El sistema de neuronas que intervienen en la transmisión nerviosa se llama arco reflejo y                                                                                                                                                                       constituye una unidad básica de actividad                                                                                                                                                             nerviosa integrada .tiene tres neuronas,                                                                                                                                                                  la neurona sensitiva que se localiza en los                                                                                                                                                        ganglios nerviosos y lleva la información desde                                                                                                                                                   los receptores hasta la sustancia gris medular y                                                                                                                                                      la neurona motora que envía un axón hasta el receptor                                                                                                                                         y La neurona de asociación en la medula.

 El impulso nervioso se produce cuando la membrana                                                                                                                                       de  la neurona presináptica transmite una corriente                                                                                                                                    eléctrica. Al llegar esta corriente al botón presináptico,                                                                                                                                  las vesículas de secreción se fusionan con la membrana                                                                                                                            plasmática y descargan en el espacio sináptico unas                                                                                                                                  sustancias químicas capaces de estimular a otras                                                                                                                                       neuronas, los neurotransmisores. Se unen a la neurona                                                                                                                              postsináptica. Así el impulso nervioso pasa de una                                                                                                                                             célula a otra sin que haya un contacto físico directo.

Botón presináptico. Son los abultamientos que hay al                                                                                                                                        final de los axones y que secreta al espacio sináptico                                                                                                                                            los neurotransmisores.

Esclerótica. Gruesa envoltura protectora que se                                                                                                                                           encuentra en el globo ocular. Se hace transparente                                                                                                                                            para permitir el paso de los rayos de luz y forma                                                                                                                                                 la córnea. Se llama también conjuntiva.

Coroides capa muy vascularizada. Proporciona                                                                                                                        nutrientes a las células de la retina.
Por delante                                                                                                                                   de la coroides se encuentra el iris, porción                                                                                                                            pigmentada situada frente a la lente del                                                                                                                                             ojo: el cristalino.
Es la parte coloreada                                                                                                                                              del ojo. El orificio central de color                                                                                                                                                         negro es la pupila.

Retina. Es la capa más interna del ojo y                                                                                                                                          contiene los conos y bastones

Conos. Tienen forma apuntada, necesitan                                                                                                                                                                           luz de cierta intensidad y permiten                                                                                                                                                                                 la visión diurna y en color.                                                                                                                                                                                                                                                                                                                Bastones. Forma de cilindro. Son muy sensibles                                                                                                                                                             a la luz y permiten la visión nocturna y                                                                                                                                                                          en blanco y negro.

Humor acuoso. Es un líquido transparente                                                                                                                                                                que rellena la cavidad que existe                                                                                                                                                                              entre la córnea y el cristalino.                                                                                                                                                                             Cristalino. Estructura esférica, transparente                                                                                                                                                             y no vascularizada situada bajo la córnea.                                                                                                                                                           Cambia su curvatura para  acomodar el                                                                                                                                                           cristalino a las diferentes distancias a las                                                                                                                                                                        que se visualizan los objetos.                                                                                                                                                                                                                                              Humor vítreo. Liquido gelatinoso situado                                                                                                                                                                      entre el cristalino y la retina