Neurons, neuroglia and organization of the nervous system

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Neurons, neuroglia and organization of the
nervous system
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Overview of the nervous system
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General definitions

• Central nervous system (CNS)- brain and spinal
  cord
• Peripheral nervous system (PNS)- links the CNS to
  target systems, including sensory organs
• Autonomic nervous system (ANS)- spans CNS and
  PNS controls visceral functions

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Functions of the nervous system

• Sensory- detection of internal and external
  stimuli (sensory neurons)
• Integrative- processing of sensory information
  (interneurons)
• Motor- response to integrated decision (motor
  neurons)

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Organization of the nervous system
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Structure and function of neurons

• Act through propagation of action potentials
• Vary considerably in size
• Dendrites, cell body and axons
• May be myelinated
• Synapse with other neuron or muscle cell release
  neurotransmitters

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Neurotransmitter action

• About 100 neurotransmitters known
• Amino acids, polypeptides, gases
• Released into synaptic cleft, bind to membrane of
  target cell

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Structural diversity in neurons

• Multipolar- many dendrites, one axon
• Most neurons in CNS
• Bipolar- one dendrite, one axon
• Sensory organs
• Unipolar- sensory
• Axon termini extend into CNS

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Neuroglial cells

• Much smaller than neurons and more numerous
• Do not propagate action potentials
• Can replace themselves

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Four types of neuroglia in CNS

• Oligodendrocytes
• Myelinating cells
• Astrocytes
• Blood-brain barrier (BBB)
• Microglia
• Phagocytes (from bone marrow)
• Ependymal cells
• Line ventricles of brain produce cerebrospinal
  fluid (CSF)

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Neuroglia of the CNS
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Neuroglia of the PNS

• Schwann cells
• Myelinating cells
• Help direct axon regeneration
• Satellite cells
• Support, protection, regulation of molecular
  exchange
• Filter out other stimuli

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Why myelin?

• Increases speed of nerve impulse transmission
• Myelinating cells leave gaps on axons (nodes of
  Ranvier)
• Schwann cells can help regenerate axons
• Demyelination can lead to loss of function (MS)

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Classification of neurons

• Exteroreceptors- deal with external environment
• Interoreceptors- deal with internal environment
• Proprioreceptors- position and movement

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Neuronal circuits
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Neural circuits in CNS vary in direction and
complexity

• One presynaptic?one postsynaptic is simplest
• One presynaptic? many postsynaptic (divergence)
  amplifies a sensory signal
• Many presynaptic ?one postsynaptic (convergence)
  brings information from many sources into one
  pathway
• May move repeatedly through circuit
  (reverberating)- repeated or learned activities
• Parallel-discharge- pathways diverge and then
  converge- complex activities requiring
  concentration?

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Repair and regeneration of neurons

• Neurons can grow new dendrites or axon termini
• Repair is limited to certain neurons in the PNS
• Growth factors promote formation of new neurons
  limited to specific regions of brain
  (hippocampus) in human adults
• Growth does occur in fetal and juvenile stages-
  what turns it off?

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Synapses (chemical)
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Electrical synapses

• Membranes of presynaptic and postsynaptic cells
  are fused
• Transmission is faster
• Can be bidirectional
• Generally associated with defensive reflexes

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Gray matter and white matter in the nervous system
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Medically significant conditions

• Multiple sclerosis (demyelination)
• Epilepsy (uncontrolled electrical discharges)
• Tumors
• Neuropathies- affect specific nerves
• Infectious disease