Communication among these neurons is facilitated by synapses.
Types of Synapses
Two main types: Electrical and Chemical Synapses.
Electrical synapses use connexons for direct current flow.
Direct, passive flow of electrical current.
Chemical synapses use neurotransmitters for cell-to-cell communication.
Chemical: Communication via neurotransmitter release.
Feature
Electrical Synapses
Chemical Synapses
Structure
Gap junctions
Connexons enable close neuron-to-neuron links
Larger synaptic cleft between neurons
Synaptic vesicles in presynaptic terminal
Function
Direct flow of electrical current
Current sourced from local potential difference
Neurotransmitters facilitate communication as chemical messengers
Key Characteristics
Permits passive electrical current flow
Influenced by presynaptic action potential
Presence of neurotransmitters in synaptic vesicles
Acts through specialized receptors on postsynaptic cell
Chemical Synapses and Neurotransmitters
Over 100 different neurotransmitters identified.
Communication process: Synthesis, packaging, release, and binding of neurotransmitters, followed by rapid removal or degradation.
Overview of Chemical Synapses
Presynaptic terminal with synaptic vesicles.
Postsynaptic cell, separated by synaptic cleft.
Filamentous elements in pre- and postsynaptic processes.
Structures in synaptic cleft, including active zone and postsynaptic density.
Signaling Transmission Sequence at Chemical Synapses
Sequence of events involved in transmission at a typical chemical synapseSynaptic Vesicle Preparation → ormation and filling of synaptic vesicles with neurotransmitter.
Rapid influx due to concentration gradient (10–3 M external vs. 10–7 M internal).
Elevates Ca2+ in presynaptic terminal.
Exocytosis and Postsynaptic Response
Exocytosis
Ca2+ elevation → Synaptic vesicles fuse with presynaptic membrane.
Neurotransmitters released into synaptic cleft.
Neurotransmitter Binding
Diffuses across cleft → Binds to postsynaptic receptors.
Opens/closes channels in postsynaptic membrane, altering ion flow.
Postsynaptic Neuron Response
Alters conductance and membrane potential.
Increases/decreases probability of firing an action potential.
Termination of Signal
Neurotransmitter Removal
Uptake into glial cells or enzymatic degradation.
Effect
Terminates neurotransmitter action.
Ensures transient information transmission from one neuron to another.
Slide 14: Overview of Chemical Synapse Transmission
Neurotransmitter Role: Chemical signals like acetylcholine play critical roles in transmitting information across neurons, as demonstrated by Otto Loewi in 1926.
Classification: Over 100 neurotransmitters identified, broadly categorized into small-molecule neurotransmitters and neuropeptides.
Slide 15: Neurotransmitter Diversity and Regulation
Diverse Functions: Neurotransmitters can excite or inhibit postsynaptic neurons, with small-molecule neurotransmitters facilitating rapid responses and neuropeptides modulating slower functions.
Co-Transmitters: Enable dynamic changes in signaling properties based on synaptic activity.
Synaptic Regulation: Synthesis, packaging, release, and removal/degradation of neurotransmitters are tightly controlled to maintain appropriate levels.
Slide 16: Synaptic Vesicle Preparation and Action Potential
Vesicle Formation: Synaptic vesicles are prepared by being filled with neurotransmitters, ready for release upon an action potential arrival.
Action Potential's Role: Triggers the opening of voltage-gated Ca2+ channels in the presynaptic terminal, leading to Ca2+ influx due to the concentration gradient.
Slide 17: Exocytosis and Neurotransmitter Release
Ca2+ Influx: Causes a rapid increase in presynaptic Ca2+ levels, facilitating the fusion of synaptic vesicles with the presynaptic membrane.
Exocytosis: Releases neurotransmitters into the synaptic cleft, where they diffuse across to bind to postsynaptic receptors.
Slide 18: Postsynaptic Response and Signal Termination
Neurotransmitter Binding: Alters postsynaptic ion flow by opening or closing channels, changing membrane potential and conductance.
Neuron Firing Probability: Influences the likelihood of the postsynaptic neuron firing an action potential.
Termination of Signal: Achieved through neurotransmitter uptake into glial cells or enzymatic degradation, ensuring the transient nature of the signal.
Synaptic Transmission at the Neuromuscular Junction (NMJ)
Neuromuscular Transmission and Nicotinic Synapses
NMJ: Connection between alpha-motor neuron axons and skeletal muscle fibers.
Mechanism: Cholinergic transmission using acetylcholine (Ach).
Relevance: Basics of neurotransmitter release applicable across all synapses.