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Chapter 4

Chapter 4 textbook notes PSYC2410 Fall -Choleris

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PSYC 2410
Elena Choleris

PSYC2410 – Chapter 4 *to note: images important in textbook are marked by green and yellow stickers! 4.1 Resting Membrane Potential Membrane potential: difference in electrical charge between the inside and the outside of a cell. Recording a membrane potential: to record a neuron’s membrane potential, position the tip of one electrode inside the neuron and the top of another electrode outside the neurone in the extracellular fluid. The intracellular electrodes are called microelectrodes: their tips are less than one-thousandth of a millimeter in diameter. Resting membrane potential: about -70mV in neurons, the neuron is said to be polarized. Ionic Basis of the Resting Potential:  The resting potential results from the fact that the ratio of negative to positive charges is greater inside the neuron than outside. Based on 4 factors: o Random motion: ions are in constant random motion and tend to become evenly distributed as they move down the concentration gradient o 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. o Passive ion distribution: the passive property of the neural membrane hat contributes to the unequal disposition of Na , K and Cl and protein ions is its differential permeability to those ions. + - +  In resting neurons, K and Cl ions pass readily through the neural membrane, Na ions pass through it with difficulty and the negatively charged protein ions do not pass trhough it at all. At rest the unequal distribution of Cl ions across the neural membrane is maintained in equilibrium by the balance between - the tendency for Cl ions to move down their gradient into the neuron and the 7- mV of electrostatic + pressure driving them out. 90 mV is electrostatic pressure is required to keep K ions from moving out of the neuron. And for Na 50 mV of pressure is required to keep them from moving into the neuron, added + to the 70 mV of pressure acting to move them in the same direction; thus 120 mV is acting to force Na ions into resting neurons. o Active ion distribution: + +  Sodium-potassium pumps: move 3 Na out, and bring 2 K in. 4.2 Generation and Conduction of Postsynaptic Potentials When neurotransmitters bind to post-synaptic membrane, have one of 2 effects:  Depolarize: decrease the resting membrane potential o Excitatory postsynaptic potentials (EPSPs): they increase the likelihood that a neuron will fire  Hyperpolarize: increase the resting membrane potential o Inhibitory postsynaptic potentials (IPSPs): decrease the likelihood of a neuron firing Both EPSPs and IPSPs are graded responses: 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. Their transmission is extremely rapid, and can be assumed to be instantaneous for most purposes. Their transmission is Decremental: they decrease in amplitude as they travel through the neuron 4.3 Integration of Postsynaptic Potentials and Generation of Action Potentials  Whether or not a neurone fires depends on the balance between the excitatory and inhibitory signals reaching its axon.  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. PSYC2410 – Chapter 4  If the sum of depolarizations and hyperpolarizations reaching the section of the axon adjacent to the hillock at any time is sufficient to depolarize the membrane to its threshold of excitataion (~ -65 mV) an action potential is generated. o Integration: combining a number of individual signals into one overall signal. Neuron’s integrate signals in 2 ways: over space and over time. o Spatial summation (p.81) o Temporal summation (p.82)  The action potential is a massive but momentary (1 millisecond) reversal of the membrane potential from about -70 mV to +50 mV. Action potentials are not graded responses, all or none, 4.4 Conduction of Actions Potentials Ionic Basis of Action Potentials: action potentials are produce and conducted through the action of voltage-activated ion channels. When the potential is reduced to the threshold potential, the voltage-activated sodium channels in the axon membrane open and Na ions rush in, driving the potential from -70 mV to +50 mV. This rapid influx activates potassium channels. K ions near the membrane are driven out of the cell through these channels, first by their relatively high internal concentration and then when the action potential is near its peark, by the positive internal charge. After about 1 millisecond, the sodium channels close. This marks the end of the rising phase of the action potential and the beginning of repolarization by the continued efflux of K ions. Once + repolarization is achieved, the potassium channels gradually close. This gradual close causes too many K ions to flow out, so the neuron becomes hyperpolarized for a brief time. Action potential only involves those ions close to the membrane, and thus has little effect on the relative concentrations of various ions inside and outside the neurone. Refractory Period: responsible for the fact that action potentials normally travel along axons in only on direction. The refractory period is responsible for the fact that the rate of neural firing is related to the intensity of the stimulation.  Absolute refractory period: 1 to 2 milliseconds after the initiation of an action potential during which it is impossible to elicit a second one.  Relative refractory period: it is possible to fire the neuron again but need higher-than-normal levels of stimulation Axonal Conduction of Action Potentials: conduction of action potentials along axons is nondecremental and are conducted more slowly than EPSPs or IPSPs because those are passive, and action potentials are active (require E). Antidromic conduction: when action potential is stimulated at the terminal buttons and transmitted back to the cell body Orthodromic conduction: conduction from cell body to axon terminal buttons (natural) Conduction in Myelinated Axons: in myelinated axons, ions can only pass through the membrane at Nodes of Ranvier, and axonal sodium channels are concentrated at the nodes. Action potentials are conducted passively and decrementally in the parts covered in myelin to the nodes of Ranvier, where the voltage-activated sodium channels open and generate another full-blown action potential, which is then conducted passively along to the next node. Myelination increases the speed of axonal conduction because conduction along the myelinated segments is passive and is instantaneous, although there is a slight delay at each node, but it is still much faster than in unmyelinated axons. Saltatory conduction: transmission of action potentials in myelinated axons. The Velocity of Axonal Conduction: conduction is faster in large-diameter axons, and in myelinated axons. Mammalian motor neurons are large and myelinated and can reach speeds up to 100 m/s. Small unmyelinated axons conduct action potentials at about 1 m/s. In humans, the max. velocity of conduction in motor neurons is about 60m/s. Conduction in Neurons without Axons: conduction in interneurons is usually passive and decremental. Hodgkin-Huxley model is based on giant squid motor neurons, and therefore must be applied to cerebral neurons with caution. It is clear that cerebral neurons are far more complex than motor neurons. 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  Axons of some cerebral neurons can actively conduct both graded signals and action potentials PSYC2410 – Chapter 4  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.  Many cerebral neurons have no axons and do not display action potentials  The dendrites of some cerebral neurons can actively conduct action potentials 4.5 Synaptic Transmission: Chemical Transmission of Signals among Neurons Structure of Synapses  Axodendritic synapses: synapses of axon terminal buttons on dendrites. Many axodendritic synapses terminate on dendritic spines (nodules of various shapes that are located on the surfaces of many dendrites).  Axosomatic synapses: synapses of axon terminal buttons on somas (cell bodies)  Dendrodendritic synapses: often capable of transmission in either direction  Axoaxonic synapses: very 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) is that they can selectively influence one particular synapse rather than the entire presynaptic neuron.  Directed synapses: synapses at which the site of neurotransmitter release and the site of neurotransmitter reception are in close proximity.  Nondirected synapses: synapses at which the site of release is at some distance from the site of reception. Neurotransmitter molecules are released from a series of varicosities (bulges or swellings) along the axon and its branches and thus are widely dispersed to surrounding targets. Often referred to as string-of-beads synapses. The Synthesis, Packaging, and Transport of Neurotransmitter Molecules  Coexistence: neurons synthesi
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