12.4 Electrophysiology of Neurons

12.4 Electrophysiology of Neurons Expected Learning Outcomes: • Explain why a cell has an electrical charge difference (voltage) across its membrane. • Explain how stimulation of a neuron causes a local electrical response in its membrane. • Explain how local responses generate a nerve signal. • Explain how the nerve signal is conducted down an axon. 50 12.4a Electrical Potentials and Currents 1 Neural communication is based on electrical potentials and currents • Electrical potential—difference in concentration of charged particles between one point and another • Type of potential energy; measured in volts • Under some conditions, can produce a current • Current—flow of charged particle from one point to another • Example: flashlight battery • Has a potential (charge) of 1.5 volts • Can produce a current of electrons that lights the bulb • With a potential (voltage), the battery is polarized • Living cells are also polarized 51 Electrical Potentials and Currents 2 Resting membrane potential (RMP)—charge difference across plasma membrane • Typically −70 millivolts (mV) in an unstimulated, “resting” neuron • Negative value indicates more negatively charged particles on inside of membrane compared to outside • Electrical currents in the body created by flow of ions (Na + , K + , etc.) through gated channels in the membrane 52 12.4b The Resting Membrane Potential 1 Ionic basis of the resting membrane potential (RMP): • Ions are unequally distributed between extracellular fluid (ECF) and intracellular fluid (ICF) • RMP results from the combined effect of: • The diffusion of ions down their concentration gradients through the membrane • Selective permeability of the membrane, allowing some ions to pass more easily than others • Electrical attraction of cations and anions to each other 53 The Resting Membrane Potential 2 Ionic basis of the resting membrane potential (continued): • Potassium (K + ) has greatest influence on RMP • K + is more concentrated in ICF compared to ECF • Cell membrane more permeable to K + (through leak channels) than to other ions • + As K leaks out, inside of membrane becomes more negative • Resulting electrical attraction brings K + back in • Equilibrium is reached: no net movement of K + occurs when tendency for K + to exit (down its concentration gradient) equals tendency for K + to enter (by electrical attraction) 54 The Resting Membrane Potential 3 Ionic basis of the resting membrane potential (continued): • Sodium (Na + ) also influences the RMP • Na + is more concentrated in ECF compared to ICF, but membrane is much less permeable to Na + than to K + + • Na diffuses into the cell, down its concentration gradient and attracted electrically, but it’s a relatively small amount • Cancels some of the negative charge, reducing the voltage across the membrane 55 The Resting Membrane Potential 4 Ionic basis of the resting membrane potential (continued): • Sodium-potassium (Na  /K  ) pump compensates for the + + Na and K continual leakage of • Moves Na + out of the cell and brings K + into the cell— maintains their concentration gradients • Contributes about -3 mV to the RMP • Works continuously and requires ATP • 70% of the energy requirement of the nervous system 56 Ionic Basis of the Resting Membrane Potential Figure 12.12 57 12.4c Local Potentials 1 Sensory neurons can be stimulated by chemicals, light, heat, or mechanical forces Stimulation triggers local, temporary change in membrane potential Example: chemical stimulant binds to a receptor on the neuron • Opens Na + channels, allows Na + to enter • Entry of positively charged Na + makes membrane potential less negative • Polarity is reduced, voltage is less negative—called depolarization • Depolarization spreads from the point of stimulation • Temporary, short-range change in voltage is a local potential 58 Local Potentials 2 Characteristics of local potentials: • Graded—vary in magnitude with stimulus strength • Stronger stimuli open more Na + channels, and they stay open longer • Decremental—get weaker the farther they spread from the point of stimulation • Reversible—if stimulation ceases, membrane voltage quickly returns to normal resting potential • Can be either excitatory or inhibitory • Depolarization is excitatory—makes a neuron more likely to fire an action potential) • Hyperpolarization (membrane more negative) is inhibitory—makes a neuron less likely to produce an action potential 59 