Concept Of Divergence Of Signal Passing Through Neuronal Pool

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Figure 46-11

Neuronal Pool

Divergence in multiple tracts

Figure 46-12

"Convergence" of multiple input fibers onto a single neuron. A, Multiple input fibers from a single source. B, Input fibers from multiple separate sources.

Divergence in multiple tracts

"Divergence" in neuronal pathways. A, Divergence within a pathway to cause "amplification" of the signal. B, Divergence into multiple tracts to transmit the signal to separate areas.

Figure 46-12

"Convergence" of multiple input fibers onto a single neuron. A, Multiple input fibers from a single source. B, Input fibers from multiple separate sources.

Divergence of Signals Passing Through Neuronal Pools

Often it is important for weak signals entering a neuronal pool to excite far greater numbers of nerve fibers leaving the pool. This phenomenon is called divergence. Two major types of divergence occur and have entirely different purposes.

An amplifying type of divergence is shown in Figure 46-11 A. This means simply that an input signal spreads to an increasing number of neurons as it passes through successive orders of neurons in its path. This type of divergence is characteristic of the corticospinal pathway in its control of skeletal muscles, with a single large pyramidal cell in the motor cortex capable, under highly facilitated conditions, of exciting as many as 10,000 muscle fibers.

The second type of divergence, shown in Figure 46-115, is divergence into multiple tracts. In this case, the signal is transmitted in two directions from the pool. For instance, information transmitted up the dorsal columns of the spinal cord takes two courses in the lower part of the brain: (1) into the cerebellum and (2) on through the lower regions of the brain to the thalamus and cerebral cortex. Likewise, in the thalamus, almost all sensory information is relayed both into still deeper structures of the thalamus and at the same time to discrete regions of the cerebral cortex.

Convergence of Signals

Convergence means signals from multiple inputs uniting to excite a single neuron. Figure 46-12A shows convergence from a single source. That is, multiple terminals from a single incoming fiber tract terminate on the same neuron. The importance of this is that neurons are almost never excited by an action potential from a single input terminal. But action potentials converging on the neuron from multiple terminals provide enough spatial summation to bring the neuron to the threshold required for discharge.

Convergence can also result from input signals (excitatory or inhibitory) from multiple sources, as shown in Figure 46-125. For instance, the interneurons of the spinal cord receive converging signals from (1) peripheral nerve fibers entering the cord, (2) propriospinal fibers passing from one segment of the cord to another, (3) corticospinal fibers from the cerebral cortex, and (4) several other long pathways descending from the brain into the spinal cord. Then the signals from the interneurons converge on the anterior motor neurons to control muscle function.

Such convergence allows summation of information from different sources, and the resulting response is a summated effect of all the different types of information. Convergence is one of the important means by which the central nervous system correlates, sum-mates, and sorts different types of information.

Neuronal Circuit with Both Excitatory and Inhibitory Output Signals

Sometimes an incoming signal to a neuronal pool causes an output excitatory signal going in one direction and at the same time an inhibitory signal going elsewhere. For instance, at the same time that an excitatory signal is transmitted by one set of neurons in the spinal cord to cause forward movement of a leg, an inhibitory signal is transmitted through a separate set of neurons to inhibit the muscles on the back of the leg so that they will not oppose the forward movement. This type of circuit is characteristic for controlling all antagonistic pairs of muscles, and it is called the reciprocal inhibition circuit.

Figure 46-13 shows the means by which the inhibition is achieved. The input fiber directly excites the excitatory output pathway, but it stimulates an intermediate inhibitory neuron (neuron 2), which secretes a different type of transmitter substance to inhibit the second output pathway from the pool. This type of circuit is also important in preventing overactivity in many parts of the brain.

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Responses

  • lalia galbassi
    What is the divergence of the signal?
    9 months ago

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