Block 6 · CNS · CNS02 · Rev 1.1 · Updated Oct 23, 2026
CNS02 · Neuroscience · Study Guide
Neuronal Communication I: Synapses & Neurotransmitters
Learning objectives
- LO 1Describe the building blocks of a chemical synapse (presynaptic terminal, cleft, postsynaptic cell and its receptors) and contrast it with an electrical synapse.
- LO 2Walk through, in order, how a chemical synapse turns an action potential into a postsynaptic response, including vesicle fusion and recycling.
- LO 3State what a molecule has to do to qualify as a neurotransmitter, and use those rules to sort the transmitter classes.
- LO 4Follow acetylcholine and glutamate from synthesis through vesicle storage, release, receptor binding and inactivation, naming the rate-limiting step of each.
- LO 5Point to the brain regions and nuclei rich in cholinergic and glutamatergic neurons and their receptors.
★ Step 1 HY
⚠ Step 1 trap
✎ Memory tool
“ Buzzwords
MAPBig picture
- AP reaches terminal → voltage-gated Ca²⁺ channels open → Ca²⁺ influx
- Ca²⁺ → SNAREs fuse vesicle → transmitter released into cleft
- Transmitter binds receptor
- Ionotropic → fast (ms)
- Metabotropic (GPCR) → slow, modulatory
- Postsynaptic cell sums EPSPs + IPSPs → fires or not
- Signal shut off
- ACh → cleaved by AChE
- Glutamate → taken up by astrocytes (→ glutamine)
- Every step = a target → toxins (botulinum, tetanus) · autoantibodies (LEMS, MG) · drugs
LO 1Anatomy of a chemical synapseThree parts, two receptor classes, and how it differs from an electrical synapse
By the end: you can name the three parts of a chemical synapse and what sits in each, compare electrical vs chemical synapses on every feature, and predict whether an ionotropic or metabotropic receptor is responsible for a fast or slow effect.
Q1. Why talk about a "synapse" rather than a single neuron?
- Neurons never work alone; they are wired into ensembles called neural circuits that together produce behavior ("brain cells fire in patterns").
- The synapse is the functional unit of that wiring: the place where one cell passes information to the next.
- Two general classes exist: electrical and chemical.
Q2. How do electrical and chemical synapses differ?
| Feature | Electrical | Chemical |
|---|---|---|
| Cytoplasmic continuity | Yes — direct, passive current flow | No — cytoplasm NOT shared |
| Structure / mediator | Gap junctions made of connexons | Neurotransmitter → receptors open/close ion channels |
| Direction | Bidirectional | Unidirectional (pre → post) |
| Speed | Extremely fast (no delay) | Slower (synaptic delay for Ca²⁺ entry + fusion) |
| Best at | Synchronizing neuron populations (e.g., brainstem neurons generating the breathing rhythm) | Flexible, amplifiable, modifiable signaling |
| Abundance | Less common | Most common synapse in the CNS |
- Cleanest discriminator: electrical → cytoplasm of both neurons connected by gap junctions (connexons); chemical → cytoplasm NOT connected.
- Electrical = bidirectional; chemical = unidirectional.
CheckBrainstem respiratory neurons must fire in tight synchrony. Which synapse type helps, and why?
Electrical synapses (gap junctions). Current passes directly and almost instantly in both directions through connexons, so coupled cells depolarize together.
Step 1 trap
"Fast" alone doesn't mean electrical: ionotropic chemical synapses are fast too (milliseconds). The words that point to electrical are bidirectional, shared cytoplasm, connexin/connexon, synchronized.
Memory tool
Electrical = a door between two rooms (walk either way). Chemical = a note slid under the door (one way, takes a moment, needs someone to read it).
Q3. What are the three parts of a chemical synapse, and what lives in each?
Presynaptic terminal
- Synaptic vesicles filled with one or more transmitters
- Small-molecule transmitters are made here (exception: neuropeptides are made in the cell body)
- Release at the active zone (usually the end of the axon terminal), triggered by a large Ca²⁺ influx
Synaptic cleft
- Space of 20–40 nm
- Cytoplasm of the two cells is NOT shared
- Transmitter must diffuse across
Postsynaptic cell
- Specialized neurotransmitter receptors
- Postsynaptic density (PSD): receptors + adaptor proteins that stabilize them; prominent at excitatory synapses
- Ion flow → depolarize or hyperpolarize
CheckOn an EM of a synapse, what is the dark band on the postsynaptic membrane, and what is it for?
The postsynaptic density: a dense scaffold holding neurotransmitter receptors and adaptor proteins that anchor them; typical of excitatory (e.g., glutamatergic) synapses.
Buzzwords
"active zone" → presynaptic docking/release site
"postsynaptic density" → receptor scaffold, excitatory synapse
"connexon" → gap junction, electrical synapse
"20–40 nm" → chemical synaptic cleft
Q4. What two kinds of receptor can the transmitter hit, and how do they differ?
