Synapse Study Guide

📖 Core Concepts Synapse – a specialized junction where a neuron transmits an electrical or chemical signal to another neuron or effector cell. Synaptic cleft – the narrow extracellular space separating presynaptic and postsynaptic membranes. Chemical synapse – signal transferred by neurotransmitter release, diffusion, and receptor binding. Electrical synapse – direct ionic current flow through gap‑junction channels; bidirectional and ultra‑fast. Excitatory vs. inhibitory – excitatory neurotransmitters open cation (Na⁺) channels → depolarization; inhibitory neurotransmitters open Cl⁻ or K⁺ channels → hyper‑polarization. Neurotransmitter release – Ca²⁺ entry via voltage‑gated channels triggers vesicle exocytosis. Synaptic plasticity – activity‑dependent strengthening (LTP) or weakening (LTD) of synaptic efficacy; basis of learning & memory. Key adhesion molecules – neurexins, neuroligins, and other SAMs that lock pre‑ and postsynaptic specializations together. --- 📌 Must Remember Voltage‑gated Ca²⁺ channels open when the presynaptic action potential arrives → Ca²⁺ influx = release trigger. Glutamate = excitatory, GABA = inhibitory (most common neurotransmitter classes). Acetylcholine & dopamine can be excitatory or inhibitory depending on receptor subtype. Endocannabinoid retrograde signaling = postsynaptic release of cannabinoids → presynaptic CB1 activation → ↓ release. SSRIs block serotonin reuptake → ↑ extracellular 5‑HT. NMDAR activation → Ca²⁺ influx → downstream LTP/LTD cascades. CaMKII autophosphorylation → kinase stays active after Ca²⁺ falls → maintains LTP. Electrical synapses use gap‑junction channels → rapid, bidirectional, synchronize networks. --- 🔄 Key Processes Calcium‑triggered neurotransmitter release Action potential → depolarizes presynaptic terminal → opens \( \mathrm{Ca^{2+}} \) channels → \([ \mathrm{Ca^{2+}} ]{i}\) rises → vesicle fusion (exocytosis). Postsynaptic receptor activation Neurotransmitter binds → transmitter‑gated ion channel opens → ion flux (Na⁺, Cl⁻, K⁺) → graded change in membrane potential. Neurotransmitter clearance Reuptake into presynaptic terminal or uptake by astrocytes or enzymatic degradation (e.g., acetylcholinesterase). Presynaptic GPCR modulation GPCR activation → inhibits voltage‑gated Ca²⁺ channels or activates K⁺ channels → ↓ vesicle release probability. Endocannabinoid retrograde signaling Postsynaptic Ca²⁺ rise → synthesize endocannabinoid → diffuse backward → bind CB1 receptors → suppress further release. LTP induction (canonical) High‑frequency stimulation → strong depolarization removes Mg²⁺ block from NMDAR → Ca²⁺ influx → CaMKII activation → AMPA receptor insertion → synaptic strengthening. LTD induction Low‑frequency stimulation → modest Ca²⁺ rise → phosphatase activation → AMPA receptor removal → synaptic weakening. --- 🔍 Key Comparisons Chemical vs. Electrical Synapse Chemical: neurotransmitter release, unidirectional, slower (ms), plasticity‑rich. Electrical: gap‑junction ions, bidirectional, ultra‑fast (<1 ms), limited plasticity. Excitatory vs. Inhibitory Neurotransmitters Excitatory: open Na⁺/Ca²⁺ channels → depolarization (e.g., glutamate). Inhibitory: open Cl⁻/K⁺ channels → hyper‑polarization or shunting inhibition (e.g., GABA). Presynaptic GPCR modulation vs. Endocannabinoid signaling GPCR: typically ligand from another neuron; reduces release by channel modulation. Endocannabinoid: retrograde messenger made by postsynaptic cell; targets CB1 to suppress release. Axo‑dendritic vs. Axo‑axonic vs. Dendro‑dendritic Axo‑dendritic: classic; axon → dendrite. Axo‑axonic: axon modulates another axon's output (often inhibitory). Dendro‑dendritic: dendrite ↔ dendrite (common in olfactory bulb, retina). --- ⚠️ Common Misunderstandings “All glutamate = excitatory” – true for most CNS synapses, but metabotropic glutamate receptors can be modulatory/inhibitory. “Electrical synapses are always excitatory” – they simply pass current; the net effect depends on the resting potentials of the coupled cells. “Acetylcholine is only excitatory” – muscle ACh is excitatory, but neuronal ACh can be inhibitory via muscarinic M2 receptors. “Calcium influx only triggers release” – Ca²⁺ also activates signaling cascades (e.g., NMDAR‑dependent plasticity). --- 🧠 Mental Models / Intuition “Key‑Lock” model: neurotransmitter (key) fits a specific receptor (lock); only the right key opens the ion channel gate. “Calcium as the spark”: Think of presynaptic Ca²⁺ as the spark that ignites the vesicle “fireworks” of release. “Weight‑lifting analogy for LTP: Repeated strong stimulation “adds bricks” (AMPA receptors) to the postsynaptic wall, making it easier to reach threshold later. --- 🚩 Exceptions & Edge Cases Dual‑action neurotransmitters (ACh, dopamine) – effect depends on receptor subtype distribution. Bidirectional electrical synapses can become rectifying (prefer one direction) if connexin composition is asymmetric. Endocannabinoid signaling can produce both short‑term depression (STD) and long‑term depression (LTD) of release, depending on stimulation pattern. --- 📍 When to Use Which Identify synapse type → look for presence of gap‑junction proteins (electrical) vs. vesicles & cleft (chemical). Predict excitatory vs. inhibitory → check neurotransmitter class (glutamate → exc, GABA → inh) and receptor subtype. Choose plasticity mechanism → high‑frequency → LTP; low‑frequency → LTD; presence of NMDAR → calcium‑dependent. Select pharmacological target → if goal is to ↑ serotonin → SSRIs (reuptake block); to ↓ excessive glutamate → NMDA antagonists. --- 👀 Patterns to Recognize “Ca²⁺ rise + NMDAR opening = plasticity” – whenever you see NMDA involvement, expect a calcium‑dependent LTP/LTD cascade. “Presynaptic GPCR activation → ↓ release” – any mention of presynaptic Gi/o‑coupled receptors (e.g., CB1, α2‑adrenergic) signals reduced vesicle release. “Gap‑junction ↔ synchronous firing” – clusters of neurons linked by electrical synapses often fire in lockstep (e.g., retinal interneurons). --- 🗂️ Exam Traps Distractor: “Electrical synapses always produce excitatory postsynaptic potentials.” – Wrong; they merely pass current; net effect depends on membrane potentials. Distractor: “All GABAergic synapses are inhibitory in adults.” – Generally true, but during development GABA can be depolarizing due to reversed Cl⁻ gradient. Distractor: “Endocannabinoids increase neurotransmitter release.” – Opposite; they suppress release via CB1 activation. Distractor: “LTP only involves postsynaptic changes.” – Incorrect; presynaptic release probability can also be enhanced during LTP. ---