Synapses Electrophysiology and Vesicle release
Major point Cell theory (cells being separated) implies that cells must communicate with each other through extracellular connections most communication is through chemical messages "synapse" - = "clasp (Sherrington)
Notes from outline Vesicles, spine, receptors ionotropic channels Gap junction electrically coupled with small cytoplasmic continuities crayfish escape, Furshpan & DDPotter, 51- Gap junctions also connect myocardial cells electrically at intercalated disk. Conductance is high - 120 ps.
More on gap junctions patch of hexamers, big channel in register connexins, diverse, molecular weight. CDLandisman & BWConners, Long-term modulation mammalian thalamus connexin36 (Cx36) in thalamic reticular nucleus (TRN) metabotropic glutamate receptors (mglurs) from neocortex. Activating corticothalamic path causes longterm decrease in electrical synapse strength like long term depression
Finally, Pore is big enough to give cytoplasmic continuity for medium sized molecules (dyes). In addition to electrical coupling, there can be communication by molecules. In EM, membranes appear very close but not fused Extracellular tracers (heavy metal Lanthanum) proves there is extracellular space.
Chemical synapse History 1906 Sir Charles S. Sherrington (England) Integrative Action 1932 Nobel prize coined "synapse. spinal motor neuron "final common pathway" Spinal reflexes spinal animals prevent descending cortical influence no inhibitory neuromuscular junctions in vertebrates spinal motor neuron fires, based on summed inhibitory and excitatory influences
Acetylcholine 1926 Otto Loewi (Austria) dreamed stimulate vagus 10th cranial parasympathetic substance slows a heart in another dish, vagus substance = acetylcholine (ACh) a monamine transmitter. 1930's Sir Henry H. Dale (England) acetylcholine share 1936 Nobel "chemical transmission of nerve impulses"
Notes and terms Sir John C. Eccles 1963 Nobel (with Hodgkin & Huxley) EPSP & IPSP Excitatory or Inhibitory PostSynaptic Potentials spinal motor neurons integrate in cell, and axon hillock "decides" whether to fire.
Excitation & Inhibition glutamate excitatory EPSP - depolarize (unless clamped positive to reversal) increase sodium and potassium conductance inferred because reversal potential near zero GABA (gamma amino butyric acid) inhibitory IPSP - hyperpolarize (unless clamped negative to reversal potential)
More on inhibition increase potassium and chloride conductance inferred because reversal potential negative to resting potential (in voltage clamp) and by changing Cl- gradient by using ion specific electrodes to inject Cl-
Sir Bernard Katz 1970 Nobel w/ von Euler (Sweden) Julius Axelrod (US) classic experiment shows transmission at the neuromuscular junction is "quantal." Quantum is one vesicle.
Summary from outline EPP (end plate potential) reduced to meep's (miniature end plate potentials, 0.4 mv) by lowering extracellular Ca2+ ion nerve stimulation elicits responses the size of 0, 1, 2, or 3 meep's according to the Poisson distribution. "end plate" potential is big and effective in generating muscle action potential. Usually 200 vesicles give 40 mv potential.
Vesicle release transmitter very concentrated, pumps sometimes part of synthesis is in vesicle Box C on toxins that affect release there are vesicle membrane proteins, target (presynaptic) membrane proteins cytoplasmic proteins Ca2+ Q or N type voltage gated channel (N stands for "neither," vs T=transient or L=long lasting N blocked by omega toxin Conus [snail])
Vesicle proteins Synaptobrevin / VAMP (vesicle-associated membrane protein) = v-snare (SNAP receptor) Botulinum and Tetanus toxin (clostridial toxins) are proteases which cleave synaptobrevin Botulism (Clostridium botulinum) (stories) Tetany sustained muscle contraction Tetanus toxin synaptobrevin in inhibitory interneurons. (stories)
Vesicle proteins continued Synaptotagmin - binds calcium synapsins get phosphorylated (by CaM Kinase II and PKA) interact with actin rhabphilin receptor
Target membrane proteins Syntaxin = t-snare = unc-18 (uncoordinated C. elegans roundworm mutant) Neurexin - black widow spider venom (alpha Latrotoxin) causes too much release Neuroexins bind to synaptotagmin
Cytoplasmic proteins NSF - N-ethylmaleimide sensitive factor (ATPase activity when complex dispersed) SNAP - soluable NSF associated protein Rab3 (like ras, small GTP binding protein) (lots of rab's, specific for transport)rabphillin