Synaptic Transmission

Overview of Synaptic Transmission

• A synapse is defined as a specialized zone of contact at which one neuron communicates with another cell.

• Quantitative parameters of the human brain:

  • Neurons in the human brain: $10^{11} - 10^{12}$ neurons.

  • Average number of synapses received per neuron: $1000$ synapses (with a wide range depending on cell type).

  • Total number of synapses in the human brain: $10^{15} - 10^{16}$ synapses.

• Main functional classes of synapses:

  • Electrical synapses: Direct physical junctions between neurons that permit direct, passive flow of electrical current.

  • Chemical synapses: Junctions where neurons communicate indirectly via the secretion of chemical neurotransmitters, which diffuse across a space to act on target cell receptors.

Electrical Synapses

• Ultrastructure of Gap Junctions:

  • Electrical synapses occur at gap junctions, where opposing cell membranes are in extremely close apposition, separated by a gap of only $3\,nm$

  • Gap junctions consist of precisely aligned, paired hexameric protein channels.

  • Molecular assembly sequence:

$6 \times \text{connexin proteins} \rightarrow 1\text{ hemichannel (connexon)}$

$2 \times \text{hemichannels (1 from each cell membrane)} \rightarrow 1\text{ gap junction channel}$

$10^3 \times \text{gap junction channels} \rightarrow 1\text{ gap junction}$

• Properties of Electrical Transmission:

  • Pore diameter: The pores of gap junction channels are relatively wide ($14 - 20\,\text{\AA}$ or $1.4 - 2.0\,nm$) and non-selective.

  • Permeability: Allows free passive diffusion of all major inorganic ions ($Na^+$, $K^+$, $Ca^{2+}$, $Cl^-$) as well as low molecular weight compounds and second messengers ($cAMP$, $IP_3$).

  • Directionality: Transmission is usually bidirectional.

  • Speed: Transmission is extremely rapid with a latency of less than $0.1\,ms$

  • Signal Sign and Amplitude: Postsynaptic potential changes retain the same sign as presynaptic potential changes:

  • Presynaptic depolarization $\rightarrow$ Postsynaptic depolarization

  • Presynaptic hyperpolarization $\rightarrow$ Postsynaptic hyperpolarization

  • Attenuation: Signal amplitude is lower in the postsynaptic cell because a fraction of the electrical signal is lost across junctional resistance and cellular capacitance.

  • Filtering: Slow potential changes are transmitted through gap junctions with greater fidelity than rapid potential changes.

• Occurrence and Physiological Roles Across Species:

  • Crayfish and Teleost Fish:

  • Location: Motor circuits (e.g., connection between crayfish giant axon and motor neurons).

  • Function: Mediates very fast escape responses (flight response).

  • Mammalian Retinal Neurons:

  • Location: Interconnected retinal neurons.

  • Function: Signal sharing improves visual sensitivity (helping the retina detect weak visual signals in dim light) and spatial discrimination (helping locate stimuli and distinguish patterns across visual space).

  • Mammalian Brain Interneurons:

  • Location: Networks of GABAergic interneurons in the central nervous system.

  • Function: Synchronization of electrical activity, allowing large groups of inhibitory neurons to fire action potentials synchronously.

Chemical Synapses: Structure and Specializations

• Common Structural Features:

  • Chemical synapses possess five defined structural components under electron microscopy:

  1. Presynaptic bouton: Terminal specialization of the sending axon where neurotransmitters are released.

  2. Synaptic vesicles: Membrane-bound spheres containing concentrated neurotransmitters.

  3. Active zone: Proteinaceous structure at the presynaptic membrane required for docking, priming, and exocytosis of synaptic vesicles.

  4. Synaptic cleft: Extracellular fluid gap measuring $20\,nm$ between presynaptic and postsynaptic membranes across which neurotransmitter diffuses.

  5. Postsynaptic specialization: Protein-dense apparatus in the receiving cell containing neurotransmitter receptors, signaling cascade enzymes, and scaffolding networks.

• Molecular Components of Synaptic Specializations:

  • Presynaptic Active Zone:

  • Contains protein complexes dedicated to synaptic vesicle docking, ATP-dependent priming, voltage-gated calcium channel tethering, and cell adhesion.

