Sturcture of Neurons

• Neurons


  neurons = structural units of the nervous system

  there r billions of these large highly specialised cells that conduct messages in the form of nerve impulses from one part to the other
• Characteristics of Neurons - Extreme Longevity

  given good nutrition, they can function optimally for a lifetime
• Characteristics of Neurons - Amitotic


  neurons r amitotic

  = as neurons assume their role as communicating links of the nervous system, they lose their ability to divide

  = neurons can't be replaced if destroyed
• Characteristics of Neurons - High Metabolic Rate


  neurons have an exceptionally high metabolic rate + require cntinous + abundant supplies of oxygen + glucose

  they can't survive for more than a few mins without oxygen
• Neuron Cell Body - Nucleus & Plasma Membrane


  the neuron cell body consists of a spherical nucleus surrounded by cyto

  in most neurons, the plasma membrane of the cell body acts as part of the receptive region that reveives info from other neurons

  the cell body is the major biosynthetic centre + metaboilc centre of a neuron
• Neuron Cell Body - Protein & Membrane-making Machinery


  neuron cell bodies have the organelles needed to synthesise proteins - RER, free ribosomes + Golgi apparatus
• Neuron Cell Body - Cytoskeletal Elements


  microtubules + neurofibrils, which r bundles of intermediate filaments (neurofilaments) maintain cell shape + integrity

  they form a network thru out the cell body + its processes
• Neuron Cell Body Anatomy


  neuron cell body

  
• Cell Bodies in CNS & PNS


  most neuron cell bodies r located in the CNS, where they r protected by the bones of the skull + vertebral column

  clusters of cell bodies in the CNS = nuclei

  clusters of cell bodies in PNS = ganglia
• Neuron Processes


  armlike processes extend from the cell of all neurons

  the brain + spinal cord (CNS) contain both neuron cell bodies + their processes

  the PNS consists chiefly of neuron processes, whose cell bodies r in the CNS
• Neuron Processes - Dendrites & Axons


  dendrites + axons differ in the structure + function of their plasma membrane
• Dendrites


  dendrites of motor neurones r short, diffusely branching extensions

  virtually all organelles present in the cell body also occur in dendrites

  they r the main receptive regions which provide an enormous SA for receiving signals from other neurons

  they convey incoming messages toward the cell body

  these electrical signals r usually not action pots but r short-distance signals = graded pots
• Dendrites Anatomy


  dendrites

  
• Axon Structure - Axon Hillock


  a neuron never has more than a single axon

  the axon arises from a cone-shped area of the cell body = axon hillock
• Axon Structure - Initial Segment


  the initial segment of the axon narrows to form a slender process that is uniform in diameter for the rest of its length

  in some neurons the axon is v short/absent, but in others it accounts for nearly the entire length of the neuron

  any long axon = nerve fibre
• Axon Structure - Axon Collaterals


  altho one axon arises from the cell body, the axon may have occasional branches along its length = axon collaterals

  = extend from the axon at more/less right angles
• Axon Structure - Terminal Branches & Terminals


  an axon usually branches profusely at its end w 10k+ terminal branches

  the knob-;ike distal endings of the terminal branches = axon terminals
• Axon Anatomy


  axon

  
• Axon = Conducting Region


  axon = conducting region of the neuron

  it generates nerve impulses + transmits them, typically away from the cell body, along the plasma membrane

  in motor neurons, the nerve impulse is generated at the initial segment of the axon + conducted along the axon to the axon terminals - the secretory region of the neuron
• Axon Terminals - Secretory Region


  when the impulse reaches the axon terminals, it causes neurotransmitters to be released into the extracellular space

  the neurotransmitters either excite/inhibit neurons (or muscles/glands) w which the axon is in close contact

  each neuron receives signals from + sends signals to scores of other neurons carrying on diff signals w many diff neurons at the same time
• Axon's Organelles


  an axon contains the same cytoplasmic organelles found in the dendrites + cell body w 2 important exceptions

  = it lacks RER + Golgi apparatus, the structures involved w protein synthesis + packaging
• Axon's Dependency on the Cell Body


  an axon depends on its cell body to renew the necessary proteins + membrane components

  also for efficient transport mechanisms to distribute them

  axons quickly decay if cut/severely damaged
• Myelin Sheath


  many long/large in diameter nerve fibres r covered w a fatty (protein-lipoid) sgemented myelin sheath

  myelin protects + electically insulates fibres + it increases the transmission speed of nerve impulses

  myelinated fibres (axons bearing a myelin sheath) conduct nerve impulses rapidly, whereas non-myelinated fibres conduct impulses more slowly