Final Exam Review

Rachel A. Kaplan and Elbert Heng

Announcements

• Your final is tomorrow; get hype!• Things you should bring:

– A calculator– Some pencils (or pens, if you want to be bold)– Your brain!

What this review today will cover:

• As your exam is tomorrow, hopefully this isn’t the first time you’re going to be reviewing material– accordingly, we will be:

• going over important topics and difficult concepts• answering any questions you have• providing moral support

Musings…

• You should study material that was tested on previous exams

• You should study material that was not tested on previous exams

• Most importantly:– You should know the big important concepts that we’ve

covered– You should also spend time to review some details – Look at our old slides for more comprehensive review

Be prepared to think…

Final Exam

THE BIG PICTURE: FIRST THIRD

Fundamentals of synaptic transmission from an electrophysiological perspectiveImportant Topics• Ion Channels! • Membrane Potentials - The Nernst Equation• The Action Potential • Membrane Properties • Synaptic Transmission

THE BIG PICTURE: SECOND THIRD

A little bit of everything, but mostly synaptic transmission from a molecular and cellular biological approach with electrophysiological implicationsImportant Topics• Vesicles: exo and endocytosis• Indirect synaptic transmission• Mechanosensation• Behavioral Neurobiology• Dendrites• Electrical Synapses

THE BIG PICTURE: THIRD THIRD

Plasticity and all its friendsIMPORTANT TOPICS

• LTP and LTD • Intrinsic Plasticity • Learning • Development • LTP and Addiction• The third paper

Ion Channels

• Ion channels pass ions• This is studied with electrophysiological techniques

• Voltage Clamp• Current Clamp• (and putting these two together) I/V Plots

• Single Channel vs. Whole Cell Recording• Single channels are constantly flickering open and shut;

the population of channels will reflect the state of the cell• Gating– Modulation of Gating

Electrophysiology

Electrophysiology

Electrophysiology

I=Vg

Electrophysiology

Gating and Modulation

• Gating: how the channel opens and closes• S4 is the voltage sensor for VG channels• e.g. Glu gates NMDARs and AMPARs

• Modulation: changes the open probability of the channel• MODULATORS:• Other subunits of the protein (beta subunits)• Second messengers• Changes in gene expression• Phosphorylation• Allosteric regulators

Nernst Equation vs. GHK

• Nernst: Single ion’s equilibrium potential. – Equivalent to Vrev if a channel is singly selective for that

ion.

• GHK: Combined equilibrium potential of all relevant (permeant) ions.– Can give you the RMP– Also can give you Vrev of multi-ion channels.

Membrane Properties

• All serve to modulate the speed of an action potential– Membrane resistance (Rm)

– Membrane capacitance (Cm)

– Axial Resistance (Ra)– Derived from these:

• Length Constant (λ)• Time Constant ( )𝜏

• All of the equations will be given to you if you would like to see the relationships written out…

Synaptic Transmission

• Llinas’ experiment– Proved that calcium was necessary and sufficient for

presynaptic transmitter release• Depolarization is not sufficient! (if no calcium, no go)

• Quantal Hypothesis– Quantum is a vesicle of neurotransmitter– Quantal content - how many vesicles resleased!– Quantal size – content of a single vesicle – how much NT is

in it• Content = mean EPP / average quantal sizeELBERT HERE

Mechanosensation

• Mechanosensitive neurons:– Generally: stretch-gated channels tethered to intra and

extracellular matrices• Fast, sensitive, adaptable (so that it can transduce a wide range of

inputs), and specialized

• Lots of receptor subtypes– E.g. Pacinian Corpuscles

• Respond to vibration because they are fast adapting• Neuron is surrounded by epithelial cells that form many layers of

gelatinous membranes called lamellae– Pressure on causes neurons to fire– Pressure off also causes neurons to fire

» In other words, they adapt

More Neurons/Proteins Involved

• Degenerin/ENaC Channels– Respond to stretch/mechanical stimulation – slow

adapting• Meaning that they will stay open if they are continuously poked

• CEP Neuron Channels– Senses viscosity of surrounding bacteria– Rapidly adapting cation channels– TRP-4: mechanosensory channel

• Other TRP Channels– Sense temperature, chemical tastants

TRP Channels

Hearing and Proprioception

• Vibrations of air are transduced by mechanosensory hair cells – Stereocilia are deflected, links between stereocilia are stretched,

allows K+ inward current to depolarize cell• Deflecting the other way will hyperpolarize the hair cell• Stereocilia adapt by tightening tip links

