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NEURONS AND NEURONS AND GLIA GLIA

NEURONS AND GLIA. Introduction “Neurophilosophy” Brain (neurons) is the origin of mental abilities Glia and Neurons Glia Insulates, supports, and nourishes

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Page 1: NEURONS AND GLIA. Introduction “Neurophilosophy” Brain (neurons) is the origin of mental abilities Glia and Neurons Glia Insulates, supports, and nourishes

NEURONS AND NEURONS AND GLIAGLIA

Page 2: NEURONS AND GLIA. Introduction “Neurophilosophy” Brain (neurons) is the origin of mental abilities Glia and Neurons Glia Insulates, supports, and nourishes

Introduction

“Neurophilosophy”Brain (neurons) is the origin of mental abilities

Glia and NeuronsGlia

Insulates, supports, and nourishes neurons90% of brain cells are glial cells

NeuronsProcess informationSense environmental changes Communicate changes to other neuronsCommand body response

Page 3: NEURONS AND GLIA. Introduction “Neurophilosophy” Brain (neurons) is the origin of mental abilities Glia and Neurons Glia Insulates, supports, and nourishes

The Neuron DoctrineThe birth of neurohistology

Microscopy inventionDiscovery of fixation method for cutting thin slicesStaining methods for selectively coloring parts of cells

The Nissl StainDeveloped by by the German neurologist Franz NisslStain the nuclei and surrounding material (Nissl body)Made it possible to distinguish neurons vs. glia and to study the arrangements of neurons in different parts of brain (cytoarchitecture)

Fig2.1

Page 4: NEURONS AND GLIA. Introduction “Neurophilosophy” Brain (neurons) is the origin of mental abilities Glia and Neurons Glia Insulates, supports, and nourishes

The Neuron DoctrineThe Golgi Stain

Camillo Golgi discovered that by soaking brain tissue in a silver chromate solution, a small percentage of neurons became darkly colored in their entiretySoma (cell body or perikaryon) and neurites (axons and dendrites)

Fig 2.3

Page 5: NEURONS AND GLIA. Introduction “Neurophilosophy” Brain (neurons) is the origin of mental abilities Glia and Neurons Glia Insulates, supports, and nourishes

The Neuron Doctrine

Cajal’s ContributionSantiago Ramon y Cajal used Golgi stain method and worked out the circuitry of many regions of the brain : Father of neuroanatomyGolgi versus Cajal

Reticular theory vs. cell theory

Neuron doctrineNeurons communicate by contact, not continuityFinal proof had to wait until EM got developed in the 1950s

Page 6: NEURONS AND GLIA. Introduction “Neurophilosophy” Brain (neurons) is the origin of mental abilities Glia and Neurons Glia Insulates, supports, and nourishes

The Prototypical Neuron

The Soma~20 um in diameterCytosol: Potassium rich watery fluid inside the cellOrganelles: Membrane-enclosed structures within the somaCytoplasm: Contents within a cell membrane (e.g., organelles, excluding the nucleus)

Fig2.7

Page 7: NEURONS AND GLIA. Introduction “Neurophilosophy” Brain (neurons) is the origin of mental abilities Glia and Neurons Glia Insulates, supports, and nourishes

The Prototypical NeuronThe Axon

Begins with axon hillock, initial tapered segment where action potentials are generated Rough ER does not extend into axon Protein composition of axon membarane is fundamentally different from that of somaNo protein synthesis in the axonMay extend from less than a millimeter to over a meter longMay branch out (generally at right angles) to form axon collaterals that could return to the same cell (recurrent collaterals)Diameter ranges from less than 1 m to 25 m in humans - The speed of nerve impulses depends on axonal diameter

Page 8: NEURONS AND GLIA. Introduction “Neurophilosophy” Brain (neurons) is the origin of mental abilities Glia and Neurons Glia Insulates, supports, and nourishes

The Prototypical NeuronThe Axon Terminal (terminal

bouton)A site where the axon comes in contact with other neurons and passes information on to themTerminal arbor or boutons en passantSynapse - To fasten togetherInnervation - making synaptic contactDifferences between the cytoplasm of axon terminal and that of axon

No microtubules in the terminalPresence of synaptic vesicles (~50 nm in diameter)Dense covering of proteins on the inside surface of the synaptic membraneLarge number of mitochondria (high energy demand)

Page 9: NEURONS AND GLIA. Introduction “Neurophilosophy” Brain (neurons) is the origin of mental abilities Glia and Neurons Glia Insulates, supports, and nourishes

The Prototypical NeuronSynapse

Pre- and Postsynaptic sides : directionality of information flowSynaptic transmissionSynaptic cleft Electrical-to-chemical-to-electrical transformationNeurotransmitter

Page 10: NEURONS AND GLIA. Introduction “Neurophilosophy” Brain (neurons) is the origin of mental abilities Glia and Neurons Glia Insulates, supports, and nourishes

The Prototypical NeuronAxoplasmic transport

Wallerian degenerationDegeneration of axon when severed (axotomy) is due to the lack of protein synthesis machinery within axon

Kandel Fig 55-18

Anterograde transport by kinesin and retrograde transport by MAP-1C (dynein)

Page 11: NEURONS AND GLIA. Introduction “Neurophilosophy” Brain (neurons) is the origin of mental abilities Glia and Neurons Glia Insulates, supports, and nourishes

The Prototypical NeuronSlow Axoplasmic transport

Paul Weiss’s experimentTied off a sciatic nerve (axon) to find that material accumulate on the proximal side of the knotWhen the knot was untied, the bulged out accumulation continued down the axon

The speed of movement was measured to be about 1 - 10 mm per day ; SLOW AXOPLASMIC TRANSPORTOnly anterograde directionSlow transport itself can be at two different speeds

Slower (0.2-2.5mm per day) : fibrillar elements of cytoskeleton (neurofilament subunits, tubulins..)Faster (about twice as fast as the slower) : various cytosolic proteins (clathrin, actin, actin-binding proteins, enzymes..)

