290 Textbook chapter 3

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Amanda( Mandy) Wintink

290 Textbook Notes [Lecture 4] Chapter 4: Neural Conduction and Synaptic Transmission How neurons conduct and transmit electrochemical signals through the nervous system 1 R ESTING M EMBRANE POTENTIAL I. Membrane potential: the difference in electrical charge between the inside and outside of a cell 1.1 R ECORDING THE M EMBRANE POTENTIAL II. Position tip of one electrode inside the neuron and tip of another electrode outside the neuron in the extracellular fluid III. Tip must be fine enough to pierce neural membrane without severely damaging it IV. Intracellular electrodes are called microelectrodes 1.2 R ESTING M EMBRANE P OTENTIAL I. When both tips are in the extracellular fluid, the voltage difference b/w them is zero II. When one tip is inserted into a neuron, a steady potential of about -70 milivolts (mV) is recorded III. Indicates that potential inside resting neuron is about 70 mV less than outside the neuron a. A steady membrane potential of about -70 mV is called the neuron’s resting potential IV. In its resting state with the -70 mV charge built up across the membrane, a neuron is said to be polarized 1.3 IONIC BASIS OF THE R ESTING POTENTIAL I. Why are resting neurons are polarized? a. The salts in the neural tissue separate into pos and neg charged particles called ions b. The resting potential results from the fact that the ratio of neg to pos charges is greater inside the neuron than outside II. This unequal distribution of charges can be understood in terms of the interaction of 4 factors: a. 2 factors act to distribute ions equally throughout the intracellular and extracellular fluid of the nervous system i. Random motion 1. Ions in neural tissue are in constant random motion 2. Particles in random motion tend to become evenly distributed b/c they’re more likely to move down their concentration gradients than up them (move from areas of high concentration to low) ii. Electrostatic pressure 1. Any accumulation of charges 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 2. Particles of dif charges are attracted to each other (pos attracted to neg) – prevents accumulation of many ions in the same area b/c they either are repelled by the same charge or attracted to a different charge in another area b. 2 features of the neural membrane counteract these homogenizing effects c. Despite homogenizing effects, no single class of ions is distributed equally on the two sides of the neural membrane i. 4 kinds of ions contribute to the resting potential a. Sodium ions (Na +) b. Potassium ions (K+) c. Chloride ions (Cl-) d. And various negatively charged protein ions 2. The concentration of both Na+ and Cl- ions are greater outside a resting neuron than inside it whereas K+ ions are more concentrated on the inside 3. The neg charged protein ions are synthesized inside the neuron and stay there III. 2 properties of the neural membrane are responsible for unequal distribution of the above 4 kinds of ions a. One property is active and involves the consumption of energy b. One property is passive (does not involve the consumption of energy) i. The passive property of the neural membrane that contributes to unequal distribution of the 4 kinds of ions is its differential permeability to those ions 1. In resting neurons, K+ and Cl- ions pass readily through the neural membrane 2. Na+ ions pass through it with difficulty 3. Negatively charged protein ions don’t pass through at all 4. Ions pass through the neural membrane at specialized pores called ion channels a. Each ion channel is specialized for the passage of particular ions c. Hodgkin and Huxley experiments i. 1950s; provided first evidence that there is an energy-consuming process involved in the maintenance of the resting potential ii. Wondered why the high extracellular concentrations of Na+ and Cl- ions and the high intracellular concentration of K+ ions were not eliminated by the tendency for them to move down their concentration gradients to the side of lesser concentration 1. Possible that the electrostatic pressure of -70 mV across the membrane was a counteracting force that maintained the unequal distribution iii. Calculated the electrostatic charge required to offset the tendency for each of the 3 ions to move down their concentration gradients 1. Cl- ions = -70 mV (same as actual resting potential) a. Concluded that when neurons are at rest, the unequal distribution of Cl- ions across the neural membrane is maintained in equilibrium by the balance b/w the tendency for Cl- ions to move down the concentration gradient into the neuron and the 70 mV of electrostatic pressure driving them out b. They end up staying in the same place, which maintains the unequal distribution (don’t move down the gradient and aren’t driven out by