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Neurophysiology Outline

by: Chantay Harris

Neurophysiology Outline BIOL 221

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Chantay Harris

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Notes taken on Neurophysiology
Anatomy & Phisiology 1
Professor Colleen Winters
anatomy, neurophysiology
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This 5 page Bundle was uploaded by Chantay Harris on Sunday April 3, 2016. The Bundle belongs to BIOL 221 at Towson University taught by Professor Colleen Winters in Spring 2016. Since its upload, it has received 11 views.


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Date Created: 04/03/16
NEUROPHYSIOLOGY ­Lecture outline Spring 2015 Reading: pages 398­425 Vocabulary Ion atoms or molecules that bear a net charge because they have unequal numbers of protons and electrons.  Current The movement or flow of charges Proton Positively charged ion ElectronNegatively charged ion Permeable characteristic of cells to let things enter or leave Equilibrium Where there is no net movement of a particular ion across the plasma membrane  ResistanceMeasure of how much the membrane restricts ion movement  Excitatory depolarizes Inhibitory hyperpolarizes Semipermeable “selectively permeable” Propagation Repeat of event  Decremental potential weakens further from stimulus  Depolarize  more positive Hyperpolarize more negative Repolarize restore back to RMP Protons and electrons have electrical charge, and ions are atoms or molecules that bear a net charge because  they have unequal numbers of protons and electrons.  The movement or flow of charges makes up an electric current which is similar to the flow of water through  pipes. When we separate positive and negative electrical charges we call this Voltage or Potential difference  • The potential difference can do work when charges are allowed to flow as a current.  o In our bodies the positive and negative charges are separated across cell membranes and we call  this the Transmembrane potential  ▪ because the inside is more negative than the outside we say the transmembrane potential  of a resting neuron (resting membrane potential) is –70 mV o What is responsible for the difference ▪ Ions are distributed unequally  ▪ inside the cell we also have negatively charged proteins  ▪ cell membranes are semipermeable  So what causes the ions to flow into or out of the cell if the membrane channels are open? • Chemical gradients Ions move from high to low concentration  • Electrical gradients Ions move to oppositely charged sides • Electrochemical gradients Net between electrical and chemical gradients (normally follows  chemical)  • Establishing the Resting Membrane Potential • To maintain RMP and return cells to RMP after a change in membrane potential • Sodium­potassium (Na ­K ) ATPase pump Membrane Channels that allow ion movement         Passive channels or leak channels    always open dendrites, axon, soma         Chemically regulated (ligand regulated) channels   – (Fig 12­10a) Ligands= key dendrites and soma local         Mechanically regulated channels   – (Fig 12­10c) physical deformation of gate dendrites and soma local         Voltage regulated channels  – (Fig 12­10b) based on transmembrane potential axon action Overview of neuron activity Local Potentials or Graded Potentials (Fig 12­11) When a neuron is stimulated by a signal from another neuron, a ligand binding to a chemical channel or a shape  change in a mechanically regulated channel it causes small local disturbances in the membrane potential. These  channels allow Na  to flow into the cell.   O       DEPOLARIZATION   o Incoming Na+ ions diffuse short distances from the initial site producing a current along  the dendrite and cell body toward the axon hillock or trigger zone; local potential – short distance o Characteristics of a local potential         Gra  Can vary in magnitude depending on strength in stimulus bell ringer         Decrementa  the further away from the stimulus, the weaker the potential ripples         Reversi  When stimulus stops, channels close, potential stops         Excitatory or inhibitory (Fig 12.13  o Excitatory:  DEPOLARIZE       o only local potential that can bring axon hillock to threshold  o Inhibitory:  HYPERPOLARIZE  (make more negative)  O   Na ­K  ATPase pumps return  cell to resting membrane  potential ­  REPOLARIZATION Action Potential (Fig 12.14) Neurons can generate an electrical signal or action potential. The ion channels that produce action potentials are voltage­gated channels, that is, their opening depends on the membrane potential.  