Communication Between Excitable Cells: The Synapse and Sensory Nerves

Communication Between Excitable Cells: The Synapse and Sensory Nerves

Introduction

• Presenter: Andrea Streit

• Institution: Centre for Craniofacial & Regenerative Biology

The Synapse

Overview

• Synapses are critical junctions for communication between neurons. In the human brain, there are approximately 500 trillion synapses.

Types of Synapses

1. Location Types

   • Axo-somatic Synapse: These are common and usually inhibitory.

   • Axo-axonic Synapse: These are rare and can be mixed in function.

   • Axo-dendritic Synapse: These are common and typically excitatory.

2. Neurotransmitter Types

   • Neuropeptides: Examples include Substance P, Oxytocin, and Neurotensin.

   • Amino Acids: Important neurotransmitters include Glutamate (excitatory), GABA (inhibitory), and Glycine (inhibitory).

   • Monoamines: Include Noradrenaline, Dopamine, and Serotonin.

Neurotransmitter Release

Key Steps in Neurotransmitter Release

1. Presynaptic Depolarization

2. Calcium Entry: Mediated by voltage-gated channels.

3. Vesicle Movement & Priming: Vesicles are prepared for release.

4. Vesicle Docking: Vesicles dock at the presynaptic membrane, proximity = 5-40 nm.

5. Vesicle Fusion: Vesicle releases neurotransmitters into the synaptic cleft.

6. Vesicle Recycling: Reuses vesicular materials for future transmission.

• Neurotransmitters (NTs) are released synchronously in discrete, measurable units called quanta, where a single quantum corresponds to the contents of one synaptic vesicle.

• The synaptic cleft measures approximately 20-40 nm.

Physiology of Presynaptic Terminal

Key Cell Components

• Axon: Conducts impulses away from the cell body.

• Dendrites: Receive signals from other neurons.

Signals Induced by Ion Movements

• Excitatory Postsynaptic Potentials (EPSPs): Induced by Na+ entry.

• Inhibitory Postsynaptic Potentials (IPSPs): Induced by K+ and Cl- movements.

• Action Potential (AP) Initiation: Occurs at the Axon Hillock through integration of EPSPs and IPSPs.

Calcium Dynamics

Calcium Entry Regulation

1. Calcium Channel Subfamilies

   • CaV Subfamily: Voltage-gated calcium channels (CaVs) involved in neurotransmitter release.

   • Cav1: Skeletal muscle, endocrine release, and retina NT release.

   • Cav2: Present in the CNS, involved in NT release (affected by toxins such as ω-agatoxin, ω-conotoxin).

   • Cav3: Pacemaker roles in various tissues.

• Local [Ca2+]i can reach over 1000 nM despite resting levels around 100 nM.

Additional Regulatory Mechanisms

• Ligand-Gated Ion Channels: Function in conjunction with G-protein coupled receptors (GPCRs) to initiate Ca2+ influx, mediated by neurotransmitters like ACh, Glutamate, and Dopamine.

Vesicle Dynamics

Vesicle Movement and Priming

• Movement: Vesicles are transported to the active zone on the presynaptic membrane.

• Priming: Prepares vesicles for rapid fusion when Ca2+ influx occurs, readied for action upon arrival of an action potential.

Vesicle Docking Process

• Achieved through SNARE proteins (e.g., Synaptobrevin, Syntaxin-1, SNAP-25) that facilitate the fusion of the vesicle with the presynaptic membrane.

• Docking position is crucially defined as being 5 to 40 nm from the presynaptic membrane.

Vesicle Fusion Mechanism

• Calcium-activated vesicle-membrane fusion is mediated by Synaptotagmin-1, which binds calcium to trigger the opening of the fusion pore, leading to the release of neurotransmitters into the synaptic cleft.

Vesicle Recycling

• Recycling Process: Following neurotransmitter release, vesicles are replenished from a reserve pool and reused for subsequent signaling.

• The recycling vesicle pool is integral to maintaining synaptic transmission efficiency.

Questions for Discussion

1. What differences exist between ion entry at the nodes of Ranvier versus the presynaptic terminal?

2. Besides CaVs, what regulates Ca2+ entry at the presynaptic site?

3. What are the different neurotransmitter (NT) vesicle pools?

4. What proteins are essential for vesicle fusion?

5. Are additional components required for vesicle fusion?

Sensory Nerves and Pain Perception

Characteristics of Sensory Nerve Fibers

Nociception and Sensory Neurons

Pathways of Nociception

• Myelinated (Aδ) and Non-Myelinated (C) Fibers: Aδ fibers are responsible for fast, sharp pain transmission, while C fibers transmit dull, chronic pain signals.

