Axon    [Loligo pealei]
                   there is a a great diversity* of Nervous Systems 

FUNCTIONS of NERVOUS SYSTEM -                                                  [ akin to SIGNAL TRANSDUCTION : reception - transduction - response ]                                              and a homeostatic regulator : receptor - controller - effector ]                1.  gathers sensory input (sense organs via Peripheral NS) --> info in                    2.  integrates information (CNS - brain & spinal cord)      3.  responds with motor output (effector organs via PNS - muscles)

STRUCTURAL PARTS of Nervous System - 
    central nervous system - brain and spinal cord...     (neural stem cell = origin of brain)
    peripheral nervous system - outside the CNS- carries signals in/out of CNS
       PNS = sensory  (affernet - in)     and    motor  (efferent - out)  neurons:
          - somatic nervous system - carries signal to skeletal muscle - under conscious control
          - autonomic nervous system - regulate homeostatic internal systems - involuntary control
                    2 complimentary    Parasympathetic (cranial & cholenergic) - calming: HR-, energy storage
                       systems:    Sympathetic (spinal & noradrenaline) - energy & arousal: HR+, glycogen--> glu
      
    
FUNCTIONAL TERMINOLOGY of Neurons  -  
            Nerve   -  bundle of individual neuron cells wrapped in connective tissue
            Ganglia -  cluster of cell bodies of individual neurons      
            Sensory neurons... (afferent neurons) - external stimuli from receptors toward CNS
            Interneurons... integrate & relay sensory input to motor neuron
            Motor Neurons... (efferent neurons) - convert signals to effector cells = response 
                                       

 
         common pathway through Nervous system*

 
        Reflex Arc - hard wired, unconscious rapid response to external stimulus
                                       involving spinal nerves & effector cell electrical impulses

                   knee-jerk reflex*   or   another spinal reflex

                                 animation: by the Animated Brain

                                 narrated explanation of spinal reflex arc:  by Sumanas Multimedia, Inc.
                                                   
            a Model experimental system of reflex for the neurophysiologist is the...

                                                     neuro*muscular junction

STRUCTURE  of an individual  NERVE  CELL

Cell Body        - is main part of neuron cell with cytoplasm & organelles    Dendrites*     - short cylindrical outgrowths of cell Body                                              carry signals (electrical impulses) into cell body    Axon*             - long outgrowth of cell body - carry signals to next neuron    Schwann cell* - cells surrounding axon in vertebrates -                                              produce myelin (sheath) membrane         (components)                                              protein + lipid-like membrane insulation surrounding axon     Nodes of Ranvier*  - space between successive Schwann cells along axon...                                                                   the inter-node area is a non-myelinated area                                                        speed of conduction -  w/o myelin speed is less (5 m/sec)                                                                                         w/myelin (100 m/sec or 200 mi/hr)                                                        Multiple Sclerosis - degenerative disease of myelin sheaths    Synaptic Knob* - enlarged ends of neuron holds neurotransmitters in synaptic vesicles    Glia cells* - (astrocytes) provide support, function as blood-brain barrier, etc...                                    [ pic1 &  pic2 ]

 
 The electrical properties of cells:    resting & action potentials

   RESTING POTENTIAL - the characteristic electric charge
                                             exhibited by a cell at rest...    most often negative (-)

             potential - (in electrical terms) is amount of electrical charge
                                at one point in an electric circuit compared to some other 
                                point in the same circuit often measured with a volt-meter (multi-meters*)

   How to measure membrane potentials* in cells - microelectrode*   pic*
                                             inside  vs.  outside of cells    

	SGA	-	65 to - 70 mVi
	Frog muscle fibers	-	90 mVi
	Nitella  (algae)	-	150 mVi
	Valonia  (algae)	+	15 mVi

 
  Causes of Resting Potential...      of -65 mV inside for SGA ... 
           ion distributions* -   all of which make inside of cell negative (-)  But HOW ?
                                             How do these ions distribute themselves across a SGA?