• Movement of head in space is transduced by similar hair cells in other organs – Utricle and sacculus – linear acceleration moves gel and crystals

(otoliths), causes opening of hair cells – Semicircular canals – rotational motion causes fluid in canals to

move ampulla and embedded hair cells

Behavioral Neurobiology

• Responses to releasing stimuli– e.g. Egg Rolling• Stimulus (egg) triggers fixed action pattern

– e.g. Seagull Chick Feeding• Stimulus (spot color) triggers pecking

• Supernormal stimuli: allows us to study nature of what an animal is actually responding to in a stimulus

Electrical Synapses

• Channels are composed of two Connexons– Connexons are Hemichannels– They are in turn composed of 6 connexins• If all 6 connexins are the same protein: homomeric• If different: heteromeric

– Most common connexons in the brain:• Cx43 – Glial cells• Cx36 – Brain neurons (perhaps the only connexon that

is expressed in brain neurons!)

Electrical Synapse Physiology

• GJ provide high conductance pathway for ionic current to pass from one cell to another– Ohmic (no voltage gating)– Bidirectional– Also pass small molecules like ATP, cyclic nucleotides

• So what would the electrophysiological recording of stimulation of a neuron that connected by a GJ to another neuron look like?

Gap Junction Evolution

• Pannexins / Innexins and Connexins are orthologues– No sequence similarity but in teritiary structure

are very similar• Invertebrates do not express connexins• Innexins and connexins can form GJs or

functional hemichannels– Pannexins only form hemichannels

Learning

• LTP and LTD are putative cellular mechanisms• Shown with lots of experiments

Rabbits?

• The process of associative learning uses this circuit– Input: sensory motor – tone- parallel fibers

• Also excites pons and deep nuclei directly (there are two pathways)

– Input: “error” signal – shock – climbing fibers– Output: motor command- eye blink – purkinje cells

• LTD occurs in parallel fibers which means less inhibition of deep nuclei from purkinje cells– Easier to express blinking behavior!

Lashley

• Searched for the engram• Equipotentiality• Mass Action

Development of Circuits is:

• Activity Independent– Sperry: chemoaffinity

hypothesis– Experiments

• Eye rotation• Retinal ablation• Stripe assay

– Mechanism• Ephrins and Eph

Receptors

• Activity Dependent– Hebb: correlation based

change– Experiments

• Rewiring of A1/V1

– Mechanism• Synapse maturation

– LTP– Depolarizing GABA

• Activity dependent gene expression

A little bit of both…

• Ocular dominance columns start to develop before eye opening but require activity to segregate more completely– Spontaneous retinal waves may be responsible– Ocular dominance shift: Monocularly deprived

animals develop ocular dominance stripes but the open eye’s stripes are much wider

Putting it all together:

• Neural development is influenced by both activity dependent and independent factors– Much of original structure is dictated by activity

dependent factors– Refinement comes from activity• This is a result of LTP-like mechanism

• But in general, it’s hard to say which causes which feature…

Activity Independent Experiments

• Eye rotation in newt– Rotation of the eye of an adult newt will cause the newt to see the

world upside-down because in the adult brain, the retino-tectal connections don’t rewire, little plasticity. Previous projections from a part of the retina now project to the “wrong” part of the tectum.

• Retinal ablation– Ablating half of the retina will cause missing connections in half of the

tectum. The persisting retinal half will not rewire to take up the whole tectum.

• Stripe Assay– Neurons from temporal retina will only grown onto membrane stripes

from the anterior , and nasal retinal neurons will project through both (as it has to to get to the posterior tectum!)

Activity Dependent Experiments

• Rewiring of ferret cortex– Rewiring of retinal projections to the MGN (after

deafening the ferret) will cause A1 to have V1’s features like orientation pinwheels and long horizontal connection.

• Formation of eye specific stripes– They don’t form if APV is perfused!

Mechanisms

• Ephrins and Eph Receptors– Chemical gradient that guides neuronal projections from (e.g. retina

to tectum) specific regions of one neural area to another specific region

• Axonal Segregation / Map Refinement– Synapses that fire together wire together, so synapses become

refined– Accomplished via LTP (requires NMDAR activity)

• Synapse Maturation– NMDA only synapses become unsilenced as a result of LTP (insertion

of AMPARs) – change in NMDAR/AMPAR ratio– Depolarizing GABA also aids in unsilencing

• Gene Expression– e.g. cpg15 is induced by neural activity and regulates synaptic

maturation