Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Human Anatomy & Physiology, Sixth Edition Elaine N. Marieb PowerPoint ® Lecture Slides prepared by Vince Austin, University of Kentucky 11 Fundamentals of the Nervous System and Nervous Tissue Part B

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings  Potential energy generated by separated charges is called voltage.  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 Electrical Current and the Body

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings  Types of plasma membrane ion channels:  Passive, or leakage, channels – always open  Chemically gated channels – open with binding of a specific neurotransmitter  Voltage-gated channels – open and close in response to membrane potential (change in charge)  Mechanically gated channels – open and close in response to physical deformation of receptors Role of Ion Channels InterActive Physiology ® : Nervous System I: Ion Channels PLAY

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Operation of chemical Gated Channel Figure 11.6a

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Operation of a Voltage-Gated Channel Figure 11.6b

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings  When gated channels are open:  Ions move along chemical gradients, diffusion from high concentration to low concentration.  Ions move along electrical gradients, towards the opposite charge. Together they are called the Electrochemical Gradient  An electrical current and Voltage changes are created across the membrane Gated Channels

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Electrochemical Gradient  The EG is the foundation of all electrical phenomena in neurons.  It is also what starts the Action Potential.

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings  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  )  The cytoplam inside a cell is negative and the outside of the cell is positive. (Polarized) Resting Membrane Potential (V r )

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings  Used to integrate, send, and receive information  Membrane potential changes are produced by:  Changes in membrane permeability to ions  Alterations of ion concentrations across the membrane  Types of signals – graded potentials and action potentials Membrane Potentials: Signals

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings  Changes are caused by three events  Depolarization – the inside of the membrane becomes less negative  Repolarization – the membrane returns to its resting membrane potential  Hyperpolarization – the inside of the membrane becomes more negative than the resting potential Changes in Membrane Potential

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Changes in Membrane Potential Figure 11.9

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings  Short-lived, local changes in membrane potential  Decrease in intensity with distance  Their magnitude varies directly with the strength of the stimulus  Sufficiently strong graded potentials can initiate action potentials Graded Potentials

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Graded Potentials  A stimuli from sensory input causes the gated ion channels to open for a short period of time.  Positive Cations flow into the cell and move towards negative locations around the stimuli.  Alternately the now negative area on the outside of the cell will flow towards the positive areas.  However, this spread of depolarization is short lived because the lipid membrane is not a good conductor and is very leaky, so charges quickly balance out.

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Graded Potentials Figure 11.10

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Graded Potentials Figure 11.11

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings  A brief change in membrane potential from - 70mV(resting) to +30mV (hyperpolarization)  Action potentials are only generated by muscle cells and neurons  They do not decrease in strength over distance  An action potential in the axon of a neuron is a nerve impulse Action Potentials (APs)

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings  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 Action Potential: Step 1 Resting State Figure

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings  The local depolarization current flips open the sodium gate and Na + rushes in.  Threshold: when enough Na+ is inside to reach a critical level of depolarization (-55 to -50 mV) threshold, depolarization becomes self-generating. Action Potential: Step 2 Depolarization Phase Figure

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Action Potential: Step 2 Cont.  Na + will continue to rush in making the inside less and less negative and actually overshoots the 0mV (balanced) mark to about +30mV.

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings  After 1 ms enough Na+ has entered that positive charges resist entering the cell.  Sodium inactivation gates close and membrane permeability to Na + declines to resting levels  As sodium gates close, voltage-sensitive K + gates open  K + exits the cell and internal negativity of the resting neuron is restored Action Potential: Step 3 Repolarization Phase Figure

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Action Potential: Step 4 Hyperpolarization  Potassium gates are slow and remain open, causing an excessive efflux of K +  This efflux causes hyperpolarization of the membrane (undershoot).  The neuron is insensitive to stimulus and depolarization during this time Figure

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings  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 Action Potential: Role of the Sodium-Potassium Pump

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Phases of the Action Potential  1 – resting state  2 – depolarization phase  3 – repolarization phase  4 – hyperpolarization

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings  When one area of the cell membrane has begun to return to resting the positivity has opened the Na+ gates of the next area of the neuron and the whole process starts over.  A current is created that depolarizes the adjacent membrane in a forward direction  The impulse propagates away from its point of origin Propagation of an Action Potential

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Propagation of an Action Potential (Time = 0ms) Figure 11.13a

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Propagation of an Action Potential (Time = 1ms) Figure 11.13b

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Propagation of an Action Potential (Time = 2ms) Figure 11.13c

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings  All action potentials are alike and are independent of stimulus intensity  Strong stimuli can generate an action potential more often than weaker stimuli  The CNS determines stimulus intensity by the frequency of impulse transmission Coding for Stimulus Intensity

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings  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 Conduction Velocities of Axons InterActive Physiology ® : Nervous System I: Action Potential PLAY

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings  Current passes through a myelinated axon only at the nodes of Ranvier  Voltage-gated Na + channels are concentrated at these nodes and Action potentials jump from one node to the next  Much faster than conduction along unmyelinated axons where the entire axon has continuous conduction. Saltatory Conduction

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Saltatory Conduction Figure 11.16