Upload
others
View
3
Download
0
Embed Size (px)
Citation preview
1
September 27, 2017
Introduction to CNS neurobiology: focus on retina
The retina is part ofthe CNS
Calloway et al., 2009)
2
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
3
Single cell recordingRemote referenceelectrode, outside ofthe cell
4
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
Depolarization: Greater than resting potential
Less than resting potential
Return toresting potential
Membrane potentials
5
Composition: Extracellular & intracellular fluid
Resting Ca2+ concentration in the cytoplasm is 10-100 nM
10-7
6
Transport across the cell membrane
11
Membrane permeability
12
7
AquaporinsWater channels
202003 Nobel Prize in Chemistry
Peter Agre & Roderick MacKinnon
tetramer
13
Primary active transport: Na+ – K+ ATPase pump uses energy to pump ions against the concentration gradients. For every cycle of the pump, 3 Na+ ions leave the cell and 2 K+ ions enter the cell
8
Ion concentrations and equilibrium potentials
The Nernst Equation
For calculating equilibrium potentials
Eion (mV)= - 61(mV) x log ( [ion conc]in/[ion conc]out)
-61 = RT/F
9
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
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
10
Ohm’s Law E=I*RE = voltageI = currentR = resistance
R= 1/g Resistance (R) = 1/conductance (g)
E=I*1/g; I=E*g
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 =
11
The Donnan EquilibriumPrior to the equilibrium
A B
[Y-] = 0.1 M[K+ ]= 0.1 M
[K+]= [Cl-] =0.1 M
Y- -- A nondiffusable large protein, e.g. albumin
The Donnan Equilibrium
Potential difference across the membrane must be equal for K+ and Cl-
K+ Em = -61 * log ([K+]A/[K+]B)
Cl- Em = -61 * log ([Cl-]B/[Cl-]A)
[K+]A/[K+]B = [Cl-]B/[Cl-]A
[K+]AX [Cl-]A = [Cl-]B X [K+]B
12
The Donnan equilibrium A B
[Y-] = 0.1 M[K+ ]= 0.133[Cl-] = 0.033
[K]= [Cl] =0.0666 M
Concentration ratios of diffusible ions are =# of positive and negative ions balance in each compartmentY- -- A nondiffusable large protein, e.g. albumin
.033 x .133 = .0044 .066 x .066 = .0044
Donnan equilibrium would cause cells to swell
More particles, i.e. higher osmolarity inside the cell (A) than outside the cell (A)
Water would move (osmosis) into the cell to offset the difference in number of particles of solute per amount of solvent
13
Primary active transport: Na+–K+ ATPase keeps the Donnan equilbrium from occurring in cells. If it did occur, the cells would burst.
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
14
• 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.
• 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 circuitshttp://hubel.med.harvard.edu/index.html
Local potential and action potential
Linear summation vs threshold
15
Action potential: Tetrodotoxin (TTX) blockade of NaVs
There is no inward sodium (Na+) current
KugelfischPuffer fish
16
Action Potential - initiated by depolarization: Conductance (g) changes in voltage-gated channels
Action potential: Sea water experiments
There is no inward sodium (Na+) current in sodium-free sea water
Only the outward potassium (K+) current remains
17
Propagation of action potentials
Hubel online book
Retina: cells and layersLocal potentials
Action potentials
18
Myelin sheath
II. Synapses for neural transmission
Electrical – gap junctions
Chemical – classical pre and postsynaptic membrane- vesicular release
19
Junctionsbetween cells
Gap Junctions
20
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)
Chemical Synapse -axodendritic
21
Chemical Synapse -axodendritic
Exocytosis - vesicular release and the importance of calcium
Sudhoff – In Ganong Review of Medical Physiology
22
Receptors
G-protein-mediated signal transduction pathways
Second messengers
Ionotropic and metabotropic receptors
23
Ionotropic & metabotropic glutamate receptors
Synaptic transmission – glutamate is the major neurotransmitter in CNS and retina
Glutamate
Ionotropic(GluR)KainateAmpaNMDA
MetabotropicmGluR
24
Ionotropic and metabotropic glutamate receptors
Ionotropic Metabotropic
Retinal glutamate receptor types
25
Ionotropic & Metabotropic Receptors(GABA receptors in this example)
Ganong, Review of Medical Physiology
Neurotransmitters
m: metabotropic i: ionotropic
26
Heterotrimeric G-proteins
Examples of G-protein–coupled receptors and common effectors
27
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)
28
Phototransduction – a well studied G-protein cascade
Rhodopsin2 adrenergic receptor
Rhodopsin is a G proteincoupled receptor (GPCR)
29
Visual pigments: seven membrane-spanning loops
Photoreceptors in primates: humans and monkeys
30
Spectral Sensitivity
Visual pigments: homologies in amino acid sequences
31
AVA-322 : gene for L-opsin - protan defects.AVA-323 : gene for M-opsin - tdeutan defects.
Breakthrough non-surgical intravitreal injection method to deliver genes directly to cone cells at the back of the eye.
Phototransduction
Leads to closure of a cation channel in the plasma membrane. This interrupts the dark current, and hyperpolarizes of the rod or cone photoreceptor
The opsin in the outer segments,rhodopsin in rods, catches light and is activated when 11-cis retinal is attached to it
32
The visual cycle – conversion of all-trans retinol(from the blood) to 11-cis retinal in the retinal pigment epithelium (RPE)
Details of the visual cycle
Retina
RPE FIGURE 3. Schematics of two visual cycles in vertebrate eye. The canonicalRPE visual cycle (left) recycles all-trans-retinol (at ROL) released from rods andcones following a bleach to 11-cis-retinal (11c ROL), which can be used byboth rods and cones for pigment regeneration. The retina visual cycle (right) relies on the Müller cells to recycle all-trans-retinol released from cones to11-cis-retinol, which only cones can move to their outer segments and oxidizeto 11-cis-retinal for regeneration of the pigment. IPM, interphotoreceptormatrix. h, photon of light
Kefalov, JBC 2012
33
Biochemical steps in the phototransductioncascade
Phototransduction cascade
34
Current flow around photoreceptors
Rod photocurrents:prolonged responses
Cone photocurrents:brief responses
Rods are 70-100 times more sensitive than cones
35
RPE cells phagocytize outer segments –entire OS turns over in less than two weeks
36
The retina has two blood supplies: outer (PCA) and inner (CRA) retinal (pg. 9)