A transmitter binds its receptor with high specificity, but one transmitter can activate both classes and several subtypes within each class (ACh → nicotinic + muscarinic; glutamate → AMPA/NMDA/kainate + mGluRs).
| Feature | Ionotropic | Metabotropic |
|---|---|---|
| What it is | Ligand-gated ion channel: transmitter opens the pore directly | G-protein-coupled receptor (GPCR): no pore; acts through G proteins and second messengers; can open neighboring channels (indirect gating) |
| Speed | Milliseconds | Seconds to hours |
| Duration | Short; ends when transmitter unbinds (binding kinetics set how long the channel stays open) | Prolonged; persists via second messengers, can change gene expression |
| What decides excite vs inhibit | Ion selectivity: Na⁺/Ca²⁺ in → depolarize (e.g., NMDA); Cl⁻ in → hyperpolarize (e.g., GABAA) | Which G protein (Gs, Gi, Gq); subtypes for one transmitter can couple to different G proteins → stimulatory or inhibitory |
| Big feature | Rapid, point-to-point excitation/inhibition | Signal amplification, modulation, plasticity, long-term effects |
| Examples | nAChR, AMPA, NMDA, kainate, GABAA, glycine receptor | mAChR (M1–M5), mGluR1–8, D2, β-adrenergic |
GPCR amplification
1 receptoractivated by transmitter
→
Many G proteinsactivated
→
Effector enzymese.g., adenylyl cyclase
→
Huge cAMP pooleach enzyme makes many
→
Kinases (PKA)phosphorylate many targets
Net result: one activated GPCR can affect hundreds to thousands of second-messenger molecules.
| G protein | Effector | 2nd messenger | Downstream | Example |
|---|---|---|---|---|
| Gs | ↑ adenylyl cyclase | ↑ cAMP | ↑ PKA | β-adrenergic, D1 |
| Gi | ↓ adenylyl cyclase | ↓ cAMP | ↓ PKA (often opens K⁺ channels) | D2, M2, presynaptic mGluR |
| Gq | Phospholipase C | IP₃ + DAG | Ca²⁺ release + PKC | M1/M3, group I mGluR |
CheckA ligand-gated channel lets Cl⁻ flow in when its transmitter binds. Predict the effect and name one.
Hyperpolarization → inhibition (membrane more negative), e.g., the GABAA receptor. Contrast Na⁺/Ca²⁺ entry through NMDA → depolarization.
CheckIf a metabotropic receptor has no pore, how does it end up changing ion flow?
Transmitter activates the GPCR → G protein → G-protein subunits or second messengers act on a separate ion channel nearby → ions flow. This is indirect gating.
Step 1 HY — receptor class drives the pharmacology
- Ionotropic examples: NMDA overactivation → excitotoxicity (stroke); benzodiazepines/barbiturates enhance GABAA (Cl⁻ channel); nicotinic receptor at the NMJ.
- Metabotropic examples: antipsychotics block D2 (Gi); atropine blocks muscarinic receptors → removes parasympathetic tone.
- Receptor → G protein: "qiss and qiq till you're siq of sqs" → α1 q, α2 i, β1 s, β2 s, M1 q, M2 i, M3 q, D1 s, D2 i, H1 q, H2 s, V1 q, V2 s.
Step 1 trap
Glutamate is not "always excitatory": its ionotropic receptors always excite, but presynaptic mGluRs inhibit release. Likewise ACh excites skeletal muscle (nicotinic) but slows the heart (M2, Gi).
Memory tool
IONotropic = ION channel. METAbotropic = goes through METAbolism (enzymes, second messengers) — and anything with a middleman is slower but louder (amplified).
Q5. How are transmitters grouped by what they do?
Point-to-point neural signaling
- Excitatory: glutamate (the major one)
- Inhibitory: GABA and glycine
- Acetylcholine
Modulators (act on large populations)
- Biogenic amines: dopamine, norepinephrine, serotonin, histamine
- Neuropeptides: orexin, NPY, enkephalin, endorphins
- Endocannabinoids
Buzzwords
"major excitatory NT" → glutamate
"major inhibitory NTs" → GABA (brain), glycine (spinal cord/brainstem)
"indirect gating" → metabotropic receptor
"signal amplification" → GPCR cascade
ApplyLO 1 application questions
A1A researcher injects a fluorescent dye into one neuron and sees it spread into a neighbor within seconds. What kind of synapse connects them?
Electrical (gap junction). Only connexon channels create cytoplasmic continuity that lets small molecules pass; chemical synapses keep the cytoplasm separate.
A2Match each to a receptor class: (a) NMDA overactivation in stroke, (b) benzodiazepines, (c) antipsychotics, (d) atropine.
(a) Ionotropic (excitotoxicity) · (b) Ionotropic (enhance GABAA) · (c) Metabotropic (D2 antagonists) · (d) Metabotropic (muscarinic antagonist).
A3A transmitter produces a postsynaptic effect that begins after ~200 ms and lasts minutes, outlasting its presence in the cleft. Which receptor class, and why does the effect persist?
Metabotropic (GPCR). Second messengers and kinases keep acting (and are amplified) after the transmitter is gone; ionotropic effects stop within milliseconds of unbinding.
A4A drug is applied to a postsynaptic membrane and the inside becomes more negative. Name two mechanisms that could explain it.
Opening Cl⁻ channels (ionotropic, e.g., GABAA/glycine receptor) or a Gi-coupled GPCR opening K⁺ channels (metabotropic, e.g., M2, GABAB). Both → hyperpolarization (IPSP).
LO 2Chemical neurotransmission, step by stepAP → Ca²⁺ → SNAREs → release → receptor → integration → recycling
By the end: you can list every step from action potential to postsynaptic response in order, name the fusion proteins and Ca²⁺ sensor, explain how vesicles are recycled, and predict what happens when any step is blocked (toxins, antibodies, drugs).
Q1. What are the steps of chemical transmission, in order?
AP arrivesterminal depolarizes
→
VGCC openCa²⁺ in, ~1000×
→
Synaptotagminsenses Ca²⁺
→
SNAREsv + t zipper → fusion
→
Receptorion flow / GPCR
→
Recycleendocytosis + clearance
- Release is Ca²⁺-dependent → trigger = VGCC opening by depolarization → intracellular Ca²⁺ rises ~1000-fold.
