neuroanatomy & audition
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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. - PowerPoint PPT PresentationTRANSCRIPT
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
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/
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
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
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
True or False
The brain contains more supporting cells (glia) than it does neurons (cells that send signals throughout the brain).
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
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.
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.
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