Neuroanatomy & Audition

June 14, 2011

Zooming In Perspective

What is a Neuron?

http://www.nikonsmallworld.com/gallery/year/2005/37/true

Neurons

Neurons control brain function on a cellular level.

There are 100 billion neurons in human brain!

Neurons come in many shapes and sizes.

Each neuron communicates with many others to coordinate various functions of the nervous system.

Image courtesy of Dr. Joshua Sanes, Harvard University, 2005

“Typical” Neuron

Soma (nucleus)

Myelin Sheath

Adapted from http://www.mhhe.com/socscience/intro/ibank/ibank/0002.jpg

The Soma

Cell body Contains

A nucleus with genetic information

Ribosomes for processing genetic information into proteins

Endoplasmic reticulum for transport of materials

Mitochondria for energy Several other important

organelles

http://faculty.washington.edu/chudler/cells.html

The Axon

Carries information AWAY from the Soma (Axons Away!) Typically only 1 Axon per Neuron Can be covered in a fatty conductive substance called

Myelin Speeds the transfer of information

http://faculty.washington.edu/chudler/cells.html

Dendrites

Bring information from other neurons to the soma

Rough surface covered with spines Unmyelinated Most neurons have MANY dendrites with

extensive branching

http://www.usc.edu/programs/neuroscience/faculty/profile.php?fid=12

And Let’s Not Forget…Glia

Glia are non-neuronal supporting cells in the brain

Although there are many more glia than neurons in the brain, they cannot generate action potentials, and also don’t communicate with neurotransmitters.

So what DO they do?

Glial Types & Functions

Astrocytes Clean up brain debris & “eat” dead neurons Bring nutrients to neurons Hold neurons in place

Microglia Digest parts of dead neurons

Oligodendrocytes Create myelin for insulated axons

Schwann Cells Also create myelin for insulating axons

Satellite Cells Provide structural support for neurons

located in the periphery

http://www.psych.ndsu.nodak.edu/mccourt/Psy460/Neurophysiology%20of%20vision/

Neuronal Modeling

http://www.enchantedlearning.com/subjects/anatomy/brain/Neuron.shtml

Brief Overview of Neuroanatomy

Frontal Lobe

Parietal Lobe

Occipital Lobe

CerebellumBrain StemTemporal Lobe

Corpus Callosum

Frontal Lobe

Found behind your forehead

Involved in: Reasoning & Planning Some parts of speech Movement Emotions Problem solving

Contains Motor Cortex

Return to brain parts

Frontal Lobe

Parietal Lobe

Found on the top of your head Contains Sensory Cortex Involved in:

Touch Pressure Temperature Pain Spatial Orientation

Parietal Lobe

Temporal Lobe

Found on the sides of head above your ears

Contains Limbic Cortex

Involved in: Speech perception Hearing Some types of memory Emotion

Return to brain parts

Temporal Lobe

Occipital Lobe

Found at the back of your head

Receives input from the eyes

Often referred to as the visual cortex

Return to brain partsOccipital Lobe

Cerebellum

Found at the at the back of your head under the cerebrum.

Means “little brain” Involved in:

Unconscious coordination Movement Balance Posture Often takes over learned

activities Cerebellum

Brainstem

Most basic part of your brain

Controls essential functions automatically

Contains 2 parts: Medulla controls breathing,

heart & blood vessel activity, digesting, eliminating waste, sleeping, maintaining body temperature…

Pons regulates breathing Also responsible for

movement

Return to brain parts

Brainstem

Cerebral Cortex

Corpus Callosum

Located centrally between the left and right hemispheres of your brain.

Thick bundle of nerve fibers that connects the left and right hemispheres.

Involved in: Creativity Problem solving Allows hemispheres to

process information together

Corpus Callosum

Create a Brain!

How Do Neurons Communicate?

The Normal, At Rest, Condition

When the neuron is at rest, there are several important ions (+ or – charged chemicals) that are carefully balanced.

Important ions K+ (Potassium) Na+ (Sodium) Cl- (Chloride) Ca2+ (Calcium)

These ions enter and leave the neuron through ion channels and pumps.

The Neuron At Rest

The balance of these ions inside and outside of the cell membrane creates a membrane potential.

For the neuron at rest, this is -70 mV. How does the neuron achieve this?

Electrical & Chemical Gradients

Outside

Inside

-70mV

Na+

Na+

K+

K+

Cl-

Cl-

Concentration Gradient

Electrical Gradient

At Rest

So at rest, the inside of the neuron is negatively charged because of the balance of ions inside and outside the cell.

