FUNCTIONAL ORGANIZATION OF NEURONS
The body has about 10 million sensory neurons, 20 billion interneurons, and one-half million motor neurons. These neurons are arranged in so many waysthat impulses generated can converge on one neuron or diverge to many neurons or even have a feedback on the neuron that originally generated the impulse. All these possibilities help the body better coordinate its activities. For some different ways that impulses can be modified, see Figure 5.10.
A diverging arrangement allows for a wide distri-bution of a specific input. For example, sensory input is distributed to other neurons in the spinal cord and the brain. Parallel processing allows the informa-tion to be processed by different neurons at the same time and produce a response in different regions of the body. For example, if you encounter a grizzly bear face-to-face on a hike, I do not know what you would do, but I would scream and run at the same time (but, please don’t do that if it actually happens). Par-allel processing would help me do that.
A converging arrangement helps more than one neuron have an effect on a postsynaptic neuron. For example, if you are carrying a hot plate, your initial response may be to drop the plate. But if the plate contained something you did not want to loose, you could force yourself to carry it to the nearest table without it dropping. It is convergence that makes this possible. The motor neurons to your muscles have synapses with sensory neurons conveying tempera-ture sensation, as well as neurons, from your brain. The activity of the motor neuron depends on the in-tegrated effect of all the neurons synapsing with it. In this way, the motor neuron can be inhibited or stim-ulated, according to situations and altering the activ-ity of the presynaptic neurons.
Serial processing affects only one neuron. Thisarrangement is seen in the way sensations are con-veyed to the brain. This helps the brain discern the exact region from which the sensation originated. For example, all sensations initiated in the left big toe reach the “toe area” in the brain on the right side (Figure 5.35).
Reverberation helps neurons down the circuit toinitiate an impulse in the presynaptic neuron. Al-though the Figure shows a simplified version, more complicated circuits, with many neurons involved, are seen in the CNS. These circuits continue on and on until the neurons are inhibited or fatigued. An ex-ample of such circuits is the respiratory center, which helps with repetitive activities such as breathing.