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Anatomy, composition and physiology of neuron, dendrite, axon,and by W5wI9I

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									Anatomy, composition and
physiology of neuron, dendrite,
axon, and synapses
Glial cells/ Astrocyets, Oligodendrocyt , Schwan cells/

   Support neuronal cells
   Produce myelin
   Act as scavengers
   Takes up released neurotransmitters
   Guide migrating neurons and the out growth of axons
   Form BBB
   Release growth factor and help nourish nerve cells
Neurons type
   Unipolar cells: single process serving as receptor and releasing terminal
                      e.g. In autonomic nervous system
   Biplar cells : two process dendrite and axon
                      e.g. in retina , olfactory nerves
   Pseudo_unipolar cells:
                      e.g. dorsal root ganglia
   Multipolar cells : most common type
                      e.g. spinal motor neurons ,pyramidal cells,pukrinje cells
   NERONS COULD ALSO BE CLASSIFIED AS :
           SENSORY
           MOTOR
           LOCAL INTERNEURON
           PROJECTION INTERNEURON
           NEUROENDOCRINE CELL
         Structure of a neuron
   Cell body
   Dendrites:
         apical and basal types,
               are input elements together with the cell body contains nucleus and gives rise to axon and dendrite


   Axons: transmitting element,
          could be longer than 3m,
         covered with myelin interrupted by node of Ranvier
          is the out put element of neuron
          single axon may form synapses with as many as 100000 neurons
   Axon hillock: initial segment of neuron
   Synapse:
          presynaptic terminal,
          synaptic cleft ,
         postsynaptic membrane
               dendrites
                cell body
               axon hillock
               muscle
         synaptic connections could be divergent or convergent
Functional organization of neurons

   Input component /receptor or synaptic potential/ :
          signal electrical
        input signal is graded in amplitude and duration,
         proportional to amplitude and duration of stimulus


   Integrative component :
        signal electrical
        action potential is generated only if input signal is greater than spike
           threshold
          stimulus intensity is represented by frequency of action potential
          duration of stimulus is represented by number of action potentials
Functional organization of neurons

   Conductile component/action potential/ :
        Signal is electrical
        action potentials are all or none
        every action potentials have same amplitude and duration
        information in the signal is represented by frequency and duration
   Out put component :
         signal chemical transmitter
        total number of action potential determine how much
           neurotransmitter should be released
Comparison of local and propagated signals

   Input signals                       Action potential
                                              amplitude          large
        amplitude        small                duration           brief
        duration         brief                summation          all/none
        summation        graded                        signal effect
        signal effect depolarizing       depolarize    propagation
        propagation      passive
                                              active
Cytology of Neurons

   Nucleus
   Nuclear envelope
   Cytoplasm
        cytosol including cytoskeletal matrix
        membranous organelle
   Plasmalemma
   myelin
Membranous organelles

   Mitochondria and peroxisomes
   Rough endoplasmic reticulum/smooth endoplasmic reticulum
   Golgi complex
   Secretory vesicles, endosomes, lysosomes
   Most of these structures are abundant in the cell body and dendrite
    and there are no synthetic function differences between cell body and
    dendrite.
   The axon has few mitochondria and smooth endoplasmic reticulum
    with abundant secratory vesicles
Synthesis and trafficking of neural proteins

   Most proteins are synthesized in the cell body
            The neuron express more the total genetic material than any other
             organ
            Neural cells are engaged in protein synthesis more often than other
             cells and hence their chromosomes uncoiled
            Ribosomal and m RNA are synthesized in the nucleus and exported
             through nuclear pore
            Some genetic information is also contained in the mitochondria
            Protein synthesis occurs in cytosol where mRNA ribosome and tRNA
             form complex
            Secretory proteins and vacuolar apparatus and plasmalemma are
             synthesized and modified in the endoplasmic reticulum
            Secretory proteins are processed further in the golgi complex and then
             transported
cytoskeleton

