plus... 100 x 10*9 neurons in our body
​neurons in CNS do not have centrioles so they cannot mitotically divide

material can move from soma to/from axon (axonal transport) along tubules by little motors called kinesin (to) and dynein (from) via fast (300mm/day) or slow (5mm/day) stream mechanism. 

   structural classification of neurons

       in addition to above structures:
perikaryon-cytoplasm (around nucleus)
neurofibrils-neurofilament bundles
nissl bodies-RER/ribosome clusters 
axoplasm-axon cytoplasm
axolemma-membrane of axon
collaterals-side branches off axon

telodendria- tiny extensions at end of axon
axon terminal-end of telodendria
​                          (synapse interface)
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c

   Neuron Structure

           types of neuroglial (glial) cells
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CNS:
1. Astrocytes:
    maintain the blood brain barrier
    stabilizes damaged tissue
    cytoskeleton provides
         CNS framework
    direct fetal neuron development 
    regulate interstitial fluid
         (ions/nutrients/Co2)
2. Oligodendrocyt​es:
    provide myelination (membranous
        wrapping) for CNS axons
        via cytoplasmic "pad"
        wrapping around axolemma =
        myelin internodes
    provide structural framework
        by binding adjacent neurons
3. Ependymal cells:
     line the ventricles/central canal
     of brain/spinal cord and
     produce CSF. 
4. Microglia: roving phagocytes that
     remove wastes and pathogens






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​PNS:
​1. Schwann cells: myelinate
​        axons of PNS neurons
2. Satellite cells: regulate
       levels of nutrients
​       and neurotransmitters





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        next up...what is an action potential and how is it initiated/propagated...

        schwann cells and myelination
the myelin sheath allows electrical impulses to travel along the length of the axon quickly. 

notice single neuron of somatic efferent/motor pathway with soma in CNS and axon running to effector

   Neural Tissue

         Types of
​   Neural Tissue

   blood brain barrier (bbb)

   Neurons

notice pre/post ganglionic neurons of        visceral motor/efferent pathway ANS

        peripheral nerve regeneration =
               Wallerian degeneration

 anaxonic neurons: many dendrites but no visible axons
                                     nobody is sure what they do...
  bipolar neurons: rare, tiny (<30micrometers) cells that are                                         found in sense organs like the eye
  unipolar neurons: sensory neurons of PNS with axons that                                              may run >1 meter long! axon starts where                                          the dendritic branches converge 
  multipolar neurons: two + dendrites and one axon
                                       most common type
                                       these are the motor neurons that convey                                             directions to effectors

1. Sensory neurons/afferent neurons receive external stimuli like light, sound, smell, contact, heat from the environment and convey this information to the brain/spinal cord. Sensory neurons are often unipolar with an axon that branches into two extensions; one dendritic that receives the stimulus and and one axon that transmits the sensory info to the spinal cord. 
Their cell bodies are found in sensory ganglia. 
Sensory neurons may be:
1. somatic: receive info about outside environment
2. visceral: receive info about internal environment
Sensory neurons exhibit:
interoceptors: monitor organ systems re stretch,                                 pressure, 
exteroceptors: monitor external environment re
                           touch, temp, pressure, taste,                                           sight, hearing, etc
proprioceptors: monitor skeletal system re
                              position/movement

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​2. Motor/efferent neurons have cell bodies in the CNS. Their axons extend outside of the CNS to control muscles via a neuromuscular junction. Motor neurons is multipolar  with
a single axon and multiple dendrites. May be:
1. somatic: innervate skeletal muscles
                    cell body in CNS...axon runs to NMJ
2. visceral: innervate all else...smooth/cardiac                               muscle and glands
                     axons of CNS motor neurons
                     synapse with second motor neuron
                     at autonomic ganglia 
                 (preganglionic -> postganglionic fibers)   



3. Interneurons are found in the CNS and serve to relay info between sensory and motor neurons. Their structure is also multipolar. 

 demyelination: destruction of myelin results in the loss of motor control and sensation.
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   in summary...

   functional classification of neurons