Transcript Brain Structures

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 Srini Narayanan
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Comp
Sci 182 / Cog Sci 110 / Ling 109
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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.
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Intelligence and Neural Computation
Embodied Intelligence

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.
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Brains ~ Computers
1000 operations/sec
 100,000,000,000
units
 10,000 connections/
 graded, stochastic
 embodied
 fault tolerant
 evolves, learns

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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
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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 14
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
Dr Rachel Swainson
NEURAL COMMUNICATION 1:
Transmission within a cell
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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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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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Artist’s rendition of a typical cell membrane
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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].
+40
Membrane
potential 0
(mV)
[C]
[B]
[A]
[D]
excitation threshold
-70
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0
1
2
3
Time (msec)
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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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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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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 energyconsuming (Na+/K+ pump).
 A faster, more efficient mechanism has
evolved: saltatory conduction.
 Myelination provides saltatory conduction.

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Membrane potential
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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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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
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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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 (NB 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+ (NB remember termination of the action
potential)
 Cl- (if already depolarized)
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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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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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Motor Control
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Hierarchical Organization of Motor
System

Primary Motor Cortex and Premotor Areas
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Primary motor cortex (M1)
Hip
Trunk
Arm
Hand
Foot
Face
Tongue
Larynx
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Leg motor control
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Two major descending pathways
Pyramidal vs. extrapyramidal
Motor cortex
Pyramidal
system
• Pathway for
voluntary movement
• Most fibers originate
in motor cortex (BA
4&6)
• Most fibers cross to
contralateral side at
the medula
Brain stem
centers
Lower motor neurons
(brain stem and spinal cord)
Striated muscles
Extrapyramidal
system
• Pathways for postural
control/certain reflex
mov’t
• Originates in brainstem
• Fibers do not cross
• Cortex can influence
this system via inputs to
brain stem
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Cortical Motor System
Pre-motor cortex
Movement planning/sequencing
• Many projections to M1
• But also many projections directly into
pyramidal tract
• Damage => more complex motor
coordination deficits
• Stimulation => more complex mov’t
• Two distinct somatotopically organized
subregions
• SMA (dorso-medial)
• May be more involved in
internally generated movement
• Lateral pre-motor
• May be more involved in
externally guided movement
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Cortical Motor System
Posterior parietal cortex (PPC)
Sensory guidance of movement
• Many projections to pre-motor cortex
• But also many projections directly into
pyramidal tract
• Damage can cause deficits in visually
guided reaching (Balint’s syndrom) and/or
apraxia
• Likely part of the dorsal visual stream
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Vision
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Vision and Action
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Cortical Motor System
Pre-motor cortex
Movement planning/sequencing
• Many projections to M1
• But also many projections directly into
pyramidal tract
• Damage => more complex motor
coordination deficits
• Stimulation => more complex mov’t
• Two distinct somatotopically organized
subregions
• SMA (dorso-medial)
• May be more involved in
internally generated movement
• Lateral pre-motor
• May be more involved in
externally guided movement
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Somatotopy of Action Observation
Foot Action
Hand Action
Mouth Action
Buccino et al. Eur J Neurosci 61
2001
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Fly Brain
Basic Atlas of the Drosophila Brain
http://flybrain.neurobio.arizona.edu/Flybrain/html/index.html
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Antennal Lobe Stereopair
This example shows the dendrites of two "uniglomerular"
neurons, each invading a single olfactory glomerulus in
the antennal lobe of the cockroach
A stereopair taken with an "Edge microscope"
(Edge Scientific Instrument Corporation, Santa Monica, CA 63
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The axons of projection neurons
from the antennal lobes.
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
Sagittal section of the antennal nerve and lobe, showing olfactory receptor axons one
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of
which is shown ending in a glomerulus that, is invaded by a local interneuron.

Mechanosensory endings from the right antennal nerve revealing three classes of
terminals: small diameter axons, that distribute lateral to and around the back of the
antennal lobe (ant lob); intermediate diameter axons that reside between these
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smaller
fibers
and
a
dense,
bifurcating,
tract
of
thicker
varicose
processes
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two neurons that invade the ellipsoid body from
dendrites within the lateral deutocerebrum
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
Optic array of inputs from the medulla to the lobula showing the layer 68
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relationship
between the medulla afferents and lobula output dendrites

Optic lobes invading the lobula plate with branches that also extend to the
inner
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Type 1 and
Type 5 T-cell

Horizontal section showing a Y-cell and a small-field Tm1 transmedullary cell
in the medulla. The latter terminates in the outer layer of the lobula amongst
dendrites of T5 neurons, several of which are shown here. Tm1 and T5 are
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essential components in the elementary motion detecting circuit
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
Drosphila optic lobes demonstrate the presence of orthogonally arranged
dendrites of horizontal and vertical neurons in the lobula plate. Optical
stacks show the vertical neurons and the three horizontal neurons. The
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axons
of
columnar
neurons
elements
converge
onto
the
giant
fibre
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
beneath the retina (ret) of the compound eye, the lamina neuropil is shown
with the cross sections of the L1 and L2 monopolar cell axons denoting the
mosaic
of optic cartridges
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
The lobula (lo) is shown connected to the medulla (me) via the second
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optic chiasma (och 2)
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Lobula
Complex
The neural architecture of the lobula (lo) and the lobula plate (lo p), both
connected to the inner layer of the medulla (me) by the second optic chiasma
(och 2). The lobula plate shows three layers of tangential neurons (1-3).
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They
receive
inputs
from
chromatic
motion-sensitive
interneurons
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