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Psychology 304: Brain and Behaviour Lecture 11 1 The Cells of the Nervous System and The Generation of Electrochemical Neural Signals 1. What are glial cells? (continued) 2. What is the neuron’s resting potential? 3. What causes a neuron to produce an action potential? 2 By the end of today’s class, you should be able to: 1. describe the neurological basis of multiple sclerosis. 2. explain how the resting potential of a neuron is maintained. 3. distinguish between EPSPs, IPSPs, and action potentials. 4. describe the electrochemical changes that trigger an action potential. 3 From last class ….. 4 Structure of Astrocytes 5 What are glial cells? (continued) • Glial cells have been implicated in multiple sclerosis: 6 Multiple Sclerosis 7 What is the neuron’s resting potential? • A neuron’s membrane potential refers to the difference in electrical charge between the inside and the outside of the cell. • The membrane potential of a resting neuron is about -70 mV (-50 to -80 mV). Thus, the resting neuron is “polarized.” 8 • Resting neurons are polarized due to the distribution of ions around the neuron’s membrane. • Sodium ions (Na+), potassium ions (K+), chloride ions (Cl-) and negatively charged protein ions are distributed unevenly across the neuron’s membrane. • The ratio of negative to positive charges is greater inside the resting neuron than outside. 9 The Resting Neuron 10 • Two processes maintain the unequal distribution of ions across the membrane of resting neurons: 1. The differential permeability of the membrane to ions (most permeable to K+ and Cl-; least permeable to negatively charged protein ions). 2. The action of sodium-potassium pumps (continually exchange three Na+ ions inside the neuron for two K+ ions outside of the neuron). 11 A Sodium-Potassium Pump in a Neuron Membrane 12 What causes a neuron to produce an action potential? • A neuron produces an action potential or “fires” when it generates and conducts an electrochemical signal. • A neuron receives electrochemical signals from thousands of adjacent neurons, in the form of “synapses” onto the dendrites or cell body of the target neuron. 13 Electron Micrograph of Synaptic Contact 14 • The terminal buttons release chemicals or neurotransmitters that bind to receptors on the dendrites or cell body of the target neuron. • The neurotransmitters can excite or inhibit the target neuron. 15 • Neurotransmitters that excite the target neuron depolarize its membrane, producing excitatory postsynaptic potentials (EPSPs). EPSPs increase the likelihood that the target neuron will fire. • Neurotransmitters that inhibit the target neuron hyperpolarize its membrane, producing inhibitory postsynaptic potentials (IPSPs). IPSPs reduce the likelihood that the target neuron will fire. 16 • The EPSPs and IPSPs are conducted to the axon hillock and integrated. • If the integrated sum of the EPSPs and IPSPs is sufficient to depolarize the membrane to the threshold of activation (-40 to -65mV), an action potential is generated. 17 Neural Integration 18 • An action potential is a momentary reversal of the membrane potential from a highly negative value (e.g., -70mV) to a highly positive value (e.g., +50 mV). 19 The Cells of the Nervous System and The Generation of Electrochemical Neural Signals 1. What are glial cells? (continued) 2. What is the neuron’s resting potential? 3. What causes a neuron to produce an action potential? 20