• Postsynaptic Specialization:

  • Contains neurotransmitter receptor proteins (ionotropic and metabotropic), cell adhesion proteins, signaling pathway molecules (such as CaMKII and kinases), and master scaffolding proteins (such as PSD-95, Shank, Homer, GKAP).

• Ultrastructural Classification (Gray Types):

  • Asymmetric Synapses (Gray Type I):

  • Characterized by a postsynaptic membrane density that is noticeably thicker than the presynaptic density.

  • Functional identity: Almost exclusively excitatory.

  • Symmetric Synapses (Gray Type II):

  • Characterized by presynaptic and postsynaptic membrane specializations of equivalent, thin thickness.

  • Functional identity: Almost exclusively inhibitory.

• Classification by Anatomical Localization:

  • Axospinous: Axon terminal forms a synapse onto a dendritic spine; predominantly glutamatergic and excitatory.

  • Axodendritic: Axon terminal forms a synapse onto a dendritic shaft; can be excitatory or inhibitory.

  • Axosomatic: Axon terminal forms a synapse onto the soma (cell body); frequently inhibitory.

  • Axo-axonic: Axon terminal forms a synapse onto another axon terminal; functions to regulate neurotransmitter release from the target axon (presynaptic inhibition).

  • Dendro-dendritic: Synaptic contact formed directly between two dendrites; typically inhibitory.

  • Neuromuscular Junction: Synaptic connection between a motor neuron axon terminal and a muscle fiber membrane; drives muscle contraction.

Neurotransmitter Release Mechanisms

• Sequence of Events during Transmission:

  1. An action potential arrives at and depolarizes the presynaptic bouton.

  2. Depolarization opens presynaptic voltage-gated calcium channels.

  3. $Ca^{2+}$ flows rapidly into the cytosol down its electrochemical gradient.

  4. Increased cytosolic $[Ca^{2+}]$ triggers exocytosis of synaptic vesicles at the active zone.

  5. Neurotransmitter is released into the cleft, diffuses across the $20\,nm$ space, and binds to postsynaptic receptors.

  6. Postsynaptic ion channels open or close, generating a postsynaptic current that alters membrane potential.

• Evidence for the Essential Role of Calcium:

  • Experiments conducted at the squid giant synapse demonstrated that presynaptic depolarization steeply regulates neurotransmitter release via voltage-gated $Ca^{2+}$ currents ($I_{Ca}$).

  • Pharmacological blockade of $Na^+$ and $K^+$ channels confirmed that presynaptic depolarization alone opens voltage-gated $Ca^{2+}$ channels to drive postsynaptic responses.

  • Manipulating extracellular calcium concentration ($[Ca^{2+}]_{out}$) directly scales postsynaptic potential (PSP) size:

$\uparrow [Ca^{2+}]_{out} \rightarrow \uparrow I_{Ca} \rightarrow \uparrow \text{PSP amplitude}$

$\downarrow [Ca^{2+}]_{out} \rightarrow \downarrow I_{Ca} \rightarrow \downarrow \text{PSP amplitude}$

• Microinjection of calcium chelators (such as EGTA or BAPTA) into the presynaptic terminal binds free $Ca^{2+}$ and completely eliminates PSPs.

• Application of voltage-gated $Ca^{2+}$ channel blockers (e.g., cadmium, $Cd^{2+}$) blocks presynaptic $Ca^{2+}$ current and abrogates PSPs.

• Direct intracellular injection of $Ca^{2+}$ into the presynaptic terminal triggers neurotransmitter release and generates a PSP even in the total absence of presynaptic action potentials.

  • Quantal Nature of Neurotransmitter Release:

• Investigated primarily at the neuromuscular junction (NMJ).

• Miniature Endplate Potentials (mEPPs): Spontaneous, action-potential-independent depolarizations recorded in muscle membranes in the absence of nerve stimulation. All mEPPs have a uniform amplitude of approximately $0.4\,mV$

• Evoked Endplate Potentials (EPPs): When motor nerves are stimulated in solutions with low extracellular $Ca^{2+}$, evoked EPPs drop in amplitude to discrete steps: either $0\,mV$ (failures), $0.4\,mV$, $0.8\,mV$, $1.2\,mV$, or $1.6\,mV$

• Quantal Hypothesis: Neurotransmitter is released in discrete, uniform packets called quanta.

