Synaptic Transmission: Electrical vs. Chemical, and the Neuromuscular Junction Model

  • Synapses: A General Overview

    • Definition: Specialized junctions between individual neurons that are crucial for neural processing, transmitting, and processing information within circuits.

    • The term "synapse" was coined by Sherrington in the 1940s.

    • Two major types: Electrical and Chemical synapses.

  • Electrical Synapses (Gap Junctions)

    • Anatomical Structure: Areas where the neuronal membranes between neurons are brought very close together.

      • Contain specialized transmembrane proteins called connexins that align perfectly on the two sides of the junction.

      • These connexins form channels that allow direct passage of charged ions.

    • Functionality: Allow for bidirectional flow of charged ions between neurons.

    • Benefit: Essentially no delay (less than a fraction of a millisecond) between electrical signals in the presynaptic and postsynaptic neurons due due to direct transfer of charge.

    • Rarity: Extremely rare in the brain.

    • Example Location: The hippocampus, a region in the brain that regulates brain waves, especially during sleep.

  • Chemical Synapses

    • Major Distinction: Unlike electrical synapses, there is no direct ion current flow from the presynaptic cell to the postsynaptic cell.

    • Anatomical Structure:

      • Presynaptic Neuron: Contains uniform-sized synaptic vesicles (e.g., depicted as green dots) that carry specific amounts of neurotransmitters.

      • Postsynaptic Neuron: Features postsynaptic neurotransmitter receptors (e.g., depicted as yellow transmembrane proteins).

      • No gap junctions; instead, there is a synaptic cleft (a gap) between the neurons.

    • Process Overview: An electrical signal (action potential) arrives at the presynaptic neuron's axon terminal. This triggers the release of chemical neurotransmitters, which diffuse across the synaptic cleft. These neurotransmitters bind to receptors on the postsynaptic membrane, generating a new electrical potential (resumption of electrical activity).

    • Synaptic Delay: Slower than electrical synapses. The delay between a membrane change in the presynaptic cell and the response in the postsynaptic cell is typically a couple of milliseconds (significantly slower than the fraction of a millisecond in electrical synapses).

  • Detailed Steps of Chemical Synaptic Transmission

    1. An electrical signal (action potential) arrives at the axon terminal of the presynaptic neuron.

    2. This action potential initiates the opening of voltage-gated $Ca^{2+}$ channels in the presynaptic terminal.

    3. $Ca^{2+}$ ions rapidly influx into the presynaptic terminal.

    4. The increased intracellular concentration of $Ca^{2+}$ triggers synaptic vesicles (which are ready in the terminal) to fuse with the presynaptic membrane. This process is called exocytosis.

    5. Neurotransmitters are released from the fused vesicles into the synaptic cleft.

    6. Neurotransmitters diffuse across the synaptic cleft and bind to specific receptor molecules on the postsynaptic membrane.

    7. Binding of neurotransmitters opens up passageways (ion channels) for ions to flow across the postsynaptic membrane.

    8. This ion current causes a change in the postsynaptic membrane potential (measured in voltage).

  • Termination of Synaptic Transmission

    • Importance: Essential to prevent continuous signaling and allow for the transmission of new, distinct signals.

    • Mechanisms (Steps 1010 and 1111):

      1. Enzymatic Breakdown: Enzymes located in the synaptic cleft can break down neurotransmitters.

      2. Reuptake: Neurotransmitters or their breakdown products can be taken up by glial cells or reabsorbed into the presynaptic terminal.

  • The Neuromuscular Junction (NMJ): A Model System

    • Definition: The synapse between a motor neuron and a muscle cell.

    • Significance: Widely used as a model for studying synapses due to its large size and accessibility.

    • Observation: An electrode placed in the muscle cell can record the postsynaptic membrane potential.

      • Stimulating the motor axon at time t=0t=0 results in a depolarization of the muscle membrane after a delay of about 1−21-2 milliseconds.

  • End-Plate Potential (EPP)

    • Definition: The postsynaptic depolarization recorded in the muscle cell following stimulation of the motor axon.

    • Etymology: "Endplate" refers to the specific site on the muscle where the motor nerve contacts it.

    • Relationship to Action Potential: If an EPP reaches threshold (a specific membrane potential, e.g., represented by a red horizontal line), it triggers a muscle action potential (e.g., represented by a black line).

      • EPPs and action potentials are physiologically distinct, as they are caused by ions flowing across different types of channels.

