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what are synapses?

specialized zone of contact at which one neuron communicated with another


how many neurons in human brain?
avg human received how many synapses?
number of synapse in human brain?

neurons= 10^11 - 10^12
1000 synapses in avg. neuron
synapses in human= 10^15 - 10^16


2 types of synapses & explain

1) electrical- junctions b/w neurons permitting direct, passive flow of electrical current

2) chemical- junctions between neurons that comminicate via secretion of neurotransmitters
- chemical agents released by presynaptic neuron produced secondary flow in postsynaptic neurons activating specific receptors


neurotransmitters undergo similar cycle (4 steps)

1) synthesis and packaging into synaptic vesicles
2) release from presynaptic cell
3) binding to post synaptic receptors
4) rapid removal and/or degradation


influx of ____ through voltage-gated channels triggers secretion of ______



rise in Ca concentration, causes what?

- synaptic vesicle to fuse with presynaptic plasma membrane and release contents into space between pre and post synaptic cells


electrical synapses are ___ _______

gap junctions


structure of gap junctions (electrical synapses)

- plasma membrane closely apposed (3nm)
- precisely aligned, paired channels= connexons (made up of connexins)


how do connexins form pore that connects cells (electrical synapse)

- 6 presynaptic connexin align with 6 postsynaptic connexins to form pore between 2 cells


(electrical synapse) gap junctions consist of wht complexes, and they are formed by?

- hexameric complexes

formed by connexons coming together (present in pre/post synaptic membranes)


components/formation of gap junction (how much of each thing)

(electrical synapse)

Connexin (x6)
connexon (hemichannel)- (x2)
gap junction channel (x 10^2 - 10^3)


different connexin isoforms determine....?

(electrical synapse)

transmission properties of the electrical synapse


what diffuses through neuronal gap junctions (2)

(electrical synapse)

- all ions (ex: K, Na, Ca, Cl)

- small molecular weight compounds (ex: 2nd messengers: cAMP, IP3)


experiment: patch clamping pair of electrical coupled neurons

- type of current injected (depol/hyper)
- when see AP?

2 electrically connected neurons
- inject depolarizing current and neuron fires AP
- polarizes to threshold and see AP


what happens at post synaptic neuron (patch clamping experiment)

- speed of Na

- see depolarization in post synaptic
- no AP

- the gap junctions (small openings), only fraction of Na can make it across the junction into postsynaptic neuron, the depolarization would be much smaller than presynaptic neuron

- very fast b/c ass soon as AP, theres Na flowing into cell


inverse experiment of patch clamping

- inject hyper-polarizing current into presynaptic current
- positive charged ions will flow from other side, hyper-polarizing neuron


what is end plate current (patch clamp experiment)

- direction current flowing, causing..?

macroscopic current resulting from summed opening of many ion channels
- current flowing= inward, causes post synaptic membrane potential to depolarize
- depolarizing change inpotential, triggers postsynaptic AP by opening voltage-gated Na and K channels


directionality of electrical synapses transmission
- what does it depend on?

- electrical synapses are rectifying due to voltage-dependent of gap junction channel opening
- current can flow in either direction across gap junction, depending on which member of coupled pairs is invaded by AP


velocity of electrical synapse transmission

- rapid
- synaptic latency in order of 1ms

- b/c passive current flow across gap junction is instantaneous


sign and amplitude of transmitted changes of electrical synapses

- which is better transmitter (slow/fast potential change)

same sign, smaller amplitude
- ex: 10mV hyperpolarization presynaptically may lead to 1mV hyperpolarization postsynaptically

- slow potential changes are better transmitted than fast potential changes (ex: AP)


general purpose of electrical synapse?

synchronize electrical activity among populations of neurons


regulation of electrical transmission
(gap junctions)

- most gap junction channels are closed, regulated to alter fraction of open channels


connexin phosphorylation

- and example

(electrical synapse- regulation of transmission)

extracellular signals activate protein kinases which phosphorylate connexins
- depending on connexin type, phosphorylation can have opposite effect and tend to open cap junction channels

ex horizontal cells: D1 receptor--> adenyly cyclase--> cAMP--> protein kinase A--> phosphorylation of connexins--> gap junction channels less open


gap junction response to intracellular Ca concentration

(electrical synapse- regulation of transmission)

gap junctions close in response to pathologically high Ca concentration


differences in membrane potential between electrically connected cells

large differences in membrane potential between electrically connected called tend to close gap junction channels
- dependent on connexin composition

- if membrane depolarized, can lead to closing of gap junction channels


electrical synapses in aplysia californica

motor neurons in control of ink discharge are connected via electrical synapses
- allowing them to fire synchronously, providing rapid and complete ink discharge


electrical synapses in fish

- mauthner neurons are large reticulospinal neurons in fish and amphibia that mediate escape responses
- in teleosts, these neurons receive mixed electrical and chemical synaptic input from auditory afferents


electrical synapse in retina (mammals)

- allow for some processing of visual info
- horizontal cells are for lateral inhibition of input, they are couples and allow to activate at the same time
- horizontal cells get uncoupled, and close electrical synapse, less inhibition, photoreceptors can transmit info to bipolar cells (allow to see in dark)


electrical synapses in neocortex

- have excitatory projection neurons and inhibitory neurons (GABA)
- interneurons, similar functions, connected via electrical synapses, activate in similar way

- if one interneuron is depolarized from incoming chemicals, sends depolarization to connecting neurons so they can depolarize the same way


(chemical synapses structure)
- presynaptic bouton

specialization of presynaptic neuron containing cellular components required for the secretion of neurotransmitter