- Vesicles dock at the active zone and fuse there.
CheckA drug blocks presynaptic VGCCs. The AP still invades the terminal. What fails?
No Ca²⁺ entry → synaptotagmin isn't triggered → no vesicle fusion → no transmitter release, even though the terminal depolarizes normally.
Memory tool
"All Children Sing Songs, Read, Recycle" → AP · Ca²⁺ channels · Synaptotagmin · SNAREs · Receptor · Recycle.
Q2. Which proteins actually fuse the vesicle, and where do toxins strike?
| Group | Where | Proteins |
|---|---|---|
| v-SNAREs | Vesicle | Synaptobrevin + synaptotagmin |
| t-SNAREs | Target = presynaptic membrane | Syntaxin + SNAP-25 |
| Ca²⁺ sensor | Vesicle | Synaptotagmin |
Ca²⁺ influxthrough VGCC
→
Synaptotagmindetects Ca²⁺
→
v-SNARE binds t-SNAREcomplex zippers
→
Fusionexocytosis into cleft
- v-SNAREs = synaptotagmin + synaptobrevin; t-SNAREs = SNAP-25 + syntaxin.
- Tetanus and botulinum toxins inhibit SNARE function → no fusion → no release.
CheckWhich SNARE proteins sit on the target membrane?
Syntaxin and SNAP-25 (t-SNAREs). Synaptobrevin is the vesicular v-SNARE.
Step 1 HY — botulism vs tetanus
- Both are clostridial zinc proteases that cleave SNAREs → block vesicle release.
- Botulinum stays at peripheral cholinergic terminals → ↓ACh → descending flaccid paralysis, diplopia, ptosis, dysphagia, dilated pupils. Infant botulism: honey, "floppy baby," constipation. Adults: home-canned food. Therapeutic Botox for dystonia, spasticity, migraine.
- Tetanus enters motor terminals and travels by retrograde axonal transport to the spinal cord → blocks GABA/glycine release from inhibitory interneurons (Renshaw cells) → disinhibited motor neurons → spastic paralysis, trismus (lockjaw), risus sardonicus, opisthotonos.
- Boards say synaptotagmin is the Ca²⁺ sensor, not a SNARE; the classic toxin targets are synaptobrevin, SNAP-25, syntaxin.
Step 1 trap
Same mechanism, opposite clinical picture. The difference is which synapse is hit: botulinum = excitatory cholinergic output (→ flaccid); tetanus = inhibitory interneurons (→ spastic). Don't pick "blocks ACh receptors" for botulinum — it is presynaptic; the receptor is fine.
Buzzwords
"floppy baby + honey" → infant botulism
"lockjaw after puncture wound" → tetanus
"Ca²⁺ sensor" → synaptotagmin
"retrograde transport to spinal cord" → tetanus toxin
Q3. What happens to the vesicle after it fuses?
- A–C: clathrin-coated pits and vesicles form, segregated from the active zone; dynamin pinches the vesicle off the membrane.
- D–E: clathrin is removed; the vesicle is modified (SNARE proteins added back to its surface).
- F: vesicle is reloaded with transmitter, ready for the next fusion.
Without recycling, a fast-firing terminal would run out of vesicles within seconds.
CheckWhich protein "pinches off" the new vesicle, and which coat protein shapes it?
Dynamin pinches (GTPase); clathrin forms the coat.
Q4. How does the postsynaptic cell decide whether to fire?
- Transmitter binding opens or closes channels → a conductance change → current flows → membrane potential changes.
- EPSP (excitatory): depolarizing, e.g., glutamate → Na⁺/Ca²⁺ in. IPSP (inhibitory): hyperpolarizing, e.g., GABA/glycine → Cl⁻ in.
- The cell sums (integrates) all EPSPs and IPSPs; if the axon hillock reaches threshold, an action potential fires.
- Temporal summation = repeated inputs at one synapse in quick succession.
- Spatial summation = inputs from many synapses at once.
CheckWhich ion channels drive the phases of the action potential that starts this whole cascade?
Rest: K⁺ leak. Depolarization: voltage-gated Na⁺ channels open (Na⁺ in). Repolarization: Na⁺ channels inactivate, voltage-gated K⁺ channels open (K⁺ out). After-hyperpolarization: K⁺ channels close slowly; Na⁺/K⁺-ATPase restores gradients.
Step 1 HY — diseases of the release step
| Feature | Lambert–Eaton (LEMS) | Myasthenia gravis |
|---|---|---|
| Antibody target | Presynaptic P/Q-type VGCC → ↓ACh release | Postsynaptic nicotinic AChR (or MuSK) |
| With repeated use | Improves (Ca²⁺ builds up) | Worsens (fatigable) |
| Muscles | Proximal limbs + autonomic (dry mouth) | Ocular (ptosis, diplopia), bulbar |
| Reflexes | ↓ / absent | Normal |
| Association | Small cell lung cancer | Thymoma / thymic hyperplasia |
| Treatment | Amifampridine (K⁺-channel blocker → longer AP → more Ca²⁺ entry) | Pyridostigmine (AChE inhibitor), thymectomy, immunosuppression |
Antiseizure drugs that act at release: gabapentin/pregabalin bind the α2δ subunit of VGCCs; levetiracetam binds vesicle protein SV2A.
Step 1 trap
LEMS vs MG: both cause weakness and both involve the NMJ, but LEMS is presynaptic (Ca²⁺ channel) and gets better with effort; MG is postsynaptic (receptor) and gets worse. Hyporeflexia + dry mouth + smoker → LEMS.
