Nero 1

Cerebral Cortex

executive functions

Hypothalamus

metabolism & autonomic functions

Cerebellum

motor control

Hippocampus

memory consolidation & spatial navigation

Corpus collosum

connection of hemispheres

Basal Ganglia

Signal integration

Substantia nigra

Reward & movement

Brain Circulation: From the Outside In

• Blood supplied by 2 carotid arteries (80%) and 2 intervertebral arteries (20%), which then pool in a redundant circulatory loop and spread over the cortices of the brain, entering the tissue from the parenchyma perpendicular to the brain surface.

• Organization important for understanding pathology of strokes and other CNS diseases

Cerebrospinal Fluid (CSF)

• ~125-150 mL of fluid surrounding the brain and spinal cord

• Contains many proteins (albumin, antibodies) & cells common in the periphery but not found within CNS tissue

• Produced by choroid plexus cells in CNS, ~500 mL/day (constant turnover)

• Drains into venous system

• Functions:

– Buoyancy

– Protection

– Chemical stability (waste, pH)

– Creates pressure gradient that facilitates perfusion

CNS Ventricles

• “Open” spaces within CNS that are filled with CSF

• Specialized epithelial cells, ependyma, form barrier between CSF and CNS.

• Passageway for CSF produced by choroid plexus

• Transfer of nutrients and components from CSF into brain

– Sampling CSF can give good clues about what’s happening in the CNS tissue

Meninges

• Three membranes that envelop the brain and spinal cord

– Dura (thick, tough)

– Arachnoid (spider-like processes)

– Pia (think, delicate)

• CSF is between arachnoid and pia, termed subarachnoid space

• Lymphatic elements and gateway of peripheral immune elements

– Peripheral immune elements don’t typically penetrate the pia and get into the CNS tissue

• Common Pathologies: Site of pathogen-induced inflammation (meningitis) and subarachnoid hemorrhages

Grey Matter (~50% volume)

• Neuronal cell bodies

• Mostly unmyelinated axons

• Muscle control, sensory perception such as seeing and hearing, memory, emotions, and speech

• High metabolic requirements (80% of brain)

White Matter (the other 50%)

• Neuronal axonal tracts

• Highly myelinated

• Relays action potentials between different brain regions

Cells of the CNS: Neurons

• Non-proliferative cells that electrically conduct impulses responsible for function of the organism

• A single neuron can have over 100K connections (synapses with) to other neurons

• No consensus on how many different types, based upon:

• Location

• Shape/morphology

• Neurotransmitter

• Synaptic connections

• Functional task

Oligodendrocytes

• Non-proliferative cell that myelinates neurons within CNS white matter

• One oligodendrocyte can myelinate dozens of different axon segments

• Very large cell with high metabolic burden

• Axonal conduction of myelinated axons is very rapid and is a key element of long-distance signal conduction

• The target cell of many diseases (MS, several leukodystrophies)

• Difficult cell to isolate and/or transplant

Astrocytes

• Most abundant cell type in CNS. Robust and resilient cell type where the best-defined functions are as a supportive cell for neurons

– Takes on an inflammatory phenotype during injury response

• Astrocytes will take up neurotransmitters from synapses, controlling the intensity and duration of a neurotransmitter in the synapse

• Becoming evident that astrocytes are the primary dysfunction of several neurodegenerative disorder

Microglia

• Function in many contexts as macrophages of the brain (inflammation, debris clearing etc..) with M1 and M2 (pro-repair) phenotypes.

• Constantly survey the local microenvironment and rapidly respond to nearby injury by activating, proliferating and migrating

• Cytotoxic secretion (H 2O 2, NO, IL-1, PGE2s, TNFα..) is aimed at destroying infected neurons, virus, and bacteria, but causes extensive (sometimes perpetuating) collateral damage.

• Activated microglia and astrocytes perpetuate the neurotoxic and neuroinflammatory effects of one another

Neural Stem Cells

• Self renewing cells that can differentiate into neurons, oligodendrocytes, and astrocytes

• Niches include the sub-ventricular zone and NSC “islands” near capillaries

Synapse is the Functional Unit of the CNS

• Impulses are transmitted by neurotransmitters.

• Neurotransmitters are formed in the neuron.

• Synaptic knobs or boutons store the neurotransmitters.

• Neurotransmitters are released across the synaptic cleft, which is the space between neurons.

