Synaptic Effectiveness

Individual synaptic events—whether excitatory or inhibitory—have been presented as though their effects are constant and reproducible. Actually, the variability in postsynaptic potentials following any particular presynaptic input is enormous. The effectiveness of a given synapse can be influenced by both presynaptic and postsynaptic mechanisms.

First, a presynaptic terminal does not release a constant amount of neurotransmitter every time it is activated. One reason for this variation involves calcium concentration. Calcium that has entered the terminal during previous action potentials is pumped out of the cell or (temporarily) into intracellular organelles. If calcium removal does not keep up with entry, as can occur during high-frequency stimulation, calcium concentration in the terminal, and hence the amount of neurotransmitter released upon subsequent stimulation, will be greater than usual. The greater the amount of neurotransmitter released, the greater the number of ion channels opened (or closed) in the postsynaptic membrane, and the larger the amplitude of the EPSP or IPSP in the postsynaptic cell.

The neurotransmitter output of some presynaptic terminals is also altered by activation of membrane receptors in the terminals themselves. These presynaptic receptors are often associated with a second synaptic ending known as an axon-axon synapse, or presynaptic synapse, in which an axon terminal of one neuron ends on an axon terminal of another. For example, in Figure 8-32 the neurotransmitter released by A combines with receptors on B, resulting in a change in the

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

Neural Control Mechanisms CHAPTER EIGHT

Neural Control Mechanisms CHAPTER EIGHT

Presynaptic Axon Terminal

FIGURE 8-32

A presynaptic (axon-axon) synapse between axon terminal A and axon terminal B. C is the final postsynaptic cell body.

FIGURE 8-32

A presynaptic (axon-axon) synapse between axon terminal A and axon terminal B. C is the final postsynaptic cell body.

amount of neurotransmitter released from B in response to action potentials. Thus, neuron A has no direct effect on neuron C, but it has an important influence on the ability of B to influence C. Neuron A is said to be exerting a presynaptic effect on the synapse between B and C. Depending upon the nature of the neu-rotransmitter released from A and the type of receptors activated by that neurotransmitter on B, the presynaptic effect may decrease the amount of neuro-transmitter released from B (presynaptic inhibition) or increase it (presynaptic facilitation).

Presynaptic synapses such as A in Figure 8-32 can alter the calcium concentration in axon terminal B or even affect neurotransmitter synthesis there. If the calcium concentration increases, the number of vesicles releasing neurotransmitter from B increases; decreased calcium reduces the number of vesicles that are releasing transmitter. Presynaptic synapses are important because they selectively control one specific input to the postsynaptic neuron C.

Some receptors on the presynaptic terminal are not associated with axon-axon synapses. Rather they are activated by neurotransmitters or other chemical messengers released by nearby neurons or glia or even the axon terminal itself. In the last case, the receptors are called autoreceptors and provide an important feedback mechanism by which the neuron can regulate its own neurotransmitter output. In most cases, the released neurotransmitter acts on autoreceptors to decrease its own release, thereby providing negativefeedback control.

Postsynaptic mechanisms for varying synaptic effectiveness also exist. For example, as described in Chapter 7, there are many types and subtypes of receptors for each kind of neurotransmitter. The different receptor types operate by different signal transduction mechanisms and have different—sometimes even opposite—effects on the postsynaptic mechanisms they influence. Moreover, a given signal transduction mechanism may be regulated by multiple neurotransmitters, and the various second-messenger systems affecting a channel may interact with each other.

Recall, too, from Chapter 7 that the number of receptors is not constant, varying with up- and down-regulation, for example. Also, the ability of a given receptor to respond to its neurotransmitter can change. Thus, in some systems a receptor responds once and then temporarily fails to respond despite the continued presence of the receptor's neurotransmitter, a phenomenon known as receptor desensitization.

Imagine the complexity when a cotransmitter (or several cotransmitters) is released with the neuro-transmitter to act upon postsynaptic receptors and maybe upon presynaptic receptors as well! Clearly, the possible variations in transmission at even a single synapse are great, and the functions of a given neuro-transmitter can be extremely difficult to identify.

Essentials of Human Physiology

Essentials of Human Physiology

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.

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