Page 234 - Anatomy and Physiology of Farm Animals, 8th Edition

Physiology of the Nervous System / 219

one synapse on the postsynaptic neuron A large and important class of insecti-
and their cumulative effects bring the
VetBooks.ir postsynaptic neuron to threshold. Spatial cides known as organophosphates act by
inhibiting the breakdown of a neuro-
summation is possible because neuron cell
bodies typically have hundreds or even transmitter called acetylcholine (ACh) at
synapses that use it. This prolongs the
thousands of presynaptic neurons. action of ACh, interfering with normal
Temporal summation occurs when a neurotransmission and ultimately killing
single presynaptic neuron releases neuro­ the insect. Organophosphates are some-
transmitter repeatedly and rapidly before times used for treating fleas, lice, and
the effect of each single release is lost. other external parasites on animals. Since
When effective summation occurs, the mammalian nervous systems also use
additive effects of the multiple releases are ACh (see later), organophosphates can
enough to produce an action potential in produce signs of poisoning in humans
the postsynaptic neuron. and animals by the same mechanism.
After a neurotransmitter has been As described earlier, a single neuron and
released and had its effect on the postsyn­ its dendrites contain multiple synaptic
aptic neuron, it must be removed to junctions and receive synaptic input from
prevent continuous stimulation of the multiple presynaptic neurons, so a single
postsynaptic neuron. The specific mecha­ neuron may receive impulses from several
nisms by which neurotransmitters are sources. This pattern or organization is
removed from neural synapses vary among described as convergence (Fig. 11‐7).
neurotransmitters. However, in general Divergence is the opposite: each axon
these can be any one or a combination of branches so that synaptic connections are
the following: (1) enzymes in the area of made with many neurons (Fig. 11‐7). These
the synapse degrade the neurotransmitter; organizational patterns permit informa­
(2) cell membrane transport systems tion to be widely distributed throughout a
absorb the neurotransmitter; or (3) the neural network (divergence) or permit
neurotransmitter diffuses away from the multiple sources of information to be
area of the synapse. The first two of these brought to focus on a single neuron for an
are by far the most important mechanisms averaged response (convergence). These
by which the synaptic signal is terminated. are very simple illustrations of how the

Divergence

Convergence

Figure 11-7. Convergence and divergence in neural networks.