• Myelination facilitates rapid progress of action potentials

• can have over 100k synapses

Neurotransmitters

• More than 46 different neurotransmitters, in four general categories:

– Amino Acids: Glutamate, GABA, aspartate..

– Monoamines: Dopamine, serotonin, histamine..

– Peptides: Somatostatin, opioids..

– Other Misc: Acetylcholine, adenosine..

• Possible effects on postsynaptic neurons

– Excitatory: Pushes post-synaptic neuron closer to threshold potential (causes localized depolarization)

• glutamate, acetylcholine…

– Inhibitory: Makes post-synaptic neuron locallyhyperpolarized, making it less likely to depolarize

• GABA

Excitatory (Ex. glutamate)

• Glutamate is release from excitatory pre-synaptic neuron

• Binds to glutamate receptors on post-synaptic neuron

– Ionotropic (AMPA, NMDA..): Ion channels that open when glutamate binds

– Metabotropic (mGluR): Non-channels that cause signaling cascade that opens other channels

• Calcium and sodium rush into the cytoplasm, altering the electric potential

Inhibitory (Ex. GABA)

• GABA is released from inhibitory pre-synaptic neuron

• Binds to GABA receptors on post-synaptic neuron

– Ionotropic (GABAA): Ion channels that open when GABA binds

– G-protein coupled (GABAB): Non-channels that cause signaling cascade that opens other channels

• Chloride rushes into the cytoplasm, locally altering the electric potential

Concept of Synaptic Plasticity

• Individual neurons have tens of thousands of connections (synapses) with other neurons. It is the combined effect of the interactions that ultimately determines whether the individual neuron will reach the threshold potential and fire an action potential.

• Synaptic connections and the influence that a single synaptic connection has on the post-synaptic neuron can change as a result of learned experience or pathology.

• This change in influence and alteration in synaptic connections is referred to as synaptic plasticity.

• This is the basis for learning & memory and pathological alterations are thought to be the basis or byproduct of neurological diseases

Synaptic plasticity occurs when synapses strengthen (enlarge) or weaken (get smaller or disappear) over time

• Synapses are strengthened if quantity and frequency of excitatory neurotransmitters released to that synapse (from the pre-synaptic neurons) increases.

• Strengthened post-synaptic “spines” of neurons have a greater effect on the change in electrical potential of the post-synaptic neurons (because they’re bigger – more volume to affect the overall electric potential)

– A strengthened synapse plays a larger role in influencing the activity of the post-synaptic neuron – caused by increased pre-synaptic input

• A weakened synapse, due to less excitatory input from pre-synaptic neuron, becomes smaller and plays a lesser role in influencing the activity of the post-synaptic neuron

– Spine shrinkage

• Physiological and pathological mechanisms alter synaptic connections, causing spine strengthening (enlargement) or weakening (shrinkage)

• Lack of sufficient neuronal input will cause apoptosis (lonely neurons..)

In order for a post-synaptic neuron to fire, it needs

a strong depolarization signal provided by many synapses (it’s not a 1:1 ratio as shown in so many figures)

Synaptic Plasticity: Glutamate & GABA Example

If a spine of a dendrite receives prolonged strong excitatory stimulus from presynaptic neurons at spine (lots of excitatory NT), the spine can enlarge, increase the amount of receptors in that spine and exert a stronger influence on the behavior of the post-synaptic neuron. Such a synapse has been strengthened.

• Often occurs by increasing the amount of excitatory receptors on the post-synaptic neuron

• More receptors = more local depolarization upon NT release

• Contribution of GABA to spine alteration is more complex. GABA, in general, does not elicit significant synaptic plasticity to the extent that occurs with excitatory neurotransmission. Some newer data challenges this.

• If blunts excitatory input, then can shrink spine.

• If GABA increases on its own, GABA-receiving spines can strengthen

Pruning Neurons without Enough Synaptic Connections - Development

• Development leads to generation of over a trillion neurons

– Adult brain has ~80’ish billion

• Most loss occurs during development and early childhood

– Loss of neurons = smarter?

