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Brain:

A Network of Neurons

BT101: Introduction to

Life Sciences

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Brains cells

Two main cell types Neurons

Glia

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Neuron

Electrically active cell

Arboreal projections

Number in human brain: 100 billion

in Aplysia nervous system = 18,000-20,000 in each segmental ganglia in the leech = 350

C. Elegans = 302

Size: 4 micron (granule cell) to 100 micron (motor neuronin cord)

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Other cells

RBC

Paramecium

Skeletal Musclewww.uic.edu/classes/bios/bio100

Bacilus

Bone marrow

Cardiac cells

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Neuron Morphologies

Average number of connections per neuron = 1000-10,000

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Neuron Morphologies

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Synapse

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-ve insideVm = -70 mV

-

+

- -

-

-

-

-

--

-

-

-

-

+

+ ++

+

+

+

+

++

+

+

+

Neurons are electrically activecells

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-

+

- - - -- - - -

+++++

- -

-

+

- - - - - - - - -

++++

Vm

Vm

Neural signals areelectrical waves

-

+

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Wave propagation over a neuron

The nature of wave propagation is different

in different parts of the neuron

Propagation over dendrites is usually lossy

Propagation over axons is non-lossy

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Wave propagation over a

dendrite

Vm

As the wave propagates down the dendrite:

- It loses amplitude

-- It spreads in time

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Action Potential generation at the

Soma (axon hillock)All-or-none Response

http://www.codeproject.com/cs/algorithms/NeuralNetwork_1.asp?print=true

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Wave propagation over an axon

As the wave propagates down the axon:

- No loss of amplitude

-- No spreading

ActionPotential

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http://www.cwru.edu/groups/ANCL/pages/02/s02_01ttl.gif

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Across the synapseAction Potentials on the

pre-synaptic terminalGet converted into

Post-synaptic Potentials (PSPs) on the

Post-synaptic terminal

NeurotransmissionA chemical step

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Neurotransmission

PSP

ActionPotential

Synapse

Pre-synaptic

Terminal

Post-synaptic

Terminal

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Post-Synaptic Potentials are ofTwo kinds

EPSP: excitatory post-synaptic potential post-synaptic cell temporarily depolarizes

IPSP: inhibitory post-synaptic potential

post-synaptic cell temporarily hyperpolarizes

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EPSP/IPSP

EPSP

IPSP

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EPSPs and IPSPs are gradedpotentials

Unlike the all-or-none action potential,PSPs are graded potentials

They come in all shapes and sizes

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Types of Synapses

Excitatory synapses: those that produceEPSPs

Inhibitory synapses: those that produce

IPSPs

Depends on the chemistry of the synapse

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EPSPs and IPSPs summate

Two kinds of summation: Spatial summation - inputs from several

neurons add together

Temporal summation - inputs add up overtime, perhaps even from the same spatial

location

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EPSPs and IPSPs summate

Spatial summation - inputs from severalneurons add together

Temporal summation - inputs add up

over time, perhaps even from the same

spatial location

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Neuron as an Input-Output System

NeuronDendrites

Axon

Axon

Collaterals

Inputs fromOther neurons Outputs toOther neurons

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Signaling inside a neuron 4 components

NeuronDendrites

3) Sig. prop.

along Axon

1) Signal

propagation

along dendrites

2) Signal summation

in cell body

4. Signaling

across

Synapse

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Condition for Neural Firing

Dendrites

Soma

(cell body)

Axon

If the net effect of inputs < threshold

no APs (no firing)

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Dendrites

Soma

(cell body)

Axon

If the net effect of inputs > threshold

APs are generated

Condition for Neural Firing

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Molecular basis of neural signaling

Neurons are electrically active cells

They communicate with each other using

electrical impulses

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CytosolVm=-70 mV

Extracellular Space

0 volts (reference)

Neuron in

Resting Condition

Cytosol

Vm(t)mV

Neuron Firing

What is the basis of this voltage?

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Equilibrium Potential

If the membrane is permeable to both K+ and Cl-, both the ionic

species will cross the membrane from left to right until the

concentrations on both sides become equal.

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Equilibrium Potential

(semi-permeable membrane)

If the membrane is permeable only to K+, then only K+will cross the membrane from left to right which introduces

a charge gradient (potential difference) opposing the flow.

The process will continue until the potential difference and

chemical gradient cancel each other.

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Cell membrane

A double layer (bilayer) of lipid molecules

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Cell membranes - EMs

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Cell membrane is semi-

premeable

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Semi-permeability comes from

Transmembrane channel proteins Membrane proteins can be

Integral proteins : these are fully incorporated into the

membrane and are in contact with both the inside and theoutside of the cell.

Surface membrane proteins : these are only associatedwith the outer of the bilipid layers and make contact withthe extracellular space.

Inner membrane proteins : these are only associated withthe inner side of the bilipid layers and make contact withthe cytoplasm (cytosol) [one of these is shown to the rightof the above diagram]

Transmembrane channel proteins : these are similar to

integral proteins but appear to possess a channelconnecting the extracellular space to the cytoplasm.

http://www.jdaross.mcmail.com/cell2.htm

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The Cell membrane which becomes semipermeable

due to Ion channels allows ions to be exchangedbetween ECS and Cytoplasm

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Nernst Potential

1

221

][][ln

XX

FZRTVVx

V1-V2 - Nernst potential for ion X

[X]1,2 concentrations of X

Zx Valence of X

R Ideal gas constantT Absolute temperature

F Faradays constant

RT/F = 26 mV at T=25oC (Zx = +1)

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Nernst Potentials of K+

i

o

x

kKK

FZRTE

][][ln

[K+]o = 20 mM[K+]i = 400 mM

Ek= -77 mV

gk=36mS/cm2

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Potassium Equilibrium Potential

Chemical force pushing ions out equalselectrical force pulling them back in

No net movement of ions

Ek= -77mv

Ek

gkEquivalent circuit of

- K+ channel and- K+ Nernst Potential

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Sodium Equilibrium Potential

Chemical force pushing ions out equals electrical forcepulling them back in

Sodium Ions are more concentrated outside the cell

No net movement of ions

ENa = 55mv

ENa

gNaEquivalent circuit of

- Na+ channel and- Na+ Nernst Potential

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Nernst Potentials of Na+

i

o

x

NaNaNa

FZRTE

][][ln

[Na+]o = 440 mM

[Na+]i = 60 mM

ENa = 50 mVgNa = 120 mS/cm2

EL = 10.6 mV

gL = 0.3mS/cm2

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Membrane Potential

In steady (resting) state, current through the capacitor is 0:

LKNa

LLKKNaNa

mggg

EgEgEg

V

Vm = -70 mV

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Equivalent ckt for the membrane