CNS Lecture Outline PDF

LECTURE OUTLINE I Development of Nerve Tissue Neurons A. Cell Body (Perikaryon or Soma) B. Dendrites II C. Axons D. Nerve Impulses E. Synaptic Communication Glial Cells & Neuronal Activity...

LECTURE OUTLINE I Development of Nerve Tissue Neurons A. Cell Body (Perikaryon or Soma) B. Dendrites II C. Axons D. Nerve Impulses E. Synaptic Communication Glial Cells & Neuronal Activity A. Oligodendrocytes B. Astrocytes III C. Ependymal Cells D. Microglia E. Schwann Cells F. Satellite Cells of Ganglia I. DEVELOPMENT OF NERVE TISSUE CNS A. Meninges IV B. Blood-Brain Barrier C. Choroid Plexus Nervous system develops from PNS ectoderm V A. Nerve Fibers - In the 3rd week of dev’t B. Nerve Organization Neural plate C. Ganglia VI Neural Plasticity & Regeneration - From the thickening of VII Study Questions notochord and ectoderm - The sides fold upward 2 divisions: and grow medially 1. CNS = brain & spinal cord - Fuses to form the neural 2. PNS tube ✓ Cranial, spinal, & peripheral ✓ Cells of this tube nerves (sensory & motor nerves) give rise to entire ✓ Ganglia → smallest aggregates CNS of nerve cells outside CNS Neural Crest Cells → 2 kinds of cells: developmentally important cells 1. Neurons → has numerous long - Separates from neuroepithelium processes and becomes mesenchymal - Differentiate into: 2. Glial cells a. Cells of PNS - “glia” = glue b. Non neuronal cells - Has short processes - Supports & protects neurons - Participate in neural activities, neural nutrition, and defense Neurons respond to stimuli by altering the ionic gradient (electrical potential) across the membrane ✓ Excitable / irritable → rapidly changes the potential in response to stimuli Membrane potential → reversal of gradient Action potential / depolarization wave / nerve impulse ✓ “Propagation” ✓ Travels long distance along neurons  -Comprise 99% of all neurons in II. NEURONS adults In CNS - Perikarya = in gray matter It is the functional unit of CNS & PNS - Axons = in white matter Neurolemma nerve cell membrane In PNS Nerve fibers processes of neurons - Perikarya = in ganglia - Axons = bundled in nerves 3 Parts of Neurons Cell body Contains nucleus & organelles “Perikaryon” The synthetic / trophic center “Soma” Up to 150 μm Elongated processes Dendrites Receives stimuli at synapses Axon Single long process “Axon = axis” Generate & conduct impulses Classification of Neurons Most common Multipolar 1 axon >2 dendrites In sensory neuron of retina, olfactory epithelium, inner ear Bipolar 1 dendrite 1 axon Unipolar / Has single process that bifurcates Pseudo- close to perikaryon unipolar Extends to peripheral ending & CNS Parkinson’s Dse. Many dendrites Charac. by tremors, loss of balance, ⬛ Regulates electrical changes of stiffness Anaxonic adjacent CNS neurons due to apoptosis of dopamine-producing neurons + True axon neurons + Action potential o Its cell bodies lie within substantia nigra Treatment: L-dopa (L-3,4- dihydroxyphenylalanine) It is difficult to classify neurons by microscope Functional division of nervous compartments: a. Sensory neurons ✓ Afferent ✓ Receives stimuli from receptors b. Motor neurons ✓ Efferent ✓ Sends impulse to effector organs ✓ Somatic motor nerves Voluntary control Innervates skeletal muscles ✓ Autonomic motor nerves Involuntary / unconscious control (glands, heart, smooth muscles) Interneurons → forms complex functional networks (circuits) in CNS - Either multipolar or anaxonic  A. Cell Body (Perikaryon or Soma) o Its morphology depends on actin filaments o Its changes are important in Contains nucleus Trophic center → produces most neuroplasticity cytoplasm o To view: Typical neuron has large, euchromatic ✓ Stain = silver stain nucleus w/ prominent nucleolus ✓ Microscope = confocal / Cytoplasm of Perikarya electron mic ✓ Many free ribosomes ✓ Highly developed RER ▪ Basophilic ▪ Chromatophilic or Nissl C. Axons substance / bodies Golgi apparatus is in cell body only Mitochondria is throughout the cell Action Potential” - Abundant in axon terminals Initiated at axon hillock Actin & intermediate filaments This is propagated along the axon a. Neurofilaments as a “wave” of membrane ▪ Neurofibrils when stained w/ depolarization produced by silver stains and