  • 1 Quantum = Neurotransmitter content of 1 individual synaptic vesicle.

  • A single mEPP represents the spontaneous exocytosis of 1 quantum ($0.4\,mV$

  • Evoked EPPs represent the multivesicular, synchronized release of multiple quanta.

Molecular Machinery of Exocytosis and Vesicle Recycling

• Multistep Vesicle Exocytosis Process:

  1. Docking: Synaptic vesicles move to and associate with the active zone plasma membrane.

  2. Priming: Vesicles undergo molecular assembly to become fusion-competent. Involves assembly of the core SNARE complex (requires ATP and auxiliary factors).

  3. Fusion / Triggering: $Ca^{2+}$ influx binds to the calcium sensor protein, triggering lipid bilayer fusion and neurotransmitter discharge.

• SNARE Complex Architecture:

  • SNARE = Soluble N-ethylmaleimide-sensitive factor Attachment Protein Receptor.

  • Core SNARE proteins:

  • Synaptobrevin (also called VAMP): Vesicle membrane SNARE (v-SNARE).

  • Syntaxin: Plasma membrane SNARE (t-SNARE).

  • SNAP-25: Plasma membrane SNARE anchored by palmitoyl chains (t-SNARE).

  • Synaptobrevin, syntaxin, and SNAP-25 assemble into a tight, parallel four-helical bundle that pulls the vesicle lipid bilayer into close contact with the plasma membrane.

  • Botulinum Neurotoxins: Bacterial zinc-endopeptidases that specifically cleave SNARE proteins (synaptobrevin, syntaxin, or SNAP-25), preventing SNARE complex assembly and completely blocking synaptic transmission.

• Synaptotagmin Calcium Sensor:

  • Synaptotagmin is a synaptic vesicle transmembrane protein containing cytosolic $Ca^{2+}$-binding C2 domains.

  • $Ca^{2+}$ binding to synaptotagmin causes it to interact cooperatively with membrane phospholipids and the SNARE complex, driving final membrane fusion.

  • Synaptotagmin has a relatively low affinity for $Ca^{2+}$ (micromolar range), requiring synaptic vesicles to be docked in immediate physical proximity to voltage-gated $Ca^{2+}$ channels.

• Endocytosis and Vesicle Recycling:

  • Following exocytosis, synaptic vesicle proteins and lipids incorporated into the presynaptic plasma membrane are retrieved via endocytosis.

  • Clathrin: Scaffolding protein that polymerizes into a triskelion lattice on the cytoplasmic surface of the membrane to shape invaginating endocytic pits.

  • Dynamin: Large GTPase enzyme that forms a ring around the neck of the invaginated coated pit and hydrolyzes GTP to pinch/cleave the vesicle from the plasma membrane.

• Vesicle Refilling via Electrochemical Gradient:

  • Refilling synaptic vesicles with neurotransmitters requires secondary active transport.

  • V-type $H^+$--ATPase (Vesicular Proton Pump): Uses ATP hydrolysis to pump $H^+$ ions into the vesicle lumen against their concentration gradient, creating an acidic interior ($\Delta \text{pH}$) and an inside-positive membrane potential ($\Delta \Psi$).

  • Vesicular Neurotransmitter Transporters: Transport neurotransmitter molecules into the vesicle lumen against steep concentration gradients by coupling uptake to the efflux of $H^+$ ions down their electrochemical gradient.

Postsynaptic Signal Transduction

• Receptor Classes:

  • Ionotropic Receptors (Ligand-Gated Ion Channels):

  • Neurotransmitter binding directly opens an intrinsic ion channel pore.

  • Permeable to cations ($Na^+$, $K^+$, $Ca^{2+}$) or anions ($Cl^-$).

  • Mediates fast postsynaptic potentials.

  • Metabotropic Receptors (G-Protein Coupled Receptors):

  • Neurotransmitter binding activates intracellular heterotrimeric G-proteins ($\alpha$, $\beta$, $\gamma$ subunits).

  • G-protein subunits or second messenger cascades indirectly modulate ion channels (frequently opening $K^+$ channels).

  • Mediates slow postsynaptic potentials.

