collateral sprouting

Following a brain injury, neurons adjacent to the damaged area may take over the functions performed by the damaged neurons in a process called collateral sprouting.

      Neurons A, B and C before damage


        ****
    ****    ****                     ooo
   *            *                oooo   oooo
  *              *              o           o
  *       A      * -- Soma --  o             o
  *              *             o             o
  *              *             o      B      o
   *            *              o             o
    ****    ****                o           o
        ********                 oooo   oooo
           *****                  oooooo
             ****                  oo
               **   -- Axons --    oo
                ***                o
                  **               o
                   *              ooooo
                   **            oo   oooo
                    **          oo       oo
                     **        oo         oo
                      **** *****           oo
                       * ************       oo
                      **    oo      **       oo
  Telodendrons ----   *     o        **       o
                     *     oo        **       oo
                    **     o        **         o
                   **      oo       *          o
                   *      @@@@@@    *          o
                   ** @@@@      @@@@*         oo
                    @@              @@        o
                   @                  @     ooo
                   @        C         @oooooo
                   @                  @
                    @@              @@
                      @@@@      @@@@
                          @@@@@@
                           @@@@
                            @@
                            @@
                             @
                             @
                             @
                             @
                            @@
                            @@
                            @@
                            @
                           @@
                          @@@@
                          @@ @@
                         @@   @@@
                         @@     @@
                         @       @@
                        @@       @@
                        @@        @
                        @@        @

    Collateral sprouting after damage 
        to neuron A

          ****
      *****  *****
    ***          ***                    ooo
    *              **               oooo   oooo
   **     A         *              o           o
   **               *             o             o
    *              **             o    B        o
     *            **              o             o
      *****  ******               o             o
          ********                 o           o
              ****   *              oooo   oooo
                **  *                oooooo
                  **                  oo
                 **                   oo
               **                     o
               *                      o
                                     ooooo
                                    oo   oooo
                                   oo       oo
                                  oo         oo
                                ooo          ooo
                            oooooo          oo oo
                         oooo  oo          oo   oo
                        oo     o          oo     o
  New telodendrons --  oo     oo        oo       oo
                       oo     o        oo         o
                      oo      oo       oo         o
                      oo     @@@@@@    o          o
                      oo @@@@      @@@@o         oo
                       @@              @@        o
                      @                  @     ooo
                      @        C         @oooooo
                      @                  @
                       @@              @@
                         @@@@      @@@@
                             @@@@@@
                              @@@@
                               @@
                               @@
                                @
                                @
                                @
                                @
                               @@
                               @@
                               @@
                               @
                              @@
                             @@@@
                             @@ @@
                            @@   @@@
                            @@     @@
                            @       @@
                           @@       @@
                           @@        @
                           @@        @

Following damage to neuron A, neuron B generates new telodendrons from its axon to form a synapse at the receptor sites on neuron C formerly occupied by neuron A.

Types of collateral sprouting:

Heterotypic collateral sprouting (aka reactive synaptogenesis): This type of collateral sprouting occurs within day and does not lead to recovery of function.

Homotypic collateral sprouting: Begins to occur after about four weeks and leads to recovery of function.

Although collateral sprouting does occur in the Central Nervous System, the damaged is often too extensive for it to have much effect. It is believed that collateral sprouting may also be one of the mechanisms behind associative learning/classical conditioning.

See also: Neurogenesis, Neuroplasticity

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Sources:
Klein, Stephen B. Biological Psychology. Prentice Hall, New Jersey: 2000.
http://www.medinfo.ufl.edu/other/profmed/slides/pm011399/slide19.html
http://www.nyu.edu/classes/azmitia/lectures/lecture13/