ApplyLO 2 application questions
A1A farmer with a puncture wound develops lockjaw; another patient who ate home-canned beans has descending flaccid paralysis. What do the toxins share, and why are the effects opposite?
Both cleave SNAREs → block vesicle release. Tetanus blocks GABA/glycine release from spinal inhibitory interneurons → disinhibition → spasm. Botulinum blocks ACh release at the NMJ → flaccid weakness.
A2A 62-year-old smoker has proximal leg weakness that briefly improves after sustained grip, dry mouth and absent reflexes. Which step of transmission is impaired?
Step 2: presynaptic VGCC opening (Lambert–Eaton; anti-P/Q Ca²⁺ channel antibodies, paraneoplastic with small cell lung cancer). Repeated stimulation lets residual Ca²⁺ accumulate → transient improvement.
A3A mutant synaptotagmin cannot bind Ca²⁺. Predict release after an action potential.
Synchronous, Ca²⁺-triggered release is lost even though Ca²⁺ enters normally, because the fusion machinery never "hears" the Ca²⁺ signal.
A4A dynamin-blocking drug is applied and the terminal is stimulated at high frequency. What happens over the next minute?
Release fades as the vesicle pool is used up: vesicles fuse but can't be pinched back off and recycled.
LO 3What makes a neurotransmitterFour rules, and the transmitter classes they produce
By the end: you can list the four criteria, link each to a drug or toxin that targets it, and say where small-molecule transmitters vs neuropeptides are made (and which transmitter is the vesicle exception).
Q1. What four things must a substance do to count as a neurotransmitter?
1 · Synthesizedin the neuron
→
2 · Releasedon presynaptic depolarization; Ca²⁺-dependent
→
3 · Receptorspecific receptors on the postsynaptic cell
→
4 · Removedspecific mechanism ends its action
Each criterion is a potential drug target. Using ACh as the example:
| Criterion | ACh example target | Agent |
|---|---|---|
| Synthesis | Choline uptake (ChT) — the rate-limiting step | Hemicholinium blocks ChT |
| Storage | Vesicular ACh transporter (VAChT) | Vesamicol |
| Release | SNARE-mediated fusion | Botulinum toxin |
| Receptor | Nicotinic / muscarinic | α-bungarotoxin (nicotinic), atropine (muscarinic) |
| Termination | Acetylcholinesterase | Donepezil, pyridostigmine, organophosphates |
CheckA substance is made in neurons and acts on receptors, but its release is unchanged when Ca²⁺ is removed from the bath. Is it a classical neurotransmitter?
No — it fails the "Ca²⁺-dependent release on depolarization" criterion.
Memory tool
"S-R-R-R": Synthesize, Release, Receptor, Remove. One drug for each R makes a good self-quiz.
Q2. Where is each class of transmitter made?
| Class | Examples | Where made |
|---|---|---|
| Small-molecule | ACh; amino acids (glutamate, GABA, glycine); purines (ATP); biogenic amines — catecholamines (DA, NE, epinephrine), serotonin, histamine | Presynaptic terminal (enzymes shipped from soma) |
| Peptide | Neuropeptide Y, vasopressin, substance P, ACTH, orexin, opioid peptides (endorphin, dynorphin, enkephalin) | Cell body (RER/Golgi) → transported to terminal |
| Lipid-based retrograde | Endocannabinoids | Made on demand in the postsynaptic cell; signal backward to the presynaptic terminal |
Life cycle of a small-molecule transmitter
Somaenzymes made in RER → Golgi
→
Axonanterograde transport (kinesin)
→
Terminalprecursor uptake → synthesis in cytoplasm
→
Vesicletransport in (NE exception)
→
Release + bindCa²⁺ exocytosis
→
Cleardiffusion / removal / inactivation
CheckWhich transmitter breaks the rule "synthesized in the cytoplasm, then pumped into vesicles"?
Norepinephrine: dopamine is pumped into the vesicle and dopamine β-hydroxylase inside the vesicle makes NE.
Step 1 HY
- Axonal transport: anterograde = kinesin (soma → terminal); retrograde = dynein (how tetanus toxin, rabies, HSV and polio reach the soma).
- Endocannabinoids (e.g., anandamide, 2-AG) act on presynaptic CB1 receptors → ↓ transmitter release.
- Catecholamine pathway: tyrosine → L-DOPA (tyrosine hydroxylase, rate-limiting) → DA → NE (in vesicle) → epinephrine (adrenal medulla, PNMT, cortisol-induced).
Step 1 trap
Morphine and oxycodone are not neuropeptides — they are plant-derived/semisynthetic opioid drugs that act on the same receptors as the endogenous opioid peptides (endorphin, dynorphin, enkephalin).
Buzzwords
"made in cell body, shipped down axon" → neuropeptide
"retrograde messenger" → endocannabinoid
"last step inside vesicle" → norepinephrine
ApplyLO 3 application questions
A1Colchicine (blocks microtubules) is injected into the axon. Which transmitters are depleted fastest at the terminal: small molecules or neuropeptides?
Neuropeptides. They are made only in the soma and must travel down the axon; small-molecule transmitters are synthesized locally from existing enzymes (those also eventually run down).
A2Give one drug/toxin acting at each criterion for ACh.
Synthesis: hemicholinium (ChT). Release: botulinum. Receptor: atropine / α-bungarotoxin. Termination: donepezil.
A3A candidate molecule is released Ca²⁺-dependently and binds receptors, but its action lasts indefinitely after a single release in a dish. Which criterion is missing?
