Neuronal Communication and Neurotransmitters

Overview

• Neurons communicate via chemical messages known as neurotransmitters.
• An action potential is generated when a neuron is sufficiently stimulated.
• The action potential travels down the axon to the nerve terminal, triggering the release of neurotransmitters into the synaptic cleft.
• Neurotransmitters bind to receptors on neighboring neurons, transmitting the signal.

Key Terms

• Pre-synaptic neuron: The neuron releasing the neurotransmitter.
• Post-synaptic neuron: The neuron receiving the neurotransmitter signal.
• Synaptic cleft: The space between neurons where neurotransmission occurs.
• Synapse types:
  • Axodendritic: Synapse with a dendrite.
  • Axosomatic: Synapse with a cell body.
  • Axoaxonic: Synapse with another axon.
• Target cells: Neurons can also form synapses with muscle or gland cells.

Classes of Neurotransmitters

• Amino acids
  • Glycine, glutamate, aspartate, GABA
• Neuropeptides
  • Beta-endorphin, substance P
• Monoamines
  • Epinephrine, norepinephrine, dopamine, serotonin, histamine
• Acetylcholine
  • Ester of choline, in its own class

Neurotransmitter Synthesis and Release

• Synthesized in presynaptic neurons and stored in synaptic vesicles in the axon terminal.
• Vesicles are docked on the plasma membrane ready for release.
• Action potential arrival causes depolarization, opening voltage-gated calcium channels, and allowing calcium influx.
• Calcium causes vesicles to fuse with the plasma membrane, releasing neurotransmitters (exocytosis).

Neurotransmitter Receptor Binding

• Some neurotransmitters open ligand-gated ion channels, directly changing the membrane potential.
• Others act through second-messenger systems.
• Neurotransmitters can be excitatory or inhibitory depending on the receptor.
  • Excitatory: e.g., Glutamate opens ion channels, making the cell more positive and likely to generate action potentials.
  • Inhibitory: e.g., GABA opens chloride channels, making the cell more negative and less likely to generate action potentials.
  • Dual function: e.g., Acetylcholine can be excitatory (nicotinic receptors on skeletal muscle) or inhibitory (muscarinic receptors on cardiac muscle).

Neurotransmitter Inactivation

• Neurotransmitter binds to its receptor for a millisecond and then diffuses away.
• Taken up by astrocytes for recycling.
• Continuous presynaptic firing leads to continuous binding and activation.
• Absence of presynaptic signals stops transmission.
• Mechanisms to prevent overstimulation:
  • Enzymatic degradation in the synapse.
  • Reuptake by transporter proteins back into the presynaptic neuron for reuse.