What happens when a signal comes along?? Ions move!!

Action Potential: Na+

When a stimulus occurs, Na+ channels open and Na+ rushes into the neuron, making it more positively charged.

This also passes a negative (depolarizing) current along to the next section of axon.

Outside

Inside

Na+

Na+

-70 mV-55 mV

Action Potentials: All or None

If Na+ outflow causes the potential to reach -55 mV, an action potential will occur and the signal will be sent.

This is known as the threshold potential. If the potential does not reach the

threshold, no action potential will occur…thus it is an “All or None” phenomenon.

Action Potential: K+

K+

K+

Outside

Inside

Once the action potential is generated, Na+ channels close and K+ channels open.

K+ moves slowly outward to bring the potential back to -75 mV (repolarization).

+55 mV-75 mV

Action Potential: Overshoot

So much K+ flows out of the neuron that the membrane potential returns to a value lower than that of its resting state.

This is called hyperpolarization. What effect do you think this might have on

the neuron’s ability to fire again and send a second message?

Refractory Period

While the neuron is hyperpolarized, it cannot fire again.

This also prevents a signal from traveling backwards.

Once the neuron regains its resting membrane potential, it will be able to send a second message.

Propagation

Action potential in one region of axon depolarizes the next region to pass along, or propagate, the action potential.

This process can be sped up by myelin coating on the axons. Nodes of Ranvier: Small segments of unmyelinated axon Action potential “jumps” from Node to Node:

much speedier! This is called saltatory conduction.

Putting it All Together

At Rest: -70 mV (membrane potential) Na+ enters the cell

If -55 mV threshold potential is reached, action potential begins

K+ leaves the cell Cell becomes hyperpolarized (-75 mV) and is temporarily

refractory.

Action potential is passed in one direction down the axon.

Whew! We finally made it down the axon!

Now What??

http://fleetfeetsportswinston-salem.blogspot.com/2010/05/moving-from-competitor-to-spectator.html

We still need to get the message to the next neuron.

http://www.georgiapainphysicians.com/l2_edu_pharma_mod1_slides.htm

Neurons Communicate at Synapses

Neurons talk to each other all the time, but never actually touch.

Two neurons meet at a place called the synapse.

Special chemicals called neurotransmitters carry the message across the synapse.

Neurons Talk at Synapses

Photo by T. Due, Harvard University, 7/2005These C. elegan worms contain a transgene encoding unc-49 gene (GABA receptor) fused to its own promoter and GFP (Harvard Medical School)

From Dr.Venkatesh N. Murthy, Harvard University, 7/2005

Neurons Talk Through Neurotransmitters & Receptors

Neurotransmitters: Chemicals that carry messages from one neuron to another

across the synapse (messages travel really fast!) Receptors:

Protein molecules that receive and translate the chemical message

Neurotransmitters

Neurotransmitters are how the brain passes messages from one neuron to the next.

Neurotransmitters can be either: Inhibitory (they prevent

other neurons from firing) Excitatory (they increase

firing in other neurons)

http://www.besttreatments.co.uk/btuk/images/epilepsy_neurotransmitter.gif

Neurotransmitters: GABA & Glu

Glutamate Primary excitatory

neurotransmitter Involved in:

Epilepsy Learning & Memory Schizophrenia

GABA Primary inhibitory

neurotransmitter Involved in:

Epilepsy Depression & Anxiety Anesthesia

http://www.cnsforum.com/imagebank/item/Neuro_path_GABA/default.aspxhttp://www.cnsforum.com/imagebank/item/Neuro_path_GLUT/default.aspx

Neurotransmitters: 5-HT & NE

Serotonin Involved in:

Depression & Mood Eating Sleep & Wake Pain

Norepinephrine Implicated in

Mood & Depression Sleep & Wake Drug Abuse Parkinson’s Disease

http://www.deplin.com/LifeWithDepression,Causes

Neurotransmitters: DA & ACh

Dopamine Involved in:

Drug Abuse Parkinson’s Disease Schizophrenia

Acetylcholine Involved in:

Muscular movement Nicotine Addiction Alzheimer’s Disease

http://www.3dchem.com/molecules.asp?ID=289 http://www.worldofmolecules.com/emotions/acetylcholine.htm

Illustrate a Neurotransmitter

Brain BeliefsJune 14, 2011

True or False?

The brain is static, unchanging, and set before you start school.

