synaptic transmission and neurotransmitters-6

SYNAPTICA

  • Course: Essentials of Cell Biology and Neuroscience

  • Instructor: Dr. Cominski

THE SYNAPSE

  • Fundamental component of the nervous system allowing communication between neurons

  • Information transfer occurs at synapses, connecting:

    • Neuron to neuron

    • Neuron to effector cell

SYNAPSE CLASSIFICATION

  • Types of synapses based on connections:

    • Axodendritic: connection between axon terminals of one neuron and dendrites of another

    • Axosomatic: connection between axon terminals and the soma (cell body)

    • Less common types include:

      • Axoaxonic: between axons

      • Dendrodendritic: between dendrites

      • Somatodendritic: between soma and dendrites

ELECTRICAL SYNAPSES

  • Less common than chemical synapses; involve direct electrical coupling through gap junctions

  • Advantages:

    • Fast communication

    • Unidirectional or bidirectional signaling

  • Common in embryonic nervous tissue and certain regions of the brain (e.g., rhythmic breathing regulation)

CHEMICAL SYNAPSES

  • Discovered in 1926 by Otto Loewi; demonstrated that stimulation of the vagus nerve releases acetylcholine to slow heart rate

  • Structure includes:

    • Presynaptic neuron: produces neurotransmitters, contains synaptic vesicles

    • Postsynaptic neuron: contains receptor region for neurotransmitters

    • Separated by synaptic cleft, facilitating chemical transmission

  • Conversion process: electrical impulse ➔ chemical ➔ back to electrical

TERMINOLOGY

  • Presynaptic neuron: neuron sending information

  • Postsynaptic neuron: neuron receiving information

  • Both neurons generally perform both functions in different contexts

SYNAPTIC CLEFT

  • Narrow gap (30-50 nm) that prevents direct nerve impulse transfer

  • Chemical transmission occurs through release & binding of neurotransmitters, ensuring one-way communication

INFORMATION TRANSFER MECHANISM AT CHEMICAL SYNAPSES

  • Steps in synaptic transmission:

    1. Action potential triggers voltage-gated Ca2+ channels to open

    2. Ca2+ influx promotes fusion of synaptic vesicles with the membrane

    3. Neurotransmitter released via exocytosis

    4. Neurotransmitter diffuses across cleft & binds to receptors on postsynaptic membrane

    5. Postsynaptic potential generated (excitatory or inhibitory)

TERMINATION OF NEUROTRANSMITTER EFFECTS

  • Occurs within milliseconds through one of three processes:

    • Reuptake: uptake by astrocytes or presynaptic neuron

    • Degradation: enzymatic action, e.g., acetylcholinesterase (AChE)

    • Diffusion: neurotransmitter moves away from synaptic cleft

GRADED POTENTIALS VS. ACTION POTENTIALS

  • GRADED POTENTIALS (GP):

    • Occur in the cell body and dendrites, can vary in amplitude, decay with distance

    • Triggered by chemical or other stimuli

  • ACTION POTENTIALS (AP):

    • Occur at the axon hillock, all-or-nothing response, travel long distances without decay

    • Triggered by depolarization reaching threshold

MECHANISMS OF POSTSYNAPTIC POTENTIALS

  • Excitatory Postsynaptic Potentials (EPSP):

    • Local depolarization towards threshold for AP generation

  • Inhibitory Postsynaptic Potentials (IPSP):

    • Local hyperpolarization, making AP less likely to occur

SYNAPTIC INTEGRATION

  • EPSPs can summate to influence whether an AP occurs

    • Temporal Summation: rapid sequence of stimuli

    • Spatial Summation: multiple simultaneous stimuli

  • Only predominant EPSPs can bring membrane potential to threshold for AP generation

PRESYNAPTIC INHIBITION AND POTENTIATION

  • Presynaptic Inhibition: reduces neurotransmitter release from presynaptic neuron via axoaxonic synapse

  • Synaptic Potentiation: increased presynaptic ability to excite postsynaptic neurons after repeated use, often involves Ca2+ signaling

  • Long-Term Potentiation (LTP): lasting increase in synaptic strength, important for learning and memory

NEUROTRANSMITTERS

  • Classified by function and chemical structure

  • Neurons often produce multiple neurotransmitters affecting various biological processes

CRITERIA FOR NEUROTRANSMITTERS

  1. Substance must be present in presynaptic neuron

  2. Must be released in response to depolarization, dependent on Ca2+

  3. Specific receptors must be present on postsynaptic cells

VARIATIONS IN NEUROTRANSMITTER EFFECTS

  • Neurotransmitters can be excitatory or inhibitory based on receptor interaction:

    • GABA and glycine are inhibitory

    • Glutamate is excitatory

    • Acetylcholine can be both (excitatory at neuromuscular junctions, inhibitory in cardiac muscle)

NEUROTRANSMITTER RECEPTORS

  • Channel-Linked Receptors: fast action, mediates ion flow directly

  • G-Protein-Linked Receptors: slower, broader effects mediated through second messengers

UNCONVENTIONAL NEUROTRANSMITTERS

  • Includes gases (e.g., nitric oxide) and endocannabinoids, acting as retrograde messengers

CLINICAL NOTES

  • Myasthenia Gravis: an autoimmune disease affecting neuromuscular connections, involves neurotransmitter interactions.