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
C. Axon terminal calcium
3. Other receptors
E. Certain drugs and diseases, which act via the above mechanisms A-D
II. Postsynaptic factors
C. Certain drugs and diseases
III. General factors
A. Area of synaptic contact
B. Enzymatic destruction of neurotransmitter
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
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)
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
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.
Was this article helpful?
This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.