Neurotransmitters and Neuromodulators

We have emphasized the role of neurotransmitters in eliciting EPSPs and IPSPs. However, certain chemical messengers elicit complex responses that cannot be simply described as EPSPs or IPSPs. The word "modulation" is used for these complex responses, and the

I. Presynaptic factors

A. Availability of neurotransmitter

1. Availability of precursor molecules

2. Amount (or activity) of the rate-limiting enzyme in the pathway for neurotransmitter synthesis

B. Axon terminal membrane potential

C. Axon terminal calcium

D. Activation of membrane receptors on presynaptic terminal

1. Presynaptic (axon-axon) synapses

2. Autoreceptors

3. Other receptors

E. Certain drugs and diseases, which act via the above mechanisms A-D

II. Postsynaptic factors

A. Immediate past history of electrical state of postsynaptic membrane (that is, facilitation or inhibition from temporal or spatial summation)

B. Effects of other neurotransmitters or neuromodulators acting on postsynaptic neuron

C. Certain drugs and diseases

III. General factors

A. Area of synaptic contact

B. Enzymatic destruction of neurotransmitter

C. Geometry of diffusion path

D. Neurotransmitter reuptake messengers that cause them are called neuromodulators. The distinctions between neuromodulators and neurotransmitters are, however, far from clear. In fact, certain neuromodulators are often synthesized by the presynaptic cell and co-released with the neurotrans-mitter. To add to the complexity, certain hormones, paracrine agents, and messengers used by the immune system serve as neuromodulators.

Neuromodulators often modify the postsynaptic cell's response to specific neurotransmitters, amplifying or dampening the effectiveness of ongoing synap-tic activity. Alternatively, they may change the presy-naptic cell's synthesis, release, reuptake, or metabolism of a transmitter. In other words, they alter the effectiveness of the synapse.

In general, the receptors for neurotransmitters influence ion channels that directly affect excitation or inhibition of the postsynaptic cell. These mechanisms operate within milliseconds. Receptors for neuromodulators, on the other hand, more often bring about changes in metabolic processes in neurons, often via G proteins coupled to second-messenger systems. Such changes, which can occur over minutes, hours, or even days, include alterations in enzyme activity or, by way of influences on DNA transcription, in protein synthesis. Thus, neurotransmitters are involved in rapid communication, whereas neuromodulators

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Neural Control Mechanisms CHAPTER EIGHT

Neural Control Mechanisms CHAPTER EIGHT

TABLE 8-7 Classes of Some of the Chemicals Known or Presumed to be Neurotransmitters or Neuromodulators

1. Acetylcholine (ACh)

2. Biogenic amines

Catecholamines Dopamine (DA) Norepinephrine (NE) Epinephrine (Epi) Serotonin (5-hydroxytryptamine, 5-HT) Histamine

3. Amino acids

Excitatory amino acids; for example, glutamate

Inhibitory amino acids; for example, gamma-aminobutyric acid (GABA)

4. Neuropeptides;for example, the endogenous opioids

5. Miscellaneous

Gases; for example, nitric oxide Purines; for example, adenosine and ATP

tend to be associated with slower events such as learning, development, motivational states, or even some sensory or motor activities.

Table 8-7 lists the major categories of substances generally accepted as neurotransmitters or neuromod-ulators. A huge amount of information has accumulated concerning the synthesis, metabolism, and mechanisms of action of these messengers—material well beyond the scope of this book. The following sections will therefore present only some basic generalizations about certain of the neurotransmitters presently deemed most important. For simplicity's sake, we use the term "neurotransmitter" in a general sense, realizing that sometimes the messenger may more appropriately be described as a neuromodulator. A note on terminology should also be included here: Neurons are often referred to as "-ergic," where the missing prefix is the type of neurotransmitter released by the neuron. For example, "dopaminergic" applies to neurons that release the neurotransmitter dopamine.

Essentials of Human Physiology

Essentials of Human Physiology

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    What is the physical function of neuromodulators?
    6 years ago
  • claudia
    What is a neuromodulator vs neurotransmitter?
    6 years ago
  • mary
    What are neuromodulators in the body ?
    6 years ago
  • gloria johnson
    What are neuromodulators and how they modulate pain?
    6 years ago
  • jasmine gamgee
    What 2 factors affect availability and function of neurotransmitters?
    5 years ago

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