electrostatic pressure) 2. K+ ions = 90 mV of electrostatic pressure would be required to keep intracellular K+ ions from moving down their concentration gradient and leaving the neuron a. 20 mV more than the actual resting potential 3. Na+ ions = situation more extreme b/c the effects of both the concentration gradient and electrostatic gradient act in the same direction a. 50 mV of outward pressure required to keep the extracellular Na+ ions from moving down their concentration gradient b. The 50 mV is added to the 70 mV of electrostatic pressure acting to move them in the same direction c. Therefore 120 mV of pressure is acting to force Na+ ions into resting neurons (add instead of subtract b/w both are trying to move them in the same direction) iv. Subsequent experiments confirmed calculations 1. K+ ions are continuously being driven out of resting neurons by the 20 mV of pressure 2. Na+ ions are continuously being driven into the neuron by 120 mV of pressure v. Why do intracellular and extracellular concentrations of Na+ and K+ remain constant in resting neurons? 1. There is an active mechanism in the cell membrane to counteract the inflow of Na+ ions into the resting neuron, by pumping Na+ ions out as rapidly as they pass in 2. T here is an active mechanism to counteract the outflow of K+ ions by pumping K+ ions in as rapidly as they pass out 3. IV. transport of Na+ ions out of neurons and transport of K+ ions into them are not independent processes a. this transport is performed by energy-consuming mechanisms in the cell membrane b. continually exchange 3 Na+ ions inside the neuron for 2 K+ ions outside c. called sodium-potassium pumps V. several other class of transporters (mechanisms in the membrane of a cell that actively transports ions/molecules across the membrane) have been discovered 2 G ENERATION AND C ONDUCTION OF POSTSYNAPTIC P OTENTIALS I. When neurons fire, they release chemicals called neurotransmitters from their terminal buttons II. NTs diffuse across the synaptic clefts and interact with specialized receptor molecules on the receptive membranes of the next neurons in the circuit III. When NT molecules bind to postsynaptic receptors, they have one of 2 effects: a. They may depolarize the receptive membrane i. Decrease the resting membrane potential b. Postsynaptic depolarizations are called excitatory postsynaptic potentials (EPSPs) i. They increase the likelihood that the neuron will fire c. May hyperpolarize the receptive membrane i. Increase the resting membrane potential ii. Postsynaptic hyperpolarizations are called inhibitory postsynaptic potentials (IPSPs) iii. Decrease the likelihood that the neuron will fire IV. Both EPSPs and IPSPs are graded responses a. The amplitudes of EPSPs and IPSPs are proportional to the intensity of the signals that elicit them b. Weak signals elicit small postsynaptic potentials and strong ones elicit large potentials c. They travel passively from their sites of generation at synapses, usually on the dendrites or cell body, in the same way that electrical signals travel through a cable d. The transmission of postsynaptic potentials has 2 important characteristics i. It’s rapid (assumed instantaneous) ii. The transmission of EPSPs and IPSPs is decremental 1. They decrease in amplitude as they travel through the neuron 3 I NTEGRATION OF POSTSYNAPTIC POTENTIALS AND G ENERATION OF A CTION POTENTIALS I. Postsynaptic potentials created at a single synapse typically have little effect on the firing of the postsynaptic neuron II. Receptive areas of neurons are covered w/ thousands of synapses; whether a neuron fires is determined by the net effect of their activity (balance b/w excitatory and inhibitory signals reaching its axon) III.Graded EPSPs and IPSPs created by the action of NTs at particular receptive sites on a neuron’s membrane are conducted instantly and decrementally to the axon hillock (conical structure at junction b/w cell body and axon) a. if the sum of depolarizations and hyperpolarizations reaching the section of the axon adjacent to the axon hillock at any time, sufficient to depolarize the membrane to a level referred to as its threshold of excitation (usually -65 mV), an action potential is generated near the axon hillock IV. action potential (AP): a massive but momentary reversal of the membrane potential from -70 mV to about +50 mV a. unlike postsynaptic potentials, APs are not graded i. magnitude not related to intensity of stimuli that elicit them ii. they’re all or none responses 1. they either occur to full extent or don’t occur at all b. each multipolar neuron adds all graded excitatory and inhibitory postsynaptic potentials reaching its axon and decides to fire or not to fire on the basis of their sum c. integration: adding/combining individual signals into one signal d. neurons integrate incoming signals in two ways: over space and over time V. spatial summation: a. local EPSPs produced silmoutaneously on different parts of the receptive membrane sum to form a greater EPSP b. silmoutaneously IPSPs sum to form greater IPSP c. silmoutaneously EPSPs and IPSPs sum to cancel each other out VI. temporal summation a. postsynaptic potentials produced in rapid succession at the same synapse sum to form a greater signal b. they add together over time b/c the postsynaptic potentials they produce often outlast them c. if a particular synapse is activated and then activated again before the first postsynaptic potential has dissipated, the effect of the second stimulus will be superimposed on the lingering postsynaptic potential produced by the first d. it’s possible for a brief subthreshold excitatory stimulus to fire a neuron if it’s administered twice in rapid succession e. an inhibitory synapse activated twice in rapid succession can produce a greater IPSP than that produced by a single stimulation VII. each neuron integrates signals over both time and space as it’s bombarded with stimuli through the thousands of synapses covering its dendrites and cell body VIII. the location of the synapse of a neuron’s receptive membrane has been assumed to be an important factor in determining its potential to influence a neuron’s firing a. b/c EPSPs and IPSPs are transmitted decrementally, synapses near the axon trigger zone have been assumed to have most influence on the firing of the neuron b. it has been demonstrated that some neurons have a mechanism for amplifying dendritic signals that originate far from their cell bodies i. in these neurons all dendritic signals reaching the cell body have a similar amplitude regardless of where they originate 4 C ONDUCTION OF A CTION P OTENTIALS 4.1 I ONIC BASIS OF A CTION P OTENTIALS I. Voltage-activated ion channels a. Ion channels that open and or close in response to changes in the level of the membrane potential II. When the membrane potential of the axon is reduced to the threshold of excitation, the voltage activated sodium channels in the axon membrane open a. Na+ ions rush in, driving the membrane potential from -70 to +50 mV b. The rapid change associated w/ the influx of Na+ ions triggers the opening of voltage activated potassium channels c. K+ ions near the membrane are driven out of the cell through these channels d. Sodium channels close, marking the end of the rising phase of the action potential and the beginning of repolarization by the continued outflow of K+ ions e. Once repolarization is achieved, potassium channels gradually close i. Due to gradual closing, many K+ ions flow out of the neuron and it is left hyperpolarized for a brief period of time f. AP involves only the ions right next to the membrane i. A single AP has little effect on the relative concentrations of various ions inside and outside the neuron ii. The sodium-potassium pumps play only a minor role in the reestablishment of the resting potential 4.2 R EFRACTORY P ERIODS I. absolute refractory period: brief 1 to 2 milisecond period after initiation of an AP during which it’s impossible to elicit a second one II. followed by the relative refractory period a. period during which it’s again possible to fire neurons but only by applying higher-than-normal levels of stimulation b. at the end of this period the amount of stimulation necessary to fire a neuron returns to baseline III. refractory period is responsible for 2 important characteristics of neural activity a. fact that APs normally travel along axons in only one direction i. an AP cannot reverse direction b/c the portions of an axon over which an AP has just travelled are left momentarily refractory b. fact that the rate of neural firing is related to the intensity of the stimulation i. if a neuron is subjected to a high level of continual stimulation, it fires and then fires again as soon as the absolute ref period is over ii. if the level of stimulation is of an intensity just sufficient to fire the neuron when it’s at rest, the neuron doesn’t fire again until both the absolute and relative ref periods run their course 4.3 A XONAL C ONDUCTION OF A CTION POTENTIALS I. Conduction of APs along an axon differs from the conduction of EPSPs and IPSPs in 2 important ways a. Conduction of APs along an axon is nondecremental i. AP’s don’t grow weaker as they travel along the axonal membrane b. APs are conducted more slowly than postsynaptic potentials II. Reason for these 2 differences is that the conduction of EPSPs and IPSPs is passive but the axonal conduction of APs is largely active a. 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 b. The arrival of the electrical signals opens these channels, allowing Na+ ions to rush into the neuron and generate an AP on this portion of the membrane c.
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