viewed under voltage-gated Na+ & K+ channels light microscope -65 mV = Axon’s resting potential Lipofuscin → pigment inclusion made o Na is increased outside up of residual bodies from lysosomal o K is increased inside +30 mV digestion o Depolarized (influx of Na) Summary of events: B. Dendrites “dendron” = tree Local anesthetics o Resting potential → -65 mV Principal signal reception and Low MW o Na channel opens → Na processing sites on neurons Binds to voltage- influx gated Na+ o Depolarization → +30 mV Short, small processes o Na channel closes channel Covered by many synapses Interferes w/ Na o K channel opens → K influx Thinner as they branch influx o Membrane Dendrite spines synapses on dendrites in Inhibits action returns to resting CNS potential potential E. Synaptic Communication Synapses o “Synapsis” = union o Sites where impulses are transmitted o Ensures unidirectional transmission o Components: “terminal bouton” Has mitochondria Presynaptic axon terminal Synaptic vesicles → where neurotrans. are released via exocytosis Initiates new impulse Postsynaptic cell Has receptors for membrane neurotransmitters o Dynamic membrane protrusions in Has ion channels also Wide intercellular space small dendritic branches Synaptic cleft Separates pre- & postsynaptic o Serve as the initial processing sites for synaptic signals o 1014 = amount in cerebral cortex Presynaptic cell → where electrical  signals are converted Postsynaptic cell → affected by the chemical signal converted from electrical signal Neurotransmitters → small molecules that bind specific receptor proteins to either open or close ion channels or initiate second messenger cascades o Released by synapses At Presynaptic region: 1. Nerve impulse causes opening of Ca2+ channel 2. Ca2+ influx 3. Neurotransmitted release 4. Neurotransmitter will diffuse across the synaptic cleft 5. Neurotransmitter will bind to the receptor at postsynaptic region 6. It will cause either an excitatory or inhibitory effect ✓ Excitatory synapses cause Selective Serotonin postsynaptic Na+ channel to Reuptake Inhibitors open = depolarization (SSRIs) ✓ Inhibitory synapses cause Used for opening of Cl- or anion treating depression & channels anxiety Acetylcholine → neurotrans. at NMJ of CNS Other neurotrans. III. GLIAL CELLS & NEURONAL ACTIVITY o Amino acids = glutamate & GABA Glial cells → supports neuronal activites o Monoamines = serotonin & catecholamines ✓ 10x more abundant than neurons (dopamine) ✓ Develop from embryonic neural plate o Small polypeptides (endorphins & P Neuropil → fibrous intercellular network of substance) CNS that resembles collagen  A. Oligodendrocytes Alzheimer’s Disease → type of dementia Affects perikaryal & synpase in cerebrum “oligos” = small Neurofibrillary tangles → accumulation of tau proteins “dendron” = tree Neuritic plaques → dense aggregates of β-amyloid protein “kytos” = cell Sheetlike & wraps repeatedly around axon Predominant glial cells in white matter o White = due to lipid C. Ependymal Cells Myelin → multiple compacted layers of cell membrane Columnar/cuboidal cells that line the o Formed from moving out of fluid-filled ventricles of brain & central cytoplasm during wrapping of axon canal of spinal cord Myelin sheath → insulates axon and facilitates Apical ends have cilia → for mov’t of CSF rapid transmission of nerve impulses Long microvilli → for absorption In light microscope = small cells w/ rounded, Joined apically by apical junctional complexes condensed nuclei & unstained cytoplasm + Basal lamina Elongated basal ends that extends to adjacent B. Astrocytes Neuropil “astro” = star Large number of long radiating, branching processes The most numerous glial cells of the brain!!! Its proximal regions are made up of glial fibrillary acid protein (GFAP) → its unique marker Distally = + GFAP Originate from embryonic neural plate Long delicate process Fibrous astrocytes Abundant in white matter D. Microglia Protoplasmic Shorter processes astrocytes In gray matter Less numerous than oligodendrocytes Small cells w/ actively mobile Functions of astrocytes processes evenly distributed o Extending processes that cover synapses throughout gray and white matter o Regulates ECF ionic conc. (buffers K+ It