Synaptic Transmission (Chapter 5) – Comprehensive Study Notes

Architecture of Synapses

  • Two fundamental designs provide inter-neuronal communication:
    • Electrical Synapses (gap junctions) – cytoplasmic continuity through connexon channels allows ions & small molecules to pass directly.
    • Chemical Synapses – physical discontinuity; information carried by neurotransmitters that bind to receptors on the postsynaptic membrane.
  • Tripartite concept: every functional synapse consists of presynaptic bouton, postsynaptic element, and surrounding astroglial processes that monitor & modulate transmission.

Electrical Synaptic Transmission

  • Connexons form aqueous pores; six connexin subunits per hemichannel; two hemichannels align to complete a gap junction.
  • Found in vertebrate brain regions (inferior olive, hippocampus, retina, spinal cord).
  • Bidirectional & fast (<1 ms latency); critical for synchronising oscillations, escape responses, hormone pulses.
  • Limited plasticity; amplitudes attenuate with distance but enable metabolic coupling.
  • Reduction or knockout of connexin-36 → epilepsy-like hyperexcitability (clinical linkage).

Chemical Synaptic Transmission

  • Sequence of events (Fig. 5.4)
    • NT synthesis & vesicular storage (40–50 nm clear vesicles for small-molecule NTs; 90–250 nm dense-core for peptides).
    • AP invades terminal → voltage-gated Ca2+Ca^{2+} channels (P/Q, N, R, L, T types) open.
    • Calcium microdomains raise local [Ca2+]i[Ca^{2+}]_{i} from ≈100 nM\approx 100\,\text{nM} to >10 μM>10\,\mu\text{M} within 100 µs.
    • Ca2+Ca^{2+} binds synaptotagmin → SNARE complex zippering → vesicle fusion & exocytosis.
    • NT diffuses (≈0.3 µs across 20–50 nm cleft) → binds postsynaptic receptors → ion channels open/close.
    • Signal terminated by uptake via transporters, enzymatic degradation (e.g., AChE), or glial buffering.
  • Unidirectional, versatile, subject to profound plasticity (LTP/LTD, neuromodulation).

Neurotransmitter Diversity & Properties

  • Classical (small-molecule): glutamate, γ\gamma-aminobutyric acid (GABA), glycine, acetylcholine (ACh), monoamines (DA, NE, 5-HT, HA), ATP/adenosine.
  • Non-classical: neuropeptides (endorphins, CRF, orexin, BDNF, hundreds more), lipid transmitters (anandamide, 2-AG), gases (NO, CO, H2SH_{2}S).
  • A single neuron may express one or several transmitters; vesicles can co-package, yielding:
    • Co-release – two transmitters released from same vesicle.
    • Co-transmission – separate vesicle pools, often with differential Ca2+Ca^{2+} sensitivity or spatial segregation.
  • Functional implications: fast + modulatory signaling, trophic support, gene regulation.

Quantal Release – Lessons from the Neuromuscular Junction (NMJ)

  • Spontaneous miniature end-plate potentials (MEPPs) represent single-vesicle quanta (≈7000 ACh molecules).
  • Evoked end-plate potentials (EPPs) are integer multiples of MEPP amplitude; distribution described by Poisson statistics.
  • Release probability PrP_{r} shows steep power-law dependence on Ca2+Ca^{2+} influx (∝ [Ca2+]4[Ca^{2+}]^{4}).
  • Experiments lowering extracellular Ca2+Ca^{2+} convert evoked EPPs to single-quantum size ⇒ validates quantal theory.