• Signals from sensory neurons project to dorsal horn neurons in the spinal cord, affecting pain perception and processing.

Key Receptors Involved in Pain Perception

1. AMPA Receptors: Ligand-gated ion channels that mediate fast synaptic transmissions and lead to excitation of the postsynaptic neuron.

2. NMDA Receptors: Co-activate with AMPA receptors, amplifying pain perceptions.

TRP Channels for Nociception

• TRPV1: Activated by heat, acid, and capsaicin.

• TRPM8: Cold temperature sensor.

• TRPA1: Activated by noxious chemicals and mechanical stimuli.

• Involvement of Acid-Sensing Ion Channels (ASICs) and P2X3 channels illustrates the complexity in pain signaling pathways.

Clinical Applications in Analgesia

Quantitative Aspects of a Glutamate Synapse

• Action potentials may reach frequencies up to 400 Hz (with a peak of 1000 Hz in the auditory system).

• Intracellular Ca2+ concentration ([Ca]i) may range between 250 nM to 1000 nM during neurotransmitter release.

• Vesicular concentration of Glutamate: 20 mM. Each synaptic vesicle can release between 35,000 to 80,000 molecules of Glutamate.

• The volume of a synaptic cleft is 2 attolitres ( \text{L} = 2 \times \text{10}^{-18} \text{L}).

• Glutamate binds to AMPA receptors with a duration of 0.2-0.6 ms, and desensitizes within 10 ms. Transporters clear Glutamate from the cleft in 1 ms.

Review Questions

1. Which sensory nerves carry nociceptive information?

2. What is the initial signal of a nociceptive nerve?

3. What is the neurotransmitter released from a sensory nerve onto a dorsal horn neuron?

4. Which class of analgesics is most commonly used by dentists?

Learning Outcomes

• Identify anatomical locations of synapses.

• Understand the primary events involved in neurotransmitter release.

• Highlight significant steps in activating sensory nerves.

• Discuss the relevance of sensory nerves for dental practice and patient care.

Recommended Reading

Neuroscience Textbooks

• Standard texts as discussed in previous lectures.

Additional Reading on Neurotransmitter Release

1. Rizo (2018) - Mechanism of neurotransmitter release coming into focus, Protein Sci. 27: 1364-1391.

2. Silva et al (2021) - Calcium Dependent Docking of Synaptic Vesicle, TiNs 44: 479-492.

3. Sauvola & Littleton (2021) - SNARE Regulatory Proteins in Synaptic Vesicle Fusion and Recycling, Front. Neurosci. 14: 1-26.

Further Reading on Sensory Nerves

1. Middleton et al (2022) - The structure of sensory afferent compartments in health and disease, J. Anatomy. 241: 1186-1210.

Communication Between Excitable Cells: The Synapse and Sensory Nerves

Introduction

• Presenter: Andrea Streit

• Institution: Centre for Craniofacial & Regenerative Biology

• Objective: Overview of cell communication via neurotransmitter release, sensory perception (nociception), and analgesia.

The Synapse

Overview

• There are approximately 500 trillion synapses in the human brain alone, facilitating vast communication networks.

Types of Synapses (By Location)

1. Axo-somatic Synapse: Synapses onto the cell body; common and usually inhibitory.

2. Axo-axonic Synapse: Target the axon of a neuron; rare and can be mixed (inhibitory or excitatory).

3. Axo-dendritic Synapse: Target dendrites or dendritic spines; common and typically excitatory.

   • Dendritic Spines: Serve to enlarge surface area for potential synaptic contacts.

Neurotransmitter Types

1. Neuropeptides

   • Examples: Substance P, Oxytocin, Neurotensin.

   • Generated from larger proteins via cleavage.

   • Mechanism: Usually act via G-protein coupled receptors (GPCRs).

   • Speed: Slower action than amino acids, providing modulated and sustained effects.

2. Amino Acids

   • Glutamate: Primary excitatory neurotransmitter.

   • GABA and Glycine: Primary inhibitory neurotransmitters.

3. Monoamines

   • Noradrenaline: Used in CNS and PNS; controls the "fight or flight" response.

   • Dopamine: Predominantly active in the brain.

   • Serotonin: Active in both the brain and the PNS.

Neurotransmitter Release

Key Steps in Release

1. Presynaptic Depolarization: Action potential arrives via absolute saltatory conduction.

2. Calcium Entry: AP activates voltage-gated calcium channels (Ca_{V}).

3. Vesicle Movement & Priming: Vesicles move to the "Active Zone."

4. Vesicle Docking: Vesicles dock near the presynaptic membrane (5-40 nm).

5. Vesicle Fusion: Membranes fuse to release contents.

6. Vesicle Recycling: Materials are recovered for reuse.

• Quantal Release: Neurotransmitters are released in discrete, measurable units called quanta, where one quantum equals the contents of a single vesicle.

