NEURO 80: Week 6

How do electrical synapses transmit signals?

Ions pass directly between cells through gap junctions.

What are two advantages of electrical synapses?

Very fast transmission and synchronization of neurons.

Can electrical synapses pass hyperpolarizing signals?

Yes; they can pass both depolarizing and hyperpolarizing currents.

Are electrical synapses usually one-way or two-way?

Often two-way; chemical synapses are usually one-way.

What can gap junctions pass besides electrical current?

Some small molecules.

What trade-off distinguishes chemical from electrical synapses?

Chemical synapses are slower but allow more diverse, modifiable responses.

What is the active zone?

The presynaptic membrane region where vesicles fuse and release transmitter.

What is the postsynaptic density?

A postsynaptic specialization containing receptors and associated proteins.

What is the synaptic cleft?

The narrow extracellular space between pre- and postsynaptic membranes.

Sketch a chemical synapse: what must you label?

Presynaptic terminal, vesicles, active zone, Ca²⁺ channels, cleft, receptors, postsynaptic density.

What is the order of evoked neurotransmitter release?

AP → voltage-gated Ca²⁺ channels open → Ca²⁺ binds synaptotagmin → SNARE-mediated fusion → release.

Which ion directly couples the presynaptic AP to vesicle fusion?

Ca²⁺ entering the presynaptic terminal.

What does synaptotagmin do?

Binds Ca²⁺ and triggers rapid fusion through the SNARE machinery.

What do SNARE proteins do?

Bring vesicle and presynaptic membranes together for fusion.

Which SNARE is on the vesicle?

Synaptobrevin.

Which SNAREs are on the presynaptic plasma membrane?

Syntaxin and SNAP-25.

What is one quantum of neurotransmitter release?

The transmitter released by fusion of one vesicle, containing many molecules.

What is quantal size?

The average postsynaptic response amplitude produced by one vesicle.

What is quantal content?

The average number of vesicles released per presynaptic AP.

What produces an mEPP, and does it need an AP?

Spontaneous release of one vesicle; no presynaptic AP is required.

How can you measure quantal size?

Average the amplitudes of spontaneous miniature postsynaptic responses.

What is an EPP versus an EPSP?

An EPP is an excitatory postsynaptic potential specifically at the neuromuscular junction.

What approximates mean evoked EPSP amplitude for small, linearly summating responses?

Quantal size × quantal content.

Quantal content = 4 and quantal size = 0.5 mV. Mean response?

2 mV.

Mean EPSP = 4.5 mV and mean mini = 0.9 mV. Quantal content?

5 vesicles per AP on average.

Lower presynaptic Ca²⁺ entry: what usually happens to quantal content and evoked response?

Both decrease; fewer vesicles are released.

Lower presynaptic Ca²⁺ entry alone: what usually happens to mini amplitude?

No change, if vesicle filling and postsynaptic responsiveness are unchanged.

More transmitter per vesicle changes quantal size or content directly?

Quantal size; not the number of vesicles released.

Blocking SNARE-mediated fusion has what effect?

Less vesicle release, lower quantal content, and a smaller evoked response.

Why is normal NMJ transmission so reliable?

High quantal content produces a large EPP that normally exceeds muscle AP threshold.

What is the transmitter at the NMJ versus most fast excitatory CNS synapses?

NMJ: acetylcholine. CNS: glutamate.

What is the basic structure of the muscle nicotinic ACh receptor?

Five subunits surrounding a central ion-conducting pore.

Why does the nicotinic ACh receptor favor cations?

Negatively charged residues help make its pore cation-selective.

How does a muscle nicotinic ACh receptor differ from a voltage-gated Na⁺ channel?

AChR is ligand-gated and passes Na⁺/K⁺; the Na⁺ channel is voltage-gated and Na⁺-selective.

What is reversal potential?

The membrane voltage at which net current through a receptor/channel is zero.

Why is AChR reversal potential between Eₙₐ and Eₖ?

Inward Na⁺ current and outward K⁺ current balance there.

At AChR reversal potential, have all ions stopped moving?

No; opposing ionic currents cancel, giving zero net current.

What equation gives synaptic current and driving force?

I = g(Vm − Erev); driving force is Vm − Erev.

Vm = −65 mV and Erev = −10 mV. What happens when the receptor opens?

Net inward current depolarizes Vm toward −10 mV.

Vm = +20 mV and Erev = −10 mV. What happens when the receptor opens?

Net outward current drives Vm downward toward −10 mV.

Is a receptor's reversal potential the same as the neuron's resting potential?

No; Erev describes zero channel current, while rest is the unstimulated membrane voltage.

Why does a chemical EPSP begin after a brief delay?

Ca²⁺ entry, fusion, diffusion, receptor binding, and channel opening take time.

Sketch a small subthreshold CNS EPSP: what should it look like?

A few-mV depolarization after a brief delay, with fast rise and slower decay over tens of ms.

Why does an EPSP usually decay more slowly than it rises?

Synaptic conductance subsides, then membrane capacitance discharges through resting conductances.

What is receptor desensitization?

Current declines even while neurotransmitter remains bound.

How do AMPA and NMDA receptors differ in ions and kinetics?

AMPA: fast, usually Na⁺/K⁺. NMDA: slower, Na⁺/K⁺/Ca²⁺.

What must happen for substantial NMDA current to flow?

Glutamate and co-agonist bind, and depolarization relieves the Mg²⁺ block.

Why is NMDA a coincidence detector?

It requires transmitter binding plus postsynaptic depolarization; Ca²⁺ entry can support plasticity.