Neurotransmitter Pathways: A Tutorial Study Guide

Nicoladie Tam

• Editorial: Nicoladie Tam, Ph.D.
• ISBN:
  9781301696666
• Categoría: Sciences biologiques
• Categoría: Neurosciences

“Neurotransmitter Pathways” is a part of the college-level Neuropsychopharmacology course series textbooks. It is a tutorial written in questions and answers format to describe the anatomy and physiology of the neurotransmitter circuitry, including the cell origins and the targets of the neurotransmitter pathways.

It is a study guide with in-depth explanations. Each section is a modular unit that is self-contained for easy reading. The principles and concepts are introduced systematically so students can learn and retain the materials intuitively.

1.1.Neurotransmitters Systems

Objectives

• Understand the functions of neurotransmitters

Concepts to Learn

• Characteristics of neurotransmitters, neurohormones and neuromodulator.
• Differences between neurotransmitters, neurohormones and neuromodulator.
• Functions of neurotransmitter
• Mechanisms of action for neurotransmitter
• Relationship between excitatory synapse and post-synaptic receptor
• Relationship between inhibitory synapse and post-synaptic receptor

Q&A

What is a neurotransmitter?

Neurotransmitter is a chemical molecule that is released from the synaptic vesicle into the synaptic cleft that will eventually bind with the post-synaptic receptor.

Usually there are receptors at the post-synaptic neuron that bind with the neurotransmitter, but sometimes there are also auto-receptors at the pre-synaptic neuron that bind with the neurotransmitter to regulate the pre-synaptic neuron’s function.

When the neurotransmitter binds with the post-synaptic receptor, it either excites or inhibits the post-synaptic neuron. Thus, the signal originated from the pre-synaptic neuron is transmitted to the next neuron.

What distinguishes a neurotransmitter from a neurohormone?

When the chemical is released from a neuron into the synaptic cleft, it is called a neurotransmitter. When the chemical is released from a neuron into the blood stream, it is called a neurohormone.

It is an artificial distinction to classify the difference between where the chemical is released to, and what the target cells are. In reality, a chemical can be both neurotransmitter and neurohormone, depending on where it is released to. For example, norepinephrine is both a neurotransmitter and a neurohormone. Dopamine is both a neurotransmitter and a neurohormone.

What is a neuromodulator?

A neuromodulator is a chemical that is similar to a neurotransmitter except that it may not satisfy all the criteria as a neurotransmitter. Usually it regulates the function of other neurotransmitters and diffuses far beyond the local synapse. Sometimes, they are called neuroregulators.

The criteria to be classified as a neurotransmitter is actually imposed on by humans in which the chemical has to be stored in a synaptic vesicle, released from a pre-synaptic neuron into the synaptic cleft, and then binds with the post-synaptic receptor locally at the synapse. If any of the criteria is not met, neuroscientists don’t call them neurotransmitters.

In many cases, the neuromodulator can diffuse away from the local synapse, and affect the release of other neurotransmitters. In such cases, neuroscientists call them neuromodulators.

In other cases, the neuromodulator may not be stored in synaptic vesicles. They can be synthesized on demand and then released into the synaptic cleft. An example is anandamide, which is synthesized on demand, released into the synapse and then binds with the cannabinoid receptor at the post-synaptic neuron.

Another example of neuromodulator is nitric oxide. It is a gas, which is synthesized on demand by nitric oxide synthetase rather than stored in synaptic vesicles. Nitric oxide diffuses into the synapse and neighboring synapses to produce its action.

Because there are a lot of chemicals released by a neuron that may not strictly satisfy all the criteria to be a neurotransmitter. As stated above, some neurotransmitters are both neurotransmitter and neurohormone, depending on where they are released to. The distinction between neurotransmitter and neuromodulator is very artificial, and can be used interchangeably.

What is a chemical synapse?

A synapse is the junction between two neurons (pre-synaptic and post-synaptic neurons). The electrical signal carried by the action potential is converted into a chemical signal (neurotransmitters) to be transmitted (relayed) to the next neuron.

Chemical synapse is evolved to provide two forms of communication – excitatory and inhibitory.

Originally, electrical synapse is used to connect two neurons in which action potentials can be transmitted from one neuron to the next directly via the gap junctions in the electrical synapse. This form of communication is excitatory only, since the post-synaptic neuron will always be excited by the pre-synaptic neuron.

Chemical synapse is an alternate mechanism to transmit signals to the next neuron that can either be excitatory or inhibitory to the post-synaptic neuron while electrical synapse always excite the post-synaptic neuron. Thus, chemical synapse is a generic mechanism for transmitting either an excitatory or inhibitory signal to the next neuron. Whether the synapse is excitatory or inhibitory depends on the post-synaptic receptor that binds with the neurotransmitter.

What is the processing function for a synapse?

A synapse is basically used to integrate all the information provided by the pre-synaptic neurons in order to produce an output represented by the firing/non-firing of action potentials.

If the synapse is an excitatory synapse, stimulation by the pre-synaptic input would increase the probability of firing for the post-synaptic neuron.

If the synapse is an inhibitory synapse, stimulation by the pre-synaptic input would decrease the probability of fi...