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Chapter 4

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Western University
Psychology 2220A/B
Scott Mac Dougall- Shackleton

Chapter 4: Neural Conduction and Synaptic Transmission • Although dopamine levels are low in Parkinson's disease, dopamine is not an effective treatment because it does not readily penetrate the blood-brain barrier • An effective treatment: L-dopa, the chemical precursor of dopamine, which readily penetrates the blood-brain barrier and is converted to dopamine once inside the brain RESTING MEMBRANE POTENTIAL • Membrane potential is the difference in electrical charge between the inside and the outside of a 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 Potential • When both electrode tips are in the extracellular fluid, the voltage difference between them is zero • However, when the tip of the intracellular electrode is inserted into a neuron, a steady potential of about -70mV is recorded • This indicated that the potential inside the resting neuron is about 70mV less than that outside the neuron • This steady membrane potential of about -70mV is called the neuron’s resting potential • Neuron is said to be polarized • Ionic Basis of Resting Potential • The resting potential results form the fact that the ratio of negative to positive charges is greater inside the neuron than outside • Unequal distribution of charges can be understood in terms of the interaction of four factors: •The intracellular and extracellular fluids of the nervous system and two features of the neural membrane that counteract these homogenizing effects • The first two of these 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 • The second factor that promotes the even distribution of ions is electrostatic pressure • No single class of ions is distributed equally on the two sides of the neural membrane • Four kinds of ions contribute significantly to the resting potential: Na+, K+, Cl-, and various negatively charged protein ions • The concentrations of both Na+ and Cl- ions are greater outside the resting neuron than inside, whereas K+ ions are more concentrated on the inside • The negatively charged protein ions are synthesized inside the neuron and, for the most part, stay there Chapter 4: Neural Conduction and Synaptic Transmission • Two properties of the neural membrane are responsible for the unequal distribution of Na+, K+, and Cl-, and protein ions in resting neurons • One of these properties is passive, that is, it does not involve the consumption of energy • The other is active and does involve the consumption of energy • The passive property is its differential permeability to those ions • In resting neurons K+ and Cl- ions pass through it with difficulty, and the negatively charged protein ions do not pass through it all • Ions pass through the neural membrane at specialized pores called ion channels, each type of which is specialized for the passage of particular ions • Hodgkin and Huxley thus concluded that when neurons are 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 concentration gradient into the neuron and the 70mV of electrostatic pressure driving them out • Hodgkin and Huxley calculated that 90mV of electrostatic pressure would be required to keep intracellular K+ ions from moving down their concentration gradient and leaving the neuron • The concentration of Na+ ions exists outside of a resting neuron is such that 50mV of outward pressure would be required to keep Na+ ions from moving down their concentration gradient into the neuron, which is added to the 70mV of electrostatic pressure acting to move them in the same direction, therefore 120mV of pressure • Hodgkin and Huxley discovered that there are active mechanisms in the cell membrane to counteract the influx (inflow) of Na+ ions by pumping Na+ ions out as rapidly as they pass in and to counteract the efflux (outflow) of K+ ions by pumping K+ ions in as rapidly as they pass out • It was subsequently discovered that the transport of Na+ ions out of neurons and the transport of K+ ions into them are not independent processes • Such ion transport is performed by energy-consuming mechanisms in the cell membrane that continually exchange three Na+ ions inside the neuron for two K+ ions outside • These transporters are commonly referred to as sodium-potassium pumps GENERATION AND CONDUCTION OF POSTSYNAPTIC POTENTIALS • When neurons fire, they release form their terminal buttons chemicals called neurotransmitters, which diffuse across the synaptic clefts and interact with specialized receptor molecules on the receptive membranes of the next neuron in the circuit • When neurotransmitter molecules bind to postsynaptic receptor, they typically have one of two effects, depending on the structure of both the neurotransmitter and the receptor • They may depolarize the receptive membrane or they may hyperpolarize it • Postsynaptic depolarizations are called excitatory postsynatic 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 Chapter 4: Neural Conduction and Synaptic Transmission • Both 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 • The transmission of postsynaptic potentials has two important characteristics • 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, just as a sound wave loses amplitude as it travels through air INTEGRATION OF POSTSYNAPTIC POTENTIALS AND GENERATION OF ACTION POTENTIALS • The postsynaptic potentials created at a single synapse typically have little effect on the firing of the postsynaptic neuron • Whether or not a neuron fires depends on the balance between the excitatory and inhibitory signals reaching its axon • Axon hillock - the conical structure at the junction between the cell body and the axon • 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 65mV - an action potential is generated near the axon hillock • The action potential (AP) is a massive but momentary reversal of the membrane potential from about -70mV to about +50V • Action potentials are not graded responses; their magnitude is not related in any way to the intensity of the stimuli that elicit them • To the contrary, they are all-or-none responses; that is, they either occur to their full extent or do not occur at all • Each multipolar neuron