Transcript Lecture 1 Brain Structure

Motor cortex
Somatosensory cortex
Sensory associative
cortex
Pars
opercularis
Visual associative
cortex
Broca’s
area
Visual
cortex
Primary
Auditory cortex
Wernicke’s
area
Brain Structures
[Adapted from Neural Basis of Thought and Language
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Feldman,
Spring 2007, [email protected]
Jerome
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Intelligence
Learning and Understanding
•
I hear and I forget
•
I see and I remember
•
I do and I understand
attributed to Confucius 551-479 B.C.
There is no erasing in the brain
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Intelligence and Neural Computation

What it means for the brain to compute and how that
computation differs from the operation of a standard digital
computer.

How intelligence can be implemented in the structure of the
neural circuitry of the brain.

How is thought related to perception, motor control, and our
other neural systems, including social cognition?

How do the computational properties of neural systems and the
specific neural structures of the human brain shape the nature of
thought?

What are the applications of neural computing?
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Nervous System Divisions

Central nervous
system (CNS)
 brain
 spinal cord
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Nervous System Divisions

Peripheral nervous
system (PNS) consists
of:
 Cranial and spinal
nerves
 Ganglia
 Sensory receptors

Subdivided into:
 Somatic
 Autonomic
– Motor component
subdivided into:


sympathetic
parasympathetic
 Enteric
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Brains ~ Computers

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

1000 operations/sec
100,000,000,000 units
10,000 connections/
graded, stochastic
embodied
fault tolerant
evolves
learns

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
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
1,000,000,000 ops/sec
1-100 processors
~ 4 connections
binary, deterministic
abstract
crashes
designed
programmed
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PET scan of blood flow for 4 word tasks
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Neurons structures
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Neurons
cell body
dendrites (input structure)
 receive inputs from other
neurons
 perform spatio-temporal
integration of inputs
 relay them to the cell body
axon (output structure)
 a fiber that carries messages
(spikes) from the cell to
dendrites of other neurons
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Neuron cells
unipolar
bipolar
multipolar
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Synapse

site of
communication
between two cells

formed when an
axon of a
presynaptic cell
“connects” with
the dendrites of a
postsynaptic cell
science-education.nih.gov
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Synapse
axon of presynaptic
neuron
dendrite of
postsynaptic
neuron
bipolar.about.com/library
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Synapse
•
•
•
•
a synapse can be excitatory or inhibitory
arrival of activity at an excitatory synapse depolarizes the
local membrane potential of the postsynaptic cell and makes
the cell more prone to firing
arrival of activity at an inhibitory synapse hyperpolarizes
the local membrane potential of the postsynaptic cell and
makes it less prone to firing
the greater the synaptic strength, the greater the
depolarization or hyperpolarization
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Visual cortex
of the rat
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Somatotopy of Action Observation
Foot Action
Hand Action
Mouth Action
Buccino et al. Eur J Neurosci 16
2001
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How does it all work?
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Amoeba eating
Artist’s rendition of a typical cell
membrane
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Neural Processing
From lecture notes by Dr Rachel Swainson
NEURAL COMMUNICATION 1:
Transmission within a cell
and from a lecture notes based on
www.unisanet.unisa.edu.au/Information/12924info/Lecture
Presentation - Nervous tissue.ppt
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Transmission of information
Information must be transmitted
 within each neuron
 and between neurons
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The Membrane
 The
membrane surrounds the neuron.
 It is composed of lipid and protein.
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Artist’s rendition of a typical cell membrane
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Cell Electrical Potential
Every neuron is covered by a membrane
The membrane is selectively permeable to the passage of
chemical molecules (ions)
The membrane maintains a separation of electrical charge across the
cell membrane.
The cell membrane has an electrical potential
Electrical potentials
Electrical charge of the membrane is related to charged ion that cross
the membrane through lipids, ion channels and protein ion-transporters.
Electrical currents (ionic flux)
The flow of electrical charge between the cell’s interior and exterior
cellular fluids
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Forces determine flux of ions
– Electrostatic forces
• Particles with opposite charges attract, Identical charges
repel
– Concentration forces
• Diffusion – molecules distribute themselves evenly –
– Protein – ion channels
• Selective Non – gated ion channels
• Selective Voltage-dependent gated ion channels
– Protein – ion transporters
– K+ Na + pump
• Cl - pump
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The Resting Potential
-
-
+
-
-