True or False

The brain contains more supporting cells (glia) than it does neurons (cells that send signals throughout the brain).

True or False?

Prior to birth, a baby gains 250,000 neurons per minute.

True or False?

Some people are left-brained and some are right-brained.

True or False?

We use only 10 percent of our brains.

True or False?

Rats have a broader hearing range than humans.

True or False?

Male and female brains are extremely different.

True or False?

Your brain is made up primarily of water and fat.

Review & View a Neuron

Review

Parts of a Neuron Lobes of the Brain Action Potentials &

Neurotransmission Neurotransmitters

http://students.cis.uab.edu/nkm188/project_back2.html

Neurons Galore!

Spinal CordPyramidal

Cortical Neuron

Purkinje Neuron in Cerebellum

Hippocampal Neuron

http://faculty.washington.edu/chudler/gall1.html

Auditory System

Anatomy of the Auditory System

http://www.hearingcarecenter.com/hearing_neural.htm

Hear Ye, Hear Ye

Sounds waves enter the outer ear (pinna), where they are amplified and localized.

The sound wave then vibrates the tympanic membrane (eardrum) and passes to the ossicles.

http://sciencewithmorton.phoenix.wikispaces.net/Sound+and+Light, http://health.allrefer.com/health/ruptured-or-perforated-eardrum-eardrum-repair-series.html

Middle Ear Ossicles

3 small bones Malleus (hammer) Incus (anvil) Stapes (stirrup)

Continue to pass along the vibration from the sound waves to the cochlea

http://health.allrefer.com/health/fusion-of-the-ear-bones-ear-anatomy.html

The Inner Ear

The cochlea is filled with fluid & converts air sounds into liquid sounds

Organ of Corti Contains hair cells on the basilar membrane Sound waves move the hair cells on the basilar

membrane against the tectorial membrane Bending these hair cells causes depolarization

and neurotransmitter release

Onward to the Brain!

Organ of Corti transmits signals to the cochlear nerve

Medulla Cochlear Nucleus to Superior

Olivary Complex Lateral Lemniscus fiber bundle

carries information to the inferior colliculus

Proceeds to the Medial Geniculate Nucleus of the thalamus and on to the auditory cortex

http://www.neuroreille.com/promenade/english/ptw/zoom1.htm

Auditory Cortex

Tonotopic Organization Different frequencies

of sound are mapped to different regions of the auditory cortex

Extends to the levelof the cochlea

Zhou, X. and M. M. Merzenich (2007). "Intensive training in adults refines A1 representations degraded in an early postnatal critical period." Proceedings of the National Academy of Sciences 104(40): 15935-15940.

Sound Localization Experiment

Auditory Acuity

Brainstorming

What factors might affect hearing? What are some possible causes of hearing

disorders?

Factors in Hearing

One ear vs. Two ears Particularly important for localization

Frequency of Sound Humans can hear sounds btw 20 to 20,000 Hz

Age Frequency range narrows with age

Competing sounds Wax or fluid build up

Why Are 2 Ears Better Than 1?

Sound arrives at different times to each ear (unless its directly ahead of us).

This phase difference is translated to the brain, where some neurons respond to sounds 90° out of phase; others respond to 180° out of phase, etc.

McAlpine, D. (2005). "Creating a sense of auditory space." The Journal of Physiology 566(1): 21-28.

Hearing Disorders

Otosclerosis Tinnitus Presbycusis Auditory

Processing Disorder

Ménière's Disease

Otosclerosis

Abnormal growth of the ossicles

Usually affects the stapes Causes conductive

hearing loss Sometimes can cause a

sensorineural hearing loss that damages sensory cells or nerve fibers

http://www.marshfieldclinic.org/patients/?page=ent_ear_otosclerosis

Tinnitus

Persistent ringing in the ears

May result from the brain’s attempt to adapt to inner ear damage by “turning up” the auditory system

Increase in unilateral brain activity on PET

http://www.newyorker.com/reporting/2009/02/09/090209fa_fact_groopman

Presbycusis

Age-related hearing loss Particularly sensitive to high-pitch

Sensorineural hearing disorder Damage to the sensory hair cells or cochlear

nerve May be due to decreased blood flow to these

regions

Auditory Processing Disorder

Difficulty paying attention to and understanding speech

Unknown cause

Ménière's Disease

Excess buildup of fluid on cochlea Interferes with ability to transmit sound from

cochlea to auditory cortex

Auditory Acuity Experiment

http://faculty.washington.edu/chudler/hearing.html