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PART 3 Integration and Control Systems
ASSESS
YOUR PROGRESS
52.
Explain the production of EPSPs and IPSPs. Why are theyimportant?
53.
What are axoaxonic synapses?
54.
Give an example of presynaptic inhibition. Describepresynaptic facilitation.
55.
Distinguish between spatial summation and temporalsummation. In what part of the neuron does summationtake place?
56.
How do EPSPs and IPSPs affect the likelihood thatsummation will result in an action potential?
11.7
Neuronal Pathways and Circuits
LEARNING OUTCOMES
After reading this section, you should be able to
A.
Contrast convergent and divergent neuron pathways.
B.
Describe a reverberating circuit.
C.
Explain a parallel after-discharge circuit.
The organizational patterns of neurons within the CNS vary fromrelatively simple to extremely complex. The simplest organizationis a
serial
pathway, where the input travels along only one path-way. However, most pathways are more complex and are called
parallel
pathways, where the input travels along several pathways.The axon of a neuron can branch repeatedly to form synapses withmany other neurons, and hundreds or even thousands of axons can
synapse with the cell body and dendrites of a single neuron.Although their complexity varies, four basic patterns of parallelpathways can be recognized: convergent pathways, divergent path-ways, reverberating circuits, and parallel after-discharge circuits.In
convergent pathways,
multiple neurons converge uponand synapse with a smaller number of neurons (figure 11.23
a
).Convergence allows different parts of the nervous system to acti-vate or inhibit the activity of neurons. For example, one part of thenervous system can stimulate the neurons responsible for making amuscle contract, whereas another part can inhibit those neurons.Through summation, muscle contraction can be activated if moreconverging neurons stimulate the production of EPSPs than con-verging neurons stimulate the production of IPSPs. Conversely,muscle contraction is inhibited if the converging neurons stimulatethe production of more IPSPs than EPSPs.In
divergent pathways,
a smaller number of presynaptic neu-rons synapse with a larger number of postsynaptic neurons toallow information transmitted in one neuronal pathway to divergeinto two or more pathways (figure 11.23
b
). Diverging pathwaysallow one part of the nervous system to affect more than one otherpart of the nervous system. For example, sensory input to the cen-tral nervous system can go to both the spinal cord and the brain.
Reverberating circuits
have a chain of neurons with synapseswith previous neurons in the chain, making a positive-feedback loop.This allows action potentials entering the circuit to cause a neuronfarther along in the circuit to produce an action potential more thanonce (figure 11.23
c
). This response, called
after-discharge,
pro-longs the response to a stimulus. Once a reverberating circuit isstimulated, it continues to discharge until the synapses involvedbecome fatigued or are inhibited by other neurons. Reverberatingcircuits play a role in neuronal circuits that control rhythmic
Inputs
Input
Input
Input
Output
(a)
Convergent pathway
Outputs
(b)
Divergent pathway
Outputs(after-discharges)(c)
Reverberating circuit
Output
(d)
Parallel after-dischargecircuit
FIGURE 11.23
Neuronal Pathways and Circuits
The direction of action potential propagation is represented by the
orange arrows.
(
a
) General model of a convergent pathway; many neurons converge and syn-apse with a smaller number of neurons. (
b
) General model of a divergent pathway; a few neurons synapse with a larger number of neurons. (
c
) Simple model of areverberating circuit; input action potentials result in the production of a larger number of output action potentials because neurons within the circuit arerepeatedly stimulated to produce action potentials. (
d
) Simple model of a parallel after-discharge circuit; several neurons in parallel that integrate complexprocesses stimulate a common output cell.