Chapter 12! Chapter 12 - Neurophysiology. Neurophysiology! pp ; !

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1 Chapter 12 Neurophysiology pp ; SECTION 12-2, Page Neurons are nerve cells specialized for intercellular communication 2 Neuron Anatomy Figure

2 Parts of a Neuron (1 of 3) Cell body (soma) Cell membrane Has different types of membrane channels Cytoplasm Perikaryon = cytoplasm surrounding nucleus Neurofilaments, neurotubules = cytoskeleton Nissl bodies RER and free ribosomes Gray matter 4 Parts of a Neuron (2 of 3) Dendrites Axon Receive information from other neurons Carry information towards cell body Transmit graded potentials, not action potentials (usually) Axolemma, axoplasm Connects to soma at axon hillock First part = initial segment Initial segment generates action potentials 5 Parts of a Neuron (3 of 3) Axon collaterals Major branches of an axon Telodendria Small branches at the end of an axon Synaptic terminals - ends of the telodendria a.k.a. boutons, synaptic end bulbs, synaptic knobs Store neurotransmitter in synaptic vesicles Release neurotransmitter in response to electrical activity 6 2

3 Structure of a Synapse Figure SECTION 12-4, page 398 The transmembrane potential is the electrical potential of the cell s interior relative to its surroundings 8 The Transmembrane Potential (1 of 2) Transmembrane potential = Electrochemical gradient Potential = voltage difference across a membrane Arises from the sum of all chemical and electrical forces acting across the cell membrane Usually reported in millivolts (mv) Inside is negative, outside positive 9 3

4 Chapter 12 - Neurophysiology The Transmembrane Potential (2 of 2) Factors determining transmembrane potential Flux = P ΔC 1. Ion concentration differences (ΔC) 2. Sodium-potassium pump (maintains ΔC) 3. Membrane permeability differences for ions Membrane channel types: a. Leak channels b. Gated channels 4. Fixed anions (non-diffusible anions; P 0) Mostly negatively-charged proteins and phosphate 10 Types of Membrane Potentials (1 of 3) 1. Resting (membrane) potential Voltage difference across the cell membrane for an unstimulated ( resting ) cell 2. Graded potentials Local changes in membrane potential due to chemical or physical changes in the membrane Do not self-regenerate or spread over long distances 11 Types of Membrane Potentials (2 of 3) 3. Action potentials Self-regenerating changes in membrane potential due to chemical or physical changes in the membrane Spread over long distances In order to understand why the transmembrane potential changes during graded and action potentials, you must understand the equilibrium potential. Next slide. 12 4

5 Types of Membrane Potentials (3 of 3) 4. Equilibrium potential (for a particular ion) The equilibrium potential is the membrane voltage at which electrical forces and the concentration difference forces acting on an ion are equal. No net diffusion of the ion occurs at this membrane potential. Understanding this concept is REALLY, REALLY important. Discussion begins on Slide Resting Potential Spotlight Figure Resting Membrane Potential (1 of 5) Concentration differences Na + /K + ATPase pump Permeability differences Electrical potential difference across membrane Overall charges 15 5

6 Resting Membrane Potential (2 of 5) Concentration differences Na + /K + ATPase pump Permeability differences Electrical potential difference across membrane Overall charges 16 Resting Membrane Potential (3 of 5) Concentration differences Na + /K + ATPase pump Permeability differences Electrical potential difference across membrane Overall charges 17 Resting Membrane Potential (4 of 5) Concentration differences Na + /K + ATPase pump Permeability differences Electrical potential difference across membrane Overall charges 18 6

7 Resting Membrane Potential (5 of 5) Concentration differences Na + /K + ATPase pump Permeability differences Electrical potential difference across membrane Overall charges 19 Resting Potential Spotlight Figure Equilibrium Potential Definition: The membrane potential at which the electrical and concentration difference forces acting on a particular ion are equal. This results in NO NET MOVEMENT of the ion across the membrane. E ion = (-58mV) log [ion]inside [ion]outside =? mv E K+ = (-58mV) log [K+]inside = -90 mv [K+]outside E Na+ = (-58mV) log [Na+]inside = +66 mv [Na+]outside 21 7

8 Equilibrium Potential for K + Figure 12-10a,b Looking only at K + ; ignore other ions. Resting membrane Imagine that the membrane suddenly becomes freely permeable to K + Net electrochemical gradient when membrane suddenly becomes leaky to K + Equilibrium potential about -90 mv; no net diffusion of K +. Electrical and concentration forces equal. 22 Equilibrium Potential for Na + Figure 12-10c,d Looking only at Na + ; ignore other ions. Resting membrane Imagine that membrane suddenly becomes freely permeable to Na + Net electrochemical gradient when membrane suddenly becomes leaky to Na + Equilibrium potential about +66 mv; no net diffusion of Na +. Electrical and concentration forces equal. 23 Changes in the Transmembrane Potential Membrane at rest is polarized Ion flow can cause changes in potential. Inside of membrane becomes more positive = depolarized Inside of membrane becomes more negative = hyperpolarized 24 8

