Neuronal Pools and Circuits

So far, we have dealt with interactions involving only two or three neurons at a time. Actually, neurons function in larger ensembles called neuronal pools, each of which consists of thousands to millions of interneurons concerned with a particular body function—one to control the rhythm of your breathing, one to move your limbs rhythmically as you walk, one to regulate your sense of hunger, and another to interpret smells, for example. At this point, we explore a few ways in which neuronal pools collectively process information.

Information arrives at a neuronal pool through one or more input neurons, which branch repeatedly and synapse with numerous interneurons in the pool. Some input neurons form multiple synapses with a single post-synaptic cell. They can produce EPSPs at all points of contact with that cell and, through spatial summation, make it fire more easily than if they synapsed with it at only one point. Within the discharge zone of an input neuron, an input neuron acting alone can make the postsynaptic cells fire (fig. 12.26). But in a broader facilitated zone, it synapses with still other neurons in the pool, with fewer synapses on each of them. It can stimulate those neurons to fire only with the assistance of other input neurons; that is, it facilitates the other input neurons. Along with other inputs, it "has a vote" on what the postsynaptic cells in the facilitated zone will do, but it cannot alone determine

Discharge Zone Facilitated Zone

Figure 12.26 Facilitated and Discharge Zones in a Neuronal Pool. In a facilitated zone, the input neuron has few synaptic contacts with each output neuron. The input neuron makes it easier for those neurons to respond to stimulation from other sources, but it cannot, by itself, make them fire. In the discharge zone, the input neuron has extensive connections with each output neuron and is capable, by itself, of making the output neurons fire.

Figure 12.26 Facilitated and Discharge Zones in a Neuronal Pool. In a facilitated zone, the input neuron has few synaptic contacts with each output neuron. The input neuron makes it easier for those neurons to respond to stimulation from other sources, but it cannot, by itself, make them fire. In the discharge zone, the input neuron has extensive connections with each output neuron and is capable, by itself, of making the output neurons fire.

what they do. Such arrangements, repeated thousands of times throughout the central nervous system, give neuronal pools great flexibility in integrating input from several sources and "deciding" on an appropriate output.

The functioning of a radio can be understood from a circuit diagram showing its components and their connections. Similarly, the functions of a neuronal pool are partly determined by its neuronal circuit—the pathways among its neurons. Just as a wide variety of electronic devices are constructed from a relatively limited number of circuit types, a wide variety of neuronal functions result from the operation of four principal kinds of neuronal circuits (fig. 12.27):

  1. In a diverging circuit, one nerve fiber branches and synapses with several postsynaptic cells. Each of those may synapse with several more, so input from just one neuron may produce output through dozens of neurons. Such a circuit allows one motor neuron of the brain, for example, to ultimately stimulate thousands of muscle fibers.
  2. A converging circuit is the opposite of a diverging circuit—input from many different nerve fibers is funneled to one neuron or neuronal pool. Such an arrangement allows input from your eyes, inner ears, and stretch receptors in your neck to be channeled to an area of the brain concerned with the sense of balance. Also through neuronal convergence, a respiratory center in your brainstem receives input from other parts of your brain, from receptors for blood chemistry in your arteries, and from stretch receptors in your lungs. The respiratory center can then produce an output that takes all of these factors into account and sets an appropriate pattern of breathing.
  3. In a reverberating circuit, neurons stimulate each other in a linear sequence such as A ^ B ^ C ^ D, but neuron C sends an axon collateral back to A. As a result, every time C fires it not only stimulates output neuron D, but also restimulates A and starts the process over. Such a circuit produces a prolonged or repetitive effect that lasts until one or more neurons in the circuit fail to fire, or until an inhibitory signal from another source stops one of them from firing. A reverberating circuit sends repetitious signals to your diaphragm and intercostal muscles, for example, to make you inhale. When the circuit stops firing, you exhale, the next time it fires, you inhale again. Reverberating circuits may also be involved in short-term memory, as discussed in the next section, and they may play a role in the uncontrolled "storms" of neuronal activity that occur in epilepsy.
  4. In a parallel after-discharge circuit, an input neuron diverges to stimulate several chains of

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

12. Nervous Tissue

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Chapter 12 Nervous Tissue 473

Diverging

Input

Output

Converging

Output

Input

Reverberating r

Output

Input

Output

Parallel after-discharge

Parallel after-discharge

Parallel After Discharge Circuits

Figure 12.27 Four Types of Neuronal Circuits. Arrows indicate the direction of the nerve signal. Which of these four circuits is likely to fire the longest after a stimulus ceases? Why?

neurons. Each chain has a different number of synapses, but eventually they all reconverge on a single output neuron. Since each pathway differs in total synaptic delay, their signals arrive at the output neuron at different times, and the output neuron may go on firing for some time after input has ceased. Unlike a reverberating circuit, this type has no feedback loop. Once all the neurons in the circuit have fired, the output ceases. Continued firing after the stimulus stops is called afterdischarge. It explains why you can stare at a lamp, then close your eyes and continue to see an image of it for a while. Such a circuit is also important to withdrawal reflexes, in which a brief pain produces a longer-lasting output to the limb muscles and causes you to draw back your hand or foot from danger.

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Responses

  • egidio
    What is the function of neuronal pools?
    2 years ago
  • mohammad
    What are the components of a neuronal pool?
    2 years ago
  • timothy
    What is neuronal pool and where they found?
    1 year ago
  • Elsie
    How is neuronal pool summation like a vote?
    1 year ago
  • marisol
    What is the function of neuronal pool?
    9 months ago
  • Neftalem
    What is the difference between neuronal circuits and neuronal pools?
    8 months ago

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