Synaptic Vesicle Cycle

  • Life-cycle stages (Fig. 3.6 / 5.11)
    1. Vesicle filling via V-ATPase proton gradient & specific transporters (e.g., VGLUT, VGAT).
    2. Docking at active zone; tethered by SNARE proteins:
    • v-SNARE: synaptobrevin (VAMP).
    • t-SNAREs: syntaxin + SNAP-25.
    1. Priming (munc13, munc18, complexin) positions vesicle nanometres from membrane.
    2. Fusion triggered by Ca2+Ca^{2+}–synaptotagmin interaction; SNARE zippering yields hemifusion → full fusion (≤1 ms).
    3. Endocytosis & recycling (10 s–100 ms):
    • Clathrin-mediated (classic; 15–20 s).
    • Ultrafast (<100 ms) – clathrin-independent, endosomal budding.
    • Kiss-and-run – transient pore, vesicle rapidly reseals (1–2 s).
    1. Uncoating by Hsc70 + auxilin; reacidification by V-ATPase.
  • Dynamin forms 50 nm helical collars to sever vesicles; mutants disrupt endocytosis (Charcot-Marie-Tooth neuropathy).

Calcium: The Master Trigger

  • Necessity: zero external Ca2+Ca^{2+} or intracellular chelators (BAPTA/EGTA) abolish transmitter release.
  • Sufficiency: photolysis of caged Ca2+Ca^{2+} or presynaptic depolarisation in voltage-clamp triggers quantal events.
  • Voltage-clamp experiments (Augustine & Eckert 1984): presynaptic Ca2+Ca^{2+} current onset precedes postsynaptic EPSC; blockade with ω\omega-conotoxin (N-type) or ω\omega-agatoxin (P/Q-type) suppresses release.

Molecular Machinery Summary

  • SNARE complex provides energy for fusion; disassembled by NSF (ATPase) + SNAPs.
  • Synapsins tether reserve vesicles to actin; phosphorylated by CaMKII to mobilise during high activity.
  • Rab3A/RIM/CAPS coordinate docking & priming; mutations cause ataxias, epilepsy.
  • Chaperones (Hsc70) recycle clathrin; defective variants → Parkinson-like endocytic deficits.

Neurotransmitter Receptors

Ionotropic (Ligand-Gated Ion Channels)

  • 4–5 subunits form central pore; rapid (≤1 ms) onset; desensitise with sustained ligand.
  • Cation-selective (ACh-nAChR, AMPA, NMDA, 5-HT3_{3}) produce EPSPs.
  • Anion-selective (GABAA_A, GlyR) produce IPSPs.
  • Dual-permeable (nAChR: Na+/K+Na^{+}/K^{+}, NMDA: Na+/K+/Ca2+Na^{+}/K^{+}/Ca^{2+}) → plasticity triggers.

Metabotropic (G-Protein-Coupled Receptors, 7-TM)

  • One polypeptide; slower (50–500 ms) but longer-lasting (seconds–minutes).
  • Two main signal routes:
    1. Direct gating of ion channels (e.g., βγ\beta\gamma subunit opens GIRK K+K^{+} channels → hyperpolarisation).
    2. Second-messenger cascades via effector enzymes.
  • Key G-protein classes:
    • GαsG_{\alpha s} → activates adenylyl cyclase → cAMPcAMP ↑ → PKA.
    • Gαi/oG_{\alpha i/o} → inhibits adenylyl cyclase → cAMPcAMP ↓.
    • G<em>αq/11G<em>{\alpha q/11} → activates PLC → splits PIP</em>2</em>2 into DAG + IP3_3 (releases intracellular Ca2+Ca^{2+}, activates PKC, stimulates NO synthase).

Second-Messenger Systems & Gene Regulation

  • First messenger = NT; second messengers amplify & diversify effects.
  • Principal systems:
    • cAMPcAMP/PKA.
    • cGMPcGMP/PKG (regulated by NO diffusion).
    • PLC pathway: IP3IP_3 binds ER receptors → Ca2+Ca^{2+} release; DAG stays membrane-bound activating PKC.
  • Kinases phosphorylate ion channels, receptors, transcription factors (CREB, Elk-1) → long-term changes (LTP, addiction, learning).

