Neurons communicate via neurotransmitters released in response to electrical impulses. These chemical messengers cross synapses, binding to receptors on other cells to excite or inhibit them. Neurotransmitters are later degraded, diffused, or recycled to regulate signaling efficiency.

• Primary Mode of Communication:
  Neurons communicate mainly through chemical signals called neurotransmitters.

• Action Potential and Neurotransmitter Release:

  • When a neuron is sufficiently stimulated, it generates an action potential (electrical impulse).

  • The action potential travels down the axon to the nerve terminal.

  • At the terminal, it triggers the release of neurotransmitters into the synaptic cleft (space between neurons).

• Synaptic Transmission:

  • Neurotransmitters bind to receptors on the neighboring (post-synaptic) neuron.

  • This binding generates a signal in the receiving neuron, transmitting information.

• Types of Synaptic Connections:

  • The presynaptic neuron releases the neurotransmitter.

  • The postsynaptic neuron receives the signal.

  • The axon of the presynaptic neuron may synapse with:

    • A dendrite: forming an axodendritic synapse

    • The cell body: forming an axosomatic synapse

    • Another axon: forming an axoaxonic synapse

• Synapses Beyond Neurons:

  • Chemical synapses can also occur between a neuron and other target cells (e.g., muscle or gland cells).

• Neurotransmitter Classification:

  • Over 100 neurotransmitters identified.

  • Classified based on chemical structure:

    • Amino acids: e.g., glycine, glutamate, aspartate, GABA

    • Neuropeptides: e.g., beta-endorphin, substance P

    • Monoamines: e.g., epinephrine, norepinephrine, dopamine, serotonin, histamine

      • Derived from amino acids with the acid group removed

    • Acetylcholine: an ester of choline; unique class by itself

• Neurotransmitter Synthesis and Storage:

  • Synthesized in the presynaptic neuron

  • Stored in synaptic vesicles at the axon terminal

  • Some vesicles are docked and ready to release on demand

• Release Mechanism:

  • Arrival of action potential → depolarization

  • Voltage-gated calcium channels open → calcium influx

  • Calcium triggers exocytosis: fusion of vesicles with membrane and neurotransmitter release

• Post-Synaptic Action:

  • Neurotransmitter binds to receptors on the postsynaptic cell

  • Two main mechanisms:

    • Ligand-gated ion channels: direct change in membrane potential

    • Second-messenger systems: indirect signaling pathways

• Excitatory vs. Inhibitory Neurotransmitters:

  • Excitatory neurotransmitters: increase likelihood of action potential (e.g., glutamate)

    • Opens channels allowing positive ions in → depolarization

  • Inhibitory neurotransmitters: decrease likelihood of action potential (e.g., GABA)

    • Opens chloride channels → hyperpolarization (more negative inside)

• Dual-function Neurotransmitter – Acetylcholine:

  • Can be excitatory or inhibitory depending on receptor type:

    • At neuromuscular junctions: binds to nicotinic receptors on skeletal muscles → contraction

    • In the heart: binds to muscarinic M2 receptors → slows heart rate (parasympathetic response)

• Neurotransmitter Clearance:

  • Neurotransmitter binds for only a millisecond

  • Rapid clearance mechanisms include:

    • Passive diffusion

    • Astrocyte uptake for recycling

    • Enzymatic degradation in the synaptic cleft

    • Reuptake via transporter proteins into the presynaptic neuron for reuse

• Signal Continuity:

  • Continuous firing of the presynaptic neuron leads to ongoing signal transmission.

  • If presynaptic firing stops, neurotransmission ceases.