How the brain perceives the outside

world

Saturday Morning PhysicsDecember 4, 2004

Presenter: Rhonda Dzakpasu

Brain blunders

Kanizsa’s Triangle

“Peripheral drift” by Akiyoshi Kitaoka Taken from Nationalgeographic.com

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W.E. Hill

Central Nervous System:Brain and spinal cord

• The Brain– Three pound

wrinkled mass– Fits into cranial

cavity surrounded by cerebrospinal fluid cushioning

– Consists of multiple areas

– Localized regions are task specific

Courtesy of Morphonix LLC, Sausalito, CA

Central Nervous System:Brain and spinal cord

• Spinal cord– Carries fibers that

relay information from brain to rest of body and from the body to the brain

– Encased in bone fortress known as the vertebrae

– Surrounded by cerebrospinal fluid

Courtesy of Morphonix LLC, Sausalito, CA

Peripheral Nervous System:nerves to rest of body

• Somatic nerves– Fibers that relay commands from

brain to all muscle groups below the neck and controls somatosensory function such as touch, pain, temperature and joint position

Courtesy of Morphonix LLC, Sausalito, CA

Peripheral Nervous System:nerves to rest of body

• Autonomic nerves– Fiber carrying

information from brain to all organs in the body

– And all this is done automatically

• Cranial nerves– 12 nerves connecting

the brain to the organs in the head; also have sensory and motor functions Courtesy of Morphonix LLC,

Sausalito, CA

The “Sense”ational Organs

Receives “raw” data from around us

Each organ handles single function

Converts external stimuli to language of the brain

Seeing is believing: The Eye

Courtesy of Morphonix LLC, Sausalito, CA

Retina is initial visual processing centerHas three basic layers of cells:

Light sensitive photoreceptorsBipolar cellsGanglion cells

Seeing is believing: The Retina

Courtesy of Morphonix LLC, Sausalito, CA

Color is coded by photoreceptor rods and cones

Intensity is coded by frequency of firing of ganglion cells

Can you hear me now?

Courtesy of Morphonix LLC, Sausalito, CA

The ear: Wide dynamic range: 20 Hz – 20000 Hz

Three major components:Outer earMiddle earInner ear

Can you hear me now?

Courtesy of Morphonix LLC, Sausalito, CA

Cochlea: neurosensory component for hearing

Hair cells convert sound pressure to “brain language”

Outermost end – high frequencies

Innermost end – low frequencies

Olfaction and Taste

Smell and taste are related (artificially separated?)

Both are chemical senses

Odors bind to receptors which send electrical signals to olfactory bulb

Tastes are dissolved in mucous fluid in oral cavity to be carried to taste receptors

Courtesy of Morphonix LLC, Sausalito, CA

Like minded ordering

Cells of similar type group according to their function

Located in well defined layers

Advantage: facilitates ease of communicaton

Disadvantage: disease or injury can wipe out entire function

Brain Structure

whyfiles.org

The Cerebral Cortex

Purves et al., Life: The Science of Biology, 4th Edition

Cortical Parallel Processing Visual system

Must decode shape,color, position and movement

Auditory system Must decode many aspects of sound,

e.g., loudness, pitch, harmonics, timing and location of multiple sounds

Olfactory System Odors are composed of different

chemical groups that bind to different receptors

The Father of Modern Neuroscience

Courtesy: The Nobel Foundation

Outlined fundamental architecture of CNS

Demonstrated basic changes neurons undergo as they function in the CNS First to isolate neurons near surface of the brain

Santiago Ramon y Cajal, 1852-1934

The Father of Modern Neuroscience

Courtesy: The Nobel Foundation

Santiago Ramon y Cajal, 1852-1934

Who are the main players?

Neuron– Generate and propagate electrical

activity– Responsible for our thinking, feeling,

hearing, seeing, moving, loving and hating!

Glial cells– Supporting cells

The Neuron

There are ~ 100 billion

Small enough for 30000 to fit on head of a pin

Connects with thousands of other neurons

Generally do not reproduce

Courtesy of Morphonix LLC, Sausalito, CA

The Neuron

Three major components:

Dendrites:

Receives information from other neurons

Courtesy of Morphonix LLC, Sausalito, CA

The Neuron

Soma:

Cell body containing the nucleus, the brain of the neuron

Courtesy of Morphonix LLC, Sausalito, CA

The Neuron

Axon:

Connects with other cells to transmit informationCan be up to several feet long!Covered in myelin allowing for fast information transfer

Courtesy of Morphonix LLC, Sausalito, CA

The Neuron

Always active!

