Interneurons are organized into functional groups called neuronal pools PDF

Preview

Summary

Interneurons are organized into functional groups called neuronal pools...

Description

Interneurons are organized into functional groups called neuronal pools Your body has about 10 million sensory neurons, 0.5 million motor neurons, and 20 billion interneurons. The sensory neurons deliver information to the CNS. The motor neurons distribute commands to peripheral effectors, such as skeletal muscles. Interactions among interneurons provide the interpretation, planning, and coordination of incoming and outgoing signals. The billions of interneurons of the CNS are organized into a few hundred to a few thousand neuronal pools—functional groups of interconnected neurons. A neuronal pool may be scattered, involving neurons in several regions of the brain, or localized, with neurons restricted to one specific location in the brain or spinal cord. Each pool has a limited number of input sources and output destinations. Each pool may contain both excitatory and inhibitory neurons. The output of the entire neuronal pool may stimulate or depress activity in other parts of the brain or spinal cord, affecting the interpretation of sensory information or the coordination of motor commands. The pattern of interaction among neurons provides clues to the functional characteristics of a neuronal pool. We refer to the “wiring diagrams” in Figure 13–13 as neural circuits, just as we refer to electrical circuits in the wiring of a house. We can distinguish five circuit patterns: divergence, convergence, serial processing, parallel processing, and reverberation. ■ Divergence is the spread of information from one neuron to several neurons (Figure 13–13a), or from one pool to multiple pools. Divergence permits the broad distribution of a specific input. Considerable divergence occurs when sensory neurons bring information into the CNS, because the information is distributed to neuronal pools throughout the spinal cord and brain. Visual information arriving from the eyes, for example, reaches your consciousness at the same time it is distributed to areas of the brain that control posture and balance at the subconscious level. In convergence, several neurons synapse on a single postsynaptic neuron (Figure 13–13b). Several patterns of activity in the presynaptic neurons can therefore have the same effect on the postsynaptic neuron. Through convergence, the same motor neurons can be subject to both conscious and subconscious control. For example, the movements of your diaphragm and ribs are now being controlled by your brain at the subconscious level. But you can also consciously control the same motor neurons, as when you take a deep breath and hold it. Two neuronal pools are involved, both synapsing on the same motor neurons. ■ In serial processing, information is relayed in a stepwise fashion, from one neuron to another or from one neuronal pool to the next (Figure 13–13c). This pattern occurs as sensory information is relayed from one part of the brain to another. For example, pain sensations on their way to your consciousness make stops at two neuronal pools along the pain pathway. ■ Parallel processing occurs when several neurons or neuronal pools process the same information simultaneously (Figure 13–13d). Divergence must take place before parallel processing can occur. As a result, many responses can occur simultaneously. For example,

stepping on a sharp object stimulates sensory neurons that distribute the information to several neuronal pools. As a result of parallel processing, you might withdraw your foot, shift your weight, move your arms, feel the pain, and shout “Ouch!” at about the same time. ■ In reverberation, collateral branches of axons somewhere along the circuit extend back toward the source of an impulse and further stimulate the presynaptic neurons (Figure 13–13e). Reverberation is like a positive feedback loop involving neurons: Once a reverberating circuit has been activated, it will continue to function until synaptic fatigue or inhibitory stimuli break the cycle. Reverberation can take place within a single neuronal pool, or it may involve a series of interconnected pools. Highly complicated examples of reverberation among neuronal pools in the brain may help maintain consciousness, muscular coordination, and normal breathing....