NERVE CELL FUNCTION   -   Reception-Transduction-Response*                is akin to SIGNAL TRANSDUCTION        : reception - transduction - response                 and a HOMEOSTATIC REGULATOR       : receptor - controller - effector                      1.  sense organs via Peripheral NS gathers sensory input  --> signals info in                           2.  integration (transduces) information (within CNS - brain & spinal cord)                   3.  responds via motor output (effector organs & PNS - muscles : )

  
    
                             a common pathway in a Nervous System*

 
                Reflex Arc - hard wired, unconscious rapid response to external stimulus
                                 involving spinal nerves & effector cell electrical impulses
        
                                  narrated explanation of spinal reflex arc.*

                                  knee-jerk reflex*   or   another spinal reflex

  
                                                   
                          To study the actions and functions of nerves and muscles    
                                    a good model experimental system is the...

                                             neuro muscular*junction
   

STRUCTURE of an individual NERVE CELL [a NEURON*]      the term neuron was coined by the German anatomist Heinrich von Walder-Hartz         in 1891 to describe an individual cell in the brain.
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* (pic)             - long outgrowth of cell body - carry signals to next neuron    &  Schwann cell      - cells surrounding peripheral neuron axons in vertebrates -                                              produce myelin (sheath) membrane     (mylein origins?*)                                                 protein + lipid-like membrane insulating material surrounding axon      Nodes of Ranvier*  - space between successive Schwann cells along axon...                                               node area is a non-myelinated area that regulates the                                              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...

 
 
   What is the informational content of neurons???
                    it's in the ELECTRICAL PROPERTIES of neuronal cells...
  What is the the electrical charge across a cell's membrane...
                    what is a  RESTING & ACTION potentials

  RESTING POTENTIAL - is a characteristic ELECTRIC CHARGE
             exhibited by cells at rest...  most often inside NEGATIVE (-) in animal cells
             which is due to the existing distribution of ions across the cell membrane.             

  electrical 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*   recording equipment* 

	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...      what is cause of inside  -65 to -70 mVi  for SGA ? 
                       homeostatic distribution of  ions in neurons & surroundings*
                       all of which result in the inside of cell being negative (-65mV)  But HOW ?
                       how are these ions distributed across a SGA - passively or actively?

   Ion transport through channel proteins establishes the Resting Membrane Potential include:
    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                      

animation*view             Summary of SGA [ions]  &  resting potential*

ACTION POTENTIAL...  change resting potential charge due to passive ion transport
                                  
  9e: c4
              -   name given to changes in electrical charges occurring during stimulation of a nerve cell,
              -   transient change in the RP voltage at one point along membrane of nerve cells...
              -   change in charge is self-propagating along an axon
              -   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, including cyanide
              ... requires Na/K pump and ion channels for Na & K  (fig*) to produce ion distributions
              ... is measured at local point using microelectrodes* impaled in cells or a nerve chamber* 
              ... has a threshold - a minimal amount of stimulus is needed to open channels & "fire" an AP
              ... is an "all-or-none" phenomena,  either yes or no,  but is always of same magnitude
              ... is very rapid - time course = 2-3 msec to elicit change in polarity

 
  EVENTS DURING an AP    oscilloscope trace*     graph of AP   
   Voltage changes the  permeability of axonal membranes and elicit an Action Potential.
   
   depolarization* -  cell goes from inside negative (-) to inside positive (+)
                                Na channel opens - Na diffusively floods in -->  -70mV toward +62mV
   repolarization*  -  Na channels close & K channels open outward [returns to inside (-)]
                                K follows its diffusive gradient & K diffuses out of cell (returns -)
   hyperpolarization  - "undershoot or overshoot" of resting potential (-75mv)
   refractory period   - time before another AP can "fire" again (about 2-3 mSec)    
              overall mechanism***   &      AP animation (Pearson)*     Current changes during an AP     
                                                                      
   
CONDUCTION of an AP along an axon     
            local spreading of electric charge depolarizes adjacent membranes 
            changes the membrane permeability of adjacent non-myleinated region
            leading to continuous propagation in an autocatalytic - "domino effect"...  figure*
     
                     AP Conduction is continuous in  non-mylinated axons        =    2-3 m/sec
                     Saltatory Conduction* in mylinated axons  [node to node]  =   100 m/sec

animation of conduction*           animation of conduction*long version - view@home

    
 
 Synaptic Transmission...   synapse animation*                     .
  
        synapse - functional space connecting two neurons allowing transmission of
                                              AP's between cells:   may be electrical or chemical
        synaptic cleft - [20 to 30 nm across] open space between neurons
                                                         across which a chemical neurotransmitter may diffuse
        synaptic knob - site of vesicles holding neurotransmitter at end of axon
        synaptic vesicle - holds neurotransmitters  (ex: acetylcholine in neuromuscular junctions)
                                            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"

     
Post-synaptic membrane potential responses...
      depends upon post-synaptic receptor type and how it responds.

  EPSP - excitatory post-synaptic potential  [goes positive from RP [-65mVi] to -55mVi]
              excitatory PSM neurons --> open Na channels --> inside + --if threshold--> AP
                                        figure*
  IPSP - inhibitory post-synaptic potential    [goes negative from RP to -80mVi]             
             inhibitory PSM neurons --> opens Cl channels -  Cl-in  ->   more -  -->  no AP
                                                   -->  opens K channels - K-out  ->  more  -  -->  no AP   
 
  AP - all or none 120mv depolarization/repolarization (from -65  to  +55 mVi)  

	multiple innervations  and  integration of EPSP & IPSP impulses*
	temporal/spatial summations*    and   summary of neural impulses*        How is Sensory Information encoded*                              Review of Action potentials & Synaptic Transmission* [<-- view @ homework ]

   


Neurotransmitters...   some stimulators and drugs.
  

Some common Neurotransmitters...     Table of transmitters

Stimulants/Depressants - are chemicals that effect the activities of neurons...

cocaine - blocks re-uptake of* dopamine by synaptic vesicles -->  continual stimulation                             of reward-motivated behavior           animation of cocaine action

flea collars - fipronil - blocks GABA-gated* Cl channels (normally inhibitory),                    thus preventing hyperpolarization = results in excessive neural excitation,                     a hyper-excitation of CNS of the fleas and death.

strychnine (a poison) - antagonist of inhibitory GLY & ACH receptors in spinal cord,                    resulting in increase muscular convulsion & asphyxia.

Sarin (a nerve agent) blocks action of acetylcholinesterase, thus overloading body's                    nervous system in toto, ultimately paralyzing the diaphragm = suffocation.

Key Concepts*    synaptic cleft - the Glut-Tang Clan video

END      copyright c2024  Charles Mallery,      Biology 150, Department of Biology,   U. of Miami,  Coral Gables, FL 33124

           copyright c2024      Charles Mallery,      Biology 150, Department of Biology,   U. of Miami,  Coral Gables, FL 33124

  SKIP the MATERIAL BELOW

         NRC-Summary        

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 
                         

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

                  Prozac and Paxil (antidepressants)...       animation
                  blocks reabsorption of serotonin from synaptic cleft
                  & boost synthesis of serotonin, epinepherine, & norepinepherine
        Parkinson's = lack of dopamine   schizophrenia = too much dopamine
        LSD/mescaline - psycho-active drugs
                    function by binding to serotonin/dopamine brain cell receptors

   facilitates juvenile brain plasticity

     
Campbell reads:      10e: c48 (pg 1061-1077) c49 (pg 1079-1083)
                                                                                                 11e: c48 (pg 1065-1079) c49 (pg 1083-1088)