Synaptic Transmission Flashcards
Pre-synaptic bouton
- place where axon terminates, specialized for NT release
- contain active zones to convert electrical into chemical signals that are released into follower cells
- contain highly concentrated of NT packets (vesicles) that are released when bouton depolarizes
dendritic spines
- RECEIVE chemical signals at the synapse, convert chemical signals to electrical signals, modulated in forms of plasticity
Neurotransmitters
small molecules that transmit impulses at chemical synapses
Two main NTs to make nervous system work
- glutamate (main EXCITATORY)
2. GABA (main INHIBITORY)
3 criteria for an NT
- present in nervous system in pre-synaptic terminals
- released in response to depolarization in (Ca2+ dependent manner)
- specific receptors exist for the NT on post-synaptic cells that generate a signal in post-synaptic cell
basically (present in PRE, can be RELEASED, can be RECEIVED to generate signal)
When does synaptic release occur?
Triggered when an AP propagating down axon reaches pre-synaptic bouton containing Ca2+ channels that open in response to depolarization
Describe a classic vesicle cycle
- import of NT
- movement of vesicle to active zone
- docking at plasma zone
- Ca2+ triggered exocytosis
- endocytosis via clathrin coated vesicles (to be used again)
synaptic vesicles can be filled, exocytosed, and recycled within a minute
Generally, how big of a change is needed to alter membrane potential and initiate AP?
Little change needed. Movement of as few as 6000 ions/um^2 needed to account for AP (compared to much much more ions needed in typical neuron)
exception: INTRACELLULAR CALCIUM
What processes are regulated by calcium?
- synaptic release of NT
- plasticity
- gene regulation
- excitation-contraction coupling of muscle
Where are Ca2+ channels located in the pre-synaptic bouton?
directly below active zone
What is a target for bacterial toxins?
fusion machinery (around synapse)
Do post synaptic NT receptors respond to voltage or ligand binding?
ligand binding
Which receptors are found at EXCITATORY synapses and open to allow cations to pass through channel?
ionotrophic glutamate receptors
What is the zero current potential of the Glu receptor channel?
Near 0mV. so opening it will drive membrane potential to depolarize
At rest, describe glutamate receptors
Relaxed and closed….to open, a torqued conformation (energetically unfavorable) is required
If glutamate receptor opening is energetically unfavorable, what keeps them open?
LIGAND BINDING. (when glutamate binds)
glutamate binds to stabilize active state
How do antagonists work?
Antagonists bind to stabilize and favor inactive state…agonists are vice versa
With regards to how neuron signals spread throughout dendrites to soma, what is the space constant?
increases with higher membrane resistance and lower axonal resistance (signal will decay exponentially is tube diameter stays constant)
What factors complicate signal transmission in dendrites?
varying branch diameters, varying distance of inputs from soma, voltage gated channels (i.e. Ca and Na boost synaptic inputs)
Why is synaptic summation needed?
many small synaptic events must sum together to reach a threshold and generate AP (each synapse is only capable of making a small change in somatic membrane potential)
What are two types of synaptic summation?
spatial summation and temporal summation
spatial summation
Two EPSP synaptic events…
A. occur together within overlapping compartment = sub-linear summation (competing for receptors)
B. occur in electrically distant compartments = linear summation to generate independent currents that work together to drive depolarization
temporal summation
potentials occuring in brief succesion can add together to produce larger combined responses (depends on rate limiting step = usually elimination of NTs within synaptic clef i.e. reuptake)
components of a typical glutamate excitatory synapse
glutamine, phosphate activated glutaminase in mitochondria (PAG), excitatory AA transporter (EAA), and vesicular proton driven transporter (VGlut)
Which two receptors are activated by glutamate?
AMPA and NMDA (respond to Ca)
components of typical GABA inhibitory synapse
GABA transaminase (GABA-T), Glu Acid Decarboxylase (GAD), Vesicular inhibitory AA transporter (VIAAT)
How do excitatory and inhibitory synapses work together?
Feedback, feedforward, lateral inhibition (“winner take all”, silence other inhibitors)
