Histo.10

• Nervous tissue

  Distributed throughout the body as an integrated communications network
• Divisions of the nervous system

  • Central nervous system (CNS)
  • Peripheral nervous system (PNS)
  • Autonomic nervous system
• Neuron

  The functional unit in both the CNS and PNS

  
• Main parts of a neuron

  • Cell body (perikaryon or soma)
  • Dendrites
  • Axon

  
• Cell body (perikaryon or soma)

  • Contains the nucleus, surrounding cytoplasm, and most of the cell's organelles
  • Serves as the synthetic or trophic center for the entire neuron

  
• Dendrites

  • Typically short, small processes emerging and branching off the soma
  • Irregular in thickness, with Nissl granules extending into them
  • Become much thinner as they branch, with cytoskeletal elements predominating in these distal regions
  • Principal signal reception and processing sites on neurons
  • Bear numerous small spines that are of variable shape
• Axons

  • Most neurons have only one axon, typically longer than its dendrites
  • Axonal processes vary in length and diameter according to the type of neuron
  • The longest axons may be as much as a meter long
  • Each axon has a uniform diameter, and is devoid of Nissl substance
  • Axons generally branch less profusely than dendrites, but do undergo terminal arborization
  • Axons of interneurons and some motor neurons also have major branches called collaterals that end at smaller branches with synapses influencing the activity of many other neurons
  • The end branches of an axon are called telodendria
  • The swollen end of a telodendron is known as the axon terminal (terminal bouton) that contacts another neuron or non-nerve cell at a synapse to initiate an impulse in that cell
• Types of neurons

  • Multipolar neurons
  • Bipolar neurons
  • Unipolar or pseudounipolar neurons
  • Anaxonic neurons
• Synapse

  Sites where nerve impulses are transmitted from one neuron to another, or from neurons and other effector cells
• Components of a synapse

  • Presynaptic axon terminal (terminal bouton) contains mitochondria and numerous synaptic vesicles
  • Postsynaptic cell membrane contains receptors for the neurotransmitter, and ion channels or other mechanisms to initiate a new impulse
  • Synaptic cleft intercellular space separates these presynaptic and postsynaptic membranes
• Types of synapses

  • Axodendritic synapse
  • Axosomatic synapse
  • Axoaxonal synapse
• Motor end plate (MEP)

  Each axonal branch forms a dilated termination situated within a trough on the muscle cell surface, which are part of the synapses termed the neuromuscular junctions
• As in all synapses the axon terminal contains mitochondria and numerous synaptic vesicles; here the vesicles contain the neurotransmitter acetylcholine
• Between the axon and the muscle is the synaptic cleft
• Adjacent to the synaptic cleft, the sarcolemma is thrown into numerous deep junctional folds, which provide for greater postsynaptic surface area and more transmembrane acetylcholine receptors