NEUS 609 - Structure of synapses

electrical synapses

• direct contact between the 2 cells

• continuous ionic flow with postsynaptic cell

• gap junctions, each made of 2 connexons,

• each connexon is made of 6 connexin subunits

• FAST signaling

• minimal plasticity

• BIDIRECTIONAL

chemical synapse

• cells kept apart creating the synaptic cleft (full of extracellular proteins)

• MOST COMMON type of synapses

• sensitive to drugs and toxins

• contains NT filled vesicles

• SLOWER signaling

• high plasticity

• UNIDIRECTIONAL (due to asymmetrical structure)

overview of chemical synapse signaling

1. arrival of action potential at presynaptic terminal opens Ca2+ voltage gated channels causing Ca2+ influx

2. vesicles filled with NTs migrate to the active zone and fuse with membrane once Ca2+ binds to synaptotagmin.

3. NTs released by exocytosis & diffuse across synaptic cleft to bind to post synaptic receptors which allow ion flow across postsynaptic membrane

4. Clathrin pulls membrane back into presynaptic membrane and dynamin pinches endosome off to re-create standard vesicle which joint he reserve pool bound to actin filaments.

vesicle

quantal unit of neurotransmission

EPPs

end plate potentials

postsynaptic response at neuromuscular junction (NMJ)

MEPPS

miniature end plate potentials

lower scale postsynaptic response at NMJ observed even in the absence of stimulation

• move in additive fashion with EPPS

• reason for the theory/discovery of quantal neurotransmission

adhesion molecules

___ span synaptic cleft to maintain close alignment of pre- and postsynaptic membranes

adhesion molecules spanning synaptic cleft

• neurexin/neuroligin

• NCAM = neural cell adhesion molecule

• SynCAM

• Cadherins

Postsynaptic density

aka PSD

large collection of proteins that produce density seen by electron microscopy

• mainly prominent in excitatory synapses

• made of:

  • NTs receptors

  • linker proteins

  • additional scaffolding proteins for stabilization

  • cytoskeleton elements

vesicle morphology

___ differ based on the synapse type

round, wide

excitatory synapses have __ vesicles, __ synaptic cleft and prominent PSD

oval, narrow

inhibitory synapses have ___ vesicles, ___ synaptic cleft, and less prominent PSD

small molecules NTs

• small, usually amino acids or biogenic amines

• amino acid transporters on cell body bring them inside the cell

• enzymes like tyrosine hydroxylase in cell body carry their synthesis (some synthesis happens in terminal)

• usually recycled

neuropeptides

• large

• made from genetic material which is transcribed into mRNA and then translated (synthesis in cell body)

• activated via peptidases

• trafficked by Golgi

• anterogradely transported in dense core vesicles

• after transmission, made into inactive metabolites by peptidases

tripartate synapse

postsynaptic & postsynaptic membranes as well as the surrounding astrocyte processes

• 3 components synapse

• astrocyte processes take in GLU, turn it back into glutamine & send it back to presynaptic terminal through transporters

functions of tripartate synapse

• clear synapse of NTs

• neuronal synchronzation

• synaptic modulation

• coordination of synaptic network

skeletal

___ muscle synapses end on motor end plate and are found at NMJ

smooth

___ muscle synapses end with boutons en passant (just like bead on a string) & synapses form along the length of the axon

sarin

• component of weaponized nerve gas

• potent inhibitor of acetylcholinesterase (which normally break down acetylcholine (ACh) at the synaptic cleft level

• causes constant stimulation, constant depolarization, stops breathing and causes seizures