Modules #1-5
What is the nervous system?
Network of cells specialized to sense inputs & generate outputs
What is signaling in the nervous system?
Electrical impulses for information propagation + synapses for conveying information between neurons
What are 2 types of electrical impulses?
Graded potentials + action potentials
What are chemical synapses?
Convert electrical signals into chemical signals for postsynaptic cells
What barrier to electric signals do synapses overcome?
Hydrophobic phospholipid bilayer can’t conduct electrical signals
What are graded potentials?
Convey intensity of stimulus
What is the disadvantage of graded potentials?
Dissipate so no long distance
Why do graded potentials dissipate?
Quick depletion of ions/charges
What are graded potentials in the retina?
Photoreceptors of receptor cells to bipolar cells & then Ganglion cells both producing graded potentials
What is stimulus intensity in retinal graded potential?
Stronger light = more neurotransmitter secretion by receptor cell = stronger depolarization of bipolar cell
When does action potential arise in the retina?
Strong enough graded potential in excitable neurons like Ganglion cells
What is an excitatory potential?
Depolarizing (excitatory postsynaptic potential = EPSP)
What is an inhibitory potential?
Hyperpolarizing (inhibitory synaptic potential = IPSP)
What are action potentials?
All-or-nothing electrical impulse up to 120 m/s along axon
When are action potentials activated?
Graded potentials activate voltage-gated sodium (NaV) & potassium (Kv) channels
What are 2 advantages of action potential?
Long distance + don’t dissipate
How far apart are cells?
~10 microns
What are NaV channels?
Depolarize membrane
What are Kv channels?
Depolarize/hyperpolarize membrane
What are 2 functions of neuronal action potentials?
Transmit signals along axons + trigger presynaptic Ca2+ influx through CaV at nerve terminals
What is presynaptic Ca2+ influx at nerve terminals?
Regulated release of neurotransmitters by exocytosis
What is the function of graded/action potentials in muscle?
Contraction
What is the function of graded/action potentials in endocrine cells?
Secretion of hormones by exocytosis
What is the challenge of understanding the nervous system?
Integrating molecular biology with electrophysiology (physics)
What made the cell theory (1838) possible?
Combined observations of light microscopy
What are 3 tenets of the cell theory?
(1) All living things composed of cells, (2) cell is basic unit of organism structure & organization, (3) cells come from pre-existing cells
What is the reticular theory of the nervous system (1861)?
Made up of single contiguous network, not separate cells
Why was reticular theory preferred over cell theory?
No discrete cells under microscope of nervous tissue
Who were 2 proponents of reticular theory?
Camillo Golgi + J. von Gerlach
What are 2 alternate names of the Golgi stain?
La reazione nera + the black reaction
What is the Golgi stain?
Potassium dichromate & silver nitrate to dead fixed neurons to randomly label a subset of neurons black in their entirety for single cell tracing
What is the (ironic) significance of the Golgi stain?
Shows neurons as individual cells
Who was Santiago Ramon y Cajal?
Histologist studying nervous system tissues with Golgi stain that disproved reticular theory in favour of neuron theory
What is the neuron doctrine (theory)?
Nervous system is made up of separate cells
Who was Henrich Waldeyer?
Synthesized nervous system organization principles to propose the neuron theory
Who was Deiters?
Protoplasmic processes renamed dendrites by Waldeyer
Who was Kolliker?
Axis cylinders renamed as axons by Waldeyer
What were Cajal vs. Waldeyer’s contributions?
Cajal et al. generated data whereas Waldeyer only synthesized into neuron doctrine
What are 4 tenets of the neuron doctrine?
(1) Neuron (cell) is functional unit of nervous system, (2) law of dynamic polarization, (3) neuron communication via synapses, (4) Dale’s law
What are 3 common structures of neurons?
Dendrites, soma, axons
What are nerve fibers?
Project from soma of single neurons
What is the neuron nucleus?
Nutritive center
What is the law of dynamic polarization (directionality)?
Dendrites & soma = receptor surfaces, single axon = effector
What is the direction of neuronal information?
Dendrites to axons
What was Cajal’s contribution to law of dynamic polarization?
Inferred directionality through diagrams of neurons (ex. retina)
What are synapses?
Regions of neuron-neuron contact
Who was Charles Scott Sherrington?
Proposed synaptic transmission through electron microscopy
What is Dale’s law?
Single neurons use single type of neurotransmitter (ex. glutamate = glutaminergic, glycine, GABA)
Which 3 scientists received Nobels for neuron doctrine?
Golgi, Cajal, Sherrington
What are 4 issues with the neuron doctrine?
(1) Gap junctions resemble reticular theory, (2) reciprocal synapses, (3) action potential backpropagation, (4) Dale’s law exceptions
What are gap junctions?
Electrical rather than chemical synapses by connecting cytoplasms through pores at membranes
What are reciprocal synapses?
Axons can act as dendrites, dendrites can act as axons
What is action potential backpropagation?
Signals travelling against polarity from soma/axon to dendrites
What are exceptions to Dale’s law?
Some neurons secrete more than 1 neurotransmitter type
What were 3 early inroads to understanding the nervous system?