Excitation of a Neuron by a Chemical Stimulus Figure 12.13 60 12.4d Action Potentials 1 Action potential—rapid up-and-down change in voltage produced by the coordinated opening and closing of voltagegated ion channels • Only occurs where there is a high enough density of voltage-gated ion channels (trigger zone of axon) • If excitatory local potential reaches trigger zone and is still strong enough, it opens enough voltage-gated Na + channels to generate an action potential 61 Action Potentials 2 Steps of an action potential: • Local potential spreads to axon hillock • Voltage at axon hillock must reach threshold—the minimum voltage to open voltage-gated channels (around −55 mV) • Voltage-gated Na + channels open quickly • Voltage-gated K + channels open more slowly • Na + enters and depolarizes membrane further, quickly surpassing 0 mV • Na + channels become inactivated, begin closing • Voltage peaks at +35 mV by the time Na + inflow ceases • Membrane polarity has reversed—now more positive on the inside and negative on the outside 62 Action Potentials 3 Steps of an action potential (continued): • The voltage-gated K + are fully open, K + flows out of cell and membrane becomes more negative again—repolarization • K + continues to exit and produces a negative overshoot hyperpolarization) 1 to 2 mV more negative than the original RMP • Membrane voltage returns to RMP as Na + leaks into the cell 63 An Action Potential Figure 12.14 64 Actions of the Sodium and Potassium Channels During an Action Potential 1 Figure 12.15(1,2) 65 Actions of the Sodium and Potassium Channels During an Action Potential 2 Figure 12.15(3,4) 66 Action Potentials 4 The concentrations of Na  and K + on either side of the membrane do not change significantly during an action potential Movement of only a few ions can have a large effect on the membrane potential • Only about one in a million ions crosses the membrane during an action potential • Only the thin layer of ions close to the membrane is affected Even after thousands of action potentials, the cytosol still has a higher concentration of K + and a lower concentration of Na + than the ECF 67 Action Potentials 5 Characteristics of action potentials: • All-or-none law—if threshold reached, neuron fires up to maximum voltage; if threshold not reached, it does not fire • Non decremental—do not get weaker with distance • Irreversible—once started, an action potential travels all the way down the axon cannot be stopped 68 12.4e The Refractory Period Refractory period—period of resistance to stimulation; has two phases: • Absolute refractory period—no stimulus of any strength will trigger another AP • Caused by inactivation of voltage-gated Na+ channels • Relative refractory period—an unusually strong stimulus is needed to trigger a new AP • During hyperpolarization, a larger depolarization (local potential) is required to reach threshold 69 The Absolute and Relative Refractory Periods in Relation to the Action Potential Figure 12.16 70 12.4f Signal Conduction in Nerve Fibers 1 Unmyelinated axons and continuous conduction: • Unmyelinated axons have voltage-gated channels along their entire length • Action potential at trigger zone causes Na + to enter axon and diffuse into adjacent regions • Depolarization opens voltage-gated channels • Opening of voltage-gated ion channels results in a new action potential which then allows Na + diffusion to excite membrane immediately distal to that • Chain-reaction continues down axon • Like a wave of falling dominoes • Called continuous conduction 71 Continuous Conduction of a Nerve Signal in an Unmyelinated Axon Figure 12.17 72 Signal Conduction in Nerve Fibers 2 Myelinated axons and saltatory conduction: • Action potentials can only be generated at the nodes, where voltage-gated ion channels are concentrated • Electrical signal must spread passively between nodes • Signal passes very quickly, but strength decreases (similar to a local potential) • When signal reaches the next node it is still strong enough to depolarize the membrane to threshold • Voltage gated Na+ channels open and a new, full-strength action potential occurs • Action potential seems to “jump” from node to node • • Moves faster through “insulated” segments covered with myelin Slows down when it reaches the bare axon of the nodes 73 Saltatory Conduction of a Nerve Signal in a Myelinated Axon 1 Figure 12.18a 74 Saltatory Conduction of a Nerve Signal in a Myelinated Axon 2 Figure 12.18b 75