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Chapter 4

PSYC62 - Chapter Four.docx

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Department
Psychology
Course Code
PSYC62H3
Professor
Suzanne Erb

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Chapter Four – Page 1 of 6 Chapter Four: Excitability and Chemical Signaling in Nerve Cells  The nervous system is divided into the CNS (brain and spinal cord) and the PNS (neurons outside the brain and spinal cord)  Neurons are also mixed in with nonneuronal cellular elements called glial cells (ie. Astrocytes, which ensheath synaptic connections between neurons and are required for synapse formation and maintenance, and oligodendrocytes, which wrap layers of myelin membrane around axons to insulate them for impulse conduction, which serve important metabolic and supportive functions  NS – 85 billion neurons  Neurons transduce info  Neurons transmit info by generating electrical charges in one part of the cell, conducting these electrical changes to distant parts of the cell, and then releasing chemical signals on to neighbouring neurons  They’re electrically active  They’re chemically active – release signaling molecules (neurotransmitters) The Neuron  Info typically flows from dendrite to soma to axon  Dendrites – short; axons – long  The enlarged region where the axon emerges from the soma – axon hillock  A short distance from their origin, many axons have a coating called the myelin sheath, which is analogous to the insulation on a wire  Gaps in the myelin sheath, where the axon comes into direct contact with the extracellular fluid, are called the nodes of Ranvier. The presence of these gaps allows for an increase in the rate of conduction down the axon. Near its end, the axon branches, and at the tip of each branch is an enlargement called a terminal (some terminals are actually at the tip of the axon, while in some, they’re strung along the axon like beads; in the latter case, the terminals are also known as varicosities)  Chemicals found within the axon terminal can be released into an exceedingly small gap between the neurons, called a synaptic cleft, allowing the neuron to affect the excitability of adjacent neurons.  The point of functional connection between neurons is called a synapse, and it consists of the presynaptic membrane of the axon terminal, the synaptic cleft, and the postsynaptic membrane of the target neuron  See figure 4.1 Electrical Excitability of Neurons: The Resting Membrane Potential  Under baseline conditions, each neuron is polarized – they have a voltage difference between the inside and the outside of the cell (resting membrane potential)  Voltage difference – inside of the cell is negative relative to the outside (-70mV)  What generates the resting membrane potential? Several important factors: Chapter Four – Page 2 of 6 o The electrical characteristics of ions o The physical forces that drive the movement of ions o The characteristics of the nerve cell membrane, including both the lipid and protein components  Any positive ion – cation  Any negative ion – anion  Two main physical forces that drive the movement of ions and other molecules: o Movement along a concentration gradient (ie. Diffusion) refers to the fact that molecules move from area of high concentration to an area of low concentration o Electrical gradient refers to the fact that ions can be driven by electrical forces because like charges repel and opposite charges attract  The nerve cell membrane consists of a phospholipid bilayer with embedded proteins. The lipid portion of the bilyar acts as a barrier to the movement of ions or polar substances. The protein components can include enzymes, receptors, channels, and transportesr  Enzymes are biological catalysts that promote biochemical reactions, including neurotransmitter synthesis and metabolism  Receptor proteins bind to NTs, essentially acting as the initial detection device for the presence of the transmitter  Channels are proteins that act as gates that can be opened or closed o When opened, channels allow for the passage of ions through the membrane o Channels are defined in terms of what ions they let through and their gating mechanism (what opens/closes them)  There are chemically gated channels, which are linked to receptors, and voltage gated channels, which are opened/closed based upon the voltage conditions of the local portion of the membrane  Transport proteins act as pumps that move substances across the membrane  See Figure 4.3  Very important transporter  Na+/K+ pump o Critical for establishing resting membrane potential o Pumps Na+ ions inside the cell, and K+ outside  In doing so, it utilizes a significant fraction of all the energy spent by the brain and represents a substantial energy input into the system that establishes the resting potential  Under baseline or resting conditions: o Cl- and K+ channels are mostly open o Na+ channels are virtually all closed o Na+/K+ pump actively transports some K+ into the cell but transports more Na+ out of the cell Chapter Four – Page 3 of 6  These conditions lead to the generation of the resting membrane potential. It’s negative on the inside because positively charged Na+ ions are pumped out, and these ions aren’t allowed back in because the Na+ channels are closed  Under resting conditions, the membrane is impermeable to sodium ions  The exact voltage difference is determined by the response of K+ and Cl- ions to these voltage conditions; that is, these ions can move back and forth across the membrane because their channels are open, and they move in such a way as to approach the equilibrium (or balance point) between the concentration and electrical gradient forces acting on them  The current of Na+ ions flows through when a “valve” is opened; in a neural membrane, any channel that allows Na+ to pass through will, when opened, allow an inward Na+ current to flow. This is how neurons change from the polarized resting or baseline state to become excited or depolarized Electrical Excitability of Neurons: Excitation, Inhibition, and the Action Potential  Three different types of electrical phenomena that are commonly recorded from nerve cells: o Excitatory postsynaptic potentials (EPSPs) o Inhibitory postsynaptic potentials (IPSPs) o Action potentials (spikes/neuronal firing) – refers to voltage change  EPSP – small transient change in the positive direction  IPSP – small change in the negative direction – hyperpolarized  Action potential – rapid and dramatic movement in the positive direction, followed by a rapid restoration of te resting potential  EPSPs and IPSPs are propagated in a graded and decremental fashion. They originate at one point and move outward in all directions across the surface of the membrane. They are said to be graded because they can vary in size, depending upon the magnitude of the stimulus  EPSPs and IPSPs convey info about the magnitude of the chemical signal that a neuron is receiving also, EPSPs and IPSPs are decremental because they diminish in size as they travel out from the original point of stimulation  Action potentials aren’t graded or decremental o When the level of excitation is enough to cross the threshold , an action potential is triggered, and action potentials are considered to be “all-or-none”; they either occur or they don’t, and when they occur, they don’t generally convey info based upon their size, action potentials encode info in terms of their frequency and overall pattern of activity o Once generated, they can travel down the axon o Speed determined by two main factors:  The diameter of the axon (larger = faster)  Whether or not it’s myelinated (myelinated = faster)  GLU is the most common excitatory transmitter, while GABA is the most common inhibitory transmitter  GLU release can induce excitation by binding to the NMDA receptor o The NMDA receptor is a GLU receptor linked to a cation channel Chapter Four – Page 4 of 6 o When GLU molecules bind to the binding site, it instigates the opening of the cation channel; positive ions flow through, and because one of the ions that
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