   Microtubules     largest diameter fibers
                     helical cylinders
                     made of protofilaments
                      undergo cycles of polymerization and
                     depolymerization
   Neurofilaments   monomers twist to form dimer
                     protofilament
                     protofibril
                     filament
   Microfilaments   polymerized actin monomers
                     smallest-diameter fibers
                     undergo cycles of polymerization and
                     depolymerization
Anterograde Axonal Transport
   Fast phase of axonal transport
        70-400 mm/d
        20-70mm/d
        4-20 mm/d
        Convey mainly plasma membrane proteins such as
             acetyl cholinesterase
             mitochondria
             multivesicular bodies and secretoty vesicles
             ATPases
        The physiological properties of fast phase axonal transport has been
         important for tracing connections in the brain
   Slow phase of axonal transport
        1-4 mm/d
        0.2-1.2mm/d
        Conveys mainly
             Neurofilaments and Microtubulins
             Actin and certain glycolytic enzymes
Retrograde Axonal Transport
   Toxins, drugs, heavy metals
   Neurotropic viruses
   Nerve Growth Factors and neurotrophines
   Mitochondria, endosomes
        Axonal vs dendrite transport
   Axonal                               Dendrite
      Nerofilaments abundant               Microtubules abundant
      Microtubules are of tau type          MAP-2 type
      Microtubules are uniformly           Microtubules are bidirectionaly
       arranged                              arranged
                                            These difference in
                                             arrangement explain the
                                             polarization of organelles
myelin
   Insulate axons and facilitates speed of action potential transmission
   Arranged in concentric bimolecular layers
   Has a composition similar to plasma membranes
   Schwan cells form myelin of peripheral nerves and Oligodendrocyts
    that of central nerves
   Schwan cells express their myelin gene in response to contact with
    axon while Oligodendrocytes depend also on the presence of
    Astrocytes
    Myelin Proteins
    Myelin Basic Protein             important for myelin compaction
                                      strongly immunogenic
                                      used to produce experimental allergic
                                                 encephalomyelitis
    Myelin Associated glycoprotein   supper family of immunoglobulin
                                      involved in cell to cell recognition
                                      is an adhesion molecule that initiate
                                                 myelination
    Protiolipids                     important for compaction of myelin
                                      mutation in the gene causes
                                      hypomyelination and degeneration

    Myelin protein zero              major protein in peripheral myelin
                                      immunoglobulin family
                                      important for myelin compaction
                                      mice that lack the protein have poor motor
                                                coordination
    Peripheral myelin protein 22     encoded by chromosome 17
                                      DNA duplication results in CMT disease
Ion channels
   Conduct ions at fast rate
   Selective for specific ions
   Ion channels are proteins that span the cell mme
   Flux of ions through the ion channel is passive
   Opening and closing of ion channel involves conformational change
   Open and close in response to specific stimulus
        Voltage –gated
        Ligand –gated
        Mechanically –gated
        Gap-junction channels
   The binding of exogenous ligands /toxins, poisons and drugs/can make channels
    open or close
   Ion channels are composed of several subunits
   Channels are also important targets of diseases
        myasthenia gravis
        hyperkalemic periodic paralysis
Synaptic transmission
   The average neuron makes 100000 connections
   Two basic forms of transmission
        Chemical
        Electrical
   Electrical transmissions are
              Short lasting
              Only excitatory
              Do not induce long lasting postsynaptic changes
              Gap junction channels
              Bidirectional transmission
   Chemical transmissions are
              Variable signaling :inhibitory or excitatory
              Produce complex behavior
              Longer lasting / delay in transmission
              Amplify signals
              Modify post synaptic receptors both functionally and anatomically
              Ionotropic receptors :conformational change that opens the channels on binding transmitter
              Metabotropic receptors: act by altering intracellular metabolic reaction
Chemical transmitters
   Classical
        Acetylcholine
        Cathecolamines
        Glutamates
        GABA
        Serotonine
        histamine
   Peptides
        Substance p
        Enkephaline
        Endorphine
        Prolactin, oxytocine, vasopresin
   Soluble gases
        NO
Cellular basis of connectionist approach

   Principle of dynamic polarization : electrical signals within a
    nerve flow only in one direction
   Principle of connectional specificity : nerve cells do not connect
    indiscriminately with one another to from a network
Specificity and modifiability of neuronal connections

   Specific networks
        Brain has at least two types of neuronal map/ motor and sensory maps/
         which are interconnected with each other by interneuron. The neurons that
         make up these map do not differ greatly in their electrical properties.
         Rather, They have different function because of the connections they make.
   Parallel processing:
        deployment of several neuron groups or several pathways to convey similar
          information
   Plasticity :
        functional transformation in neurons as a result of appropriate stimulation
How dose nerve cells differ ?

   Lack of axon
   Location of synaptic in puts on the cell
        cell body
        dendrite
         axon hillock
        type of target cell
   Difference in
        cell body size and shape,
        distribution of axon and dendrite tree
   Expressing different combination of ion channels
         providing them with different thresholds ,excitability and firing patterns.
         Thus ,neurons with different ion channels encode the same class of
         synaptic potential into different firing patterns and thereby convey different
         signals
Cont.
   Chemical transmitter
        The type of neurotransmitter they use
        The type of Receptors they have
   Myelin content
   Location in the nervous system –central/peripheral
   These differences and others may account for different patterns of
    disease

								
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