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10.5I: Types of Neurotransmitters by Function

Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse.

 

LEARNING OBJECTIVES

 

Describe types of neurotransmitters by their function

 

KEY TAKEAWAYS

Key Points

 

  • Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse.
  • Release of neurotransmitters usually follows arrival of an action potential at the synapse, but may also follow a graded electrical potential.
  • The major types of neurotransmitters include acetylcholine, biogenic amines, and amino acids.
  • Biogenic amines include the catecholamines such as dopamine, norepinephrine (NE), and epinephrine, as well as indolamines such as serotonin and histamine.

 

Key Terms

 

  • neurotransmitter: An endogenous chemical that transmits signals from a neuron to a target cell across a synapse.
  • acetylcholine: This neurotransmitter acts on the neuromuscular juncture and is
    synthesized from acetic acid and choline.
  • Biogenic amines: Neurotransmitters distributed in the brain, where they play a role in emotional behavior and help in regulating the biological clock.
  • glutamate: An amino acid that
    promotes excitatory effects by  increasing the probability that the target cell will fire an action potential.

Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Although some neurons produce and release only one kind of neurotransmitter, most make two or more and may release one or more at any given time. The coexistence of more than one neurotransmitter in the synapse makes it possible for the cell to exert several influences at the same time.

Neurotransmitter Structure

This diagram of the workings of a neurotransmitter includes the terms neuron, dendrites, axon, electrical impulses, neurotransmitter molecules, receptor, synapse.

Major elements in neuron-to-neuron communication: Chemical synapses are specialized junctions through which neurons signal to each other and to non-neuronal cells such as those in muscles or glands.

Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane in the axon terminal on the presynaptic side of a synapse. They are released into and diffuse across the synaptic cleft, where they bind to specific receptors in the membrane on the postsynaptic side of the synapse. Release of neurotransmitters usually follows arrival of an action potential at the synapse, but may also follow a graded electrical potential. Low-level baseline release also occurs without electrical stimulation.

Neurotransmitters are synthesized from plentiful and simple precursors such as amino acids, which are readily available from the diet and require only a small number of biosynthetic steps to convert.

Classification of Neurotransmitters

Neurotransmitters fall into several chemical classes based on the molecular structure. The major types of neurotransmitters include acetylcholine, biogenic amines, and amino acids.  The neurotransmitters can also be classified based on function (excitatory or inhibitory) and action (direct or neuromodulatory).

Acetylcholine

Acetylcholine, which acts on the neuromuscular junction, was the first neurotransmitter identified. It is synthesized from acetic acid and choline. Once released, acetylcholine binds to post-synaptic receptors and is degraded by acetylcholinesterase. Acetylcholine-releasing neurons are also found in the central nervous system (CNS).

Biogenic Amines

Biogenic amines include the catecholamines, such as dopamine, norepinephrine (NE), and epinephrine, as well as indolamines such as serotonin and histamine. Dopamine and NE are synthesized from amino acid tyrosine. Serotonin is synthesized  from tryptophan. Histamine is synthesized from amino acid
histidine.

Biogenic amines are distributed in the brain, where they play a role in emotional behavior and help in regulating the biological clock. Additionally, some motor neurons of the ANS release catecholamines like NE. NE, dopamine, and histamine can be excitatory or inhibitory depending on the receptor type. Addictive drugs such as cocaine and amphetamine exert their effects primarily on the dopamine system, while addictive opiates and functional analogs of opioid peptides which regulate dopamine levels.

Amino Acids

Glutamate and gamma-aminobutyric acid (GABA) are amino acid-based neurotransmitters. The most prevalent transmitter in the human brain is glutamate, which promotes excitatory effects by  increasing the probability that the target cell will fire an action potential. The next most prevalent is GABA, which is inhibitory at more than 90% of the synapses that do not use glutamate.

Neuropeptides such as Substance P and endorphins are strings of amino acids that are important in the mediation of pain signals. Enkephalin activity increases dramatically in pregnant women in labor. Endorphins are released in the so called “runner’s high.” There is also evidence that neuropeptides such as gut-brain peptides are produced by non-neural tissues in the gastrointestinal tract.