A specific termination mechanism (enzyme or reuptake). Without one it can't carry discrete signals.
LO 4Life cycles of acetylcholine and glutamateSynthesis → storage → release → receptors → termination
By the end: you can draw each transmitter's cycle, name the enzymes, transporters and rate-limiting step, compare nicotinic vs muscarinic and AMPA vs NMDA receptors, and connect each step to a drug or disease.
Q1. How is ACh made, stored, released and cleared?
- ChAT (choline acetyltransferase) is made in the soma and shipped to the terminal — only cholinergic neurons make it (marker of cholinergic cells).
- Acetyl-CoA comes from mitochondria.
- Choline is actively transported in by the high-affinity, Na⁺-dependent choline transporter (ChT). Rate-limiting step = choline availability/uptake. (Choline comes from liver synthesis and diet.)
- ChAT makes ACh in the cytoplasm: choline + acetyl-CoA → ACh + CoA.
- ACh is actively transported into vesicles (by VAChT, the vesicular ACh transporter).
- Release: AP → VGCC → Ca²⁺ → exocytosis; ACh binds postsynaptic receptors.
- Termination: acetylcholinesterase (AChE) hydrolyzes ACh → acetate + choline; choline is taken back up and reused. AChE is made by and tethered to the postsynaptic cell.
- AChE is a drug target in Alzheimer disease (donepezil).
- ACh rate-limiting step = how much choline can be brought into the presynaptic terminal (ChT).
- Contrast glutamate → rate-limiting enzyme = glutaminase.
CheckWhat limits how fast a cholinergic terminal can make ACh?
Choline uptake into the terminal by ChT — not the activity of ChAT.
Step 1 HY — cholinergic pharmacology
- Organophosphate poisoning (insecticides, nerve gas): irreversible AChE inhibition → muscarinic DUMBBELSS (diarrhea, urination, miosis, bronchospasm/bronchorrhea, bradycardia, emesis, lacrimation, salivation, sweating) + nicotinic fasciculations → paralysis. Treat: atropine (muscarinic) + pralidoxime (regenerates AChE before "aging").
- AChE inhibitors in Alzheimer: donepezil, rivastigmine, galantamine. In MG: pyridostigmine (doesn't cross BBB); edrophonium (old diagnostic test).
- Physostigmine (tertiary, crosses BBB) reverses anticholinergic toxicity.
Step 1 trap
- ChT and VAChT are transporters, not enzymes. ChAT is the only synthetic enzyme; AChE is the degrading enzyme.
- AChE does not recycle ACh intact — it splits it; only choline is recycled.
- AChE is anchored on the postsynaptic side (made by the postsynaptic cell), not shipped from the presynaptic neuron.
Q2. What receptors does ACh act on?
| Feature | Nicotinic (nAChR) | Muscarinic (mAChR) |
|---|---|---|
| Named after | Nicotine (tobacco) | Muscarine (mushroom toxin) |
| Class | Ionotropic — the receptor is the ion channel | Metabotropic (GPCR) |
| Ions / response | Na⁺ in, K⁺ out → fast, brief | Slower, longer-lasting |
| Subtypes | Combinations of α and β subunits (pentamer); α4β2 common in brain; muscle type α1₂β1δε | M1–M5 (M1, M3, M5 → Gq; M2, M4 → Gi) |
| Location | Postsynaptic (muscle and neuronal forms) | Pre- and post-synaptic |
| Classic antagonist | α-bungarotoxin (snake venom; binds very tightly) | Atropine |
CheckWhich ACh receptor subtype does varenicline target for smoking cessation?
α4β2 neuronal nicotinic receptor (partial agonist).
Memory tool
Nicotine is a quick hit (fast, ionotropic); a mushroom takes a while to kick in (slow, metabotropic).
Q3. How is glutamate made, stored and cleared?
- Precursor = glutamine, derived from glucose/Krebs cycle and made in astrocytes.
- Glutaminase converts glutamine → glutamate in the terminal — rate-limiting step.
- Glutamate is actively transported into vesicles by VGLUT.
- Release by Ca²⁺-triggered exocytosis.
- Termination: reuptake mainly by excitatory amino acid transporters (EAATs) on astrocytes; inside the astrocyte, glutamine synthetase converts glutamate → glutamine.
- Glutamine goes back to the neuron for reuse = glutamate/glutamine cycle.
| Step | Acetylcholine | Glutamate |
|---|---|---|
| Precursor | Choline + acetyl-CoA | Glutamine (from astrocytes) |
| Synthetic enzyme | ChAT | Glutaminase |
| Rate-limiting | Choline availability (ChT uptake) | Glutaminase |
| Vesicle transporter | VAChT | VGLUT |
| Termination | Enzymatic: AChE → choline reuptake | Reuptake: EAAT into astrocytes → glutamine synthetase |
CheckWhich enzyme sits in the astrocyte and which in the neuron?
Glutamine synthetase = astrocyte (glutamate → glutamine). Glutaminase = neuron (glutamine → glutamate).
Step 1 HY — astrocytes and ammonia
Glutamine synthetase also detoxifies ammonia (glutamate + NH₃ → glutamine). In hepatic encephalopathy, ↑NH₃ → astrocytes accumulate glutamine and swell (Alzheimer type II astrocytes) → cerebral edema, asterixis, confusion. Treat with lactulose/rifaximin.
Step 1 trap
Glutamate's action is not ended by an enzyme in the cleft (unlike ACh) — it is ended by reuptake, mostly into astrocytes. "Glutaminase" and "glutamine synthetase" sound alike: -ase makes glutamate in the neuron; synthetase makes glutamine in the astrocyte.
