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Chapter 4

psy290-chapter 4 summary

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Chapter 4 4.1) Resting membrane potential - the Membrane potential is the difference in electrialcharge between the inside and the outside of the cell % Recording the membrane potential - To record a neuron s membrane potential, it is necessary to position the tip of one electrode inside the neuron and the tip of another electrode outside the neuron in the extracellular fluid. -The intracellular electrodes are called microelectrodes; their tips are less than one-thousandth of a millimeter in diameter % Resting Membrane potentail - This steady membrane potential of about 70 mV is called the neuron s resting potential. In its resting state, with the 70 mV charge built up across its membrane, a neuron is said to be polarized. % Ionic basis of the resing potentail - The first of the two homogenizing factors is random motion. The ions in neural tissue are in constant random motion, and particles in random motion tend to become evenly distributed because they are more likely to move down their concentration gradients than up them; that is, they are more likely to move from areas of high concentration to areas of low concentration than vice versa. -The second factor that promotes the even distribution of ions is electrostatic pressure. Any accumulation of charges, positive or negative, in one area tends to be dispersed by the repulsion among the like charges in the vicinity and the attraction of opposite charges concentrated elsewhere 4.2) Generation and Conduction of Postsynaptic potentials - When neurotransmitter molecules bind to postsynaptic receptors, they typically have one of two effects, depending on the structure of both the neurotransmitter and the receptor in question. - They may depolarize the receptive membrane (decrease the resting membrane potential, from 70 to 67 mV, for example) or they may hyperpolarize it (increase the resting membrane potential, from 70 to 72 mV, for example) - Postsynaptic depolarizations are called excitatory postsynaptic potentials (EPSPs) because, as you will soon learn, they increase the likelihood that the neuron will fire. Postsynaptic hyperpolarizations are called inhibitory postsynaptic potentials (IPSPs) because they decrease the likelihood that the neuron will fire. Both EPSPs and IPSPs are graded responses. This means that the amplitudes of EPSPs and IPSPs are proportional to the intensity of the signals that elicit them: Weak signals elicit small postsynaptic potentials, and strong signals elicit large ones. - EPSPs and IPSPs travel passively from their sites of generation at synapses, usually on the dendrites or cell body: First, it is rapid so rapid that it can be assumed to be instantaneous for most purposes. Second, the transmission of EPSPs and IPSPs is decremental: EPSPs and IPSPs decrease in amplitude as they travel through the neuron 4.3) Integration of postsynaptic potential and generation of action potentials - The graded EPSPs and IPSPs created by the action of neurotransmitters at particular receptive sites on a neuron s membrane are conducted instantly and decrementally to the axon hillock. If the sum of the depolarizations and hyperpolarizations reaching the section of the axon adjacent to the axon hillock at any time is sufficient to depolarize the membrane to a level referred to as its threshold of excitation usually about 65 mV an action potential is generated near the axon hillock. The action potential (AP) is a massive but momentary lasting for 1 millisecond reversal of the membrane potential from about 70 to about *50 mV. - they are all-or-none responses; that is, they either occur to their full extent or do not occur at all. - spatial summation. It shows how local EPSPs that are produced simultaneously on different parts of the receptive membrane sum to form a greater EPSP, how simultaneous IPSPs sum to form a greater IPSP, and how simultaneous EPSPs and IPSPs sum to cancel each other out. - temporal summation. It shows how postsynaptic potentials produced in rapid succession at the same synapse sum to form a greater signal. The reason that stimulations of a neuron can add together over time is that the postsynaptic potentials they produce often outlast them. Thus, if a particular synapse is activated and then activated again before the original postsynaptic potential has completely dissipated, the effect of the second stimulus will be superimposed on the lingering postsynaptic potential produced by the first 4.4) Conduction ofAction Potentials % Ionic basis of actioin potential -voltage-activated ion channels ion channels that open or close in response to changes in the level of the membrane potential % Refractory periods - There is a brief period of about 1 to 2 milliseconds after the initiation of an action potential during which it is impossible to elicit a second one. This period is called the absolute refractory period. The absolute refractory period is followed by the relative refractory period the period during which it is possible to fire the neuron again, but only by applying higher-than-normal levels of stimulation. The end of the relative refractory period is the point at which the amount of stimulation necessary to fire a neuron returns to baseline. -The refractory period is responsible for two important characteristics of neural activity. First, it is responsible for the fact that action potentials normally