o Local potential at axon hillock increases until it rises to threshold (­60 mV) o Neuron produces an action potential; voltage­regulated Na  channels open; more + + and more Na  gates open as Na  enters the cell; K  gates open more slowly when threshold is reached (rapid depolarization) + o When 0mV is reached/passed, Na  gates are; voltage peaks at approx. +35mV (0mV in some, +50mV in others) + + o K  gates now fully open; K  leaves the cell  repolarizing the membrane; causing shift back to negative inside and positive outside + + + + o    K  channels remain open a little longer than the Na  channels and more K  leaves than Na  came  in causing a 1 or 2 mV overshot or hyperpolarization o Characteristics of action potentials          All or none   Either it reaches threshold or it doesn't           No signal degradat  Same strength the whole way          Irrever  ble o Starts at axon hillock Refractory Period (Fig 12.14) o    During an action potential and a few msec after, it is difficult or impossible to stimulate or produce another action potential  – Refractory period o Two phases of refractory period o Absolute refractory period Runs from when Na+ channels open at threshold to when they reset o Relative refractory period Runs from Na+ channel reset to when RMP is restored ▪ A new action potential can be made, but needs a stronger stimulus than normal o because the local current must deliver enough Na+ to counteract the exit of K+  ions  o and because the membrane is hyper polarized to some degree during the relative  refractory period  Signal propagation in nerve fibers  Unmyelinated fibers: (Continuous propagation) (Fig 12.15)  • No insulation, action potential runs out faster Myelinated fibers (Saltatory propagation) (Fig 12.16) Axon diameter and propagation speed  ­TYPE A: LARGE DIAMETER (4­20  ?m) MYELINATED • 440 ft per second  • sensory­ balance, position, touch • motor­skeletal muscle  ­TYPE B: MEDIUM DIAMETER (2­4  ?m) MYELINATED • 60 ft per second • Sensory: temperature, pain, touch • Motor: smooth muscle, cardiac muscle, glands  ­ TYPE C: SMALL DIAMETER (< 2  ?M) UNMYELINATED • 3 ft per second Clinical Note:  Multiple sclerosis Patchy loss of myelination in the brain and spinal cord, replaced with scar tissue Slower and less effective signaling speed Slows action potentials Spasms, Weakens limbs, bladder dysfunction, sensory loss Videos to help you: Review questions: 1. What causes K  to diffuse out of a resting cell?Chemical gradient (hi­low) What attracts it into the cell?  Electrical gradient (K+ moves to negative inside) + 2. What happens to Na+ when a neuron is stimulated on its dendrite? It enters the cell Why does the  movement of Na  raise the voltage on the plasma membrane? Na+ is a postive ion, so it makes the  membrane potential more positive as it enters the cell  3. What does it mean to say a local potential is graded The stronger the stimulus, the stronger the potential , decremental The further away from the initial stimulus, the weaker the potential, and reversible Once  stimulus stops, channels close and potential stops? 4. How does the plasma membrane at the trigger zone differ from that on the soma?The plasma membrane  has voltage gated channels whereas the soma does not. How does it resemble the membrane at a node of  Ranvier? 5. What makes an action potential rise to +35 mV? What makes it drop again after this peak? 6. List four ways in which an action potential is different from a local potential.  • Irreversible • Not graded • Starts at axon hillock not dendrites/ soma • All or none rule  7. Explain why myelinated fibers conduct signals much faster than unmyelinated fibers. In myelinated  fibers, the action potential “jumps” from node to node rather than move along the axon in tiny steps 8. Hyperkalemia is an excess of potassium in the extracellular fluid. What effect would this have on the  resting membrane potentials of the nervous system and on neural excitability? It would increase the  membrane potential and since there were no K+ leaving, it would depolarize the cell, getting it closer to  threshold, making it easier to excite.  9. Suppose a poison were to slow down the Na ­K  ATPase pumps of nerve cells.  How would this affect  the resting membrane potentials of neurons? Would it make the neurons more excitable than normal, or  make them more difficult to stimulate? Why? 


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