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Neuro I Or: What makes me do that Voodoo that I Do so Well! Neurons and More Neurons The root of it all…... The Brain Responsible for all behavior Sensation – Sensory (Afferent) Neurons Movement – Motor (Efferent) Neurons Integration of info – Interneurons The Brain Donald Hebb Proposed that the brain is not merely a mass of tissue – but a highly integrated series of structures that perform specific functions cell assemblies Cell Assemblies Groups of connected neurons that perform certain functions Cell Assemblies: The Neuron A specialized cell that receives, processes and/or transmits information – Modulatory Characteristics Modulatory Characteristics Depolarize – Make a neighbor more likely to be active – Make it less receptive to a signal (NT) Hyperpolarize – Make a neighbor less likely to fire Change the dynamics of a receptor Affect synthesis, movement and release of NT to another neuron Moduation Neuronal Structure Spinal Motor Neuron Variations on a Theme Basket Cell (Cerebellum) Golgi Type II (Cortex) Sensory Neurons Bipolar (Vision) Unipolar (Pain/Touch) Neuronal Structure Spinal Motor Neuron Soma Contains the nucleus and machinery – Life Processes Neuronal Structure: Dendrites Spinal Motor Neuron Dendrites (Tree) Highly Aborized Receive “messages” from other neurons – Some have dendritic “spines” Input sites – Separated from neighbor by a synapse (space) Caveat: They can transmit signals as well Dendritic Spines Neuronal Structure: Axon Spinal Motor Neuron The Axon Tube-like structure – Micrometers to meters – Covered by the “Myelin Sheath” Axon The Axon Tube-like structure – Carries a signal from the soma to the terminal buttons Axon Signal = Action Potential (AP) (electrical/chemical event) Myelin Sheath Myelin Sheath Surrounds many (but not all) axons Formed by Oligodendrocytes (CNS) and Schwann Cells (PNS) There are gaps between adjacent cells – Several micrometers – Called “Nodes of Ranvier” – Internode region Neuronal Structure: Terminal Buttons Spinal Motor Neuron Terminal Buttons Found at the end of the axon – When an AP reaches the terminal Release chemical into the synapse – Neurotransmitter (NT) Neurotransmitters This Info can be excitatory or inhibitory to a neighboring neuron Cell Assemblies Signaling in the Neuron Electrical Potentials Most work done with the Giant Squid Axon – Neurons work by electrical and chemical activity Electrical Potential Inside is more negative than the outside -70 mv Membrane resting potential Ions Molecules that have given up or taken on an electron – Gives the molecule a charge – Some move more readily across the membrane then others Dependent on circumstances Ion Distribution Ion Concentrations ION INSIDE OUTSIDE RATIO K+ 400 10 40:1 Na++ 50 460 1:9 Cl- 40 540 1:13 A- 400 ------ ------ Ca++ 0.4 10 1:25 The number is not as important as the ratio Ion Concentration More positive charge on the outside then on the inside of the neuron The Active Neuron The Action Potential (AP) Its hard to know what’s going on Difficult to isolate ions – Everything is occurring at once – The charge is changing Impacts ion movement Reaching Threshold Excitatory Input (Depolarization) – Causes the influx of positive ions (Na+) into the cell by opening Na+ channels Voltage gated channels – Great variety in threshold level – If enough positive charge comes in The threshold is reached – More NA+ channels open – Making the cell more positive – All or none Caveat Takes many excitatory inputs to reach thresholds – Temporal summation – Spatial summation Repolarization After time – The Na+ channels automatically close – K+ channels begin to open K+ leaves the cell carrying with it the positive charge – Repolarization Overshoot Too much K+ leaves causing the cell to be hyperpolarized Back to Resting State The Na+/K+ pump restores the normal ion concentrations and distributions Axonal Conduction This measurement takes place at one point on the giant squid axon – The signal must travel distances to reach its destination Signal Decrement Weak depolarization = loss of signal AP Propagation Strong depolarization = strong signal Neuronal Structure AXON HILLOCK Spinal Motor Neuron Axon Hillock Has a high concentration of low threshold Na+ Channels – Very sensitive to changes in ion movement – Activation results in a autocataclysmic response All Or none Neuronal Structure AXON HILLOCK Spinal Motor Neuron Myelin Sheath Act as an insulator – Prevents things from moving in and out of the cell Including Ions Oligodendrocytes Nodes of Ranvier Nodes of Ranvier Gaps in the sheath High concentration of Na+ channels – Reenergizes the signal so it can reach the axon terminal Neuron: Axon Terminal Axon Terminal: Synaptic Vesicles Synaptic Transmission Cell Assemblies Synaptic Transmission: Caveat In conclusion: Neurons are good. They excite or inhibit. They produce 1 neurotransmitter (in mammals). Transmission is essential. Neuromodulators can change everything (more on that later)