adds together al the graded excitatory and inhibitory postsynaptic potentials reaching its axon and decides to fire or no to fire on the basis of their sum • Adding or combining a number of individual signals into one overall signal is called integration • Neurons integrate incoming signals in two ways: over space and time • Three possible combinations of 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 • How simultaneous EPSPs and IPSPs sum to cancel each other out • Temporal summation show how postsynaptic potentials produced in rapid succession at the same synapse sum to form a greater signal • Each neuron continuously integrates signals over both time and space as it is continually bombarded with stimuli through the thousands of synapses covering its dendrites and cell body Chapter 4: Neural Conduction and Synaptic Transmission • The location of a synapse on a neurons receptive membrane has long been assumed to be an important factor in determining its potential to influence the neurons firing • Because EPSPs and IPSPs are transmitted decrementally, synapses near the axon trigger zone have been assumed to have the most influence on the firing of the neuron • However, it has been demonstrated that some neurons have a mechanism for amplifying dendritic signals that originate far from their cell bodies; thus, in these neurons, all dendritic signals reaching the cell body have a similar amplitude, regardless of where they originate CONDUCTION OF ACTION POTENTIALS • Ionic Basis of Action Potentials • Action potentials are produced and conducted along the axon through the action of voltage-activated ion channels - ion channels that open or close in response to changes in the level of the membrane potential • The voltage-activated sodium channels in the axon membrane open wide, and Na + ions rush in, suddenly driving the membrane potential from about -70mV to about +50mV • The rapid change in the membrane that is associated with the influx Na+ ions then triggers the opening of voltage activated potassium channels • At this point 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 peak, by the positive internal charge • Once repolarization has been achieved, the potassium channels gradually close • Refractory Periods • There is a brief period 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 fir 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: •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 of Action Potentials • The conduction of action potentials along an axon differs from the conduction of EPSPs and IPSPs in two important ways Chapter 4: Neural Conduction and Synaptic Transmission •First, the conduction of action potentials along an axon is nondecremental: action potentials do not grow weaker as they travel the axonal membrane •Second, action potentials are conducted more slowly than postsynaptic potentials • The reason for these two differences is that conduction of EPSPs and IPSPs is passive, whereas the axonal conduction of action potentials is largely active • Once an action potential has been generated, it travels passively along the axonal membrane to the adjacent voltage-activated sodium channels, which have yet to open • The arrival of the electrical signal opens these channels • This signal is then conducted passively to the next sodium channels, where another action potential is actively triggered • Because there are so many ion channels on the axonal membrane and they are so close together, it is usual to think of axonal conduction as a single wave of excitation spreading actively at a constant speed along the axon, rather than as a series of discrete events • 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 antidromatic conduction • Axonal conduction in the natural direction - from cell body to terminal buttons - is called orthodromic conduction • Conduction in Myelinated Axons • In myelinated axons, ions can pass though the axonal membrane only at the nodes of Ranvier • Indeed, in myelinated axons, axonal sodium channels are concentrated at the nodes of Ranvier • When an action potential is generated in a myelinated axon, the signal is conducted passively - that is, instantly and decrementally - along the first segment of myelin to the next node of Ranvier • Although the signal is somewhat diminished by the time it reaches that node, it is still strong enough to open the voltage-activated sodium channels at the node and to generate another full blown action potential • Myelination increases the speed of axonal conduction • Because conduction along the myelinated segments of the axon is passive, it occurs instantly, and the signal thus “jumps” along the axon from node to node • The transmission of action potentials in myelinated axons is called saltatory conduction • The Velocity of Axonal Conduction • Conduction is faster in large=diameter axons • Mammalian motor neurons are large and myelinated; thus, some can conduct at speeds of 100 meters per second • Conduction in Neurons Without Axons Chapter 4: Neural Conduction and Synaptic Transmission • Action potentials are the means by which axons conduct all-or-none signals nondecrementally over relatively long distances • The Hodgkin-Huxley Model in Perspective • The problem is that the simple neurons and mechanisms of the Hodgkin-Huxley model are not representative of the variety, complexity, and plasticity of many of the neurons in the mammalian brain • Thus, the Hodgkin-Huxley model must be applied to cerebral neurons with caution • 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 • The axons of some cerebral neurons can actively conduct both graded signals and action potentials • 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 SYNAPTIC TRANSMISSION: CHEMICAL TRANSMISSION OF SIGNALS AMONG NEURONS • Structure of Synapses • Axodendritic synapses - synapses of axon terminal buttons on dendrites • Many axondendritic synapses terminate on dendritic spines • Axosomatic synapses - synapses of axon terminal buttons on somas • Although axondendritic and axonsomatic synapses are the most common synaptic arrangements, there are several others • 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 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
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