Constantly integrating and generating information

Hotbed of activity! Neurons don’t hibernate!

Courtesy of Morphonix LLC, Sausalito, CA

How is information transmitted?!

The Resting Potential

Primarily due to concentration of K+ions inside and outside cell membrane

Typically between -80 mV to –55 mV

Action Potential:Generation

Excitatory input from synapses to dendrites depolarizes cell

Voltage activated Na+ channels open

Inside of cell becomes positive, ~20mV

Change in voltage creates current that depolarized adjacent region in axon

Impulse travels down axon Lasts ~2 ms

Action Potential: Propagation

Neuronal Communications

Neurons are like islands: no physical contacts

Chemical signals are converted to electrical ones

The supporting cells

Glial cellsOutnumber neurons by 10:1 margin

Two types:Astrocytes

Oligodendrites and Schwann cells

The supporting cells

Astrocytes:Provide physical supportby forming a mesh around neuronsFormation of blood-brain barrierRemove dead cellsTransport of nutrientsDestroyneurotransmitters in extra cellular space

The supporting cells

Oligodendrites (CNS) and Schwann cells (PNS):

Prevents abnormalcommunication between neuronsContain myelin and wraps around neuronal axons to boost communications, up to 200 mph!

Where’s the Physics?!

The “big” questions?

How do high-level psychological processes come about from basic neurophysiological effects?

How does the complex organization of brain cells, give rise to behavior?

Where’s the Physics?!

Computational models of neuronal behavior How can neural activity be defined? What types of coding mechanisms are

involved? Models from single ion channel and

synaptic models to “black-box” models describing psychological phenomena

The “Standard” Model

• Hodgkin-Huxley (1950’s)– First model of action potential

propagation

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Hodgkin-Huxley Model

• Simulates action potential from giant axon of squid

Hodgkin, A. L. and Huxley, A. F. (1952) Journal of Physiology 117: 500-544

Where’s the Physics?!

Study of spatio-temporal patterns

What new types of behaviors occur in large-scale models?

Synchronization Oscillations Are they important to brain function?

Patterns of Emergence

Large scale behavior of networks of neurons

Makeig, S., et. al., Science, 295, 690, 2002

Patterns of Emergence

Temporal and spatial changes in networks

Seidemann, E., et. al., Science, 295, 862, 2002

Where’s the Physics?!

Imaging techniques

How can we “see” inside the brain? Structural methods Functional methods

Imaging Techniques

Clinical application– PET scans– CT scans– MRI

Research applications– Fluorescence based imaging– Nuclear magnetic resonance (NMR)

imaging

What can we do when the brain fails?

Why should you care that we want to understand how the brain works???

Hybrid Brain Machine Interfaces*

Real-time direct interfaces between brain, electronics and mechanical devices

Can be used to restore lost or impaired sensory and motor function

* Nicolelis, M., Nature, 409, 403,2001

Prosthetic Ear: Cochlear Implants

Converts acoustic signal to electrical stimuli

Couples to array of implanted electrodes to auditory nerves

Stimulates cochlea:High frequencies – basalLow frequencies – apical

Mimics normal auditory processing Reprinted with permission from Rauschecker,

J.P., and Shannon, R.V., Science, 295, 1025-1029, (2002) Copyright 2002, AAAS

A Bionic Eye? Retinal Prostheses

Reprinted with permission from Zrenner, E., Science, 295, 1022-1025, (2002) Copyright 2002, AAAS

A Bionic Eye? Retinal Prostheses

Type 1: subretinal implantReplaces photoreceptors with microphotodiodes and electrodes

Type 2: epiretinal implantNo light-sensitive areasUses camera and processor outside of bodyImplanted electrodes stimulate axons of ganglion cells

It’s all Monkey Business!

Reprinted with permission, Nicolelis, M., Nature Reviews, 4, 417, 2003

It’s all Monkey Business!

Monkeys learn to produce complex hand movementsImplanted microwire arrays record activity of hundreds of neurons

Linear and non-linear models are used to extract motor control signals

Outputs used to control movement of the robot arm

Reprinted with permission, Nicolelis, M., Nature Reviews, 4, 417, 2003

Stay tuned!Next week’s lecture

• Imaging techniques

– Clinical methods– Research methods