Transmission and Processing of Signals in Neuronal Pools

The central nervous system is composed of thousands to millions of neuronal pools; some of these contain few neurons, while others have vast numbers. For instance, the entire cerebral cortex could be considered to be a single large neuronal pool. Other neuronal pools include the different basal ganglia and the specific nuclei in the thalamus, cerebellum, mesen-cephalon, pons, and medulla. Also, the entire dorsal gray matter of the spinal cord could be considered one long pool of neurons.

Each neuronal pool has its own special organization that causes it to process signals in its own unique way,

Neuronal Pools Diagram
Figure 46-8

Translation of signal strength into a frequency-modulated series of nerve impulses, showing the strength of signal (above) and the separate nerve impulses (below). This is an example of temporal summation.

thus allowing the total consortium of pools to achieve the multitude of functions of the nervous system. Yet despite their differences in function, the pools also have many similar principles of function, described in the following pages.

Relaying of Signals Through Neuronal Pools

Organization of Neurons for Relaying Signals. Figure 46-9 is a schematic diagram of several neurons in a neuronal pool, showing "input" fibers to the left and "output" fibers to the right. Each input fiber divides hundreds to thousands of times, providing a thousand or more terminal fibrils that spread into a large area in the pool to synapse with dendrites or cell bodies of the neurons in the pool. The dendrites usually also arborize and spread hundreds to thousands of micrometers in the pool.

The neuronal area stimulated by each incoming nerve fiber is called its stimulatory field. Note in Figure 46-9 that large numbers of the terminals from each input fiber lie on the nearest neuron in its "field," but progressively fewer terminals lie on the neurons farther away.

Threshold and Subthreshold Stimuli—Excitation or Facilitation.

From the discussion of synaptic function in Chapter 45, it will be recalled that discharge of a single excitatory presynaptic terminal almost never causes an action potential in a postsynaptic neuron. Instead, large numbers of input terminals must discharge on the same neuron either simultaneously or in rapid succession to cause excitation. For instance, in Figure 46-9, let us assume that six terminals must discharge almost simultaneously to excite any one of the neurons. If the student counts the number of terminals on each one of the neurons from each input fiber, he or she will see that input fiber 1 has more than enough terminals to cause neuron a to discharge. The stimulus from input fiber 1 to this neuron is said to be an excitatory stimulus; it is also called a suprathreshold stimulus because it is above the threshold required for excitation.

Input fiber 1 also contributes terminals to neurons b and c, but not enough to cause excitation. Nevertheless, discharge of these terminals makes both these neurons more likely to be excited by signals arriving through other incoming nerve fibers. Therefore, the stimuli to these neurons are said to be subthreshold, and the neurons are said to be facilitated.

Similarly, for input fiber 2, the stimulus to neuron d is a suprathreshold stimulus, and the stimuli to neurons b and c are subthreshold, but facilitating, stimuli.

Figure 46-9 represents a highly condensed version of a neuronal pool because each input nerve fiber usually provides massive numbers of branching terminals to hundreds or thousands of neurons in its distribution "field," as shown in Figure 46-10. In the central portion of the field in this figure, designated by the circled area, all the neurons are stimulated by the incoming fiber. Therefore, this is said to be the discharge zone of the incoming fiber, also called the excited zone or liminal zone. To each side, the neurons are facilitated but not excited, and these areas are called the facilitated zone, also called the subthreshold zone or subliminal zone.

Inhibition of a Neuronal Pool. We must also remember that some incoming fibers inhibit neurons, rather than exciting them. This is the opposite of facilitation, and the entire field of the inhibitory branches is called the inhibitory zone. The degree of inhibition in the center of this zone is great because of large numbers of endings in the center; it becomes progressively less toward its edges.

Facilitation Neuronal Pools

Basic organization of a neuronal pool

Discharge" and "facilitated" zones of a neuronal pool

Basic organization of a neuronal pool

Discharge" and "facilitated" zones of a neuronal pool

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