9 Membrane Channel Types Ions cross the membrane through: 1. Leak channels - always open 2. Gated channels - open or closed a. Voltage-gated channels b. Chemically-gated (ligand-gated) channels c. Mechanically-gated channels 25 Leak Channels (Passive Channels) Always open Important for establishing resting potential Ions leak down their electrochemical gradients e.g. K + leak channels, Na + leak channels Size, charge, etc. determine which ion(s) can pass through a channel Determine resting permeabilities for membrane E.g. P K+ at rest, x greater than P Na+ 26 Gated Channels A.K.A. active channels (does not refer to ATP use) Can exist in three states: Open (activated) Closed and cannot be opened (inactivated) Closed, but can be opened 27 9

10 Chemically-gated (Ligand-gated) Channels Open after binding a specific chemical (ligand) Most abundant on cell body, dendrites and motor end plate Acetylcholine receptor Binding of ACh changes shape of receptor. Channel becomes permeable to small ions like Na + and K +. Which ion will move most readily through this channel? What will be the effect on the membrane potential? 28 Voltage-gated Channels Figure 12-11b Channel opens in response to changes in membrane potential - threshold Important in action potential conduction, neurotransmitter release from end bulbs E.g. voltage-gated K +, Na + and Ca 2+ channels Resting membrane; Closed, but can open Open Closed and inactivated; cannot be opened 29 Mechanically-gated Channels Figure 12-11c Open or close in response to physical distortion. e.g. touch, pressure receptors 30 10

11 Gated Channels - Summary Figure Graded Potentials A change in membrane potential that decreases with distance Caused by ions entering cell through channels Local depolarization or hyperpolarization Does not spread very far from site of stimulus (unlike action potential) Does not involve voltage-gated channels Why don t graded potentials travel very far? Cytoplasm resists ion flow The cell membrane is LEAKY TO IONS 32 Graded Potentials Figure Neurotransmitter Leak channels present, but not shown Chemically-gated Na + channels present Voltage-gated Na + channels absent 33 11

12 Depolarization and Hyperpolarization Fig Depolarization Inside more positive than at rest e.g. Na + enters cell Hyperpolarization Inside more negative than at rest e.g. K + leaves cell e.g. Cl - enters cell 34 Graded Potentials Summary Table SECTION 12-5 An action potential is a nerve impulse 36 12

13 Action Potentials - Introduction A sudden major change in membrane potential An all-or-none phenomenon: Either happens or does not happen Occurs when membrane reaches a specific membrane voltage called threshold Does not degrade over long distances (unlike graded potentials) Depends upon the presence of voltagegated Na + and K + channels At threshold, voltage-gated Na + channels open 37 Action Potential Recording (Table 12-3: 8 th edition) 38 Action Potential Recording Figure

14 Key to Action Potential Figures Figure Voltage-gated channels Na + Na + channel Activation gate Inactivation gate K + channel Activation gate K + 40 The Generation of an Action Potential Summary Figure Figure Voltage-gated Channels - Resting Membrane Resting membrane Voltage-gated Na + channels Activation gate closed Inactivation gate open Voltage-gated K + channels Closed Leak channels for both ions are open and are unaffected by the voltage changes to come

15 Voltage-gated Channels - Step 1 Local depolarization begins e.g. chemically-gated Na + channels have opened nearby. Local current flow depolarizes membrane towards threshold, but threshold has not been reached. (Note that the number on my voltmeter is -65, not -60, as in text.) Voltage-gated Na + channels: Activation gates closed Inactivation gates open Voltage-gated K + channels: closed 43 Voltage-gated Channels - Step 2 Threshold was reached at -60 mv. Voltage-gated Na + channels: Activation gate open Inactivation gate open Na + entry is depolarizing cell membrane Voltage-gated K + channels: closed 44 Voltage-gated Channels - Step 3 Step 3 Membrane depolarized to +30mV Voltage-gated Na + channels: Activation gate open Inactivation gate closed No further influx of Na + Voltage-gated K + channels: Change in voltage causes these to open K + efflux begins repolarization 45 15

16 Voltage-gated Channel - Step 4 Step 4 Re- and Hyperpolarization Membrane potential greater (more negative) than at rest Voltage-gated Na + channels: Activation gates closed Inactivation gates back open Voltage-gated K + channels: Open until voltage reaches about -90 mv Continued efflux of K + until gates close causes hyperpolarization 46 Voltage-gated Channels - Back to Rest Back to resting conditions Voltage-gated Na + channels: Activation gate closed Inactivation gate open Voltage-gated K + channels: closed Note that leak channels have remained open throughout this process 47 Action Potential Continuous Propagation Figure

17 Action Potential Saltatory Conduction Figure Graded vs. Action Potentials Table Action Potentials and Muscle Cells Motor end plate = muscle cell membrane at neuromuscular junction Contains few voltage-gated Na + channels Does not generate action potentials (graded potentials only) Local current flow spreads to adjacent sarcolemma where action potentials are produced. About 100 vesicles, each containing 100,000 ACh molecules, are released into synapse to produce muscle action potential

18 Cholinergic Synaptic Activity Figure