migrates levels) Removes damaged or effete synapses o Guide & supports mov’t of neurons during Major mechanism of immune defense in CNS CNS dev’t Secretes cytokines o Fibrous processes w/ perivascular feet → +++Originate from neural progenitor cells covers endothelial cells & modulate blood flow & help move nutrients, wastes Originate from monocytes (macrophage, o Glial limiting membrane → lines meninges APC) at external CNS H&E Stain → to visualize nuclei ▪ Barrier of expanded protoplasmic processes Immunohistochemistry (IC) → demonstrates o Astrocytic scar → formed from filling tissue microglial processes defects after CNS injury Assumes morphology of APC when… Gap junctions → where astrocytes communicate o Activated by damage / microorganisms directly w/ one another o They retract their processes  E. Schwann Cells IV. CNS Named for Theodor Schwann Major structures: ✓ Cerebrum “neurolemmocytes” CNS = oligodendrocytes ▪ Cerebral nuclei → localized, PNS = Schwann cells Found only in PNS darker areas that contains large Precursor → neural crest cells number of aggregated cell Forms myelin sheaths around 1 axon only bodies Trophic interactions ▪ Cerebral cortex - Has 6 layers - Pyramida neurons → most conspicuous D. Satellite Cells of Ganglia - For integration of sensory info Precursor → neural crest cells - Initiation of voluntary motor responses Forms thin, intimate glial layer around cell body in ganglia of PNS ✓ Cerebellum Has trophic / supportive effect ▪ Coordinates muscle activity Thick, outer layer Molecular Has neuropil & scattered cell layer bodies Thin, middle layer Named after Jan Purkinje Purkinje cells Its dendrites extend to molecular layer → “Branching basket of nerve fibers” Thick, inner layer Granular Has small, densely packed layer neurons Granule cells & little neuropil ✓ Spinal cord ▪ White matter = peripheral ▪ Gray matter = deeper, “H-shaped” mass ✓ Anterior horns → has cell bodies of very large neurons ✓ 2 posterior horns → has interneurons that receives sensory fibers from spinal ganglia (dorsal root) ✓ Central canal → in the middle - Develops from lumen of neural tube - Continuous w/ ventricles - Lined by ependymal - Has CSF Meninges → CT that covers the CNS Little collagen White matter → found in deeper regions ✓ Composed of: ▪ Myelinated axons ▪ Grouped together as “tracts” ▪ Myelin-producing oligodendrocytes Gray matter → makes up the thick cortex / surface layer of cerebrum & cerebellum ✓ Composed of: - Abundant cell bodies (synapses occur)  A. Meninges I. DURA MATER = thick, external layer “Dura mater” = tough mother Dense irregular CT Organized as an outer periosteal layer continuous with the periosteum of the skull and an inner meningeal layer - Fuses to form dural venous sinuses Epidural space → separates dura mater from periosteum in spinal cord Thin subdural space → separates dura mater from arachnoid II. ARACHNOID MATER “Arachnoeides” = spider web-like 2 components: 1) Sheet of CT in contact w/ dura 2) Loosely arranged trabeculae of collagen & fibroblasts (continuous w/ pia) Subarachnoid space → sponge-like cavity o Filled w/ CSF o Cushion and protect CNS o Communicates w/ ventricles of brain Has avascular CT → + nutritive capillaries Pia-arachnoid → refers to its association w/ pia as a single membrane Arachnoid villi → CSF-filled protrusions o Sites for absorption of CSF to the blood III. PIA MATER = innermost “Pia mater” = tender mother Has flattened, mesenchymal cells + Direct contact to nerve cells Glial limiting membrane / Glia limitans - Thin superficial layer of astrocytic processes that separates pia from neural elements Perivascular spaces → where BV penetrates CNS B. Blood-Brain Barrier Controls passage of substances from blood to CNS Protects neurons from toxins Capillary endothelium → main structural component of BBB o Where cells are tightly sealed w/ occluding junctions o Little to no transcytosis activity  Limiting layer of perivascular astrocytic feet Peripheral nerves → bundle of nerve fibers surrounded by Schwann cells and CT o Envelopes basement membrane of capillaries in CNS +++ Present on the ff.: o Hypothalamus (where plasma components A. Nerve Fibers are monitored) o Posterior pituitary (where hormone is I. MYELINATED