+

There is an electrical charge across the membrane.
This is the membrane potential.
The resting potential (when the cell is not firing) is a
70mV difference between the inside and the outside.
+

outside
inside
+
+
-
Resting potential of neuron = -70mV
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Ions and the Resting Potential


Ions are electrically-charged molecules e.g. sodium (Na+),
potassium (K+), chloride (Cl-).
The resting potential exists because ions are concentrated on
different sides of the membrane.
 Na+ and Cl- outside the cell.
 K+ and organic anions inside the cell.
Na
+
Na
Organic anions (-)
K+
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Cl-
+
Na+
Na+
K
Organic anions (-)
+
Cl-
outside
inside
Organic anions (-)
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Maintaining the Resting Potential


Na+ ions are actively transported (this uses energy)
to maintain the resting potential.
The sodium-potassium pump (a membrane
protein) exchanges three Na+ ions for two K+ ions.
Na
Na+
+
Na+
outside
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K+
K+
inside
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Neuronal firing: the action potential
The action potential is a rapid
depolarization of the membrane.
 It starts at the axon hillock and passes
quickly along the axon.
 The membrane is quickly repolarized to
allow subsequent firing.

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Course of the Action Potential



The action potential begins with a partial depolarization (e.g.
from firing of another neuron ) [A].
When the excitation threshold is reached there is a sudden
large depolarization [B].
This is followed rapidly by repolarization [C] and a brief
hyperpolarization [D].
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The Action Potential
action potential is “all-or-none”.
 It is always the same size.
 Either it is not triggered at all - e.g. too
little depolarization, or the membrane is
“refractory”;
 Or it is triggered completely.
 The
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Action potential
2 phases:
 Depolarisation
– graded potentials move
toward firing threshold
– if reach threshold voltage
regulated sodium channels
open
– reversal of membrane
permeability
 Repolarisation
– sodium channels close
– potassium channels open
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Before Depolarization
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Action potentials: Rapid
depolarization



When partial depolarization reaches the activation
threshold, voltage-gated sodium ion channels
open.
Sodium ions rush in.
The membrane potential changes from -70mV to
+40mV.
Na+
+
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-
Na
+
Na+
+
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Depolarization
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Action potentials: Repolarization



Sodium ion channels close and become refractory.
Depolarization triggers opening of voltage-gated
potassium ion channels.
K+ ions rush out of the cell, repolarizing and then
hyperpolarizing the membrane.
Na+
Na
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+
K
+
Na+
K+
K
+
+
-
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Repolarization
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Conduction of the action potential




Passive conduction will ensure that adjacent
membrane depolarizes, so the action potential
“travels” down the axon.
But transmission by continuous action potentials is
relatively slow and energy-consuming (Na+/K+
pump).
A faster, more efficient mechanism has evolved:
saltatory conduction.
Myelination provides saltatory conduction.
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Action Potential
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Propagation of the Action Potential
•
Action Potential spreads
down the axon in a chain
reaction
•
Unidirectional
– it does not spread into the
cell body and dendrite due to
absence of voltage-gated
channels there
– Refraction prevents spread
back across axon
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Myelination



Most mammalian axons are myelinated.
The myelin sheath is provided by oligodendrocytes
and Schwann cells.
Myelin is insulating, preventing passage of ions over
the membrane.
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Saltatory Conduction



Myelinated regions of axon are electrically
insulated.
Electrical charge moves along the axon rather than
across the membrane.
Action potentials occur only at unmyelinated
regions: nodes of Ranvier.
Myelin sheath
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Node of Ranvier
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Summary of axonal conduction

Unmyelinated fibres
 continuous conduction

Myelinated fibres
 saltatory conduction
– High density of voltage
gated channels at Nodes
of Ranvier

Larger diameter axons
propagate impulses faster

Stimulus intensity encoded
by:
 frequency of impulse
generation
 number of sensory
neurons activated
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Synaptic transmission



Information is transmitted from the presynaptic
neuron to the postsynaptic cell.
Chemical neurotransmitters cross the synapse,
from the terminal to the dendrite or soma.
The synapse is very narrow, so transmission is fast.
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Structure of a synapse