Memory tool
Astrocyte = the recycling center. It takes the used glutamate, repackages it as safe glutamine, and ships it back to the factory (neuron).
Q4. What receptors does glutamate act on, and why is NMDA special?
- Ionotropic (glutamate-gated cation channels, named for their synthetic agonists): AMPA (GluA1–4), NMDA (GluN1, GluN2A–D, GluN3A–B), kainate (GluK1–5). Activation always excitatory.
- Metabotropic (mGluR1–8): 3 classes, 8 subtypes.
| Feature | AMPA | NMDA |
|---|---|---|
| Assembly | Tetramer (4 subunits) | Tetramer (4 subunits) |
| Kinetics | Fast; desensitizes rapidly | Slower |
| Permeability | Na⁺, K⁺; mostly not Ca²⁺ — most contain GluA2, whose mRNA is edited, changing pore charge | Na⁺, K⁺ and Ca²⁺ |
| To open, needs | Glutamate | Glutamate + co-agonist glycine or D-serine + depolarization to expel Mg²⁺ |
| Downstream | Depolarizes the spine | Ca²⁺ activates intracellular signaling (plasticity, LTP) |
Coincidence detector
Glutamateopens neighboring AMPA receptors
→
Na⁺ influxdepolarizes the spine
→
Mg²⁺ expelledfrom NMDA pore
→
Ca²⁺ enterssignaling, plasticity
NMDA is the unusual ligand-gated channel that is also voltage-dependent and needs a co-agonist. That is why AMPA and NMDA receptors co-localize at glutamatergic synapses: AMPA provides the depolarization NMDA needs.
Metabotropic glutamate receptors
- Dimers of two identical subunits; 3 classes, 8 subtypes (mGluR1–8).
- Located on pre- and postsynaptic neurons and glia.
- Presynaptic mGluRs → presynaptic inhibition (↓ release; a negative-feedback brake).
- Postsynaptic → can excite or inhibit, depending on the G protein, downstream signaling and nearby ion channels → very varied roles.
- Ionotropic glutamate receptors (AMPA, NMDA) are always excitatory.
- NMDA needs three things: glutamate + glycine/serine + depolarization to remove Mg²⁺.
- Metabotropic: presynaptic → inhibitory; postsynaptic → excitatory or inhibitory.
CheckGlutamate and glycine are both bound to an NMDA receptor on a cell held at −70 mV, yet no current flows. Why?
Mg²⁺ block. At resting potential Mg²⁺ sits in the pore; depolarization is needed to push it out.
Step 1 HY — NMDA in the clinic
- NMDA antagonists: ketamine (dissociative anesthetic, rapid antidepressant), PCP (violence, nystagmus), memantine (moderate–severe Alzheimer), dextromethorphan; felbamate also acts here.
- Excitotoxicity: ischemia → ATP loss → failed glutamate reuptake → NMDA overactivation → Ca²⁺ overload → proteases, free radicals → neuronal death (stroke, trauma, seizures). Hippocampal CA1 is especially vulnerable.
- Anti-NMDA receptor encephalitis: young woman, psychiatric symptoms → seizures, dyskinesias, autonomic instability; antibodies to GluN1; look for ovarian teratoma.
- Riluzole (ALS) ↓ glutamate release.
Step 1 trap
The NMDA co-agonist is glycine (or D-serine) — the same molecule that is an inhibitory transmitter at its own Cl⁻ channel in the spinal cord. On NMDA, glycine is not inhibitory. Also: memantine blocks NMDA, but donepezil blocks AChE — two different Alzheimer mechanisms.
Buzzwords
"Mg²⁺ block, voltage-dependent" → NMDA
"coincidence detector" → NMDA
"edited GluA2" → Ca²⁺-impermeable AMPA
"ovarian teratoma + psychosis + seizures" → anti-NMDA receptor encephalitis
"dissociative anesthetic" → ketamine
ApplyLO 4 application questions
A1A farm worker sprayed with insecticide has pinpoint pupils, drooling, wheezing, bradycardia and muscle twitching. Which step of ACh signaling is blocked, and what do you give?
Termination (AChE) — organophosphates irreversibly inhibit it → ACh piles up at muscarinic and nicotinic synapses. Give atropine + pralidoxime.
A2A drug blocks the high-affinity choline transporter. What happens to ACh output during sustained stimulation?
It falls — choline uptake is the rate-limiting step (e.g., hemicholinium), so the terminal can't resynthesize ACh fast enough.
A3In a mouse lacking EAATs on astrocytes, predict what happens after repeated glutamate release.
Glutamate lingers in the cleft → prolonged NMDA activation → Ca²⁺ overload → excitotoxic neuron death and seizures; less glutamine is returned to neurons.
A4A group II/III mGluR agonist is applied to a glutamatergic synapse. Predict the effect on glutamate release.
↓ Release. These mGluRs are mostly presynaptic and Gi/o-coupled → presynaptic inhibition.
A5Why does an AMPA antagonist also reduce NMDA receptor Ca²⁺ influx, even though it doesn't bind NMDA?
Without AMPA-driven depolarization, Mg²⁺ stays in the NMDA pore, so glutamate + glycine alone can't open it.
LO 5Where ACh and glutamate live in the brainCholinergic nuclei, receptor distribution, glutamatergic regions
By the end: you can name the cholinergic cell groups and their targets, explain the Alzheimer connection, list where glutamatergic neurons are densest, and match brain regions to the ACh receptor types they carry.
Q1. Where are the cholinergic neurons, and what do they do?