Page 12: NEURONS AND GLIA. Introduction “Neurophilosophy” Brain (neurons) is the origin of mental abilities Glia and Neurons Glia Insulates, supports, and nourishes

The Prototypical NeuronFast Axoplasmic transport

Bernice GrafsteinInjected radioactive amino acids into somataTraced the synthesized (hot) proteins along the axon

Large membraneous organelles are transported via fast transportIncludes vesicles of the constitutive secretory pathways, synaptic vesicles precursor membranes, mitochondria, smooth ER elements..ATP dependent but not protein synthesis dependent (once synthesized)Soma-independent (isolated axon still can transport

Page 13: NEURONS AND GLIA. Introduction “Neurophilosophy” Brain (neurons) is the origin of mental abilities Glia and Neurons Glia Insulates, supports, and nourishes

The Prototypical NeuronDendrites

Greek for ‘tree’Dendritic tree for all the dendrites of a neuron“Antennae” of neurons - covered with thousands of synapsesDendritic membrane (postsynaptic membrane) contains many specialized receptors for neurotransmittersDendritic spines

Some neurons have these structures for receiving some types of inputsDiscovered by CajalBelieved to isolate various chemical reactionsDynamic structures affected by the type and amount of inputs and developmental changes of environment

Fig 2.17

Fig 2.18

Page 14: NEURONS AND GLIA. Introduction “Neurophilosophy” Brain (neurons) is the origin of mental abilities Glia and Neurons Glia Insulates, supports, and nourishes

Mental Retardation and dendritic spinesBrain function depends on the highly precise synaptic connections, which are formed during the fetal period and are refined during infancy and early childhood95% of population falls within two standard deviations from the mean of IQ (around 70 when the mean is set to be 100). Some 2-3% of humans with intelligence score below are considered to be mentally retarded IF the cognitive impairment affects the person’s ability to adapt their behavior to the setting in which they live

Fig A

Can have many causesGenetic disorders such as PKU or Down syndromeAccidents or infection during pregnancy or early childhoodPoor nutrition during pregnancyEnvironmental impoverishment such as the lack of good nutrition, socialization, sensory stimulation during infancy

Some with clear physical correlates (retarded growth, abnormal structures of head, hands, and body), most with only behavioral manifestationsDendritic spine abnormality has been found to be correlated with mental retardation

Page 15: NEURONS AND GLIA. Introduction “Neurophilosophy” Brain (neurons) is the origin of mental abilities Glia and Neurons Glia Insulates, supports, and nourishes

Classifying NeuronsClassification Based on the Number of Neurites

Unipolar cell Found in invertebrate nervous system - single process with different segments serving as receptive surfaces or releasing terminals

Bipolar cell Two neurites

Multipolar cellMost neurons in the brain are multipolar

Kendal fig 2-4

Page 16: NEURONS AND GLIA. Introduction “Neurophilosophy” Brain (neurons) is the origin of mental abilities Glia and Neurons Glia Insulates, supports, and nourishes

Classifying Neurons

Classification Based on Dendritic and Somatic Morphologies

Often unique to a particular region of the brain

Cortex - Stellate cells (star-shaped) and pyramidal cells (pyramid-shaped)

Spiny or aspinous

Page 17: NEURONS AND GLIA. Introduction “Neurophilosophy” Brain (neurons) is the origin of mental abilities Glia and Neurons Glia Insulates, supports, and nourishes

Classifying Neurons

Further ClassificationBased on connections within the CNS

Primary sensory neuronsmotor neuronsinterneurons

Based on axonal lengthGolgi Type I - projection neurons that extend their axons to other parts of the brain (e.g. pyramidal neurons in the cortex)Golgi Type II - local circuit neurons that have short axons that do not extend beyond the vicinity of cell body (e.g. stellate cells in the cortex)

Based on neurotransmitter typeCholinergic, glutamatergic, GABAergic…

Page 18: NEURONS AND GLIA. Introduction “Neurophilosophy” Brain (neurons) is the origin of mental abilities Glia and Neurons Glia Insulates, supports, and nourishes

Glia‘Sleeping Giants’ ?Function of Glia

Supports neuronal functions Without glia brain cannot function!

AstrocytesMost numerous glia in the brainFill spaces between neuronsImporatant regulator of the chemical contents of extracellular spaces (Not much left after filling up)Envelop synaptic junctions - restrict the spreading of released neurotransmittersPossess their own neurotransmitter receptors!!

Page 19: NEURONS AND GLIA. Introduction “Neurophilosophy” Brain (neurons) is the origin of mental abilities Glia and Neurons Glia Insulates, supports, and nourishes

Glia

Myelinating GliaOligodendroglia (in CNS) and Schwann cells (in PNS)

Insulate axons by wrapping axons around Myelin sheath One Oligodedroglia can provide insulation to several axons but each Schwann cell does to a only a single axon

Node of RanvierRegion where the axonal membrane is exposed

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Page 21: NEURONS AND GLIA. Introduction “Neurophilosophy” Brain (neurons) is the origin of mental abilities Glia and Neurons Glia Insulates, supports, and nourishes

Glia

Other Non-Neuronal CellsMicroglia as phagocytes (immune)Ependymal cells provide lining of fluid-filled ventricles and directs cell migration during brain developmentVasculature