• Plasticity is the underlying process

– Ability to reorganize neuronal pathways throughout lifespan as a result of experience or pathology

Pruning primary results from:

• Too many neurons (insurance)

• Lack of connectivity to other “connected” neurons

• Exposure to repulsive cues

Neurotransmission & Excitotoxicity

• Normal excitatory neurotransmission causes a temporary and controlled calcium level spike in post- synaptic neurons

• Excitotoxicity occurs when excitatory neurotransmitters are at levels that are too high and/or are too persistent

– Glutamate is the primary excitotoxic neurotransmitter

Plays a role in reperfusion injury, where some reperfused neurons will dump large quantities of their neurotransmitters and keep releasing higher than normal levels. If the released/dumpted NT is excitatory, it potentiates the excitotoxicity to neighboring cells

Excitotoxicity: Ca+

• If cytoplasmic Ca2+ is too high for too long, cytotoxicity occurs

• High Ca 2+ activates caspases & messes up mitochondrial function

– Mito soaks up excess Ca 2+

– Ca 2+ sensitive TCA enzymes generate more electrons for transport chain.

• ROS

– Causes mito to release pro-apoptotic proteins

• Isn’t instantaneous, takes hours or days to play out

– Type of secondary injury

The Blood Brain Barrier

• Capillaries are composed of endothelial cells and pericytes that are enveloped in a basement membrane.

• Endothelial cells express tight junctions that prevent passage of all but small lipid soluble molecules into the brain.

• Astrocytes “endfeet” almost completely envelop capillaries and also regulate the entry of molecules

Immuno-different

• High regulation of entry for cells and molecules via the BBB insulates the sensitive biochemical environment

• Lymphatic system connected to CSF, but not within tissue of brain

• T’s, B’s, macrophages, dendritic cells, neutrophils can be found in CSF, but generally not very many and presence is an indication of a problem

• Microglia instead of macrophages

Immuno- different consequences

• Lacks specialized cells for diverse immune functions

– Astrocytes and microglia have important functions for normal CNS activity, but are also the major mediators of the inflammatory response.

• Traumatic breach of BBB can be disastrous (lets in the riff raff)

– Alters function of CNS cells

Typical Sequence of Events for Neuroimflammation

1. Injury/infection activates CNS cells

– Microglia via PRRs, DRRs

– Other cells via direct damage, pathogen fragments

2. Cytokines cause endothelial activation

3. Peripheral immune cells, proteins & molecules come into the CNS as BBB is made permeable

– Leukocytes, complement

4. Inflammation widens and the problem is eliminated

5. Resolution & restoration of BBB

– M2 microglia, anti-inflammatory cytokines, Tregs (if present)

The risk with neuroinflammation: dysfunction & collateral damage

Inflammatory process disrupts normal function of CNS cells, that are already a very sensitive population in a specialized environment. Persistent neuroinflammation will spiral into progressive excitotoxicity, fibrosis and neuronal dysfunction

Breach of BBB

– Glutamate and other substances in the plasma can act as uncontrolled source of neurotransmitters .

– Inflammatory components (cytokines, complement) from plasma now get into the CNS, altering cell function

• Tend to make neurons more likely to fire & can cause cell death.

– Peripheral immune cells enter the CNS.

– Fluid enters the CNS & lymph system can’t as effectively compensate, resulting in edema

Hypersensitivity

– CNS cells are tuned to function in a highly controlled microenvironment very different than the periphery. Alteration in this homeostasis has disproportionate effects.

– Glia (astrocytes & microglia) can be quick to activate, but slow to calm down. Molecules common in the periphery can induce them to activate and stay active.

• Can form a glial scar

Gliosis & the Glial Scar

• Gliosis: Activation and fibrous proliferation of glial cells in injured areas of the CNS.

• Fibrous proliferation can form a semi-permanent structure – the glial scar

• Scar serves an important purpose to reestablish barrier function – a necessary task after injury.

• Composition of the Scar:

– Reactive astrocytes, microglia, vasculature & scar ECM (fibrosis..)

• Mechanisms of inhibition by scar

– Interlocking astrocytes, microglia and ECM form a physical barrier

– Expression of inhibitory molecules that negatively affect neurons & perpetuate reactivity of glia in the scar

• May persist for months or year

Glial Scar

• A prolonged inflammatory reaction and formation of a glial “scar”

• Microglia quickly activate and migrate to the site of the injury

• Astrocytes form a tight barrier around the injured area

• Astros & microglia continually express molecules that are neurotoxic or inhibit axonal growth into the area

CNS Edema

• Swelling and enlargement of ventricles/CSF compresses brain tissue, risking:

– Ischemia or anoxia (remember placement of cerebral vasculature & lymph!)

– Causes dysfunction and death of CNS cells outright or after reperfusion

• ROS

• Excitotoxicity

• Neuroinflammation