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Michael Inzlicht

CHAPTER 3: CELLS OF THE NERVOUS SYSTEM Fugu/Puffer Fish: • Delicacy that has deadly tetrodotoxin (TTX) • 100s die each year bc it blocks APs (specifically, blocks Na ion movement) Neurons and Glia • 1 : 10-50 = neuron: glia ratio Structure of Neurons: • Membrane: o Double phospholipid (fatty phosphate-containing molecs) layer o 2 main types of embedded proteins  1)ion channels • Voltage-gated • Ligand-gated (chemical-based, whether drugs or natural)  2)ion pumps • Na-K pumps o Maintain diffs in chem. comp btwn intra and extra o Prisoner exchange of sending 3 Na out: 2 K in o Costs a lot of energy (20-40% of energy req’d by brain) • Ca pumps o Only Ca out o Release of neurotrans depends on low levels on intra Ca • Cytoskeleton: o Maintains shape of neuron o Filaments x 3:  1) microtubules: • Largest (25nm diameter), hollow tubes • Move various materials within cells o Anterograde (= forward) transport: soma  axon terminal o Retrograde (= backward) transport: axon terminal  soma • Alzheimer’s: o Memory loss, cog + physical decline,  death o Neurofibrillary tangles consisting of tau protein bc high tau lvls  Normally, tau connects adjacent microtubules and holds them in place o Bc enzymes  high tau lvls  neuron adds phosphate  tau disconnects and forms tangles  neuron collapses in on itself o Amyloid plaques (amyloid = abnormal protein)  Beta amyloid = specific type that works with tau to break apart microtubules  2) neurofilament • Most common • 10nm, hair-like • Structural support  3) microfilament • Smallest, 3-5nm • Mostly found in neuron branches • Change shape and length of branches during dev and for learning • Soma: o Nucleus: contains DNA, directs cell fxns  Nucleolous: substructure that builds ribosomes  Ribosomes= produce proteins on their own or w/ the ER o Endoplasmic reticulum:  RER: ribosomes on surface, proteins constructed here  SER: no ribosomes, transports proteins to GA o Golgi Apparatus: packages the completed proteins into vesicles (made out of membrane material) o Mitochnodria: extracts O2 and pyruvic acid from sugar in intra fluid and  ATP (adenosine triphosphate) for energy source • Dendrites: o Greater surface area of a dendrite, the more connections/synapses can be formed o Dendritic spines: knobs sometimes formed by dendrites which provide even more space for synapses to occur  can change shape based on amount of activity occurring at synapse  related to learning + memory  abnormal ~’s  mental retardation • unusually long and thin (how they are in early dev)  FMRP = Fragile X mental retardation protein = necessary for normal maturation • Axon: o Axon hllock:  Cone-shaped segment that lies at junction of axon and cell body  APs start here and then go down length of axon o Larger axons = faster at signalling  But, myelin allows for rapid signalling even though smaller axon diameter • Most of CNS neurons + PNS motor neurons = myelinated o Only some PNS sensory are myelinated, more likely for the larger diameter ones • Myelin is formed by certain types of glia • Does not cover length of an axon o Hillock is uncovered and between each sement of myelin, there is a bare space of axon membrane called node of Ranvier o Larger axons have thicker myelin and greater distances between nodes of Ranvier • Advantages of myelin: o Allow axons to be smaller without sacrificing transmission speed o Reduces energy req’s of neurons by decreasing amount of work done by Na/K pumps o Since myelin segments wrap around so tightly, there is little to no extracellular fluid between myelin and axon membrane. Therefore, no need for ion channels under myelin!  Only the axon hillock and nodes of ranvier will have ion channels • Multiple Sclerosis: o Progressive demyelination of nervous sys o Neural signalling won’t work properly o range of symptoms from increased fatigue to vision/mobility problems to death o Humans = 1-25 micrometers, squid = 1mm o Local circuit neuron: neuron with an axon that doesn’t extend far from soma o Projection neuron: neuron with very long axon o Collaterals: where the end of an axon divides into branches  Allowing a neuron w/ 1 axon to communicate with many other cells o Axon terminal: swelling at the end of each axon collateral that contains many mitochondria and synaptic vesicles that contain chem. messengers  Synaptic vesicles: made of double phospholipid layer of cell membrane, 50nm Structural Variations of Neurons: • Unipolar = only have a single branch extending from cell body o Typical of invertebrates’ nervous systems o But, for vertebrates:  Found in sensory sys and autonomic NS  Branch to CNS and PNS  Near spinal cord typically and can help with somatosensory • Bipolar: o Have one axon and one dendrite o Very important for sensory sys o Von Economo Neuron (VEN): a special bipolar neuron , developed recently in primate evolution  Are larger and more plentiful in humans than apes  Occur only in the ACC, junction of frontal lobe and insula area of temporal lobe  Fxn = fast, intuitive assessments of complex situations • Multipolar o Most common type of neuron in vertebrates o Many branches (usually 1 axon, many dendrites) o Further classified by shape:  Pyramidal: • Cerebral cortex + hippocampus  Purkinje: • Cerebellum • dramatic dendrite trees (1 cell can form as many as 150,000 synapses) Fxnl Variations in Neurons: • sensory: receive info from world • motor: transmit CNS commons to muscles and glands • inter: most neurons = interneurons = not specialized for either m/s but bridge the two • GLIA: o Support neuron activity o Categorized by size: o 1)macroglia = largest :  A) Astrocytes: • Star-shaped • most common type of glia in brain • Variety of support fxn in CNS o Structural matrix o Form connections with blood supply to transfer glucose + other nutrients to the neurons o Debris cleanup o Synapse isolation o Participating in chem. signalling (by releasing glutamate + ATP)  Glutamate = excitatory  ATP = inhibitory o Contribute to blood-brain barrier o Collect the excess potassium that neurons release when they send messages o May also be imp in learning + memory • Sometimes astrocytes are not helpful: o When CNS neurons are damaged, some astrocytes will move there and digest the dead neurons forming scar tissue that prevents axons and dendrites from rebuilding connections o May be somewhat helpful in containing some damage, it does prevent neural regrowth  B) Oligodedrocytes: • Supply myelination of axons in CNS • Contributes to structural stability bc 1 ~ can myelinate axons from many different neurons • Can’t help regrowth of damaged axons  C) Schwann cells: • Supply myelination of axons in PNS • Takes about 1 schwann cell per 1 myelin segment on a peripheral axon • Help guide regrowth of damaged axons o 2) microglia = smallest:  Debris (dead cells) cleanup in CNS  At rest, ~ reach out and sample immediate enviro to detect molecules related to cell damage and then they travel there to digest debris  Uncontrolled activation of ~ can damage brain bc of release of substances that produce inflammation  Also, they sometimes digest healthy cells located near damaged ones  May be a contributor to Alzheimer’s, Parkinson’s, and MS Generation of the Action Potential • Not all messages sent down the axon are from APs. Some complex chem. rxns can also work Ionic Comp of Intra and Extra Fluids • Both contain water • Ions remain separate in sol’n bc attraction to water is stronger than attraction to each other • Extracellular: o Lots of Na and Cl o Not much K o More Ca o Similar to sea water o More positive bc no proteins • Intracellular: o Lots of K o Not much Na and Cl o Less Ca o Literally the opposite in terms of ion comp as extracellular o But: also, large proteins that are negatively charged o More negative bc proteins • Difference in charge between intra and extra is roughly 70mV o Convention to assign outside: 0v  Inside/”across membrane”: -70mV = resting potential of cell Ion Movement: • 2 forces: o Diffusion: tendency for molecs to distribute thems
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