5. Neuron Signalling

• 86 billion


  Number of neurons in the human brain
• 3.5 nanometers


  Distance of an electrical synapse
• In an electrical synapse communication is possible in both directions as ions and signals flow through the gap junctions
• Electrical synapses allow for extremely fast, almost instantaneous transmission of charge, and so are able to be very interconnected and move all at once once specific signals are given
• Electrical synapses are common in animals where instantaneous reflexes are needed for survival
• presynaptic cells give signals to the postsynaptic cells
• 20 nm - 40 nm 


  length of a chemical synapse
• Some chemical signals are excitatory and cause depolarization, others are inhibitory and cause hyperpolarization
• Small vesicles


  40 nm diameter, 30 micron volume, capable of transporting most of the smaller neurotransmitters we think about, tens of thousands at a time
• Large dense-core vesicles


  Diameters of 100 - 250 nm, transports peptides that are much larger than the other neurotransmitters
• Vesicle


  Tiny spheres of membrane lipids made for transporting things
• Vesicular transports work because they use the outgoing H+ ions from their highly acidic inside environment to drag in the necessary neurotransmitters
• Voltage-gated calcium channels


  Large protein complexes that usually remain closed but open up and let Ca2+ flood in with its electrochemical gradient as soon as depolarization occurs
• Full fusion 


  Vesicular membrane becomes fully exocytosed into the membrane and releases its contents
• Kiss-and-run fusion


  Incomplete fusion of the vesicle and the cell membrane where only some of the neurotransmitters are let free due to diffusion, then the vesicle detaches itself
• Receptors


  Proteins capable of sending a signal to change the function or activity of a neuron
• Ionotropic receptors can be very selective because the amino acid residues on their active sites can repel certain ions (e.g. negative residue repels negative ions)
• G proteins can break down GTP into GDP through catalysis, which powers the metabotropic receptor to open
• Usually the alpha g-protein subunit becomes soluble while the beta and gamma subunits remain bound to the membrane
• Dopamine receptor classes:

  • D1 and D5 are excitatory
  • D2 through D4 are inhibitory
• Dopamine is also needed for motor control. When dopamine producing regions in the substantia nigra pars compacta (SNpc) are damaged, Parkinson's disease results, and it currently treated with exogenous dopamine
• L-DOPA induced dyskinesia (LID)


  Hyperkinesias caused by L-DOPA treatment of Parkinson's disease
• Deep brain stimulation


  Implanting a device that stimulates the brain to give back motor control to Parkinson's patients
• There are 5 types of norepinephrine:

  • Alpha 1 and 2
  • Beta 1, 2, and 3