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Chapter 4

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University of Toronto Scarborough
Suzanne Erb

Chapter 4- Excitability and Chemical Signaling in Nerve Cells  CNS (utilizing 20%--adults, 60%--infants of resting E consumption)  PNS  Non-neuronal cellular elements – glial cells - astrocytes (synaptic connection between neurons and required for synapse formation and maintenance) - oligodendrocytes (wrap layers of myelin around axons for insulation and conduction)  Neurons transduce information from physical/chemical environment and they convert one form of E into another, and transmit information typically by generating electrical changes in one part of the cell and releasing chemical signals into neighboring cells  Neurons are electrically active, chemically active (release of NT)  Common NT: glutamate, GABA, dopamine, norepinephrine, serotonin  Neuron o Soma (integration centre and communication) o Dendrites (receivers of info – usually many extending from one neuron) o Axons (relatively long, only one per neuron, transmits info) - axon hillock (enlarged region where the axon emerges from soma) - myelin sheath coating - gaps in myelin = nodes of Ranvier, axon comes into direct contact with ECF, increase in the rate of conduction - axon terminal = varicosities - chemicals found in varicosities can be released into synaptic cleft (excitability) o Synapse (functional connection between neurons), has presynaptic membrane of axon terminal, the synaptic cleft, and postsynaptic membrane of the “target” neuron o Neurons are electrically active, polarized even at baseline, the voltage difference between inside and outside of cell is the resting membrane potential (-70 mV) o Resting membrane potential is based on (a) Electrical characteristics of ions (b) Physical forces that drive ion movement (c) Nerve cell membrane composition (lipid, protein components) -phospholipid bilayer with imbedded proteins -protein components = enzymes, receptors, channels, transporters -channels are defined in terms of what ions are let through, gating mechanism (chemically gated, voltage gated) -transporters for various transmitters ex. Na+/K+ dependent pump (3 Na+ Ions inside for 2 K+ outside) o Under baseline/resting conditions: -Cl- and K+ channels are mostly open - Na+ channels are virtually all closed - Na+/K+ pump actively transports more Na+ out of the cell than K+ transported in o Resting/baseline condition is electrically unstable because anything that increases the permability of Na+ would allow make the inside of the membrane more positive = depolarized o Exact voltage difference is achieved by K/Cl responses to voltage conditions Electrical, Excitability of Neurons: Excitation, Inhibition, and the Action Potential  EPSPs (excitatory postsynaptic potentials)  IPSPs (inhibitory postsynaptic potentials) - cell becomes more negative, hyperpolarized  Action potential (spikes/neuronal firing) - not graded/decremental - all or none - when level of excitation is great enough to cross the threshold, an AP is triggered - maintain size as they travel along the axonal membrane - encode information in terms of frequency (how many are fired per unit of time) and overall pattern (e.g., bursts or single firings) of activity - speed of AP related to diameter of axon, and whether it is myelinated/not  EPSPs and IPSPs are propagated in a graded and decremental fashion, they originate at one point at a synapse(s) and move outwards in all directions acorss the membrane surface; they are graded because they can vary in size, depending upon the magnitude of the stimulus, convey the magnitude of the chemical signal, diminish in size as they travel  GLU most common excitatory NT - Binding to NMDA receptor, linked to cation channel which allows positive flow and an increase in Na+ permeability – resulting in inner membrane being more positive and is recorded as EPSP  GABA most common inhibitory NT - binds to receptor subtype GABA(a) linked to Cl- channel, inward (-) flow and hyperpolarization that leads to IPSP  It is the mechanism that the receptor is linked to and not the molecule that binds to the receptor that makes it excitatory vs. inhibitory  Action potential - ascending limb (when voltage shoots up in the positive direction) is instigated because of the opening of Na+ channels – influx of Na+ inner membrane = depolarization - influx of positive inner membrane makes K+ efflux (drives membrane potential to resting potential) = descending limb
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