EE141 1 Brain Structures [Adapted from Neural Basis of Thought and Language Jerome Feldman, Spring 2007, feldman@icsi.berkeley.edu Broca’s area Pars opercularis.
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Brain StructuresBrain Structures[Adapted from Neural Basis of Thought and Language Jerome Feldman, Spring 2007, feldman@icsi.berkeley.edu
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
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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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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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Somatotopy of Action ObservationSomatotopy of Action Observation
Foot ActionFoot Action
Hand ActionHand Action
Mouth ActionMouth Action
Buccino et al. Eur J Neurosci 2001
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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.
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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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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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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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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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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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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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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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