  • Miniature End-Plate Potentials (MEPPs)

    • Definition: Small, spontaneous, and random depolarizations observed in the muscle cell, even in the absence of motor axon stimulation.

    • Characteristics: They are literally "mini" EPPs, occurring randomly over time.

    • Experimental Evidence: Subthreshold EPPs (observed in preparations with low extracellular calcium concentration) are very similar in size and shape to spontaneous MEPPs.

  • Role of Calcium and Quantal Release

    • Calcium's Critical Role: $Ca^{2+}$ ions play a fundamentally important role in synaptic transmission, primarily by dictating the release of neurotransmitters.

    • Visualization: Fluorescence microscopy shows a significant rise in intracellular $Ca^{2+}$ concentration (imaged as an increase in pink intensity) in the presynaptic cell when an action potential arrives.

    • Active Zone: An electron micrograph of the NMJ shows a specific region called the active zone (red circle) where many synaptic vesicles are clustered close to the presynaptic membrane, poised for release.

    • Neurotransmitter Content: Each synaptic vesicle contains a fixed and specific amount of neurotransmitter. For example, at the NMJ, each vesicle contains approximately 10,00010,000 molecules of acetylcholine (ACh).

    • MEPPs and EPPs Explained:

      • MEPPs: Result from the spontaneous fusion of a single synaptic vesicle with the presynaptic membrane, releasing its fixed 10,00010,000 ACh molecules. This leads to a very characteristic, fixed size for each MEPP.

      • EPPs: Result from the simultaneous fusion of many synaptic vesicles when the motor neuron is stimulated. An EPP is effectively the sum of many MEPPs occurring at the same time.

    • Quantal Release: Neurotransmitters are released in discrete, fixed "packets" or "quanta" (each corresponding to the content of one synaptic vesicle). Neurotransmitters are not released in half-packets.

      • There is a linear and fixed relationship between the number of vesicles (quanta) that fuse and the magnitude of the EPP, often expressed as a multiple of MEPP size.

  • Neurotransmitter Reuptake (Endocytosis on the Presynaptic Side)

    • Process: After the exocytosis of synaptic vesicles, the membrane added to the presynaptic terminal is quickly reuptaken via endocytosis to form new synaptic vesicles.

    • Experimental Demonstration: Horseradish peroxidase (a marker) introduced into the synaptic cleft is taken up into the presynaptic terminal via endocytosis and can be found within newly formed vesicles.

  • Criteria for a Neurotransmitter
    A molecule must meet several criteria to be classified as a neurotransmitter:

    1. It must be found in the presynaptic neuron.

    2. It must be generated (synthesized) by the presynaptic neuron.

    3. It must be released upon presynaptic stimulation.

    4. It must exert an effect on the postsynaptic neuron.

    5. There must be mechanisms for its inactivation or removal from the synaptic cleft.

  • Postsynaptic Receptors and Currents

    • Two Main Types of Postsynaptic Neurotransmitter Receptors:

      1. Ionotropic Receptors (Ligand-Gated Ion Channels):

        • Structure: The receptor protein itself contains both the binding site for the neurotransmitter and forms the ion channel.

        • Mechanism: When a neurotransmitter binds, it directly opens the ion channel, allowing ions to flow across the postsynaptic membrane.

      2. Metabotropic Receptors (G-Protein Coupled Receptors):

        • Structure: The neurotransmitter binding site and the ion channel are separate entities.

        • Mechanism: Neurotransmitter binding causes a conformational change in the receptor, which is coupled to a G-protein. The activated G-protein then activates a secondary messenger, which, in turn, opens a separate ion channel, leading to a change in the postsynaptic membrane potential.

    • Patch Clamp Technique:

      • Purpose: A technique used to measure ion current flowing through a single ion channel or a small number of channels on the membrane.

      • Setup: A pipette interacts with a small part of the membrane.

      • Experimental Outcome:

        • When acetylcholine (ACh) (the neurotransmitter) is introduced, it activates the ion channel.

        • This activation results in a measurable inward current (e.g., sodium flowing into the cell), typically represented by a downward deflection on the current readout (measured in amps or kiloamps).

        • If multiple channels are open simultaneously, a larger inward current is observed (e.g., 1010 kiloamps for multiple channels compared to 11 kiloamp for a single channel).

        • This current typically tapers off over time as neurotransmitters are removed or channels close.