NURS 230 TRW Topic 7b Nervous System PDF

Topic 7b: Introduction to the Nervous System CHAPTER 11 IN TEXTBOOK Important Topics and Pages Introduction to the Nervous System (Electrical Potentials) Chapter 11 1. Why do neurons have a resting membrane potential and what is the cause of this potential? P 400-404 2. What is a graded potential?...

Topic 7b: Introduction to the Nervous System CHAPTER 11 IN TEXTBOOK Important Topics and Pages Introduction to the Nervous System (Electrical Potentials) Chapter 11 1. Why do neurons have a resting membrane potential and what is the cause of this potential? P 400-404 2. What is a graded potential? P 404-405 3. Describe the process through which an action potential generated P 405-410 4. How do nerves code for stimulus intensity? P410 5. What influences conduction velocity? P410-412 Membrane Potentials Like all cells, neurons have a resting membrane potential Unlike most other cells, neurons can rapidly change resting membrane potential Neurons are highly excitable Neurons use action potentials to send signals https://en.wikipedia.org/wiki/Membrane_potential  Membrane Potentials Differences in charge (positive and negative) on different sides of cell membrane create potential energy Flowing ions creates electrical currents Energy is liberated when opposite charges move toward each other Voltage is a measure of potential energy separated by charge ◦ Greater difference in charge, higher the voltage ◦ Measured in volts or millivolts (mV) ◦ 1 mV = 0.001 V Membrane Ion Channels 3 main types of gate channels: ◦ Chemically gated ion channels ◦ Voltage gated ion channels ◦ Mechanically gated channels Figure 11.7  Generating the Resting Membrane Potential Resting membrane potential of resting neuron approximately –70 mV ◦ Cytoplasmic side of membrane is negatively charged relative to the outside ◦ The actual voltage difference varies from –40 mV to –90 mV ◦ The membrane is said to be Figure polarized 11.8 Generating the Resting Membrane Potential: Differences in Ionic Composition Outside of cell has higher concentration of Na + ◦ Balanced chiefly by chloride ions (Cl −) Inside the cell has higher concentration of K+ ◦ Balanced by negatively charged (anionic) proteins K+ plays most important role in membrane potential Focus Figure 11.1 Generating the Resting Membrane Potential: Differences in Plasma Membrane Permeability Impermeable to large anionic (negatively charged) proteins Slightly permeable to Na+ (through leakage channels) ◦ Sodium diffuses into cell down concentration gradient 25 times more permeable to K+ than Na+ (more leakage channels) ◦ Potassium diffuses out of cell down concentration gradient Quite permeable to Cl– Generating the Resting Membrane Potential: Differences in Plasma Membrane Permeability More K+ diffuses out than Na+ diffuses in ◦ As a result, the inside of the cell is more negative ◦ Establishes resting membrane potential Sodium-potassium pump (Na+/K+ ATPase) stabilizes resting membrane potential ◦ Maintains concentration gradients for Na+ and K+, despite constant leaking ◦ Three Na+ are pumped out of cell while two K+ are pumped back in Generating the Resting Membrane Potential Focus Figure Changing the Resting Membrane Potential Membrane potential changes when: ◦ Concentrations of ions across membrane change ◦ Membrane permeability to ions changes Changes produce two types of signals ◦ Graded potentials: Incoming signals operating over short distances ◦ Action potentials: Long-distance signals of axons Changes in membrane potential are used as signals to receive, integrate, and send information, particularly in neurons  Changing the Resting Membrane Potential Depolarization: Decrease in membrane potential ◦ Becoming less negative ◦ Na+ ions flood into cell ◦ Increases probability of stimulus Figure 11.9 Changing the Resting Membrane Potential Hyperpolarizati on: Increase in membrane potential ◦ Becoming more negative ◦ Decreases probability of stimulus Figure 11.9 Graded Potentials Short-lived, localized changes in membrane potential ◦ The stronger the stimulus, the more voltage changes and the farther current flows ◦ Decrease in magnitude with distance Graded: magnitude varies with stimulus strength Triggered by stimulus that opens gated ion channels Named according to location and function: ◦ Receptor potential (generator potential): graded potentials in receptors of sensory neurons ◦ Postsynaptic potential: neuron graded potential, synapse driven Graded Potentials Once gated ion channel opens, depolarization of small section of plasma membrane Depolarization spreads from one area of membrane to next as ions migrate Cations move towards negative ions (direction of flow) Figure 11.10 Graded Potentials Current flows but dissipates quickly and decays Graded potentials are signals only over short distances Leakage of potential across plasma membrane ◦ Decremental Graded potentials initiate action potentials Figure 11.10 Action Potentials Principal way neurons send signals, long distance Occur only in muscle cells and axons – need excitable membranes Brief reversal of membrane potential with a change in voltage of ~100 mV (-70mV to +30mV) Action potentials (APs) do not decay over distance In neurons, also referred to as a nerve impulse Involves opening of specific voltage-gated channels ◦ Initially activated