Neuroanatomy & Audition June 14, 2011
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
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
Parietal Lobe Frontal Lobe Occipital Lobe Corpus Callosum Temporal Lobe Cerebellum Brain Stem
FrontalLobe 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
Parietal Lobe Parietal Lobe • Found on the top of your head • Contains Sensory Cortex • Involved in: • Touch • Pressure • Temperature • Pain • Spatial Orientation
Temporal 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
Occipital 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 parts
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 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
Corpus Callosum 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
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 Na+ Cl- K+ Outside Inside -70mV Na+ Cl- K+ 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+ 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 -55 mV -70 mV Na+
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+ • 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). Outside Inside +55 mV -75 mV K+
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, 7/2005 These 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 ThroughNeurotransmitters& 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 GABA • Primary inhibitory neurotransmitter • Involved in: • Epilepsy • Depression & Anxiety • Anesthesia Glutamate • Primary excitatory neurotransmitter • Involved in: • Epilepsy • Learning & Memory • Schizophrenia http://www.cnsforum.com/imagebank/item/Neuro_path_GABA/default.aspx http://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
Dopamine Involved in: Drug Abuse Parkinson’s Disease Schizophrenia Acetylcholine Involved in: Muscular movement Nicotine Addiction Alzheimer’s Disease Neurotransmitters: DA & ACh http://www.3dchem.com/molecules.asp?ID=289 http://www.worldofmolecules.com/emotions/acetylcholine.htm
Brain Beliefs June 14, 2011
True or False? • The brain is static, unchanging, and set before you start school. FALSE
True or False • The brain contains more supporting cells (glia) than it does neurons (cells that send signals throughout the brain). TRUE!
True or False? • Prior to birth, a baby gains 250,000 neurons per minute. TRUE!
True or False? • Some people are left-brained and some are right-brained. FALSE