An action potential causes neurotransmitter release
from the presynaptic membrane.
Neurotransmitters diffuse across the synaptic cleft.
They bind to receptors within the postsynaptic
membrane, altering the membrane potential.
terminal
extracellular fluid
synaptic cleft
presynaptic membrane
postsynaptic membrane
dendritic spine
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Neurotransmitter release


Synaptic vesicles, containing neurotransmitter,
congregate at the presynaptic membrane.
The action potential causes voltage-gated calcium
(Ca2+) channels to open; Ca2+ ions flood in.
vesicles
Ca2+
Ca2+
Ca2+ Ca2+ 2+
Ca
Ca2+ Ca2+
Ca2+
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Neurotransmitter release



Ca2+ causes vesicle membrane to fuse with
presynaptic membrane.
Vesicle contents empty into cleft: exocytosis.
Neurotransmitter diffuses across synaptic cleft.
Ca2+
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Opening
and closing of the channel in synaptic membrane
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Ionotropic receptors



Synaptic activity at ionotropic receptors is fast and
brief (milliseconds).
Acetyl choline (Ach) works in this way at nicotinic
receptors.
Neurotransmitter binding changes the receptor’s
shape to open an ion channel directly.
ACh
ACh
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Ionotropic Receptors
4 nm
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Metabotropic Receptors (G-Protein)
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Postsynaptic Ion motion
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Excitatory postsynaptic potentials
(EPSPs)

Opening of ion channels which leads to
depolarization makes an action potential more likely,
hence “excitatory PSPs”: EPSPs.
 Inside of post-synaptic cell becomes less negative.
 Na+ channels (remember the action potential)
 Ca2+ . (Also activates structural intracellular changes ->
learning.)
Ca2+
+
Na+
outside
inside
-
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Inhibitory postsynaptic potentials
(IPSPs)

Opening of ion channels which leads to
hyperpolarization makes an action potential less
likely, hence “inhibitory PSPs”: IPSPs.
 Inside of post-synaptic cell becomes more negative.
 K+ (remember termination of the action potential)
 Cl- (if already depolarized)
-
K+
+
Cl-
outside
inside
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Integration of information




PSPs are small. An individual EPSP will not produce
enough depolarization to trigger an action potential.
IPSPs will counteract the effect of EPSPs at the
same neuron.
Summation means the effect of many coincident
IPSPs and EPSPs at one neuron.
If there is sufficient depolarization at the axon
hillock, an action potential will be triggered.
axon hillock
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Requirements at the synapse
For the synapse to work properly, six basic
events need to happen:
1.
2.
3.
4.
5.
6.
Production of the Neurotransmitters
Storage of Neurotransmitters
Release of Neurotransmitters
Binding of Neurotransmitters
Generation of a New Action Potential
Removal of Neurotransmitters from the Synapse
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Three Nobel Prize Winners on
Synaptic Transmission
Arvid Carlsson discovered dopamine is a neurotransmitter.
Carlsson also found lack of dopamine in the brain of
Parkinson patients.
Paul Greengard studied in detail how neurotransmitters
carry out their work in the neurons. Dopamine activated a
certain protein (DARPP-32), which could change the function
of many other proteins.
Eric Kandel proved that learning and memory processes
involve a change of form and function of the synapse,
increasing its efficiency. This research was on a certain
kind of snail, the Sea Slug (Aplysia) that has relatively low
number of neurons (20,000 ).
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Neural circuits

Divergence
 Single presynaptic neuron synapses
with several postsynaptic neurons
– Example: sensory signals spread in
diverging circuits to several regions
of the brain

Convergence
 Several presynaptic neurons
synpase with single postsynaptic
neuron
– Example: single motor neuron
synapsing with skeletal muscle fibre
receives input from several pathways
originating in different brain regions
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Neural circuits

Pulsing circuit
 Once presynaptic cell stimulated
causes postsynaptic cell to transmit a
series of impulses
– Example: coordinated muscular activity

Parallel after-discharge circuit
 Single presynaptic neuron synapses
with multiple neurons which synapse
with single postsynaptic cell
– results in final neuron exhibiting multiple
postsynaptic potentials

Example: may be involved in precise
activities (eg mathematical calculations)
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