Neuron morphology, projection, (synaptic) connectivity
What facilitated early inroads to understanding the nervous system?
Technological advances like microscopy & staining techniques (ex. Golgi stain)
What was Cajal’s influence?
Understanding nervous system structure & connectivity = understanding information flow & directionality in nervous system
What is Cajal’s neural connectomics paradigm?
Significant ongoing research focused on new or improved neural imaging & tracing techniques for understanding NS function
What is a connectome?
Complete system of neural pathways in nervous system
What is the disadvantage of neural connectomics?
Studying electrical signalling necessary for NS function in biological context (mapping roadways in city not enough to understand traffic patterns during different times of day)
What are fluorescent dyes?
Inject into neuron to diffuse or transport throughout (some through gap junctions) for labelling live neurons
What is the advantage of fluorescent dyes over Golgi stain?
Label live neurons to track signals vs. dead fixed neuron
What is the mechanism of fluorescent dyes?
Electrons absorb light & emit fluorescence at different wavelength as they drop to lower energy orbitals (ex. Lucifer yellow emits yellow wavelength)
What is the relationship between excitation & emission wavelength?
Emission wavelength > excitation wavelength (emission energy < excitation energy)
What are 4 dyes labelling sub-cellular structures?
MitoTracker (mitochondria), LysoTracker (lysosomes), Hoechst & DAPI (nucleus)
What is dye conjugation?
Conjugate dyes to molecule for targeting specific cell part (ex. toxins, like phallacidin labels F-actin)
What is BODIPY FL Phallacidin?
Green fluorescent dye preventing depolymerization of F-actin for actin location in cell
What is protein and mRNA localization?
Genes to label & identify neurons
What are marker genes?
Expressed only in cells of interest
What is a marker gene for glutaminergic neurons?
VGLUT1, transporter for glutamate into vesicles
What is a marker for GABAergic neurons?
GAD1, synthesis of GABA
What is mRNA detection in cells/tissues?
In situ hybridization through complementary antisense RNA conjugated to fluorescent dye / other label as probe
What is the disadvantage of mRNA detection?
mRNAs mostly located in soma so often no location of translated protein
What is protein detection in cells/tissues?
In situ immunolabeling where protein epitopes generate antibodies that bind target proteins for detection
What are epitopes in protein detection?
Piece of protein injected into animal for antibody production, extraction & conjugation with label
What is the advantage of protein detection?
Determine function by location (ex. pre-synaptic protein)
What are fluorescent proteins?
Encoded by gene, absorb light & emit fluorescence like fluorescent dyes
What is green fluorescent protein (GFP)?
Cloned from jellyfish Aequorea victoria
What is the GFP chromodomain?
Absorbs & emits light
How is GFP genetically expressed in specific neurons?
Use cell-specific promoter for genome-inserted GFP expression in neurons of interest (ex. labelling tubulin structure during microtubule assembly)
What is the labelling capacity of GFP?
Labels entire neuron, like fluorescent dyes
What are fusion proteins of GFP?
Fuse GFP to protein of interest to visualize location in/out of cell
What is multicolour fluorescent protein labelling?
Mutated GFP & mRFP (monomeric red fluorescent protein) into many colours for simultaneous labelling
What is the nomenclature for GFP derivatives?
XFPs (ex. YFP = yellow fluorescent protein)
What is a brainbow?
In vivo transgenic technicolour (multicolour) Golgi stain developed by Jeff Lichtman at Harvard
What is the visual representation of neurons in brainbow?
Individual neurons = different coloured balls = different combo of fluorescent proteins
What is the approach of brainbow?
Randomly integrate 3 XFP gene copies (CFP, YFP, or RFP) into mouse genome so that neurons randomly express different combo of FPs for unique fluorescence profile
What is the promoter for brainbow?
Synthetic virus promoter inserted into mouse genome
What is Cre-mediated recombination in brainbow?
Cre recombinase cuts Lox sites between FP genes for recombination to control which FPs are expressed (ex. cutting RFP from RFP-YFP-CFP leads to either YFP-CFP or just CFP)
What are 3 examples of brainbow in other species?
Zebrafish, Drosophila, plants
What is the limitation of brainbow?
Ineffective at high magnification at synaptic connectivity due to tightly-packed neuron projections of CNS
Why is brainbow ineffective at high magnification?
Light microscopy resolution limit of >200 nm using photon beams vs. ~10s nm of synapses
What are static tracers of synaptically-connected neurons?
Injected into & transported only within cell in either anterograde or retrograde direction
What is the anterograde direction of neural circuit tracer?
Direction for law of dynamic polarization (ex. Phaseolus vulgaris-leucoagglutinin)
What is the retrograde direction of neural circuit tracer?
Backward direction for law of dynamic polarization (ex. Cholera toxin B)
What are adeno-associated viruses (AAVs) for neural circuit tracing?
Modified AAVs can label single neurons either anterograde or retrograde by promoting expression of detectable gene product
What are monosynaptic tracers?
Jump across single synapse
What are polysynaptic tracers?
Jump across series of synapses
What are trans-synaptic tracers?
Modified virus derivatives that can label neural circuits
What is an example of anterograde trans-synaptic tracers?
Modified herpes simplex virus