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Chapter 4

Behavioural Neuro Psych Ch.4

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PSYC 3030

Chapter 4 11/30/2013 4:56:00 PM Membrane potential is the difference in electrical charge between the inside and outside of a cell. Recording Membrane Potential Must position the tip of one electrode inside the neuron and the tip of another electrode outside the neuron in the extracellular fluid. Intracellular electrode must be thin and precise: Microelectrode Resting Membrane Potential  If both electrodes are in the ECF: Potential= 0mV If electrode is in the intracellular neuron:Potential= -70mV KNOWN as Resting Potential of a Neuron Ionic Basis of the Resting Potential Salts in neural tissues separate into positive and negative ions Potential is a result of an IMBALANCE between +ve / -ve ions. This occurs because… 1) Random Motion: ions in neural tissues are always in constant motion 2) Concentration Gradient: particles in random motion tend to become evenly distributed because they move DOWN concentration gradients. (Normally). HIGHLOW 3) Electrostatic Pressure: accumulation of charges in one area tends to be dispersed by the repulsion of like charges and attraction of opposite charges. 4 IONS Sodium (Greater OUTSIDE neuron) Chlorine (Greater OUTSIDE neuron) Potassium (Greater INSIDE neuron) Negatively Charged Proteins (Tends to not leave the inside) Unequal distribution of ions occurs from two properties 1)Passive: No energy needed 2)Active: Energy is required Influx Inflow Efflux Outflow K+/Cl- pass readily Na+ pass but with difficulty Proteins do not pass at all Na/K Pump allows for homeostasis of the neuron along with action potentials. 3 Na OUT 2 K IN Generation & Conduction of Post-Synaptic Potentials  when a neuron fires, they release chemicals from the terminal bouton called neurotransmitters. these cross synaptic cleft and interact with specialized receptors When these molecules bind they either  DEPOLARIZE (decrease membrane potential)  HYPERPOLARIZE (increase membrane potential) Post-synaptic depolarizations are called excitatory post-synaptic potentials (EPSPs). INCREASE likelihood of firing Post-synaptic hyperpolarizations are called inhibitory post-synaptic potentials (IPSPs). DECREASE likelihood of firing. Both graded responses because the amplitudes of EPSPs/IPSPs are proportional to their intensities  WEAK SIGNALSSMALL POTENTIAL  STRONG SIGNALSLARGE POTENTIAL ACTION POTENTIALS action potentials are generated ADJACENT to the axon hillock If the SUM of the depolarizations and hyperpolarizations reaching this section at any time is sufficient to DEPOLARIZE the membrane.  Threshold of excitation -65mV this action potential is STRONG but MOMENTARY  all or none response  They occur to their Full Extent or Not at All Adding all Potentials is known as integration  Spatial Summation: o EPSPs produced simultaneously= GREATER EPSPs o IPSPs produced simultaneously= GREATER IPSPs o EPSPs + IPSPs simultaneously= CANCELLED OUT  Temporal Summation o Shows how post-synaptic potentials produced in rapid succession at the same synapse sum to form a greater signal. o Ex. One fires, before it dies off it fires again. Makes a stronger signal Conduction of Action Potentials Voltage-activated ion channels ope/close in the response to changes in the level of the membrane potential. Refractory Period There is a brief period about 1-2 milliseconds after the initiation of an action potential during which it is impossible to elicit a second one  ABSOLUTE Refractory Period This is followed by the RELATIVE Refractory Period  The period during which it is possible to fire a neuron again, but only by firing with a HIGHER stimulation then the one previous. 1) Responsible for the fact that action potentials normally travel along axons in only ONE DIRECTION. CANNOT reverse direction, allowing a refractory period. 2) Refractory period is responsible for the fact that the rate of neural firing is related to the intensity of the stimulation.  If a neuron is subjected to a high level of continual stimulation, it fires and then fires again after the absolute period.  If a neuron is subjected to a level just sufficient enough to fire, it won’t fire again until the absolute AND relative refractory periods are over. The conduction of action potentials along an axon is NONdecremental; Aps do not grow weaker as they travel along the axonal membrane they are SELF-PROPOGATING. Action potentials are conducted more slowly than post-synaptic potentials  This is due to the fact that EPSPs/IPSPs are PASSIVE and action potentials are ACTIVE. Antidromic Conduction: If electrical stimulation of sufficient intensity is applied to the terminal end of the axon, an AP will be generated and will travel along the axon back to the cell body. Orthodromic Conduction: Axonal conduction in the NATURAL direction – Cell BodyTerminal Boutons Conduction in Myelinated Axons axons of many neurons are insulated from ECF by fatty tissues = MYELIN In myelinated axons, ions pass through the axonal membrane only at NODES OF RANVIER.  The transmission of action potentials in myelinated axons is called SALTATORY CONDUCTION. (Jumping) LARGER myelinated axons conduct at faster speeds. SMALLER myelinated axons conduct at slower speeds. Internuerons do not have axons, conduction in these is typically passive and DECREMENTAL. Hodgkin-Huxley Model Many cerebral neurons fire continuously even when they receive no input. Axons of some cerebral neurons can actively conduct both graded and action potentials. Many cerebral neurons have NO axons and do not display action potentials. The dendrites of some cerebral neurons can actively conduct action potentials.. CEREBRAL NEURONS= MORE COMPLEX THAN MOTOR NEURONS Chapter 4 11/30/2013 4:56:00 PM Structure of Synapses Axodendritic synapses of axon terminal boutons to dendrites. Many axodendritic synapses terminate on dendritic spines. Axosomatic Synapses of axon terminal boutons on somas (cell bodies). Dendrodendritic Capable of transmission in either direction. Axoaxonic can mediate presynaptic facilitation and inhibition. Directed Synapses: synapses at which the site of neurotransmitter release and the site of neurotransmitters reception is in CLOSE PROXIMITY NON-Directed Synapses: are synapses at which the site of release is at some distance from the site of reception.  In this type of arrangement, neurotransmitter molecules are released from a series of varicosities along the axon and its branches and thus are widely dispersed to surrounding targets. Because of their appearance, they are reffered to as string-of- beads synapses. Synthesis, Packaging and Transport of Neurotransmitters Small Molecules: synthesized in cytoplasm, packaged in synaptic vesicles by Golgi. Large Molecules: (Neuropeptides) Short amino acid chains (~3) Many neurotransmitters can produce 2 different effects on different receptors. This is known as coexistence. Exocytosis: the process of neurotransmitter s release. 
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