Postsynaptic Potentials: Ion Fluxes & Reversal Potentials

  • End-plate current (EPC) at NMJ driven by both Na+Na^{+} influx & K+K^{+} efflux through same ACh-gated channel.
  • Reversal potential ErevE_{rev} ≈ 0 mV0\,\text{mV} because channel non-selective for Na+Na^{+}/K+K^{+}.
  • Rule:
    • If E<em>rev>V</em>thresholdE<em>{rev} > V</em>{threshold} (≈−40 mV-40\,\text{mV}) ⇒ excitatory (EPSP).
    • If E<em>rev<V</em>thresholdE<em>{rev} < V</em>{threshold} ⇒ inhibitory (IPSP).
  • IPSPs may still be depolarising when E<em>revE<em>{rev} lies between rest and threshold (e.g., immature neurons with elevated [Cl−]</em>i[Cl^{-}]</em>{i}).

Spatial & Temporal Summation (Fig. 5.20)

  • Spatial: coincident inputs at multiple dendritic sites combine.
  • Temporal: successive inputs at same synapse accumulate if inter-spike interval < membrane time constant (τ).
  • Net membrane potential at axon hillock = arithmetic sum weighted by electrotonic distance; integration determines spiking.
  • Inhibition can subtract (shunting) or divide (gain control) EPSPs.

Tripartite Synapse – Role of Astrocytes

  • Glial Ca2+Ca^{2+} waves modulate transmitter clearance, release gliotransmitters (D-serine, ATP) that potentiate or depress synaptic efficacy.
  • Astrocytic dysfunction implicated in epilepsy, ALS, Alzheimer’s; therapeutic target for neuromodulation.

Key Numerical & Formulaic References

  • Gap-junction delay: <0.1 ms<0.1\,\text{ms}.
  • Chemical synaptic cleft width: 20–50 nm20–50\,\text{nm}; vesicle diameter: ≈40 nm\approx 40\,\text{nm}.
  • Resting intracellular Ca2+Ca^{2+}:  100 nM~100\,\text{nM}; fusion triggered at >10 μM>10\,\mu\text{M}.
  • NMJ MEPP amplitude: ≈0.5 mV\approx 0.5\,\text{mV}.
  • Equilibrium potentials (frog muscle in 115 mM Na+Na^{+}, 2.5 mM K+K^{+}): E<em>Na≈+70 mV,  E</em>K≈−100 mV,  ECl≈−50 mVE<em>{Na}\approx +70\,\text{mV},\;E</em>{K}\approx -100\,\text{mV},\;E_{Cl}\approx -50\,\text{mV}.

Ethical / Clinical Connections

  • Toxins targeting presynaptic proteins:
    • Botulinum & tetanus neurotoxins cleave SNAREs → flaccid paralysis / spastic paralysis.
  • Channelopathies (P/Q-type Ca2+Ca^{2+} channel mutations) cause familial hemiplegic migraine & episodic ataxia.
  • Autoimmune attack on AChR (myasthenia gravis) validates receptor function; anticholinesterases provide symptomatic relief.

Concept Integration & Real-World Relevance

  • Principles of quantal release underpin EEG rhythmy (each miniature event contributes to background synaptic noise).
  • SNARE-driven secretion conserved in endocrine & immune cells; disruptions affect insulin release, exocytosis of cytokines.
  • GPCR pharmacology is foundation of >30 % of current drugs (e.g., β\beta-blockers, antipsychotics, opioids).

Study Checklist

  • Trace every step from AP arrival to postsynaptic response.
  • Memorise identities & roles of core SNARE proteins, Ca2+Ca^{2+} sensor, vesicle recycling pathways.
  • Be able to compute direction of ion fluxes given membrane potential & E<em>revE<em>{rev} using I=g(V</em>m−Erev)I=g(V</em>m-E_{rev}).
  • Distinguish EPSP vs. IPSP by comparing ErevE_{rev} to threshold.
  • Explain how co-release expands computational repertoire of single synapses.
  • Relate astrocytic modulation to homeostatic balance and neuropathology.