MEMBRANE POTENTIAL / NERVE CONDUCTION

I. INTRODUCTION

   A. neurons initiate and conduct nerve impulses

   B. neurons exhibit both excitability and conductivity

      1. excitability- ability to be stimulated

      2. conductivity- ability to transmit a wave of excitation
         from one point to another

   C. nerve impulse= an electrical signal; a wave of electrical
      fluctuation that travels along the plasma membrane

 

II. MEMBRANE POTENTIAL

    A. = difference in electrical charge between inside and
         outside of the plasma membrane

    B. this difference is called a "potential" because it
       represents potential energy (a type of stored energy)

    C. this difference in charge is based on a difference in the
       concentration of ions across the membrane

    1. there is a slight excess of positive ions on the outside of the membrane (mostly Na+ ions)

      2.there is a slight excess of negative ions on the inside
        of the membrane (phosphates and proteins)

    D. a membrane that exhibits a membrane potential is said to
       be polarized- the membrane has a negative pole (inside)
       and a positive pole (outside) 

    E. The ionic Basis of Nerve Impules Conduction

       1. situation at rest (neuron is not conduncting an impulse)    

          a. Na+ ions are most abundant outside the resting
             nerve cells

          b. K+ ions and organic anions are most abundant inside

          c. anions remain inside because they are too large to
             diffuse out

          d. negative charges inside the cell tends to draw K+
             and Na+ ions into the cell

          e. sodium-potassium pump (a type of active transport)
             produces a slight excess of positive ions on the
             outer surface of the membrane

             -transports sodium and potassium ions in opposite
              directions and at different rates

             -three sodium ions are pumped out of the neuron for
              every two potassium ions pumped in

          f. as a result, the predominant charge outside resting
             cells is positive, while inside it is negative

             -THIS MEMBRANE POTENTIAL= THE RESTING MEMBRANE
              POTENTIAL (RMP)

             -the potential can be measured in volts or millivolts

             -typically, the RMP = about -70 mV (-65 to -85 mV)

          

        2.situation upon stimulation (action potential)
 
          a. stimulation (excitation) occurs when a stimulus
             triggers the opening of additional sodium channels

          b. this permits more sodium ions to enter cell; sodium
             rushes toward the negative interior

          c. as a result, the magnitude of the membrane potential
             is reduced (the magnitude moves toward zero)
            = depolarization
 
          d. if the threshold potential (ca. -59 mV) is reached,
             additional sodium channels are opened

             (otherwise, membrane recovers back to resting)

          e. as a result, the membrane depolarizes further (the
             membrane moves past 0 mV to a peak of + 30mV); this
             reverses the polarity to inside-positive/ outside-
             negative

          f. sodium channels close

          g. repolarization begins when potassium channels open,
             allowing outward diffusion of potassium (K ions
             diffuse out / they are also repelled by Na ions
             inside)

             repolarization = phase of the action potential in
             which the membrane potential changes from its
             maximum degree of polarization toward the resting
             state potential

             repolarization reverses the polarity back to an
             inside-negative state

          h. sometimes too many potassium ions rush out of the
             cell causing a brief period of hyperpolarization

          i. finally, the resting potential is restored by
             the sodium-potassium pump

    F. refractory period

       1. is a brief period during which a local area of a neuron's
          membrane resists restimulation

       2. the refractory period guarantees conduction in only
          one direction

       3. two parts- absolute r.p. and relative r.p.

           a. absolute refractory period

              -the half a millisecond or so during which the
               local area of the membrane has surpassed the
               threshold potential and will not respond to any
               stimulus no matter how strong 

              -time between the opening of additional Na gates and
               the closing of the Na gates

           b. relative refractory period

              -the few milliseconds after the absolute refractory
               period

              - the time during which the membrane is depolarizing
                and restoring the resting potential

              - the membrane will respond to VERY STRONG STIMULI

    G. conduction of the action potential

       1. at peak of action potential, polarity is the reverse of
          the resting potential

       2. this reversal of polarity increases the permeability
          of the adjacent part of the membrane; this next
          segment exhibits an action potential; and so on

       3. thus the impulse is self-regenerating until it reaches
          the end of the cell

       4. in myelinated fibers conduction is saltatory

          -Na channels are concentrated in the nodes of Ranvier

          -electrical charges in the membrane only occur at
           the nodes of Ranvier

          -current flows across the myelin sheath from node to node

          -the action potential "leaps" from node to node

       5. speed of nerve conduction depends on the diameter of
          the nerve fiber and on the presence or absence of a
          myelin sheath

          -the larger the diameter, the faster the conduction

          -myelinated fibers conduct impulses more rapidly than
           unmyelinated fibers (up to 130 meters/sec)

III. SYNAPTIC TRANSMISSION

    1. synapse-membrane to membrane junction between a

      2. presynaptic neuron and a postsynaptic neuron, effector
      cell,or sensory cell

    2. a synapse consists of three structures

        1. synaptic knob

        2. a synaptic cleft

        3. the plasma membrane of the postsynaptic neuron

      C. mechanism of synaptic transmission 

         1. action potential reaches synaptic knob; calcium
            channels open  

         2. increase in intracellular Ca triggers vesicles to
            move to membrane of synaptic knob

         3. vesicles fuse with membrane and release contents; the
            neurotransmitter molecules diffuse across cleft

         4. neurotransmitter binds to receptors on postsynaptic
            membrane- certain ion channels open

         5. opening of ion channels causes a postsynaptic
            potential, either an excitatory postsynaptic potential
            (EPSP) or an inhibitory postsynaptic potential(IPSP)

         6. once a neurotransmitter binds to its postsynaptic
            receptors, its action is quickly terminated by
            one of two mechanisms:

            -molecules are transported back into synaptic knobs
             (repackaged in new vesicels)

            -molecules are metabolized into inactive compounds
             by enzymes

      D. neurotransmitters


         1. are chemicals by which neurons communicate

         2. can be classified by:

            a. function - ex.- excitatory or inhibitory

            b. chemical structure-the mechanism by which
               neurotransmitters cause change- four classes:

               -acetylcholine

               -amines

               -amino acids

               -neuropeptides

         3. acetylcholine

            a. has a unique chemical structure= acetate (acetyl-
               coenzyme-A) + choline

            b. deactivated by acetylcholinesterase / choline is
               reused to make new acetylcholine

            c. present at various locations- sometimes used in
               an excitatory role, other times, inhibitory

        4. Amines

           a. synthesized from amino acid molecules

           b. found in various regions of the brain

           c. examples- serotonin, histamine, dopamine,
              epinephrine, norepinephrine (the last 3 are
              catecholamines)

        5. amino acids

           a. believed to be among the most common
              neurotransmitters of the CNS

           b. in the PNS, amino acids are stored in synaptic
              vesicles and used as neurotransmitters

           c. examples- glutamate, GABA (Gamma-aminobutyric acid)

        6. neuropeptides

           a. made up of polypeptides

           b. examples- enkephalins, endorphins