* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download …and now, for something completely different.
Survey
Document related concepts
Nonsynaptic plasticity wikipedia , lookup
Neuropsychopharmacology wikipedia , lookup
Patch clamp wikipedia , lookup
Biological neuron model wikipedia , lookup
Nervous system network models wikipedia , lookup
Node of Ranvier wikipedia , lookup
Single-unit recording wikipedia , lookup
Electrophysiology wikipedia , lookup
Molecular neuroscience wikipedia , lookup
Action potential wikipedia , lookup
Membrane potential wikipedia , lookup
Stimulus (physiology) wikipedia , lookup
Transcript
…and now, for something completely different. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Serratus Anterior…in Action! Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Basic Principles of Electricity: Opposite charges attract The coming together of opposite charges liberates (releases) energy Thus, situations in which there are separated (by a membrane) electrical charges of opposite sign(+, -) have potential energy. The potential, or possibility, to release enegry. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Electricity Definitions Voltage (V): Measure of potential energy (volts, millivolts) Measured between 2 points (called potential difference) or simply potential The greater the difference in charge between 2 points, the greater the voltage Current (I): The flow of electrical charge between two points that can be used to do work The amount of charge that can travel between 2 points depends on: Resistance (R) – the hindrance to charge flow (e.g. insulators have high hindrance, conductors have low hindrance) Voltage (V) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Electrical Current and the Body Reflects the flow of ions rather than electrons There is a potential on either side of membranes when: The number of ions is different across the membrane The membrane provides a resistance to ion flow Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Ohm’s Law Current (I) = voltage (V)/Resistance (R) Here: I is proportional to V I is inversely proportional to R No net voltage (0V) means no net current In the body, instead of electrons moving down a copper wire, we have ions passing thru membranes Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Role of Membrane Ion Channels Large proteins in membranes that allow ions to pass Gated channels change shape and open and close in response to a specific signal Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Types of plasma membrane ion channels: Passive, or leakage, channels – always open but selective to the ion they let in Chemically gated/ligand gated channels – open with binding of a specific chemical (neurotransmitter) Voltage-gated channels – open and close in response to changes in membrane potential Mechanically gated channels – open and close in response to physical deformation of receptors (e.g. touch or pressure receptors) PLAY InterActive Physiology ®: Nervous System I: Ion Channels Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Operation of a Gated Channel When gated ion channels are open, ions diffuse quickly across the plasma membrane in the direction of their electro-chemical gradient, creating electrical currents and voltage changes across the membrane according to Ohms’s law: V+IxR Thus, electro-chemical gradients underlie all electrical phenomena in neurons Example: Na+-K+ gated channel Closed when a neurotransmitter is not bound to the extracellular receptor Na+ cannot enter the cell and K+ cannot exit the cell Open when a neurotransmitter is attached to the receptor Na+ enters the cell and K+ exits the cell Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Operation of a Gated Channel Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.6a Operation of a Voltage-Gated Channel Example: Na+ channel Closed when the intracellular environment is negative Na+ cannot enter the cell Open when the intracellular environment is positive Na+ can enter the cell Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Operation of a Voltage-Gated Channel Lys, Arg, His: All positively charged amino acids Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.6b Electrochemical Gradient Ions flow along their chemical gradient when they move from an area of high concentration to an area of low concentration Ions flow along their electrical gradient when they move toward an area of opposite charge Electrochemical gradient – the electrical and chemical gradients taken together Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Measuring Membrane Potential Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.7 Resting Membrane Potential (Vr) The potential difference (–70 mV) across the membrane of a resting neuron It is generated by different concentrations of Na+, K+, Cl−, and protein anions (A−) Ionic differences are the consequence of: PLAY Differential permeability of the neurilemma to Na+ and K+ Operation of the sodium-potassium pump maintaining Na+ and K+ concentrations InterActive Physiology ®: Nervous System I: Membrane Potential Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings At Rest… Membrane is impermeable to large anionic cytoplasmic proteins Membrane is slightly permeable to Na+ Membrane is 75x more permeable to K+ Membrane is freely permeable to Cl- (which balances the Na+ & K+ charge) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Resting Membrane Potential (Vr) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.8 Membrane Potentials: Signals Changes in membrane potential is used to communicate signals and send information about environment by neurons Membrane potential changes are produced by: Anything that changes membrane permeability to ions Anything that alters ion concentrations across the membrane 2 types of signals are produced by changes in membrane potential: Graded potentials (operate over short distance) Action potentials (operate over long distance, e.g. axons) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Changes in Membrane Potential Changes are caused by three events Depolarization: reduction in membrane potential, e.g. inside of membrane side becomes less negative (moves closer to 0 mV) than the resting potential. -45mV Repolarization – the membrane returns to its resting membrane potential E.g. -70mV E.g. -45mV -70mV Hyperpolarization – the inside of the membrane side becomes more negative (moves further from 0mV) than the resting potential E.g. -70mV Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings -90mV Changes in Membrane Potential Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.9 Graded Potentials Short-lived, local changes in membrane potential Decrease in intensity with distance Magnitude varies directly with the strength of the stimulus Triggered by change in environment (stimulus) that causes gated ion channels to open Essential for initiating action potentials Sufficiently strong graded potentials can initiate action potentials Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Graded Potentials Shu f Inward flow of ions Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Shu fflin fl i n g of g of cha rge s ou t si d e th e ce ll cha rges insi de t he c ell Figure 11.10 Graded Potentials Voltage changes are decremental Current is quickly dissipated due to the leaky plasma membrane Only travel over short distances ESSENTIAL FOR INITIATING ACTION POTENTIALS!! Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Graded Potentials Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.11 Action Potentials (APs) Long distance signalling by neurons Only excitable membranes, e.g. neurons and muscle cells, can generate action potentials. A brief reversal of membrane potential with a total amplitude of 100 mV Unlike graded potentials, APs do not decrease in strength over distance They are the principal means of neural communication An action potential in the axon of a neuron is a nerve impulse PLAY InterActive Physiology ®: Nervous System I: The Action Potential Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Action Potentials (APs) In neurons, AP is generated in the axon In summary, stimulus changes the permeability of the neuron’s membrane by opening specific voltage gated ion channels on the axon Thus, VGICs are activated by local currents (Graded Potentials) that spread toward the axon along the dendritic cell body membranes Often, the transition from GP to AP occurs at the axon hillock Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Action Potential: Resting State Na+ and K+ channels are closed Leakage accounts for small movements of Na+ and K+ Each Na+ channel has two voltage-regulated gates Activation gates – closed in the resting state Inactivation gates – open in the resting state Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.12.1 Action Potential: Resting State Maintaining -70mV Depolarization opens and then inactivates Na+ channels Both must be open for Na+ to enter Only one must be closed to close Na+ channels Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Action Potential: Depolarization Phase Na+ permeability increases; membrane potential reverses Na+ gates are opened; K+ gates are closed Threshold – a critical level of depolarization (-55 to -50 mV) At threshold, depolarization becomes self-generating and progresses by positive feedback Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.12.2 Action Potential: Repolarization Phase Slow Na+inactivation gates close A) Membrane permeability to Na+ declines to resting levels AP spike stops rising B) As sodium gates close, slow voltage-sensitive K+ gates open K+ exits the cell and internal negativity of the resting neuron is restored (-70mV) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.12.3 Action Potential: Hyperpolarization Potassium gates remain open, causing an excessive efflux of K+ before K+ channels close This efflux causes hyperpolarization of the membrane (undershoot) The neuron is insensitive to stimulus and depolarization during this time Na+ channels reset Na/K pump redistributes ions Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.12.4 Action Potential: Role of the Sodium-Potassium Pump Repolarization Restores the resting electrical conditions of the neuron Does not restore the resting ionic conditions Ionic redistribution back to resting conditions is restored by the sodium-potassium pump Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Phases of the Action Potential 1 – resting state 2 – depolarization phase 3 – repolarization phase 4 – hyperpolarization Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.12 Propagation of an Action Potential When Na+ channels close, the AP must propagate away from that portion of membrane. How?? AP is initiated at one end of the axon and is conducted away toward the axons terminus. How? When Na+ channels close (and K+ channels open), we see a repolarization event “chase” the depolarization event. This only occurs in unmyelinated axons. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Propagation of an Action Potential (Time = 0ms) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.13a Propagation of an Action Potential (Time = 2ms) Ions of the extracellular fluid move toward the area of greatest negative charge A current is created that depolarizes the adjacent membrane in a forward direction The impulse propagates away from its point of origin (the origin is still repolarizing!) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Propagation of an Action Potential (Time = 2ms) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.13b Propagation of an Action Potential (Time = 4ms) The action potential moves away from the stimulus Where sodium gates are closing, potassium gates are open and create a current flow Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Propagation of an Action Potential (Time = 4ms) Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.13c Threshold and Action Potentials Threshold – membrane is depolarized by 15 to 20 mV Established by the total amount of current flowing through the membrane Weak (subthreshold) stimuli are not relayed into action potentials Strong (threshold) stimuli are relayed into action potentials All-or-none phenomenon – action potentials either happen completely, or not at all Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Coding for Stimulus Intensity All action potentials are alike and are independent of stimulus intensity Strong stimuli can generate an action potential more often than weaker stimuli, E.g. rate or frequency of stimuli Amplitude does not increase (always constant) The CNS determines stimulus intensity by the frequency of impulse transmission Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Stimulus Strength and AP Frequency Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.14 Absolute Refractory Period Time from the opening of the Na+ activation gates until the closing of inactivation gates The absolute refractory period: Prevents the neuron from generating an action potential Ensures that each action potential is separate Enforces one-way transmission of nerve impulses PLAY InterActive Physiology ®: Nervous System I: The Action Potential, page 14 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Absolute and Relative Refractory Periods Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.15 Relative Refractory Period The interval following the absolute refractory period when: Sodium gates are closed Potassium gates are open Repolarization is occurring The threshold level is elevated, allowing strong stimuli to intrude into the relative refractory period and increase the frequency of action potential events PLAY InterActive Physiology ®: Nervous System I: The Action Potential, page 15 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Conduction Velocities of Axons Conduction velocities vary widely among neurons Rate of impulse propagation is determined by: Axon diameter – the larger the diameter, the faster the impulse Presence of a myelin sheath – myelination dramatically increases impulse speed PLAY InterActive Physiology ®: Nervous System I: Action Potential, page 17 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Saltatory Conduction Current passes through a myelinated axon only at the nodes of Ranvier Voltage-gated Na+ channels are concentrated at these nodes Action potentials are triggered only at the nodes and jump from one node to the next (about 1mm) 30x faster than conduction along unmyelinated axons Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Saltatory Conduction Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.16 Multiple Sclerosis (MS) An autoimmune disease that mainly affects young adults Symptoms: visual disturbances, weakness, loss of muscular control, and urinary incontinence Nerve fibers are severed and myelin sheaths in the CNS become nonfunctional scleroses Shunting and short-circuiting of nerve impulses occurs Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Multiple Sclerosis: Treatment The advent of disease-modifying drugs including interferon beta-1a and -1b, Avonex, Betaseran, and Copazone: Hold symptoms at bay Reduce complications Reduce disability Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings