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Session 4

The Neuron

PS111 Brain & Behaviour

Module 1: Psychobiology

What are neurons good for?

• In complex organisms, cells... on the inside of the body are not in direct contact

with the outside world... live in different ‘environments’... have become specialised

• In order for the organisms to function, cell activities must be co-ordinated

• Why do more complex organisms need a nervous system?

a) Endocrine system: • specialised to secrete chemicals (‘hormones’) into

the bloodstream• provide slow, overall co-ordination of cell activities

• Two systems to co-ordinate cell activities:

b) Nervous system: • specialised to transmit electrical impulses between

two or more cells• provide fast and precise co-ordination

What are neurons good for?

What are neurons good for?

Hi, Mike!

What are neurons good for?

Hi,Mike!

• QUESTIONS:• How are neural impulses generated? • How are they transmitted?• What is their function?

What are neurons good for?

• Neural impulses (‘signals’) provide constant & rapid communication between cells.

• Signals from one (group of) cells change properties of receiving cell=> i.e., change the way the receiving cell ‘behaves’

In other words:

• Neural impulses provide constant & rapid control & adjustment of ongoing cell activities

1. Function:

• Generation & transmission of electrical impulses

Neurons are special!

• Electrical impulses reach specific targets• Modifies activity of the target cells• Allows selective control of specific target structures

• Electrical activity modulated by integrated input from other cells

• ‘Input’ used to adjust ‘output’• Combination & integration of signals from different sources• Structured communication

• Rapid• Over great distances• Point-to-point

Smooth muscle cell Skin cells

Ovary cell Blood cells

Neural (pyramidal) cell

2. Form & Size:

Neurons are special!

• Glucose (sugar) & oxygen must be constantly supplied• Without supply, neurons

• stop working within seconds• die within minutes

4. Life span: Neurons do not divide (they develop from ‘neural stem cells’)

• Neurogenesis virtually completed around 5 months after conception: • after this, dead neurons can not be replaced (mostly)

• Neuron death part of normal brain development: • 20% to 80% of all neurons die during maturation

because neurons are so special…

3. Special requirements: Virtually no possibility to store energy

Neurons are special!

Provide ‘protected environment’ for neurons to survive

Develop – like neurons – from neural stem cells

About 10 times as many glia as neurons, on average 1/10 the size of a neuron

Glia Cells

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• star-shaped • physical & nutritional support for neu-

rons (part of Blood-Brain-Barrier): • transport nutrients from blood vessels to

neurons and• waste products away from neurons• hold neurons in place

• Play a role in neural signal transmission as well!

• small• mobile for defensive function:

• produce chemicals that aid repair of damaged neurons

• digest dead neurons (phagocytosis)

Glia Cells• Astrocytes:

• Microglia:

http://i.livescience.com/images/060105_astrocyte_02.jpg

Xu, Pan, Yan, & Gan, NatNeuro,10, 549-551. http://www.nature.com/neuro/journal/v10/n5/images/nn1883-F2.jpg

• large, flat branches• wrapped around

axons• consist of fatty sub-

stance (myelin)• insulating the axon

Other types of glia exist, but will not be discussed here...

• Oligodendroglia:

Glia Cells

Neurons

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Neurons

Axon

Axon Hillock

Axon terminals

Soma

Dendrites

Neural Signal Transmission...

Membrane

• Neurons are not empty, and do not exist in a vacuum: • Thick chemical ‘soup’ of electrically charged particles

• fills the neuron (‘intra-cellular fluid’)• surrounds the neuron (‘extra-cellular fluid’)

• (now recall that the membrane has holes)

Positively charged ions

Negatively charged ions

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HOW??

1. Basic principles:

Electrical activity - Resting potential

ConcentrationGradient

ElectricalGradient

Cl

K+ Na+ Cl- A-

K+ A-

+

-

Na+ Cl-

K+ Na+ Cl-ConcentrationGradient

ElectricalGradient

• Protein channels in cell membrane • allow ions to enter or leave the cell:

• Electrical potential remains static => no electrical activity

2. Ion gradients:

Electrical activity - Resting potential

K+ Na+ Cl- - - A-

K+ A-

Na+ Cl-

K+ Na+ Cl- +

- (-70 mV)

• Ion concentrations differ between the inside and the outside of the cell:

K+ Na+ Cl- A-

K+ A-

Na+ Cl-

K+ Na+ Cl-

• Active channels work against the equilibrium:

+

- (-70 mV)

3. Sodium/potassium pump and membrane potential:

Na+

Na+ Na+

K+ K+

• Neurons need energy just to maintain their resting potential!

