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Topic 6.5 Neurons and Synapses 2016 Parts of the Nervous System • Central nervous system (CNS) – Brain and spinal cord • Both contain fluid-filled spaces which contain cerebrospinal fluid (CSF). – The central canal of the spinal cord is continuous with the ventricles of the brain. – White matter is composed of bundles of myelinated axons – Gray matter consists of unmyelinated axons, nuclei, and dendrites. • Peripheral nervous system (PNS) – Everything outside the CNS. Nervous System: https://www.youtube.com/watch?v=x4PPZCLnVkA Einstein’s Brain: https://www.youtube.com/watch?v=rnlE9q5IEuI Do we really use only 10% of our brain: https://www.youtube.com/watch?v=JDF2rjFd8hY Peripheral Nervous System (PNS) Neuron Anatomy Neuron Anatomy Membrane Potential • Membrane Potential: a term used to illustrate there is an electrical potential difference between the inside of the cell and the surrounding extracellular fluid. • -70 mV is the resting membrane potential of a neuron, which means that the inside of the cell is negative compared to the outside. Membrane Potential: http://www.sumanasinc.com/webcontent/animations/content/electricalsignaling.html Normal Levels • Sodium-Potassium Pump: is used in establishing the membrane potential in neurons (1) it makes the [Na] high in the extracellular space and low in the intracellular space (2) it makes the [K+] high in the intracellular space and low in the extracellular space (3) it creates a negative voltage in the intracellular space compared to the extracellular space. Hyperpolarization • Gated K+ channels open K+ diffuses out of the cell the membrane potential becomes more negative Depolarization • Gated Na+ channels open Na+ diffuses into the cell the membrane potential becomes less negative Action Potential • Action Potential: All or Nothing Depolarization • If graded potentials sum to -55mV a threshold potential is achieved. • This triggers an action potential. • Axons only Action Potential: http://www.sumanasinc.com/webcontent/animations/content/action_potential.html Action Potential w/ Graph: http://bcs.whfreeman.com/thelifewire/content/chp44/4402002.html Action Potential Diagram • Step 1: Resting State. Fig. 48.9 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Step 2: Threshold. Fig. 48.9 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Step 3: Depolarization phase of the action potential. Fig. 48.9 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • Step 4: Repolarizing phase of the action potential. Fig. 48.9 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Moving Potential Saltatory Conduction • In myelinated neurons, only unmyelinated regions of the axon depolarize. Signal jumps from one node to the next, making the impulse travel 100x faster than on a unmyelinated neurons. Khan Academy: https://www.khanacademy.org/science/biology/human-biology/neuron-nervous-system/v/saltatory-conduction-neurons Saltatory Conduction: http://wps.aw.com/bc_goodenough_boh_3/104/26721/6840613.cw/content/index.html Fig. 48.11 Synapses • Electrical Synapses. – Action potentials travels directly from the presynaptic to the postsynaptic cells via gap junctions. Chemical Synapses • More common than electrical synapses. • Postsynaptic chemically-gated channels exist for ions such as Na+, K+, and Cl-. • Depending on which gates open the postsynaptic neuron can depolarize or hyperpolarize. Fig. 48.12 Routes of Nerve Transmission