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September 12, 2018

Introduction to neurobiology of the CNS: focus on retina

The retina is part ofthe CNS

Calloway et al., 2009)

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Retinal circuits: neurons and synapses

Sherry, 2002

Rods and Cones

Bipolar cellsHorizontal cells(Mueller Glia)Amacrine cells

Retinal ganglioncells

1. Dendrites

2. Cell body -Soma

3. Axon hillock Axon initial segment

4. Synaptic terminal

Dendrites of post-synaptic cell

Neuron

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Single cell recordingRemote referenceelectrode, outside ofthe cell

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Membrane potentials

• Resting potential (example: -70 mV)

• Hyperpolarize (more negative than resting potential, e.g. -90 mV)

• Depolarize (less negative than resting, e.g -40 mV) – may lead to action potentials.

• Repolarize – return to resting potential

Spiking activity

Depolarization: Greater than resting potential

More neg. than resting potential

Return toresting potential

Membrane potentials

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Integral transmembrane proteins form channels for substances to cross the membrane

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Extracellular & intracellular fluid have different composition of ions (different concentrations)

Resting Ca2+ concentration in the cytoplasm is 10-100 nM

10-7

A cell

Transport across the cell membrane Mechanisms to create and use ion gradients forelectrical signaling

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Membrane permeability – diffusion through the membrane itself is very selective

Non-ion channels

Aquaporins – H2O

Aquaglyceroporinsgasesglycerolsmall uncharged

polar molecules

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AquaporinsWater channels

202003 Nobel Prize in Chemistry

Peter Agre & Roderick MacKinnon

tetramer

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Diffusion, channels and pumps

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Membrane permeability: simple diffusion, channels, carriers, pumps

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Primary active transport: Na+ – K+ ATPase pump uses energy to pump ions against the concentration gradients for the two cations. For every cycle of the pump, 3 Na+ ions leave the cell and 2 K+ ions enter the cell

Secondary active transport uses the gradient created by the pump:co-transport (symport) in this example

OutsideInside

or Glucose

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Active transport: primary and secondary contributes to movement of ions and sugar and amino acids into and out of cells

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Ion concentrations and equilibrium potentials

Berne & Levy

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The Nernst Equation

For calculating equilibrium potentials

Eion (mV)= - 61(mV) x log ( [ion conc]in/[ion conc]out)

-61 = RT/F

The Nernst Equation

Electrochemical equilibriumfor an ion

Eion (mV)= - 61(mV) x log ( [ion conc]in/[ion conc]out)

potassium (K+): -61 X log (150 mM/5 mM) = -91 mVsodium (Na+): -61 X log (14.5 mM/145 mM) = +61 mVchloride (Cl-): -61 X log (115 mM/3.6 mM) = -90 mV

(for neg ion, Cl-) ( [ion conc]in/[ion conc]out

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What is conductance, “g” ?conductance “g” is the inverse of

resistance (R)(g=1/R)

If membrane ion channels are open, resistance (R) is low, and conductance (g) is high

Ohm’s Law E=I*RE = voltageI = currentR = resistance

R= 1/g Resistance (R) = 1/conductance (g)

E=I*1/g; I=E*g

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The chord equation for membrane potential

g

+gNa+ X ENa+

g

The membrane potential will depend on the relative conductances for the major ions and the equilibrium potential for those ions.

gK+ X EK+

Em =

Membrane potentials

• Resting potential (example: -70 mV)

• Hyperpolarize (more negative than resting potential, e.g. -90 mV)

• Depolarize (less negative than resting, e.g -40 mV) – may lead to action potentials.

• Repolarize – return to resting potential

Spiking activity

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Graded (local) potentials and action potentials

Graded potentials are generated via ligand gated channels, They are small and can be hyperpolarizing or depolarizing and they scale in amplitude with the strength of the input.

Action potentials are “all or none” events, that have a threshold, and rely on the presence of voltage-gated channels

Types of channels in membranes Ligand or receptor-activated

Voltage-activated

Stretch-activated

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• Neurons sum and integrate information from their inputs and pass information to the next cell.

• Action potentials (brief impulses) are necessary for signals to travel long distances (e.g. retina to LGN).

• Information is coded in local potentials when axons are short, such as for all cells within the retina except for retinal ganglion cells whose axons form the opticnerve

Impulses and circuits (Hubel no longer on the internet)

Action potential: dominant features

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Local potential and action potential

Linear summation vs threshold

Types of channels in membranes Ligand or receptor-activated

Voltage-activated

Stretch-activated

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Action potential: Tetrodotoxin (TTX) blockade of NaVs

There is no inward sodium (Na+) current

KugelfischPuffer fish

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Action Potential - initiated by depolarization: Conductance (g) changes in voltage-gated channels

Propagation of action potentials

Hubel off line book

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Retina: cells and layersLocal potentials in INL

Action potentials in GCL

Myelin sheath

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II. Synapses for neural signal transmission from cell to cell

Electrical – gap junctions

Chemical – classical pre and postsynaptic membrane- vesicular release

Junctionsbetween cells

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Gap Junctions

Copyright ©2009 The American Physiological Society

Abd-El-Barr, M. M. et al. J Neurophysiol 102: 1945-1955 2009;doi:10.1152/jn.00142.2009

Schematic diagram of 6 rod and cone synaptic pathways(note the gap junctions (ww)

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Chemical Synapse - axodendritic

Chemical Synapse -axodendritic

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Exocytosis - vesicular release and the importance of calcium

Sudhoff – In Ganong Review of Medical Physiology

Receptors

G-protein-mediated signal transduction pathways

Second messengers

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Ionotropic and metabotropic receptors

Ionotropic & metabotropic glutamate receptors

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Synaptic transmission – glutamate is the major neurotransmitter in CNS and retina

Glutamate

Ionotropic(GluR)KainateAmpaNMDA

MetabotropicmGluR

Ionotropic and metabotropic glutamate receptors

Ionotropic Metabotropic

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Retinal glutamate receptor types

Neurotransmitters

m: metabotropic i: ionotropic

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Heterotrimeric G-proteins

Examples of G-protein–coupled receptors and common effectors

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Signal transduction cascades:at each stage, amplification may occur

Gs and Gi: stimulation or inhibition of AC, and formation of cAMP

Beta receptorsEpinephrineNorepinephrine

Dopamine receptors(D1, D3)

Alpha-2Norepi

Dopamine r (D2,D4)

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Phototransduction – a well studied G-protein cascade

Rhodopsin2 adrenergic receptor

Rhodopsin is a G proteincoupled receptor (GPCR)

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Biochemical steps in the phototransductioncascade

Current flow around photoreceptors

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Resources;

Human physiology texts books.

Example:

Physiology 6th edition Chapter 1

Costanzo (In the Health Sciences library, and in the Clinicalkey Database at the TMC library)