water and solutes into the space between the capillary and the choroid epithelium. These cannot pass into the ventricles because of these choroid epithelial cells being connected to each other by tight junctions.

Question 22

Blood-cerebrospinal fluid barrier

Answer 22

The tight junctions made by the choroid epithelial cells that do not let water and/or solutes into the ventricles.

Question 23

What is CSF and how is it made?

Answer 23

The CSF is not a simple ultrafilrant of blood. It is formed by an active process involving transport of substances by carrier proteins in the choroid epithelium. The active transport of Na and and glucose produces an osmotic gradient that draws water into the ventricles.
Other ions are dissolved in the water and enter the CSF passively; still others undergo ion exchange.
Lipid soluble substances pass freely into the CSF.

Question 24

Composition of CSF

Answer 24

It is highly regulated to maintain a constant extracellular environment. It changes little in the healthy state but can alter radically during disease, giving it diagnostic value.
Approx. 500mL of CSF is produced each day. The total volume of the ventricles and subarachnoid space is less than 150mL. Hence, CSF is replenished 3-4 times a day.

Question 25

Ependymal cells

Answer 25

Line the ventricles outside the choroid plexus and have no tight junctions. Hence, once the CSF reaches the ventricles, it passes freely into and out of the brain tissue

Question 26

Capillaries outside the choroid plexus

Answer 26

The endothelial cells are joined by tight junctions. This creates the blood brain barrier.

Question 27

Blood-Brain Barrier

Answer 27

Prevents the free passage of solutes and gives precise control over the brain’s extracellular composition.
Substances that are small gases (O2, CO2), lipophilic (barbiturates), or actively transported (glucose, amino acids) can enter the brain.
Water soluble, highly charged, and large substances (antibiotics, antibodies) cannot pass freely.

Question 28

Peripheral capillaries

Answer 28

(like those in the choroid plexus) are fenestrated and allow relatively indiscriminate passage of substances across the capillary wall

Question 29

Flow of CSF

Answer 29

Lateral -> third -> cerebral aqueduct -> fourth.
Through the ventricles, the CSF supplies deep brain structures. The supply surface structures, CSF must gain access to the subarachnoid space from holes in the fourth ventricle

Question 30

Median aperture & lateral aperatures

Answer 30

The holes in the fourth ventricle that allow the CSF to gain access to the subarachnoid space

Question 31

Cisterns

Answer 31

Where some regions of the subarachnoid space widen out. This is because the arachnoid follows the dura and not the pia. CSF is sampled from these for diagnostic purposes.

Question 32

Arachnoid villi

Answer 32

Within the sinuses, the arachnoid membrane evaginates through the dura forming this (or larger granulations).
Greater pressure in the subarachnoid space drives fluid into the venous sinus.
Emptying of CSF is a one-way bulk flow process, possibly through giant vacuoles.

Question 33

Hydrocephalus

Answer 33

The dilation of ventricles due to a blockage of CSF circulation, usually in the cerebral aqueduct or apertures.
In an adult, this can lead to a life-threatening increase in intracranial pressure.
Children with this often exhibit a greatly enlarged head, as the skull bones have not yet fused.

Question 34

Treatment of hydrocephalus

Answer 34

Can be treated surgically implanting a drainage tube from the lateral ventricle into the peritoneal cavity or atrium.
Similar external shunts can be performed in adults

Question 35

Superior sagittal sinus

Answer 35

Where surface veins generally empty along the top of the falx cerebri, and from there into the transverse sinuses along the back of the tentorium cerebelli

Question 36

Transverse sinuses

Answer 36

Along the back of the tentorium cerebelli where surface veins empty from the superior sagittal sinus

Question 37

Inferior sagittal sinus

Answer 37

Deep veins feed into this structure along the base of the falx cerebri and into the straight sinus along the top of the tentorium cerebelli.

Question 38

Straight sinu

Answer 38

Along the top of the tentorium cerebelli

Question 39

Sigmoid sinuses

Answer 39

From the straight sinus, where deep veins feel into, as well as the transverse sinuses

Question 40

Internal jugular vein

Answer 40

Ultimately where all the sinuses drain into

Question 41

Anoxia

Answer 41

The brain receives 15-20% of the cardiac output. The autonomic mechanisms ensure blood flow is maintained in order to prevent this.

Question 42

Two major blood vessels supplying the brain

Answer 42

Paired internal carotid and vertebral arteries

Question 43

Vertebral arteries

Answer 43

They ascend along the ventral surface of the medulla and give off small branches to supply the spinal cord: the anterior spinal artery and the paired posterior spinal arteries.
These ____ merge just below the pons to form the basilar artery

Question 44

Basilar artery

Answer 44

Formed when the vertebral arteries merge just below the pons.
Branches of the ____ supply the brainstem, the cerebellum, and the choroid plexus of the 4th ventricle.

Question 45

Posterior cerebral artery

Answer 45

A major branch of the basilar artery that supplies the occipital lobe, inferior temperal lobe, parts of the diencephalon, and the midbrain

Question 46

Anterior cerebral artery

Answer 46

A major branch off the internal carotid that supplies the anterior and medial cortical structures.

Question 47

Middle cerebral artery

Answer 47

A major branch off the internal carotid that supplies most of the lateral cortical hemispheres and the bulk of the basal ganglia (pallidum), amygdala, and hippocampus, and the choroid plexus of the lateral and third ventricles

Question 48

Circle of Willis

Answer 48

Made up of the proximal posterior cerebral arteries, the posterior communicating arteries, the internal carotid arteries just before their bifurcations, the proximal anterior cerebral arteries, and the anterior communicating artery.
This provides for potential redistribution of blood when one of the major arteries is blocked. This mechanism operates more so when slowly developing occlusions and not sudden blockage (an embolism)

Question 49

Posterior communicating arteries

Answer 49

Where the internal carotid and vertebral artery systems are joined at the base of the brain

Question 50

Anterior communicating artery

Answer 50

Communicating artery between the anterior cerebral arteries

Question 51

Basal ganglia supply

Answer 51

Supplied by the internal carotid system, specifically by branches off the middle cerebral artery and the anterior chorodial artery

Question 52

Diencephalon supply

Answer 52

Supplied by both the internal carotid and the vertebral systems: thamalus by the posterior cerebral artery and hypothalamus by the posterior and anterior cerebral arteries