MANY Student questions of the dayMANY Student questions of the day……..Q1: Why do neurons have axons: wouldn’t

synapses between lots of small cells be just as good?

A1: Chemical synapses are slow, and transmission speedA1: Chemical synapses are slow, and transmission speed

down a neuron would plummet (by orders of magnitude).down a neuron would plummet (by orders of magnitude).

Q2: Why not have electrical synapses?

A2: These are rapid, but as any electrical synapse A2: These are rapid, but as any electrical synapse

would generate an action potential in the postsynaptic neuron, would generate an action potential in the postsynaptic neuron,

different inputs cannot be integrated.different inputs cannot be integrated.

Q3: Can one neuron get both excitatory and inhibitory inputs?

A3: Yes, these generally come from separate synapsesA3: Yes, these generally come from separate synapses

onto the same neuron. The neuron then summates these inputs onto the same neuron. The neuron then summates these inputs

to decide whether to generate an action potential.to decide whether to generate an action potential.

Q4: Are amphetamines neurotransmitters?

A4: No. Amphetamines alter neurotransmitter release/re-uptake.A4: No. Amphetamines alter neurotransmitter release/re-uptake.

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4.23.07

Systems I

Nervous System 3

STARTSTARTFOUNDATIONSFOUNDATIONS

How-to 1How-to 1 FO

RM

AT

ION

FO

RM

AT

ION

How-to 2How-to 2

SYSTEMSSYSTEMS

PR

OB

LE

MS

PR

OB

LE

MS

BIOCHEM GENETICS CELL BIO.MOL. BIO

STEM

CELLS,

CLONING

REC. DNA

CELL TYPE &

POSITION

3DSTRUCTURE

GENOMICS

VIRUSES

CANCER

HUMAN

DISEASE

LIFELIFE

NERVOUS

SYSTEMIMMUNE

SYSTEMS

BIOLOGY

FUTUREFUTURE

excitatory/inhibitory

synapses

excitatory/inhibitory

synapses

sensory neuron interneuron motor neuron

input

output

CIRCUITaction potential

yes or no? action potential

yes or no?

action potential

yes or no?

H. Sive MIT 2007

Action potentials = message

Synapses = connections which regulate decision to “fire”

Circuits = which neurons connect

Input = neuronal, environmental: Output = movement, other

Circuits, synapses and action potentials1

Circuits

Zebra finch song circuit

2

Viewing words Listening to words

Speaking words Generating words

Purves 46.16: multiple brain nuclei (regions)

work together = language (PET scan)

3

Do neurons know where to go?

Do neurons know where to (re-)connect?

eye rotated 180o

NasalTemporal

4 Sperry’s frog retina rotation experiment

Experiment: rotate retina 180o, assay where axons regrow

5

H. Sive MIT 2007

retina optic tectum (brain)

T N R Cnormal

axon trajectories

R CN T

possible outcome:

axons find

incorrect targets

Conclusion: axons “know” where to grow

actual outcome:

axons find their

normal targetsR C

180o

rotated

retina

N T

Molecules involved in retinal axon migration

Nasal retinal axons choose to migrate on

rostral and not caudal tectal membranes

(stripe assay)

rostral

rostral

rostral

rostral

caudal

caudal

caudal

Tectal membrane6

retina optic tectum

N T R Cnormal

axon trajectories

Explaining Sperry’s retina rotation experiment

actual outcome:

axons find their

normal targets

T N

180o

rotated

retina

low

ephrin

ligand

high

ephrin

ligand

low eph receptor

high eph

receptor

R C

low

ephrin

ligand

high

ephrin

ligand

low eph receptor

high eph

receptor

Conclusion: N/T axons express different Eph receptor levels

and follow an ephrin gradient

Not covered

in lecture.

For your interest

only!!!

The growth cone

7

From Molecular Cell Biology/ Lodish

Zone of actin polymerization

Front/

leading edge

Direction of

movement

nucleus

Rear/

trailing

edgeLamellipodia/filopodia

Actin polymerization during cell movement

Remember this?! (Formation III)

axon

microtubules

microfilaments

(F-actin)

Filopodium/ lamellipodium

(thin) (thicker)

GROWTH

CONE

= Receptor

for short range/

long range guidance molecule

H. Sive MIT 2007

8

Axon guidance

via long/short range attractive/repulsive signals

Ballabore et al, 2005

9

growth cone

Axonal growth cone

10

Collapse of the axonal growth cone due to a repulsive signal

11

Guidance signals

Axonal outgrowth:

choice of laminin

versus

polylysine

substrate

12

Axon guidance in the

spinal cord: transverse section

floor plate

roof plate

trochlear

axons

grow

away from

floor plate

dorsal

ventral

commissural

axons grow

towards

and turn

at floor plate

Cell body

Growth cone

H. Sive MIT 2007

13

Spinal cord section

Commissural axons (green) , cell bodies (red)

Cell bodiesCell bodiescell bodies

axons

14

z

dorsal

spinal cord

floor

platecommissural

axon outgrowth

roof

plate

dorsal

spinal cordno outgrowth

Explant assay to test floor plate/roof plate activity

on commissural axonsH. Sive MIT 2006

15

Conclusion: floor plate attracts commissural axons

Explant assay for axon guidance activity

Dorsal spinal cord

Floor plate

Dorsal spinal cord

Tissue culture cells

growing

axons

no axon

outgrowth

16

Netrin1 RNA is

expressed

in floor plate

Netrin1 protein is

expressed in a gradient

extending from floor plate

Tessier-Lavigne et al

17

Wild type Netrin1-/-

Loss of function: Netrin is required for

normal commissural neuron (red) outgrowth

18

Netrin receptors promote attraction or repulsion

DCC-

DCC

netrin

attractive

Tyrosine kinases

GTPases (Rho)

Phosphoinositol

Cytoskeletal remodeling

(F-actin, microtubules)

Ca2+

influx

CAM Kinase

Growth cone extension/turning

Tyrosine kinases

Growth cone collapse

Cytoskeletal remodeling

G-actin

DCC-

UNC5

DCC-

UNC5

repulsive

netrin

H. Sive MIT 2007

plasma

membrane

19

Stabilizing connections

Nerve growth factor is a neuronal survival signal

(Prof. Rita Levi-Montalcini)

- NGF

+ NGF

20