by graded potentials Voltage-Gated Channels Focus Figure 11.2 Generating an Action Potential: Four Main Steps Focus Figure 11.2  Generating an Action Potential: Four Main Steps 1. Resting state: All gated Na+ and K+ channels are closed: ◦ Only leakage channels for Na+ and K+ are open ◦ Maintains the resting membrane potential ◦ Each Na+ channel has two voltage-sensitive gates ◦ Activation gates: closed at rest; open with depolarization ◦ Inactivation gates: open at rest; block channel once it is open Focus Figure ◦ Both gates need to be open for Na+ to enter, only one needs to be closed to 11.2 close channel Generating an Action Potential: Four Main Steps 2. Depolarization: ◦ Depolarizing local currents open voltage- gated Na+ channels ◦ Decreases membrane potential ◦ Na+ activation and inactivation gates open Focus Figure 11.2 Generating an Action Potential: Four Main Steps Step 2 (cont): Na+ influx causes more depolarization, opens more Na+ channels At threshold (–55 to –50 mV), positive feedback causes opening of all Na+ channels ◦ Results in large action potential spike ◦ Membrane polarity jumps to +30 mV Focus Figure 11.2 Generating an Action Potential: Four Main Steps 3. Repolarization: ◦ Na+ channel inactivation gate closes ◦ Membrane permeability to Na+ declines to resting state ◦ Slow voltage-gated K+ channels open, K+ exits cells ◦ Repolarization: membrane returns to resting membrane Focus Figure potential 11.2 Generating an Action Potential: Four Main Steps 4. Hyperpolarization : ◦ Some K+ channels remain open, allowing excessive K+ efflux ◦ Inside of membrane becomes more negative than in resting state ◦ This causes hyperpolarization of the membrane (slight dip below resting voltage) ◦ Na+ channels also begin Focus Figure to reset 11.2 Generating an Action Potential Repolarization resets electrical conditions, not ionic conditions After repolarization, Na+/K+ pumps (thousands of them in an axon) restore ionic conditions Focus Figure Threshold and the All- or-None Phenomenon Not all depolarization events produce APs For an axon to “fire,” depolarization must reach threshold voltage to trigger AP At threshold: ◦ Membrane is depolarized by 15 to 20 mV ◦ Na+ permeability increases ◦ Na+ influx exceeds K+ efflux ◦ The positive feedback cycle begins All-or-None: An AP either happens completely, or does not happen at all Propagation of an Action Potential Propagation allows AP to be transmitted from origin down entire axon length toward terminals Na+ influx through voltage gates in one membrane area cause local currents that cause opening of Na+ voltage gates in adjacent membrane areas ◦ Leads to depolarization of that area, which in turn causes depolarization in next area  Propagation of an Action Potential Once initiated, an AP is self-propagating Since Na+ channels closer to the AP origin are still inactivated, no new AP is generated there Figure 11.11 ◦ AP occurs only in a forward direction Coding for Stimulus Intensity All action potentials are alike and are independent of stimulus intensity CNS tells difference between a weak stimulus and a strong one by frequency of impulses ◦ Frequency is number of impulses (APs) received per second ◦ Higher frequencies mean stronger Figure stimulus 11.12  Refractory Periods Refractory period: time in which neuron cannot trigger another AP ◦ Voltage-gated Na+ channels are open Two types: ◦ 1. Absolute refractory period ◦ Time from opening of Na+ channels until resetting of the channels ◦ Ensures that each AP is an all-or- none event ◦ Enforces one-way transmission of nerve impulses Figure 11.13  Refractory Periods 2. Relative refractory period: ◦ Follows absolute refractory period ◦ Most Na+ channels returned to resting state ◦ Some K+ channels still open ◦ Repolarization is occurring ◦ Only very strong stimulus could stimulate an AP Figure 11.13 Conduction Velocity APs occur only in axons, not other cell areas AP conduction velocities in axons vary widely, 2 factors: Axon diameter ◦ Larger-diameter fibers have less resistance to local current flow, so have faster impulse conduction Degree of myelination ◦ Myelin sheath increases speed of propagation ◦ Two types of conduction depending on presence or absence of myelin ◦ Continuous conduction ◦ Saltatory conduction  Conduction Velocity Continuous conduction: slow conduction that occurs in nonmyelinat ed axons Figure 11.14 Conduction Velocity Saltatory conduction: occurs only in myelinated axons and is about 30 times faster ◦ Myelin sheaths insulate and prevent leakage of charge ◦ Voltage-gated Na+ channels are located at myelin sheath gaps ◦ APs generated only at gaps ◦ Electrical signal appears to jump rapidly from gap to gap Figure 11.14 Action potential summary video https://www.youtube.com/watch?v=FEHNIELPb0s Review Questions Multiple sclerosis is an autoimmune disease which leads to destruction of myelin. What effect should this have in the nervous system? Why? Certain local anesthetics block voltage-gated Na+ channels. Why does this cause anesthesia? How does the CNS know if a stimulus is strong or weak? What factors lead to resting membrane potential? Is it negative or positive? At what stage is a neuron not able to trigger an action potential? What is the difference between depolarization and hyperpolarization? Which steps do they occur during an action potential?