EE141 1 Brain Structures [Adapted from Neural Basis of Thought and Language Jerome Feldman, Spring 2007, [email protected] Broca’s area Pars opercularis.

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Brain StructuresBrain Structures[Adapted from Neural Basis of Thought and Language Jerome Feldman, Spring 2007, [email protected]

Broca’sarea

Parsopercularis

Motor cortex Somatosensory cortex

Sensory associativecortex

PrimaryAuditory cortex

Wernicke’sarea

Visual associativecortex

Visualcortex

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Intelligence Intelligence Learning and UnderstandingLearning 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 ComputationIntelligence 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 DivisionsNervous System Divisions

Central nervous system (CNS) brain spinal cord

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Nervous System DivisionsNervous 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 ~ ComputersBrains ~ Computers

1000 operations/sec 100,000,000,000 units 10,000 connections/ graded, stochastic embodied fault tolerant evolves learns

1,000,000,000 ops/sec 1-100 processors ~ 4 connections binary, deterministic abstract crashes designed programmed

EE1417PET scan of blood flow for 4 word tasks

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Neurons structuresNeurons structures

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NeuronsNeurons

cell body

dendrites (input structure) receive inputs from receive inputs from

other neuronsother neurons perform spatio-perform spatio-

temporal integration of temporal integration of inputsinputs

relay them to the cell relay them to the cell bodybody

axon (output structure) a fiber that carries a fiber that carries

messages (spikes) from messages (spikes) from the cell to dendrites of the cell to dendrites of other neuronsother neurons

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Neuron cellsunipolarbipolarmultipolar

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SynapseSynapse

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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SynapseSynapse

axon of presynapticneuron

dendrite ofpostsynapticneuron

bipolar.about.com/library

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SynapseSynapse

• a synapse can be excitatory or inhibitory• arrival of activity at an excitatory synapse

depolarizesdepolarizes the local membrane potential of the postsynaptic cell and makes the cell more prone to firing

• arrival of activity at an inhibitory synapse hyperpolarizeshyperpolarizes 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 ObservationSomatotopy of Action Observation

Foot ActionFoot Action

Hand ActionHand Action

Mouth ActionMouth Action

Buccino et al. Eur J Neurosci 2001

EE14117How does it all work?How does it all work?

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Artist’s rendition of a typical cell membrane

Amoeba eating

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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

Neural ProcessingNeural Processing

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Transmission of informationTransmission of information

Information must be transmitted within each neuron and between neurons

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The MembraneThe Membrane

The membrane surrounds the neuron. It is composed of lipid and protein.

EE14122Artist’s rendition of a typical cell membrane

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Cell Electrical PotentialCell Electrical Potential

Every neuron is covered by a membraneThe 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 potentialsElectrical 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 ionsForces 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 PotentialThe 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.

inside

outside

Resting potential of neuron = -70mV

+

-

+

-

+

-

+

-

+

-

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Ions and the Resting PotentialIons 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.

inside

outsideNa+Cl-Na+

K+

Cl-

K+

Organic anions (-)

Na+Na+

Organic anions (-)

Organic anions (-)

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Maintaining the Resting PotentialMaintaining 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.

inside

outside

Na+

Na+

K+K+

Na+

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Neuronal firing: the action potentialNeuronal 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 PotentialCourse 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 PotentialThe Action Potential

The 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.

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Action potentialAction potential2 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 DepolarizationBefore Depolarization

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Action potentials: Rapid Action potentials: Rapid depolarizationdepolarization 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+

Na+

Na+

-

+

+

-

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DepolarizationDepolarization

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Action potentials: RepolarizationAction 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.

K+ K+

K+Na+

Na+

Na+

+

-

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RepolarizationRepolarization

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Conduction of the action potentialConduction 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 PotentialAction Potential

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Propagation of the Action PotentialPropagation 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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MyelinationMyelination

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 ConductionSaltatory 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.

Node of RanvierMyelin sheath

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Summary of axonal conductionSummary 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 transmissionSynaptic 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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terminal

dendritic spine

synaptic cleftpresynaptic membrane

postsynaptic membrane

extracellular fluid

Structure of a synapseStructure 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.

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Neurotransmitter releaseNeurotransmitter 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+

Ca2+Ca2+ Ca2+

Ca2+

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Neurotransmitter releaseNeurotransmitter 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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EE14150Opening and closing of the channel in synaptic membrane

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Ionotropic receptorsIonotropic 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 ReceptorsIonotropic Receptors

4 nm

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Metabotropic Receptors (G-Protein)Metabotropic Receptors (G-Protein)

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Postsynaptic Ion motionPostsynaptic Ion motion

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Excitatory postsynaptic potentials Excitatory postsynaptic potentials (EPSPs)(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.)

inside

outsideNa+ Ca2+

+

-

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Inhibitory postsynaptic potentials Inhibitory postsynaptic potentials (IPSPs)(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- +

- inside

outside

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Integration of informationIntegration 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 synapseRequirements at the synapse

For the synapse to work properly, six basic events need to happen:

1. Production of the Neurotransmitters

2. Storage of Neurotransmitters

3. Release of Neurotransmitters

4. Binding of Neurotransmitters

5. Generation of a New Action Potential

6. Removal of Neurotransmitters from the Synapse

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Three Nobel Prize Winners on Three Nobel Prize Winners on Synaptic TransmissionSynaptic 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 neurotransmitterscarry 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 circuitsNeural 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 circuitsNeural 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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