Synapses

I.          Types

            A.        electrical

                        1.         description: gap is smaller than chemical synapses, current flows directly across interstitial fluid

                        2.         components: 2 connexons, 6 connexins each (12 connexins altogether)

                        3.         benefits: very, very fast stereotyped with little energy expenditure

            B.        chemical

                        1.         fast

                                    a.         ionotropic

                                    b.         first messenger system—called transmitter-gated

                                    c.         mechanism: NT binds, changes shape of channel, ion can  now cross membrane

                                    d.         uses amino acids and amines as NT

                        2.         slow

                                    a.         metabotropic

                                    b.         second messenger system—called G-protein coupled

                                    c.         mechanism: NT binds, changes shape of ion channel, ion can now cross membrane (i.e., 1^st messenger activated);                                                                       STEPS:

§         Receptor binds, activates a G-Protein molecule

§         G-protein binds with GTP molecule (guanosine triphosphate)

§         GTP-G-Protein complex actually moves along inner face of cell membrane to connect with and activate adenylate cyclase (enzyme)

§         Adenylate cyclase causes ATP (adenosine triphosphate) to be converted to cAMP

§         cAMP is the 2^nd messenger

§         cAMP activates protein kinases which then changed the shape of the protein channel, causing it to remain open for a longer time

§         phosphodiesterase = enzyme that breaks down cAMP

d.        produces a major amplification of signal

e.        uses amino acids, amines, and peptides as NT

II.          Structure

            A.        Presynaptic membrane

                        1.         structures in terminal

                                    a.         mitochondria

                                    b.         synaptic vesicles

                                    c.         Golgi apparatus

                                    d.         microtubules

                        2.         active zones

                                    a.         site of NT release

                                    b.         proteins jutting into the cytoplasm

                                    c.         organize vesicles and prepare them for NT release

            B.        Synaptic Cleft

                        1.         interstitial (extracellular) fluid

                        2.         enzymes and other molecules are present

            C.        Postsynaptic membrane

                        1.         receptor—may be transmitter-gated 1^st messenger system or a more complex 2^nd messenger system

                        2.         Action at Postsynaptic Membrane

                                    a.         EPSP

                                    b.         IPSP

                                    c.         Central Summation

§         Temporal

§         Spatial

III.         Types of Connections

            A.        CNS

                        1.         axoadendritic

                        2.         axosomatic

                        3.         axoaxonic

                        4.         dendrodendritic

            B.        PNS

                        1.         neuromuscular junction (NMJ)

                                    a.         communication junction between motor neuron and muscle fiber

                                    b.         more active zones

                                    c.         uses motor end plate on muscle fiber