Nerve- Myelination/ Meninges and CSF

Question 1

What are myelinated axons?

Answer 1

•Many axons are surrounded by myelin.

•The myelin sheath is produced by neuroglialcells:
Schwann cells –peripheral nervous system
Oligodendrocytes-central nervous system

• Myelin acts as an insulator, allows for faster transmission of nerve signals and helps with repair of axons.
• Each axon is myelinated along its length by chains of glial cells. (umbrella term for supporting cell)
• The gap between one such myelinating cell and the next = Node of Ranvier

Question 2

What are the steps involved in myelination?

Answer 2

1. Schwann cell starts to wrap around a portion of the axon
2. Schwann cell cytoplasm and plasma membrane begin to form consecutive layers around the axon
3. The overlapping inner layers of the Schwann cell plasma membrane form the myelin sheath
4. Eventually, the Schwann cell cytoplasm and nucleus are pushed to the periphery of the cell as the myelin sheath is formed

Diagrams in lecture slides- remember Schwann cell is very large and appears to wrap itself around the axon.

Question 3

What are Nodes of Ranvier and what are their function?

Answer 3

Nodes of Ranvier- Gaps between Schwann cells which allow diffusion of ions in and out of axons, propogating electrical signals along axon (saltatory conduction), where AP “hops” along nodes which increases conduction velocity.

Question 4

MS- Clinical significance of problems with myelination

Answer 4

• In multiple sclerosis (MS) the myelin sheaths surrounding axons are damaged by an autoimmune mechanism that interferes with the activity of the affected neurons.
• T lymphocytes and microglia, which phagocytose and degrade myelin debris, play major roles in progression of this disease. In MS destructive actions of these cells exceeds the capacity of oligodendrocytes to produce myelin and repair the myelin sheaths.

Question 5

How are unmyelinated axons supported?

Answer 5

Although some axons do not possess a myelin sheath they still travel supported by schwanncells in the PNS. The axons sit in invaginations of cytoplasm.

Schwann cells envelop multiple axons- the axons are said to be enveloped by the cells, but are unmyelinated as they do not have a myelin sheath that wraps around each axon.

Question 6

What are the 3 layers of the meninges?

Answer 6

Dura mater ( 2 layers- periosteal layer (continuous w skull periosteum), and the meningeal layer)
-Dense CT continuous with periosteum of skull

Arachnoid mater
-2 components: a layer in contact with dura mater and a system of trabeculae connecting the layer with the pia mater The cavity between the trabeculae is the subarachnoid space filled with CSF (hydraulic cushion)

Pia mater
-Loose CT containing many blood vessels and covered by squamous cells. It is very thin, and tightly adhered to the surface of the brain and spinal cord. It is the only covering to follow the contours of the brain (the gyri and fissures).

Question 7

What are the functions of the meninges?

Answer 7

Functions:
-Provide a supportive framework for the cerebral and cranial vasculature.

• Acting with cerebrospinal fluid to protect the CNS from mechanical damage.
• The meninges are often involved cerebral pathology, as a common site of infection (meningitis), and intracranial bleeds.

Question 8

Meninges- clinical relevance

Answer 8

Injuries involving the meninges, can result in a haemorrhage or hematoma.

• A subarachnoid haemorrhage is acute bleeding under the arachnoid.
• A subdural hematoma (collection of blood) located in a separation of the arachnoid from the dura mater.
• An epidural hematoma, bleeding between the dura mater and the skull.

Other medical conditions that affect the meninges: Meningitis
Meningiomas (tumors)

Question 9

What is the choroid plexus and what is its function?

Answer 9

The choroid plexus is a collection of cells that produce the cerebrospinal fluid in the ventricles of the brain. The choroid plexus consists of modified ependymal cells. There are four choroid plexuses in the brain, one in each of the ventricles.

Question 10

What is cerebrospinal fluid (CSF)?

Answer 10

CSF serves several purposes:
•Buoyancy 1500g → 25-50g
•Protection
•Prevention of brain ischemia
•Homeostasis
•Clearing waste 
Hydrocephalus –abnormal accumulation of CSF “water on the brain”
500ml produced per day, but only 100-160ml in circulation