    1.  active transport of Na & K*   high Na outside [3]  & high K inside [2] via NaK-ATPase
    2.  differential permeability*  diffusion of  K (faster out)  &  Na (slower in) = inside (-)
    3.  lots of  protein anions*  (-) at cell pH,  thus  inside more (-) then outside
    4.  normal  diffusion of*  Cl^- into the cell                     

Summary of SGA [ions] & resting potential*

 ACTION POTENTIAL - 
              -  self-propagating transient change in the RP voltage across membrane of nerve cells...
              -  name given to changes in electrical charges occurring during stimulation of a nerve cell, 
              -  usually visualized graphically from an oscilloscope* recording [graph of AP*]

    PROPERTIES of an AP... 
              ... requires a living cell, i.e., requires O₂ for metabolism 
                                                          is eliminated by metabolic poisons, such as cyanide
              ... is measured using microelectrodes impaled into cells 
              ... has a threshold - minimal amount of stimulus is needed to "fire" an AP
              ... is an "all-or-none" phenomena,  either yes or no, no in-between
              ... is very rapid - time course = 2-3 msec

 
EVENTS DURING an AP    oscilloscope trace*     graph     [Gasser & Erlanger]     
   depolarization* - cell goes from inside negative (-) to inside positive (+)
                                Na channel opens - Na diffusively floods in -->  -70mV toward +58mV
   repolarization*  - Na channels close & K channels open [returns to inside (-)]
                                K follows its diffusive gradient & K diffuses out of cell
   hyperpolarization  - "undershoot" of resting potential (-75mv)
   refractory period - time before another AP can 'fire'     
            current changes during an AP*     and     overall mechanism*
                                                                                                AP animation (Blackwell Publishing)*

CONDUCTION of an AP along an axon     
            local spreading of electric charge depolarizes adjacent membranes 
            change in membrane permeability of adjacent non-myleinated region
            leads to an autocatalytic - "domino effect"....... figure* [nonmylinated = 5 m/sec]

	myleinated Saltatory Conduction*     node to node    = [100 m/sec]
	Saltatory Conduction Animation - Blackwell Publ.

  
 Synaptic Transmission...
        synapse - functional space connecting two neurons allowing transmission of
                                                        AP's between cells:   can be electrical or chemical
        synaptic cleft - open space between neurons [ 10 to 20 nm across ]
                                                       across which a chemical neurotransmitter may diffuse
        synaptic knob - site of vesicles holding neurotransmitter at end of axon
        synaptic vesicle - holds neurotransmitters  (ex: acetylcholine)
                                                         action potentials trigger release of neurotransmitters*
        pre-synaptic side - releases neurotransmitter
        post-synaptic side - has a receptor which binds transmitter and ...
        ... ion channels open - leads to change of potential charge 
                                                      on the post-synaptic membrane ----> new AP
        removal of stimulus - an enzyme destroys neurotransmitter:  ex. "ACH-ase"

	Synaptic transmission animation*
	Sumanas, Inc - chemical synapse transmission    [ <-- view for homework]

     
Post-synaptic responses:

  EPSP - excitatory post-synaptic potential  (from RP [-65mVi] to -15mVi)
             excitatory PSM neurons --> open Na channels --> inside + --may--> AP
  
  IPSP - inhibitory post-synaptic potential    (from RP to -75mVi)              figure*
             inhibitory PSM neurons --> opens Cl channels -  Cl-in  ->  more -  -->  no AP
                                                   -->  opens K channels - K-out ->  more  -  -->  no AP   
 
  AP - all or none 120mv depolaorization/repolarization (-65  to  +62 mVi)
 
 

	integration of impulses*with multiple innervations
	summation of impulses*    &   temporal/spatial summations*

  Neurotransmitters & Other stimulators and drugs ...
     Some common Neurotransmitters... Table of transmitters*    Harvey Project    Dr. King's site

Neurotransmitter actions on PSM

Stimulants/Depressants - chemicals that effect activities of neurons
  
    cocaine - blocks re-uptake of dopamine by synaptic vesicle -->  continual stimulation
                   animation of cocaine action
 
    caffeine - Caffeine's principal mode of action is as an antagonist of adenosine receptors in the
                   brain; reduced adenosine = increased dopamine = stimulation of blood pressure,
                   breathing, HR.    (caffeine archive)
   
    flea collars - fipronil - blocks GABA-gated Cl channels (normally inhibitory),
                    thus preventing hyperpolarization - results in excessive neural 
                    excitation,  hyper-excitation of CNS, of the fleas and death.
  
    barbiturates & valium - intensify the GABA (inhibitory) effects, thus are inhibitory
  
    poisons like strychnine - prevent loss of transmitter (Ach) = tetanus
                                                                                             Key Concepts*

        go to SENSORY PHYSIOLOGY*