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Neurons, Synapses, & Signaling Campbell and Reece Chapter 48
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Neurons nerve cells that transmit information within the body communication between neurons consists of: ◦ long distance electrical signals ◦ short distance chemical signals
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Neurons use pulses of electrical current to receive transmit regulate the flow of information over long distances w/in the body
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Neuron Organization
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Nervous System
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Types of Neurons Sensory Neurons ◦ transmit information (senses) from body brain ◦ are afferent ◦ specialized dendrites that initiate action potential when stimulated
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Types of Neurons 2. Motor Neurons transmit signals to muscle fibers & glands are efferent
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Types of Neurons 3. Interneurons majority of neurons in brain ◦ form local circuits connecting neurons
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Synapse junction between axon terminal & next cell (another neuron, muscle fiber, gland cell) neurotransmitters are chemical messengers released @ most synapses that pass action potential to receiving cell
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Synapse presynaptic cell: cell releasing neurotransmitter & passing on action potential postsynaptic cell: receiving neurotransmitter synaptic cleft: physical space between the 2; neurotransmitter released into this space & diffuses across it attaching to receptors on postsynaptic cell
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Synapse
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Glial Cells cells that support neurons Greek: glue aka neuroglia nourish neurons insulate axons regulate ECF surrounding neurons
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Ion Pumps ions unequally distributed across plasma membrane inside of cell slightly (-) compared to outside cell source of potential nrg called the membrane potential resting potential : the membrane potential of neuron @ rest = -60 to –80 mV
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Resting Potential
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Formation of Resting Potential Na+/K+ pump generates & maintains the ionic gradients of membrane potential 1 turn of pump ◦ 1 ATP ◦ 3 Na+ out ◦ 2 K+ in
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Membrane Potential
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Ion Channels pores that span the membrane allowing ions to diffuse across (in or out) membranes are selectively permeable and variations in how easily any particular ion can cross a membrane depends on the # of channels & how often they are open
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Types of Ion Channels
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Action Potentials neurons have gated ion channels that open or close in response to stimuli ◦ open/close changes permeability for that ion neurons have K+ channels ◦ when open K+ diffuses out of cell ◦ changes resting potential from: -60 mV to -90 mV
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K+ Ion Channels in Neurons
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Resting & Action Potentials http://bcs.whfreeman.com/thelifewire/conten t/chp44/4401s.swf http://bcs.whfreeman.com/thelifewire/conten t/chp44/4401s.swf http://www.dnatube.com/video/1105/Unders tanding-Action-Potential-and-Nerve-Impulses http://www.dnatube.com/video/1105/Unders tanding-Action-Potential-and-Nerve-Impulses
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Hyperpolarization when K+ channels open & resting potential decreases to -90 mV inside of cell becoming more (-) than normal resting potential called: hyperpolarization
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Depolarization when Na+ ion channels open Na+ diffuse into cell making inside less (-) compared to outside cell membrane potential shifts toward (+) mv this reduction in magnitude of membrane potential called depolarization
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Graded Potentials any shift in membrane potential magnitude of shift varies with strength of stimulus induce a small electrical current that flows along the membrane leaking out of the cell so only lasts short distance from source
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Action Potential electrical signal that propagates along the membrane of a neuron as a nongraded (all or nothing) depolarization have a constant magnitude & can regenerate in adjacent regions of the membrane travel long distances
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Voltage-Gated Ion Channels ion channels that open/close based on membrane potential passing a particular level Na+ channels in neurons are voltage gated: open when depolarization occurs Na+ diffuses into cell becomes more depolarized more Na+ channels open (+ feedback)
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http://outreach.mcb.harvard.edu/an imations/actionpotential_short.swf http://outreach.mcb.harvard.edu/an imations/actionpotential_short.swf http://highered.mcgraw- hill.com/sites/0072495855/student_vie w0/chapter14/animation__the_nerve_im pulse.html http://highered.mcgraw- hill.com/sites/0072495855/student_vie w0/chapter14/animation__the_nerve_im pulse.html Interactive site to try at home:
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Threshold Action potentials occur when a depolarization increases the membrane voltage to a particular value (the threshold) for mammals the threshold is a membrane potential ~ -55mV once started the action potential has a magnitude independent of the strength of triggering stimulus
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+ feedback loop of depolarization & channel opening triggers an action potential whenever the membrane potential reached the threshold membrane depolarization opens both Na+ & K+ channels but Na+ opens faster initiating the action potential Na+ channels become inactivated as action potential proceeds (gates close) & remain so until after membrane returns to resting potential
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Refractory Period (-) membrane potential restored by inactivation of Na+ channels, which increases K+ outflow This is followed by a refractory period: ◦ no matter how strong the stimulus to initiate next action potential is cannot initiate one during refractory period
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Conduction of Action Potentials
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Myelin Sheaths glial cells oligodendrocytes (CNS) and Schwann cells (PNS) form layers of electrical insulation along length of axons
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Saltatory Conduction
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Neurotransmitters >100 neurotransmitters belonging to 5 groups: 1. Acetylcholine 2. Amino Acids 3. Biogenic Amines 4. Neuropeptides 5. Gases
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