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Course Introduction: The Brain, chemistry, neural signalingJerome FeldmanSrini NarayanCS182/Ling109/CogSci110Spring [email protected]
http://inst.eecs.berkeley.edu/~cs182/sp08/
Lecture Overview
Course introduction Neural Processing: Basic Issues Neural Communication: Basics
Instructor Contact Instructor : Srini Narayanan
Office Hours : Email: [email protected]
Instructor : Jerome FeldmanOffice Hours : Monday 1 – 2, Thur. 2:30-3:30Email : [email protected]
TA: Leon BarrettOffice Hours :Email: [email protected]
The Neural Theory of Language and Thought This is a course on the current status of interdisciplinary
studies that seek to answer the following questions: How is it possible for the human brain, which is a highly
structured network of neurons, to think and to learn, use, and understand language?
How are language and 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 and language?
What are the applications of neural computing and embodied language?
Learning
I hear and I forget
I see and I remember
I do and I understand
attributed to Confucius 551-479 B.C.
Tinbergen’s Four Questions
How does it work?
How does it improve fitness?
How does it develop and adapt?
How did it evolve?
Single Cell (Protozoan) Behaviors
No Nervous System Foraging Behavior (move toward food)
Positive chemotaxis
Defensive/Avoidance Behavior Negative chemotaxis
Reproduction Asexual and Sexual reproduction using chemical
messenger proteins (pheromones)
Earliest Nervous Systems
Earliest neurons dispensed hormones Hydra, jellyfish, corals, sea anemones Basic neural cell (Neuron) Early differentiation into 3 types of neurons
STIMULUS
SensoryNeuron
Inter-Neuron
MotorNeuron E
ffec
tor
Neural Processing
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
postsynapticneuron
science-education.nih.gov
Synapse
• site of communication between two cells
• formed when an axon of a presynaptic cell “connects” with the dendrites of a postsynaptic cell
Synapseaxon of presynaptic
neuron
dendrite ofpostsynapticneuron
bipolar.about.com/library
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
UNIPOLAR BIPOLAR
MULTIPOLAR CELLS
Brains ~ 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
Broca’sarea
Parsopercularis
Motor cortex Somatosensory cortex
Sensory associativecortex
PrimaryAuditory cortex
Wernicke’sarea
Visual associativecortex
Visualcortex
PET scan of blood flow for 4 word tasks
Somatotopy of Action ObservationSomatotopy of Action Observation
Foot ActionFoot Action
Hand ActionHand Action
Mouth ActionMouth Action
Buccino et al. Eur J Neurosci 2001
Neural Communication: 1 Processing within the cell
Transmission of information
Information must be transmitted within each neuron and between neurons
The Membrane
The membrane surrounds the neuron. It is composed of lipid and protein.
Artist’s rendition of a typical cell membrane
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.
inside
outside
Resting potential of neuron = -70mV
+
-
+
-
+
-
+
-
+
-
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.
inside
outsideNa+Cl-Na+
K+
Cl-
K+
Organic anions (-)
Na+Na+
Organic anions (-)
Organic anions (-)
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.
inside
outside
Na+
Na+
K+K+
Na+
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.
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+
Na+
Na+
-
+
+
-
Depolarization
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 the membrane.
K+ K+
K+Na+
Na+
Na+
+
-
Repolarization
The 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.
Action Potential
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 enables saltatory conduction.
Myelination
Most mammalian axons are myelinated. The myelin sheath is provided by oligodendrocytes and
Schwann cells. Myelin is insulating, preventing passage of ions through
the membrane.
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.
Node of RanvierMyelin sheath
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.
terminal
dendritic spine
synaptic cleftpresynaptic membrane
postsynaptic membrane
extracellular fluid
Structure of the 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.
Neurotransmitter release Ca2+ causes vesicle membrane to fuse with
presynaptic membrane. Vesicle contents empty into cleft: exocytosis. Neurotransmitter diffuses across synaptic
cleft.
Ca2+
Ionotropic receptors (ligand gated) Synaptic activity at ionotropic receptors
is fast and brief (milliseconds). Acetylcholine (Ach) works in this way
at nicotinic receptors. Neurotransmitter binding changes the
receptor’s shape to open an ion channel directly.
ACh ACh
Ionotropic Receptors
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.)
inside
outsideNa+ Ca2+
+
-
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)
K+
Cl- +
- inside
outside
Postsynaptic Ion motion
Requirements at the synapse
For the synapse to work properly, six basic events need to happen: Production of the Neurotransmitters
Synaptic vesicles (SV) Storage of Neurotransmitters
SV Release of Neurotransmitters Binding of Neurotransmitters
Lock and key Generation of a New Action Potential Removal of Neurotransmitters from the Synapse
reuptake
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 coincident IPSPs
and EPSPs at one neuron. If there is sufficient depolarization at the axon
hillock, an action potential will be triggered.
axon hillock
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 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). With its relatively low number of 20,000 neurons, this snail is suitable for neuron research.
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.
How does it all work?
One page written assignment
Due in class: January 29 2008
