Week 3: Supporting the brain Flashcards by Amber Lockton

The structure and function of cells that support the nervous system:

▪ Meninges
▪ Glia
▪ Ventricular system
▪ Cerebrospinal fluid
▪ Blood brain barrier

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Meninges

Tough connective tissue covering the entire nervous system (supports & transfers CFS)

Dura mater
Arachnoid membrane
Pia mater
Subarachnoid space

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Dura matter

Outer layer

thick, tough and flexible (but not stretchable)

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Arachnoid membrane

middle layer, located between dura mater & pia mater – soft & spongy (stops CFS fluid leakage)

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Pia matter

clings to surface of brain & spinal cord; thin & delicate, smaller surface blood vessels found here (also stops CFS leakage)

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Subarachnoid space

Fluid-filled space between arachnoid membrane & pia mater - cushions brain - CFS flows

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Meninges: CNS & PNS

CNS covered by 3 layers of meninges:
- Dura mater
- Arachnoid membrane
- Pia mater

PNS – two layers fuse:
- Dura and pia mater fuse
- Sheath protects spinal and cranial nerves and the autonomic ganglia
- (Arachnoid membrane (CSF) NOT present)

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Ventricular system: Ventricles & CFS

Ventricles: hollow spaces within brain, filled with cerebrospinal fluid
(from the middle of the brain, to the spine)

▪ Interconnected

Filled with Cerebrospinal fluid (CSF): clear fluid (similar to blood plasma)

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Four ventricles (names)

lateral ventricles
3rd ventricle
cerebral aqueduct
4th ventricle

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Lateral ventricles: where?

sited in centre of telencephalon – largest

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3rd ventricle

sited at midline in centre of diencephalon

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Cerebral aqueduct

is a long tube in mesencephalon connects 3rd & 4th ventricles

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4th ventricle

found between cerebellum & pons

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CSF Flow

▪ Produced by the choroid plexus of lateral ventricles

▪ Flows to 3rd ventricle where more is produced 

▪ Flows through cerebral aqueduct to 4th ventricle 

▪ Leaves ventricles to flow into subarachnoid space around CNS – where it is reabsorbed into blood stream through arachnoid granulations

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Cerebrospinal Fluid

Produced constantly from choroid plexus (in lateral ventricles)

Consists of ions, water, protein, glucose

Total volume of CSF is ~125 ml

Takes 3 hrs for half CSF to be replaced (half-life - so half replaced in 3 hours).

(55% plasma, 45% RBCs, <1% WBCs and platelets)

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The CSF has four vital functions:

Protection
1. Buoyancy (floats, support)
2. Waste reduction/ removal
3. Transport (of nutrients & hormones)

Hydrocephalus (2 types)

▪ Accumulation of CSF within the cerebral ventricles (specifically cerebral aqueduct)

▪ Leads to ventricular dilatation (pressure)

▪ Classified into two types 
	* Obstructive hydrocephalus  * Communicating hydrocephalus

Glia (4 types)

Support cells of the nervous system:

astrocytes
microglia
oligodenrocytes
schwann cells

Astrocytes

Physical support – “neuron glue”
▪ Nourish neurons: wrap blood vessels to receive, store & release nutrients to neurons
▪ Help control chemical composition (homeostasis) of extracellular fluid
▪ Surround & ISOLATE synapse (limit NT dispersion)

Astrocytes & phagocytosis:
▪ Clean up debris (e.g., dead cells) in the brain
▪ special astrocytes move around CNS engulfing & digesting debris (phagocytosis)
▪ Form scar tissue in place of dead tissue

Amyotrophic lateral sclerosis (ALS): Lou Gehrig’s disease

▪ Most common form of motor neuron disease (e.g. Stephen Hawkings)
▪ Rapidly progressive, leading to fatality
▪ Attacks nerve cells
▪ Lose all voluntary muscle control
* respiration
5,000 people affected at one time

Astrocytes and memory

Recent discovery
- Know important for homeostasis
- May also be important for cognitive functions - like memory
- Studied Einstein’s brain - only difference found was the number of astrocytes, he had more
- EEG looks at oscillations - found astrocytes influenced the frequency of gamma oscillations with cognitive functions
  = Highlights importance of astrocytes

Microglia

▪ Smallest glial cells
▪ Phagocytes function (engulf pathogens/ debris)
▪ Representative of immune system in brain
▪ Primarily responsible for inflammatory reaction in response to brain damage

▪ Thought to have role in:
	* Neurodegenerative disorders (Alzheimers & Parkinsons Disease) * Viral infections (HIV) (attack microglia = immune system affected)

Oligodendrocytes and Schwann cells

▪ Schwann Cells (peripheral NS)
▪ Oligodendrocytes (central NS)

▪ Contain fatty tissue called myelin that wraps around neuron axons 
▪ Forms insulating coating: myelin sheath 

▪ Schwann cells - wrap individual axons (PNS)

▪ Oligodendrocyte - wrap several axons (CNS)

What does new research suggest about glial cells?

Many types of glial cells
HIV doesn’t affect neurones, it affects glial cells

Why wasn’t glial cell communication discovered sooner?

Glial cells don't use electrical communication so their function was missed by EEGs

What kind of diseases could glial research help with?

- Viral diseases Motor neural diseases

Blood supply to the brain

Brain blood supply ▪ Brain receives 15-20% of body's blood supply ▪ Nutrients & oxygen carried to brain by blood vessels

Blood function

1. Blood brings materials necessary for brain function * Oxygen * Nutrients (carbohydrate, amino acids, fats, vitamins) * Hormones (endocrine system - hormone effects last longer than nervous system) 2. Blood removes materials from the brain (via ventricles) * Carbon dioxide * Lactate * Hormones (e.g. if too much adrenaline) * Ammonia

Blood-brain barrier

Semi-permeable barrier between blood & Brain * Some substances cross barrier & others can’t (Maintain homeostasis for action potentials) (Only CFS wanted around the brain, blood needs a barrier) In brain, no spaces exist between endothelial cells & unwanted substances cannot pass over capillary wall - less permeable

The BBB 3 vital functions:

1. Protects brain from "foreign substances" in blood that may injure brain 2. Protects brain from hormones & neurotransmitters in the rest of body 3. Maintains a constant environment for the brain (homeostasis)

Substances & BBB

▪ Lipid soluble molecules are able to penetrate through BBB relatively easily via lipid membranes of cells ▪ Water soluble molecules can only use specialised carrier-mediated transport mechanisms (Needs ATP) ▪ Active transport allows some substances to move across capillary walls (e.g., glucose transporters) ▪ Barrier is weaker in some areas (to detect toxins)

What layer of the meninges is not present in the PNS?

Arachnoid membrane

What type of cell is not located in CNS?

Schwann cell

Reading: where does the CSF leave the brain?

Leaves through the 4th ventricle via small openings which connect with the subarachnoid space surrounding the brain where it is then reabsorbed into the blood via arachnoid granulations - The brain floats in the CFS contained within the subarachnoid space (reduces weight)