Lecture 7: Electrical Synapses

How do neurons communicate?

• reticular theory: all one connected web, bidirectional communication, electrical synapse ONLY

• most believed this but boom after Cajal’s work everyone switched to neuron doctrine

Neurons can communicate via electrical synapses

• some cases where cytoplasm of 2 cells are continuous with each other(no gaps and no synapses to connect)

• ex. cray fish have easy to access aka LARGE electrical synapses

• can have electrode in pre and post synaptic fibers → you can control and record from both sides of synapse

• not much delay since the cells are coupled

• post-synaptic potential called gap potential since it moves through gap junctions

Gap junctions at electrical synapse

• gap between two cells is 3-4nm

• each gap junction protein has a pore

Gap junction channels(aka “protein pores”)

• makes membrane pores

• brings membranes together, connects cytoplasm

• connexon: come together to form channel pore

• 2 connexons connected = gap junction channel

• allow ions and small molecules through

Chemical vs Electrical Synapse

Chemical

• cells separated by space(synaptic cleft)

• unidirectional communication

• only positive presynaptic current creates a signal

• probabilistic

• action potential required for communication

• metabolically expensive

• speed varies but usally slow

Electrical

• cytoplasm of 2 cells is continuous

• bidirectional communication

• positive and negative presynaptic current passed between cells

• reliable

• action potential NOT required for communication

• metabolically inexpensive

• very fast

Electrical synapses allow

passive flow of current through gap junctions

Electrical synapses: Gap junction

• aligned paired channels called connexons that create pores for ions to diffuse between the two cells

• cytoplasm is continuous between both cells

• small gap between the neurons

Structure of gap junction

• one connexon(hemi-channel): made of 6 connexins

• each connexin has 4 transmembrane regions

• each cell expresses connexons that bind together to form a gap junction

Electrical synpases

• gap junctions allow the flow of electrical current in either direction(bidirectional)

• current flows regardless of amplitude(action potential is NOT necessary)

• gap potential: potential recorded in post-synaptic neuron

• very fast: almost no delay(<0.1 ms) between AP in pre-synaptic neuron and postsynaptic potential

• good way to test if 2 neurons are connected by electrical synapses(will follow each other)

Gap junction channels

• homomeric:all same connexins in one connexon

• heteromeric:alternatic connexins in one connexon

• homomeric homotypic:all same in gap junction

• homomeric heterotypic:2 connexons each have the same in their respective connexon

• heteromeric homotypic: alternating connexins but they align

• heteromeric heterotypic: alternating connexins that do not align

• heterotypic may be rectifying

Physiological properties of electrical synapses

• non-rectifying: ratio of responses are equal in both directions

• rectifying: responses are unequal-one direction has different response ratio than the other

• from pre to post synaptic: same pattern of depolarization

  • same current not the same amplitude

• spikelets: little depolarizations that aren’t action potentials

Rates of membrane voltage change at electrical and chemical synapses

• action potential coincides in time closely with post-synaptic spikelet in electrical synapse

• much slower EPSP in chemical synapse

Electrical synapses form between groups of neighboring neurons

• green: dye is too large to leave so only lights up original neuron

• only red: small molecule dye moves through gap junction proteins

  • labels electronically linked cell

• red and yellow: different wavelength of light images both dyes

  • green+red(double labeled)=yellow

  • red shows all connected electrically with original cell

Modulation of electrical synapse signal

• cells can add/remove gap junction proteins

• cell can open/close connexon pore

Electrical synapses can coordinate activity among groups of neurons

• two connected neurons have APs at the same time

• two neurons that are not connected have uncoordinated AP times

Neurotransmitter release can modulate connectivity of electrical synapses/gap junctions

• connectivity can change within one day: starlight(average coupling) vs twilight which has higher coupling of amacrine cells vs bright daylight(average coupling)

Electrical synapse’s role in sleep/wake circadian rhythms

• mutated: no connexins=not electrically coupled → random activity

• wildtype=generally consistent activity

Gap junctions important for

development of cortical columns