ACh is made by both:
- Long projection neurons
- Nucleus basalis of Meynert → widespread cortex
- Medial septal nucleus → hippocampus
- Brainstem tegmental group (pedunculopontine / laterodorsal tegmental) → thalamus, brainstem
- Local circuit interneurons — e.g., striatal interneurons; also cortex
Regions to know: cortex, nucleus basalis of Meynert, medial septal nucleus, hippocampus, brainstem, striatal interneurons.
ACh modulates emotional states, sensory input, learning and memory, wakefulness and sleep.
CheckWhich basal forebrain nucleus supplies ACh to the hippocampus, and which to the neocortex?
Medial septal nucleus → hippocampus; nucleus basalis of Meynert → cortex.
Step 1 HY — Alzheimer disease
- Degeneration of cholinergic neurons, especially the nucleus basalis of Meynert → ↓ACh in hippocampus and cortex → memory loss; ↓ChAT activity is a classic finding.
- Drugs: AChE inhibitors (donepezil, rivastigmine, galantamine) ↑ synaptic ACh; memantine (NMDA antagonist) for moderate–severe disease.
- Side effects of AChE inhibitors are cholinergic: nausea, diarrhea, bradycardia.
- Anticholinergic drugs (diphenhydramine, TCAs, oxybutynin) worsen confusion in the elderly.
Memory tool
"Meynert → Memory": the nucleus basalis of Meynert feeds memory circuits and is lost in Alzheimer's. Septum → Sea horse (hippocampus).
Q2. Where are the ACh receptors?
Take-home: ACh receptors are widespread. The localization map sorts regions into three groups:
Muscarinic only
Midbrain, medulla, pons (brainstem)
Nicotinic only
Substantia nigra, locus coeruleus, septum
Both
Striatum, cortex, hippocampus, thalamus, hypothalamus, cerebellum
CheckA region has both nicotinic and muscarinic receptors and is central to memory. Name it.
Hippocampus (also cortex, striatum, thalamus, hypothalamus, cerebellum carry both).
Step 1 trap
Don't confuse the source of a transmitter with its receptors: the substantia nigra and locus coeruleus are dopamine and NE sources, yet they carry nicotinic ACh receptors. Board questions about the substantia nigra still point to dopamine/Parkinson.
Q3. Where are glutamatergic neurons, and what diseases involve glutamate?
Most abundant excitatory (and amino acid) transmitter — "basically everywhere." Densest in:
- Cerebral cortex
- Hippocampus
- Amygdala
- Basal ganglia
- Brainstem
- Spinal cord
Physiological roles
- Neuroplasticity
- Neuronal development
- Learning and memory
- Motor function
Pathological roles
- Epilepsy
- Anxiety
- Addiction
- Ischemic brain damage / stroke
CheckName three regions where glutamatergic neurons are densest.
Any three of: cerebral cortex, hippocampus, amygdala, basal ganglia, brainstem, spinal cord.
Step 1 HY
Glutamate is the transmitter of cortical pyramidal (upper motor) neurons, the corticostriatal and subthalamic → GPi pathways (basal ganglia circuit) and hippocampal circuits (LTP). Antiseizure drugs that reduce glutamate signaling: perampanel (AMPA antagonist), topiramate (blocks AMPA/kainate), felbamate (NMDA).
Buzzwords
"nucleus basalis of Meynert" → cholinergic, Alzheimer
"medial septal nucleus" → cholinergic input to hippocampus
"most abundant excitatory NT" → glutamate
ApplyLO 5 application questions
A1A 78-year-old with progressive memory loss is started on donepezil. Which nucleus has degenerated, and why does this drug help?
Nucleus basalis of Meynert (and septal cholinergic neurons) → ↓ACh in cortex and hippocampus. Donepezil inhibits AChE so the remaining ACh lasts longer.
A2A lesion of the medial septal nucleus would most affect cholinergic input to which structure and function?
Hippocampus → learning and memory (hippocampal theta rhythm).
A3After a cardiac arrest, a patient has selective memory loss. Which transmitter mechanism best explains damage to hippocampal neurons?
Glutamate excitotoxicity: ischemia → glutamate accumulates → NMDA-mediated Ca²⁺ overload; the hippocampus (CA1) is rich in glutamatergic synapses and highly vulnerable.
MCQPractice questionsClick an answer to see the explanation.
Score: 0 / 0
1A 4-month-old infant is brought in with constipation, a weak cry, poor feeding and generalized hypotonia. His parents sweeten his pacifier with honey. The toxin responsible acts by which mechanism?
C. Infant botulism. Botulinum toxin is a zinc protease that cleaves SNAREs (SNAP-25, synaptobrevin, syntaxin) at cholinergic terminals → ↓ACh release → flaccid weakness. A = α-bungarotoxin/curare-like; B = organophosphates (cholinergic excess, not weakness with constipation); D = Lambert–Eaton; E = tetanus (spastic).
2A 35-year-old gardener who stepped on a rusty nail two weeks ago presents with jaw stiffness, a fixed grin and painful back-arching spasms. The toxin reached its site of action by retrograde transport. Which transmitter's release is directly reduced?
A. Glycine (and GABA). Tetanus toxin cleaves synaptobrevin in spinal inhibitory interneurons (Renshaw cells) → loss of inhibition → motor neurons fire unchecked → spasm. Blocking ACh (B) would cause flaccid paralysis (botulism). Glutamate, dopamine and NE release are not the primary targets.
3A 64-year-old man with a 50-pack-year smoking history has difficulty climbing stairs and a dry mouth. Hand-grip strength increases over several seconds of sustained effort, and deep tendon reflexes are absent. The antibody in this disorder most directly prevents which event?
