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Lecture 3

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Suzanne Erb

Lecture 3: Neuronal Conduction and Transmission and Neuroactive Ligands Cell types in the CNS Neurons primary messengers receive information and transmit information primary communicator distinguished from glia by axons neurons are extremely densely packed in the cerebellum (more than in the cerebrum) approximately 90 billion neurons in the central nervous system Glia (e.g., astrocytes) Types of glia: astrocytes, oligodendrocytes, and microglia about 10 to 50 times more glia than neurons important functions : - Migration of neurons - Nutritive functions - Myelin - Blood brain barrier - Communication - Genesis of disease states - the other brain book on glia -Presynaptic cells convey information to postsynaptic cells that receive information The main features of nerve cells -Features of neurons: Soma (body): main metabolic center Dendrites: main apparatus for receiving input Axons: main conducting unit - longer than dendrites - can be almost 1 m in the PNS Axon Hillock: area of soma where axon attaches - Area where all signals converge and cell decides whether to fire an action potential Myelin Sheath: insulating axons - speeds up the rate of action potential Nodes of Ranvier: gaps between myelin sheaths where action potential jump over  Terminal Branches: with terminal endings (or boutons) where the signal is passed to the adjacent cell  Synapse: gap where the first cell’s signal passes to the next cell Two major processes involved in communication between neurons CONDUCTION TRANSMISSION • Changes within neurons to allow information • Changes within one neuron caused by the release of transmission within neurons. chemicals from adjacent neurons. • Electrical - changes in electrical signals • Chemical -chemical cause variable changes • “All-or-none” – must reach a certain • Graded - charges converge to see if the neuron will threshold in order to fire (same speed and fire distance etc. no matter how much electrical transmission) 1. Neuronal conduction Conduction: If enough electrical charge is working to get it to fire - primarily electric (actually electrochemical event) - relative concentrations of the four ions Ions: charged molecules that have to use specialized channels to pass through membranes. -At rest, the charge inside the neuron in is -70 mV more negative than outside Forces maintaining Resting Potential: 1. Selective permeability: the ability of some molecules to pass much more freely through the membrane than others. - When closed, sodium cannot enter the cell because sodium channels are closed. 2. Sodium-potassium pump: transports three sodium ions out of the cell while simultaneously drawing two potassium ions into the cell (3 Na+ out and 2 K+ in), to result in a net movement of positive ions out of the cell. 3. Electrical and concentration gradients for potassium: the differences in positive and negative charges and potassium concentrations, respectively, across the membrane. - At rest, both the electrical gradient and the concentration gradient are almost balanced. So the electrical gradient wins so K+ comes in with its electrical gradient and against the concentration gradient. This contributes specifically to a resting potential at -70 mV. At rest: -If the resting potential reaches -60 mV then the action potential will fire every time. At -60mV: -the Na+ channels open and sodium rushes in with its electrical and concentration gradients At +30 mV: - Na+ channels close -K+ open and rush out with electrical and chemical gradient Refractory Period: - Absolute Refractory Period: No matter how much excitation, no action potential will fire - Relative Refractory Period: if more excitation than normal, then action potential will fire Propagation of an action potential -Propagation of an Action Potential: as the signal moves through the axon (the area where the signal is) Na+ rushes in and when it passes then Na+ channels close and the signal passes on. 2. Neurotransmission -Neurotransmission: when chemicals are released and cause a graded potential • Neurotransmission involves graded postsynaptic potentials. • Postsynaptic potentials can be excitatory (EPSP) or inhibitory (IPSP).  hyperpolarization • A neuron’s excitability is determined by the sum of its EPSP’s and IPSP’s at any given time. If overall Inhibitory Post Synaptic Potential (IPSP) then the cell will not fire If overall Excitatory Post Synaptic Potential (EPSP) then the cell will fire • EPSP’s and IPSP’s are summated through two processes that affect the membrane voltage of the postsynaptic cell: 1. Temporal summation -Many signals coming to the same point one after another -If the signal arrives before the membra
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