travel along axons in only one direction. Because the portions of an axon over which an action potential has just traveled are left momentarily refractory, an action potential cannot reverse direction. Second, the 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 as soon as its absolute refractory period is over % Axonal conduction ofAction Potentials - First, the conduction of action potentials along an axon is nondecremental; action potentials do not grow weaker as they travel along the axonal membrane. Second, action potentials are conducted more slowly than postsynaptic potentials.: The reason for these two differences is that the conduction of EPSPs and IPSPs is passive, whereas the axonal conduction of action potentials is largely active. - The wave of excitation triggered by the generation of an action potential near the axon hillock always spreads passively back through the cell body and dendrites of the neuron. - If electrical stimulation of sufficient intensity is applied to the terminal end of an axon, an action potential will be generated and will travel along the axon back to the cell body; this is called antidromic conduction. Axonal conduction in the natural direction from cell body to terminal buttons is called orthodromic conductio - 1) psp are elcited ont eh cell body and dendrites -2) psps are conducted decrementally to the axon -3) when the summated psps exceed the threshold of excitation at the axon, anAPis triggered -4) theAPis conducted nonecrementally down the axon to the terminal button -5) arrival of theAPat the terminal button triggers excytosis % Conduction in Myelinated Axons: -In myelinated axons, ions can pass through the axonal membrane only at the nodes of Ranvier the gaps between adjacent myelin segments. - The transmission of action potentials in myelinated axons is called saltatory conduction (saltare means to skip or jump ). Given the important role of myelin in neural conduction, it is hardly surprising that the neurodegenerative diseases (diseases that damage the nervous system) that attack myelin have devastating effects on neural activity and behavior % Velocity of Axonal conduction -Conduction is faster in large-diameter axons, and as you have just learned it is faster in those that are myelinated.Mammalian motor neurons (neurons that synapse on skeletal muscles) are large and myelinated; thus, some can conduct at speeds of 100 meters per second (about 224 miles per hour). In contrast, small, unmyelinated axons conduct action potentials at about 1 meter per second. - 100 meters per second in cats and was then assumed to be the same in humans: It is not. The maximum velocity of conduction in human motor neurons is about 60 meters per second % Conduction in neurons withoutAxons - interneurons % The Hodgkin-Huxley model in perspective - The Hodgkin-Huxley model was based on the study of squid motor neurons. Motor neurons are simple, large, and readily accessible in the PNS squid motor neurons are particularly large. - The following are some properties of cerebral neurons that are not shared by motor neurons: + Many cerebral neurons fire continually even when they receive no input (Lisman, Raghavachari, & Tsien, 2007; Schultz, 2007; Surmeier,Mercer, & Chan, 2005). + The axons of some cerebral neurons can actively conduct both graded signals and action potentials (Alle & Geiger, 2006, 2008). +Action potentials of all motor neurons are the same, but action potentials of different classes of cerebral neurons vary greatly in duration, amplitude, and frequency (Bean, 2007). + Many cerebral neurons have no axons and do not display action potentials. + The dendrites of some cerebral neurons can actively conduct action potentials - Clearly, cerebral neurons are far more complex than motor neurons, which have traditionally been the focus of neurophysiological research, and thus, results of studies of motor neurons should be applied to the brain with caution 4.5) Synpatic transmission: chemical transmission of signals among neurons - axodendritic synapses synapses of axon terminal buttons on dendrites.Notice that many axodendritic synapses terminate on dendritic spines (nodules of various shapes that are located on the surfaces of many dendrites) - Also common are axosomatic synapses synapses of axon terminal buttons on somas - there are dendrodendritic synapses, which are interesting because they are often capable of transmission in either direction - Axoaxonic synapses are particularly important because they can mediate presynaptic facilitation and inhibition. - an axoaxonic synapse on, or near, a terminal button can selectively facilitate or inhibit the effects of that button on the postsynaptic neuron. The advantage of presynaptic facilitation and inhibition (compared to EPSPs and IPSPs, which you have already learned about) is that they can selectively influence one particular synapse rather than the entire presynaptic neuron. % Release of Neurotransmitter molecuels -When a neuron is at rest, synaptic vesicles that contain small-molecule neurotransmitters tend to congregate near sections of the presynaptic membrane that are particularly rich in voltage-activated calcium channels (see Rizzoli & Betz, 2004, 2005). When stimulated by action potentials, these channels open, and Ca2* ions enter the button. The entry of the Ca2* ions causes synaptic vesicles to fuse with the presynaptic membrane and empty their contents into the synaptic c
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