• Reversal Potential ($E_{rev}$):

  • Reversal potential ($E_{rev}$) is the membrane potential ($V_m$) at which net synaptic current is zero.

  • Current direction rules:

$\text{If } V_m < E_{rev} \rightarrow \text{Net inward current} \rightarrow \text{Depolarization}$

$\text{If } V_m > E_{rev} \rightarrow \text{Net outward current} \rightarrow \text{Hyperpolarization}$

$\text{If } V_m = E_{rev} \rightarrow \text{Zero net current} \rightarrow \text{No change in } V_m$

• Distinguishing Equilibrium Potential ($E_x$) and Reversal Potential ($E_{rev}$):

  • Equilibrium Potential ($E_x$): Defined for a single ion species using the Nernst equation:

$E_x = \frac{R T}{z F} \ln \left( \frac{[x]_{out}}{[x]_{in}} \right)$

* **Reversal Potential ($E_{rev}$)**: Defined for a channel or synapse. If a channel is permeable to multiple ions (e.g., $Na^+$ and $K^+$), $E_{rev}$ is the weighted average of the individual ion equilibrium potentials, determined by the chord conductance equation:

$E_{rev} = \left( \frac{g_{Na}}{g_{tot}} \right) E_{Na} + \left( \frac{g_K}{g_{tot}} \right) E_K$

• Worked Example:

  • An ionotropic channel conducts $Na^+$ and $K^+$ equally ($g_{Na} = g_K$). Given $E_K = -90\,mV$ and $E_{Na} = +70\,mV$:

$E_{rev} = \frac{1}{2}(-90\,mV) + \frac{1}{2}(+70\,mV) = -10\,mV$

• Excitatory vs. Inhibitory Postsynaptic Potentials:

  • Excitatory Postsynaptic Potentials (EPSPs):

  • Defined by an $E_{rev}$ that is more positive than the action potential threshold ($V_{thr}$).

  • Drives $V_m$ toward threshold, increasing the probability of firing an action potential.

  • Inhibitory Postsynaptic Potentials (IPSPs):

  • Defined by an $E_{rev}$ that is more negative than the action potential threshold ($V_{thr}$).

  • Decreases action potential firing probability via:

    1. Hyperpolarizing $V_m$ away from threshold.

    2. Shunting Inhibition: Clamping $V_m$ near $E_{rev}$ below threshold, which increases total membrane conductance and dissipates concurrent inward excitatory currents.

• Nernst Equation Calculations for Chloride Channels:

  • Formula at $37^\circ C$ for $Cl^-$ ($z = -1$):

$E_{Cl} = -61.5\,mV \times \log_{10}\left( \frac{[Cl^-]_{out}}{[Cl^-]_{in}} \right)$

• Given $V_{rest} = -70\,mV$ and $V_{thr} = -38\,mV$:

  • Condition A ($[Cl^-]_{in} = 6\,mM$, $[Cl^-]_{out} = 120\,mM$):

$E_{Cl} = -61.5\,mV \times \log_{10}(20) = -61.5\,mV \times 1.3 = -80\,mV$

  * Impact: Since $E_{Cl} = -80\,mV < V_{rest} = -70\,mV$, channel activation hyperpolarizes the membrane and reduces excitability.
* **Condition B** ($[Cl^-]_{in} = 12\,mM$, $[Cl^-]_{out} = 120\,mM$):

$E_{Cl} = -61.5\,mV \times \log_{10}(10) = -61.5\,mV \times 1.0 = -61.5\,mV$

  * Impact: Since $E_{Cl} = -61.5\,mV > V_{rest} = -70\,mV$, channel activation depolarizes the membrane toward $-61.5\,mV$
  * Crucial Distinction: Because $-61.5\,mV$ remains below the action potential threshold ($-38\,mV$), this synapse is **inhibitory** despite depolarizing the cell, operating via shunting inhibition.

• Integration of Postsynaptic Potentials:

  • An individual EPSP produced by a single synapse is subthreshold ($0.5 - 2.0\,mV$) and cannot trigger an action potential alone.

  • Action potentials are triggered at the axon hillock through summation:

  • Spatial Summation: Simultaneous activation of multiple distinct excitatory synapses across different spatial locations on the dendritic tree.

  • Temporal Summation: High-frequency repetitive firing of a single excitatory synapse within a short time interval.