• If channels were passive ‘holes’, membrane would depolarise (electrical potential would disappear):

• Again, there would be no electrical activity!

K+ Na+ Cl- A-

Na+ A-

K+ Cl- 0

0

K+ Na+ Cl-

2. Ion gradients:

Electrical activity - Resting potential

• Based on movement of electrically charged particles (ions):

• Ion-specific channels in cell membrane are GATES that can open (they are not open all the time!)

• either by chance or in response to stimulation

• Positive or negative ions enter or leave the cell

• Depolarisation:• Positive ions in, or negative ions out: • Inside less negative than usually

• Hyperpolarisation:• Negative ions in, or positive ions out: • Inside more negative than usually

Electrical activity – Signal Transmission

Electrotonic Action Potential

Synaptic

• Passive: Ions move inside the cell along electrical & concentra-tion gradients.

• Some ions will get lost on their way: Signal decays over time

Electrical activity – Signal Transmission

Electrotonic Action Potential

Synaptic

• Active (self-replicating, no decay): ions move locally through cell membrane.

• Generated at axon hillock, moves down the axon towards ter-minal buttons

Electrical activity – Signal Transmission

• Sequence of events:1. Membrane depolarised (inside less negative)

5. All nearby Na+ channels open

4. Membrane depolarises further -- THRESHOLD?

3. Na+ ions enter the cell

2. Some Na+ channels open

6. Membrane fully depolarised (more positive on the in- than on the outside!)

K+ Na+ Cl- A-

A-

K+ Cl-

K+ Na Cl-

+

- (-70 mV)

Na+

Na+ Na+

K+ K+

Resting potential: Na+ Na+ Na+Na+

Na+

Na+ Na+Na+ Na+

Na+Na+

Na+

K+ Na+ Cl- A-

A-

K+ Cl-

K+ Na Cl-

(-70 mV)

Na+

Na+ Na+

K+ K+

Electricalstimulation: Na+ Na+ Na+Na+

Na+

Na+ Na+Na+ Na+

Na+Na+

Na+

+ +

1. Voltage gated membrane channels:

• Na+ channels open or close in response to electrical changes at the membrane

K+ Na+ Cl- A-

A-

K+ Cl-

K+ Na Cl-

(-50 mV)

Na+

K+ K+ Na+

Na+ inflow: Na+ Na+ Na+Na+

Na+

Na+ Na+Na+ Na+

Na+

Na+

Na+

K+ Na+ Cl- A-

A-

K+ Cl-

K+ Na Cl-

(-50 mV)

Na+

K+ K+

Na+

Threshold:Na+ Na+ Na+Na+

Na+

Na+

Na+Na+ Na+

Na+

Na+

Na+

K+ Na+ Cl- A-

A-

K+ Cl-

K+ Na Cl-

(+50 mV)Na+ K+ K+ Na+

Na+ Na+ Na+Na+

Na+

Na+

Na+ Na+

Na+

Na+Na+

Na+

Electrical activity – Action Potential

2. Threshold potential and the Hodgkin-Huxley cycle:

• If membrane depolarises further: • more and more Na+ channels will open, • resulting in more and more depolarisation

Na+ inflow Na+ channels open

Membrane depolarises

Electrical stimulation

• If membrane potential at axon hilock reaches threshold: • all Na+ channels in depolarised area open simultaneously,• generating an action potential

• If membrane potential at axon hillock remains below threshold, resting potential returns

Electrical activity – Action Potential

• Threshold has been reached:• so many Na+ ions enter the cell that inside becomes more positive

than outside (complete depolarisation)

Na+ channel

open

Na+

IN

3. Electrochemical processes during an AP

• Complete depolarisation causesa) Closing of Na+ channels:

• No more Na+ ions enter cellb) Opening of K+ channels:

• K+ ions rush out of cell:• membrane repolarises

Na+ channel close, K+ channel open

K+ OUT

• K+ channels close when resting potential is restored

• briefly, less K+ ions inside than outside cell:

• membrane hyperpolarized (inside more negative than usual)

Electrical activity – Action Potential

4. Conduction of the action potential

• Originates at axon hillock & travels down the axon• Each burst of depolarisation acts as a trigger, • opening Na+ channels in adjacent regions of the axon

• Why does the action potential not travel backwards?

