Transcript Course Introduction: The Brain, chemistry, neural signaling

Course Introduction:
The Brain, chemistry,
neural signaling
Srini Narayanan
CS182/Ling109/CogSci110
Spring 2006
[email protected]
Overview
Course introduction
 Neural Processing: Basic Issues
 Neural Communication: Basics

Instructor Access

Instructor : Srini Narayanan
 Office
Hours : Tuesday 2 - 3
 Email
: [email protected]

TA: Joseph Makin
 Office
Hours :
 Email

: [email protected]
TA: Johno Bryant
 Office
 Email
Hours:
: [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?
Schedule
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 (swim toward food)
 Positive

chemotaxis
Defensive/Avodiance Behavior
 Negative

chemotaxis
Reproduction
 Asexual
and Sexual reproduction using chemical
messenger proteins (pheromones)
Earliest Nervous Systems
Hydra, jellyfish, corals, sea anemones
 Basic neural cell (Neuron)
 Early differentiation into 3 types of neurons
STIMULUS
Sensory
Neuron
InterNeuron
Motor
Neuron
Effector

Overview
Course introduction
 Neural Processing: Basics
 Neural Communication: Basics

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
postsynaptic
neuron
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
Synapse
axon of presynaptic
neuron
dendrite of
postsynaptic
neuron
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
MULTIPOLAR CELLS
BIPOLAR
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
Motor cortex
Somatosensory cortex
Sensory associative
cortex
Pars
opercularis
Visual associative
cortex
Broca’s
area
Visual
cortex
Primary
Auditory cortex
Wernicke’s
area
PET scan of blood flow for 4 word tasks
Somatotopy of Action Observation
Foot Action
Hand Action
Mouth Action
Buccino et al. Eur J Neurosci 2001
Neural Communication: 1
Communication 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.

The Resting Potential
-
-
-
+
+
-
Resting potential of neuron = -70mV
-
+

+

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
Artist’s rendition of a typical cell membrane
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+
Cl-
+
Na+
Na+
K
Organic anions (-)
+
Cl-
outside
inside
Organic anions (-)
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+
Cl-
+
Na+
Na+
K
Organic anions (-)
+
Cl-
outside
inside
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.
Na
Na+
+
Na+
outside
K+
K+
inside
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.

Before Depolarization
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
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 and then
hyperpolarizing the membrane.
Na+
Na+
K
+
Na+
K+
K+
+
-
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 provides 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 over
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.
Myelin sheath
Node of Ranvier
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.
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.
terminal
extracellular fluid
synaptic cleft
presynaptic membrane
postsynaptic membrane
dendritic spine
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
Metabotropic Receptors (G-Protein)
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.)
Na+
Ca2+
-

+

outside
inside
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)
Cl-
K+
-

+

outside
inside
Postsynaptic Ion motion
Requirements at the synapse
For the synapse to work properly, six basic events need to happen:
 Production of the Neurotransmitters


Storage of Neurotransmitters




SV
Release of Neurotransmitters
Binding of Neurotransmitters


Synaptic vesicles (SV)
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 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
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 neurotransmitters
carry 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?