• Synaptic Cleft: Measures approximately 20-40 nm.

Physiology of the Presynaptic Terminal

Action Potential (AP) Characteristics

• Amplitude: Approximately 100\text{ mV}.

• Duration: Typically < 200\text{ }\mu\text{s}, but the AP "widens" at the presynaptic terminal because calcium channels are slow to open, causing slower propagation along the presynaptic membrane.

Integration

• Signals integrated at the Axon Hillock.

• EPSPs: Positive signals (Na+ entry).

• IPSPs: Negative signals (K+ or Cl- movement).

Calcium Dynamics

Calcium Channel Subfamilies

• Cav1: Skeletal/cardiac muscle (contraction), endocrine release, and retina neurotransmitter release.

• Cav2: Primarily CNS; critical for NT release. Targeted by agonists/antagonists like \omega-agatoxin.

• Cav3: Pacemaker roles (rhythmic activities in heart/nervous system).

Regulation and Feedback

• Positive Feedback: Ligand-Gated Ion Channels (activated by ACh, Glutamate, ATP) can trigger further Ca2+ influx.

• Negative Regulation: G-Protein Coupled Receptors (GPCRs) interact with receptor proteins to release inhibitory G-proteins, which inhibit calcium influx.

• Local Concentration: Can reach over 1000\text{ nM} (10x resting level of 100\text{ nM}).

Vesicle Dynamics and SNARE Proteins

Vesicle Pools

• Ready-to-Release Pool: Vesicles docked or about to dock on the plasma membrane.

• Reserve Pool: Vesicles sitting further away, ready to be mobilized.

SNARE Machinery

• SNARE Domain: A sequence of 65 amino acids common to these proteins.

• Synaptobrevin: A transmembrane protein spanning the vesicle lipid bilayer.

• SNAP-25: A transmembrane protein on the presynaptic plasma membrane.

• Syntaxin-1: Not a transmembrane protein, but interacts with the other two to form the complex that docks the vesicle.

• Clinical Note: Botulinum Toxin (Botox) targets SNARE proteins to prevent neurotransmitter release, inhibiting muscle activation.

Vesicle Fusion Mechanism

• Synaptotagmin-1: The calcium sensor.

  • Binds to phospholipids in the plasma membrane via positively charged amino acids.

  • Upon calcium influx, calcium binds to Synaptotagmin, causing a conformational change in the SNARE complex.

  • This change facilitates the fusion of the vesicle and cell membranes, opening a fusion pore.

Sensory Nerves and Pain Perception

Characteristics of Sensory Nerve Fibers

Pathways of Nociception

• Primary Afferents: Free nerve endings in the periphery (skin, gums) detect stimuli. Cell bodies sit in the Dorsal Root Ganglion.

• Projection: Axons project to the Dorsal Horn of the spinal cord (gray matter).

  • A-delta Fibers: Target Layer 1 (outer layer).

  • C-fibers: Target Layers 1 and 2.

Receptors and Analgesia

Postsynaptic Receptors

1. AMPA Receptors: (Glutamate analog name); Na+ entry; mediates fast excitation.

2. NMDA Receptors: (N-methyl-D-aspartate); Na+ and Ca2+ entry; amplifies pain perception.

Nociceptive Receptors (Periphery)

• TRPV1: Heat, acid, capsaicin (chili).

• TRPM8: Cold.

• TRPA1: Mechanical stimuli, pungent chemicals (garlic, mustard oil).

• P2X3: Activated by ATP released from damaged cells during tissue injury.

Clinical Analgesics

• Local Anesthetics: Target voltage-gated sodium channels.

• Paracetamol: Targets TRPA1.

• Capsaicin/RTX: Targets TRPV1.

• Ketamine: Targets glutamate receptors.

• Fentanyl: Targets opioid receptors on target cells.

Quantitative Aspects of a Glutamate Synapse

• Frequency: Up to 400\text{ Hz} (Retina) or 1000\text{ Hz} (Auditory).

• Vesicular Glutamate: Conc. 20\text{ mM}; 35,000 - 80,000 molecules per vesicle.

• Cleft Volume: 2\text{ attolitres} (2 \times 10^{-18}\text{ L}).

• Clearing: Transporters clear glutamate in 1\text{ ms}; AMPA receptors desensitize in 10\text{ ms}.