Electrical activity – Action Potential

• During hyperpolarisation, mem-brane more difficult to depo-larise

• But adjacent part of axon (where AP has not yet occurred) easy to depolarise

Electrical activity – Action Potential

5. Properties of the action potential:

• No decay:• always strong enough to depolarise adjacent membrane

• ‘All-or-nothing’ phenomenon:• either generated or not• can not be generated with different intensities!

• Discontinuous:• minimal time between subsequent APs: 2-5ms

• Fast:• approx. 1-10 m/s

However, for some purposes, this might not be fast enough

• In mammals, the axons of sensory and motor neurons are myelinated

• Electrical charges transported inside the axon• no need to produce an AP

• Myelin insulates, preventing ion inflow and outflow

6. Saltatory conduction

Electrical activity – Action Potential

Axon+

+ +

++

++

++

+

+

+++ +

+++

+

+++

+

++

+ + + +

+ +++++++++++ + + + +

+++++++++++ + + + +Myelin

++++++++++ + + + +

++++++++++ + + + +

• In mammals, the axons of sensory and motor neurons are myelinated

• Nodes of Ranvier: • gaps that interrupt insulation every 1-2 mm

• Electrical charges transported inside the axon• no need to produce an AP

• Myelin insulates, preventing ion inflow and outflow

6. Saltatory conduction

Electrical activity – Action Potential

Node of Ranvier

+

++

++++

++

+

+

+

+++

+ ++

+

+

++ +

+ +++++++++++ + + + +

+++++++++++ + + + +

++++++++++ + + + +

++++++++++ + + + +

• In mammals, the axons of sensory and motor neurons are myelinated

• Nodes of Ranvier: • gaps that interrupt insulation every 1-2 mm

• Electrical charges transported inside the axon• no need to produce an AP

• Myelin insulates, preventing ion inflow and outflow

6. Saltatory conduction

Electrical activity – Action Potential

7. Signal transmission and information:

• Electrical impulses can not be modified!

• How are different types of information ‘coded’?

• Qualitative: by location• the place in the brain where the signal is received• (cf. last lecture)

• Quantitative (how strong a stimulus is): by ‘firing rate’ • a strong input causes a neuron to send out APs in quicker

succession

Electrical activity – Action Potential

timevolt

age

Weak stimulus:

timevolt

age

Strong stimulus:

Electrotonic Action Potential

Synaptic

Signal Transmission

(details in the next lecture…)

QUESTION TIME

QUESTION TIME

1. In the figure below, the number 3 indicates the

a) pons

b) thalamus

c) corpus callosum

d) limbic system

e) cerebellum

2

1

3

4

5

QUESTION TIME

2. Relative to its environment, the neuron during its resting state is ___ charged; depolarisation means that it becomes more ___ than during resting state, hyperpolarisation means that it becomes more ___ than during resting state

a) Negatively; negative; positive

b) Positively; negative; positive

c) Negatively; positive; negative

d) Positively; positive; negative

e) Neutrally; negative; positive

QUESTION TIME

3. The function of myelin is to

a) Form part of the blood-brain barrier

b) Remove waste products from neurons

c) Provide structural stability and support

d) Electrically insulate axons

e) Participate in synaptic signalling

QUESTION TIME

4. Which of the following is NOT a function of microglia?

a) Remove waste products from neurons

b) Provide structural stability and support

c) Electrically insulate axons

d) Participate in synaptic signalling

e) None of these is a function of microglia

QUESTION TIME

5. Correctly label the parts in Figure 1:

a) 1. Axon terminals; 2. Axon; 3. Soma; 4. Dendrites

b) 1. Nodes of Ranvier; 2. Dendrite; 3. Soma; 4. Axon terminals

c) 1. Axons; 2. Dendrite; 3. Axon hillock; 4. Myelin

d) 1. Dendrites; 2. Axon; 3. Axon hillock; 4. Spines

e) 1. Nodes of Ranvier; 2. Myelin; 3. Cell body; 4. Dendrites

QUESTION TIME

6. The direction of signal transmission in the neural network shown in Figure 2 is

a) From A to B and C

b) From C to A and B

c) From B to A to C

d) From A and B to C

e) From C and B to A

QUESTION TIME

7. Signals from the ears enter the forebrain at the

a) 1 – pons

b) 2 – thalamus

c) 3 – corpus callosum

d) 4 – occipital lobe

e) 5 - cerebellum

2

1

3

4

5

QUESTION TIME

8. Damage to which structures might cause blindness?

a) 1 & 2

b) 1 & 5

c) 2 & 4

d) 1, 2, & 3

e) 4 & 5

2

1

3

4

5