Course Introduction: The Brain, chemistry, neural signaling Jerome Feldman Srini Narayan...

Course Introduction: The Brain, chemistry, neural signalingJerome FeldmanSrini NarayanCS182/Ling109/CogSci110Spring 2008feldman@icsi.berkeley.edu

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: snarayan@icsi.berkeley.edu

Instructor : Jerome FeldmanOffice Hours : Monday 1 – 2, Thur. 2:30-3:30Email : jfeldman@cs.berkeley.edu

TA: Leon BarrettOffice Hours :Email: barrett@icsi.berkeley.edu

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