D. Lambert–Eaton syndrome: anti-P/Q-type VGCC antibodies (paraneoplastic, small cell lung cancer) → less Ca²⁺ entry → ↓ACh release; repeated effort lets Ca²⁺ accumulate → improvement. A = myasthenia gravis (worsens with use, normal reflexes). B, C, E are not antibody targets in this disease.
4In a slice preparation, adding hemicholinium causes ACh release to fall progressively during repetitive stimulation, while ChAT enzyme activity measured in homogenate is unchanged. This shows that ACh synthesis is limited mainly by which factor?
B. The rate-limiting step for ACh is choline availability, set by the high-affinity choline transporter (ChT), which hemicholinium blocks. ChAT (C) is not limiting; acetyl-CoA (A) is plentiful; VAChT (D) is vesamicol's target; AChE (E) degrades, not synthesizes.
5A 40-year-old crop-duster is found confused with pinpoint pupils, profuse sweating, salivation, wheezing, heart rate 44/min and diffuse muscle fasciculations. In addition to atropine, which drug directly restores the function of the inhibited enzyme?
E. Pralidoxime regenerates organophosphate-inhibited acetylcholinesterase (if given before "aging") and helps nicotinic symptoms that atropine can't reverse. Physostigmine (A) and donepezil (C) are AChE inhibitors and would worsen toxicity; hemicholinium (B) and vesamicol (D) block choline uptake and vesicle loading.
6A hippocampal neuron is voltage-clamped at −70 mV. Glutamate and glycine are applied together with an AMPA-receptor blocker. Almost no current flows through NMDA receptors. When the cell is clamped at −20 mV, a large inward current carried partly by Ca²⁺ appears. What explains the difference?
B. NMDA receptors are ligand-gated and voltage-dependent: Mg²⁺ blocks the pore at rest; depolarization expels it. Glycine is a required co-agonist here (A wrong). Rapid desensitization (C) and GluA2 editing (D) describe AMPA receptors. Ligand binding (E) doesn't require depolarization — channel opening does.
7A 70-year-old woman has a left MCA stroke. In the penumbra, neurons continue to die for hours after the occlusion. Which event is the most direct trigger of this delayed neuronal death?
D. Excitotoxicity: ATP failure → glutamate reuptake fails → sustained NMDA activation → Ca²⁺ overload → proteases, free radicals, death. GABA (A) is protective; AChE (B) is irrelevant; presynaptic mGluRs (C) reduce release; reduced glutaminase (E) would lower glutamate.
8A 76-year-old man has 3 years of progressive short-term memory loss and is started on donepezil. Loss of neurons in which structure best explains the rationale for this therapy?
C. The nucleus basalis of Meynert is the main cholinergic source to the cortex; its degeneration in Alzheimer disease ↓ACh in cortex and hippocampus, so inhibiting AChE (donepezil) prolongs remaining ACh. A = dopamine (Parkinson); B = norepinephrine; D = serotonin; E = glutamatergic basal ganglia nucleus (hemiballismus).
9A 22-year-old woman develops anxiety and hallucinations, then confusion, a generalized seizure, orofacial dyskinesias and labile blood pressure. Pelvic imaging shows an ovarian mass with fat and calcification. Autoantibodies are most likely directed against which receptor?
E. Anti-NMDA receptor encephalitis, classically paraneoplastic with an ovarian teratoma; antibodies target GluN1. D2 (B) explains psychosis but not seizures/dyskinesias/autoantibody; P/Q channels (C) = Lambert–Eaton; M1 and GluA2 are not the classic targets.
10A 55-year-old man with cirrhosis becomes confused and has a flapping tremor. Serum ammonia is elevated. Brain astrocytes swell. Which astrocytic enzyme, normally part of the glutamate recycling pathway, is responsible for this swelling when ammonia is high?
A. Astrocytic glutamine synthetase combines glutamate + NH₃ → glutamine; excess glutamine is osmotically active → astrocyte swelling (Alzheimer type II astrocytes). Glutaminase (B) is the neuronal enzyme making glutamate; GAD (C) makes GABA; D and E belong to the ACh cycle.
11Two neurons in the inferior olive are recorded simultaneously. A current pulse injected into either cell produces an immediate small depolarization in the other, with no measurable delay, and the coupling is abolished by a connexin-blocking drug. Which feature also characterizes this type of synapse?
B. This is an electrical synapse (gap junctions/connexons): direct, very fast, bidirectional current flow with shared cytoplasm, ideal for synchrony. A, C, D and E all describe chemical synapses.
12An investigational antiseizure drug is a selective agonist at a glutamate receptor that is a dimer of two identical subunits, couples to Gi/o and is located mainly on presynaptic terminals. What is the expected effect at glutamatergic synapses?
E. This describes a (group II/III) metabotropic glutamate receptor; presynaptic mGluRs cause presynaptic inhibition → ↓ release (useful against seizures). A–C describe ionotropic receptors; D is the opposite effect.
13A 32-year-old woman has drooping eyelids and double vision that worsen by evening, and she has trouble chewing at the end of meals. Reflexes are normal. CT shows an anterior mediastinal mass. Her antibodies most directly target which structure?
A. Myasthenia gravis with thymoma: antibodies against the muscle nicotinic AChR (a ligand-gated ion channel) → fatigable ocular/bulbar weakness. Muscarinic receptors (B) are not at the NMJ; C = Lambert–Eaton (improves with use, hyporeflexia); D = botulinum target; E is the target of pyridostigmine, the treatment.