FIBERS released) formed from Schwann cells engulfing o Choroid plexus (where CSF is produced) axons in PNS Mesaxon → area where plasma membrane of each covering Schwann cell C. Choroid Plexus fuses with itself. Myelin Sheath → multiple layers of Schwann cells - Large lipoprotein complex - Insulates axon - In TEM = thick, electron dense axonal covering Major dense lines → fused protein rich cytoplasmic surfaces of the Schwann - Along myelin sheath, this separates periodically Myelin clefts - “Schmidt-Lanterman” clefts\ - Where major dense lines dsappear Nodes of Ranvier / Nodal Gaps - Axon is partially covered by Schwann - Axolemma is exposed - Has higher conc. of Na-channels Highly vascular ▪ Produces saltatory conduction Location: ▪ “saltare” = jump o Roofs of 3rd & 4th ventricles ▪ Internodal segment → length of axon o 2 lateral walls ensheathed by one Schwann o All regions where ependymal lining is in direct contact to pia ▪ UNMYELINATED FIBERS Removes water from blood and release it as CSF ⬛ ⬛ ⬛ Still has Schwann cell Villus of choroid +++ Wrapping to form myelin sheath o Thin layer of vascularized pia mater +++ Nodes of ranvier o Covered by cuboidal ependymal cells +++ Saltatory conduction CSF Have evenly distributed Na-channel o Clear o Little to no protein Slower impulse conduction ˙ - o Has Na, K, Cl ions o Has lymphocytes Arachnoid villi → main pathway for absorption back to venous circulation Hydrocephalus → progressive enlargement of head “hydro” = water “kephale” = head ↓↓↓ CSF absorption OR blockage of outflow from ventricles during fetal dev’t V. PNS Composed of: o Nerves o Ganglia o Nerve endings  B. Nerve Organization Nerves have a whitish, glistening appearance because of their myelin and collagen content C. Ganglia Endoneurium → around external lamina of schwann cells In PNS o Made up of reticular fibers, scattered fibroblasts, and Ovoid structures that has neuronal cell bodies capillaries and glial cells Fascicles → bundled axons & schwann Supported by delicate CT Perineurium → covers fascicles Surrounded by denser capsule o Has flat fibrocytes with their Serve as relay stations to transmit nerve edges sealed together by tight impulses junctions The direction will determine if it is sensory Epineurium → dense, irregular fibrous coat or autonomic Blood-nerve barrier → helps maintain the fibers’ microenvironment I. SENSORY GANGLIA Afferent fibers → from body to CNS o Receive afferent impulse Efferent fibers → from CNS to effector organs o In cranial nerves and dorsal roots Sensory nerves → only sensory fibers o Satellite cells → small glial cells Motor nerves → has fibers that carry o Pseudounipolar neurons impulse to effectors o Relay information from the Mixed nerves → both motor and ganglion’s nerve endings to the gray sensory matter of the spinal cord via  synapses with local neuron Regeneration of axons… II. AUTONOMIC GANGLIA Only when schwann are directed properly In mixed cell o “Autos” = self o In sensory fibers grow to columns formerly occupied by o “nomos” = law motor fibers → + fxn will not be reestablished o Small, bulbous dilations → multipolar Neuroma → source of spontaneous pain o INVOLUNTARY = smooth o Due to extensive gap muscles, glands, heart rate o Involved in regulation of homeostasis o Makes up the autonomic nervous system A. Sympathetic ▪ Pregang. Sympa nerves cell bodies are in: ✓ thoracic & lumbar segment of spinal cord ▪ Sympa. 2nd neurons = in small ganglia of vertebral column B. Parasympathetic ▪ Pregang. parasympa. nerves are in ✓ medulla ✓ midbrain ✓ sacral portion of spinal cord ▪ Parasympa. 2nd neurons = in small ganglia near effector organ o Intramural ganglia → in walls of GI tract o 2 circuits: (1) Preganglionic fiber → In CNS ▪ Its axon synapse w/ postgang. (2) Postganglionic fiber → in peripheral ganglion system o Acetylcholine → neurotrans. in pregang. VI. NEURAL PLASTICITY & REGENERATION ▪ Controlled by neurotrophins - Promote anabolic events of axon regeneration ▪ Neuronal stem cells - Located in ependyma ▪ Astrocytes can proliferate at injured sites ▪ Chromatolysis - Cell body swells - Nissl is diminished - Nucleus migrates to peripheral position in perikaryon - New schwann cells align → guide for regrowing axons